ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

New Astronomy Reviews xxx (2004) xxx–xxx www.elsevier.com/locate/newastrev

2 science with SKA F a, b c d 3 B.J. Butler *, D.B. Campbell , I. de Pater , D.E. Gary

4 a NRAO, Socorro, NM, USA 5 b Cornell University, Ithaca, NY, USA 6 c University of California at Berkeley, Berkeley, CA, USA 7 d New Jersey Institute of Technology, Newark, NJ, USA

9 Abstract

10 Radio wavelength observations of solar system bodies reveal unique information about them, as they probe to 11 regions inaccessible by nearly all other remote sensing techniques and wavelengths. As such, the SKA will be an impor- 12 tant telescope for planetary science studies. With its sensitivity, spatial resolution, and spectral flexibility and resolution, 13 it will be used extensively in planetary studies. It will make significant advances possible in studies of the deep atmos- 14 pheres, magnetospheres and rings of the giant planets, atmospheres, surfaces, and subsurfaces of the terrestrial planets, 15 and properties of small bodies, including comets, asteroids, and KBOs. Further, it will allow unique studies of the . 16 Finally, it will allow for both indirect and direct observations of extrasolar giant planets. 17 2004 Published by Elsevier B.V. TED PROO 18

19 1. Introduction probed. Probing these depths yields unique in- 29 sights into the bodies, their composition, physical 30 20 Radio wavelength observations of solar system state, dynamics, and history. The ability to resolve 31 21 bodies are an important tool for planetary scien- this emission is important in such studies. The 32 22 tists. Such observations can be used to probe re- VLA has been the state-of-the-art instrument in 33 23 gions of these bodies which are inaccessible to all this respect for the past 20 , and its power 34 24 other remote sensing techniques. For solid sur- is evidenced by the body of literature in planetary 35 25 faces, depths of up to meters into the subsurface science utilizing its data. With its upgrade (to the 36 26 are probed (the rough rule of thumb is that depths EVLA), it will remain in this position in the near 37 27 to 10 wavelengths are sampled). For giant planet future. However, even with that upgrade, there 38 28 atmospheres, depths of up to 10Õs of bars are are still things beyond its capabilities. For these 39 studies, the SKA is the only answer. We investi- 40 gate the capabilities of SKA for solar system stud- 41 * Corresponding author. ies below, including studies of the Sun. We also 42 E-mail address: [email protected] (B.J. Butler).

1387-6473/$ - see front matter 2004 Published by Elsevier B.V. doi:10.1016/j.newar.2004.09.031UNCORREC ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

2 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx

43 include observations of extra-solar giant planets. Table 2 44 Such investigations have appeared before (for Sensitivities for SKA in nJy and K in 1 h of observing 45 example, in the EVLA science cases in a general m (GHz) DF300 DTB300 DF1000 DTB1000 DF3000 DB3000 46 sense, and more specifically in de Pater (1999)), 0.5 97 2.3 81 22 73 170 47 and we build on those previous expositions here. 1.5 56 1.3 47 12 42 100 5 31 0.7 26 6.8 23 55 25 34 0.8 29 7.6 26 62 48 2. Instrumental capabilities the expected flux density and brightness tempera- 79 49 For solar system work, the most interesting fre- ture noise values, as shown in Table 2. F 80 50 quencies in most cases are the higher ones, since 51 the sources are mostly blackbodies to first order 52 (see discussion below for exceptions). We are very 3. Giant planets 81 53 interested in the emission at longer wavelengths, of 54 course, but the resolution and source detectability Observations of the giant planets in the fre- 82 55 are maximized at the higher frequencies. To frame quency range of SKA are sensitive to both thermal 83 56 the discussion below, we need to know what those and non-thermal emissions. These emissions are 84 57 resolutions and sensitivities are. We take our infor- received simultaneously, and can be distinguished 85 58 mation from the most recently released SKA spec- from each other by examination of their different 86 59 ifications (Jones, 2004). spatial, polarization,PROO time (e.g., for lightning), 87 60 The current specifications for SKA give a max- and spectral characteristics. Given the sensitivity 88 61 imum baseline of 3000 km. Given that maximum and resolution of SKA (see Table 3), detailed 89 62 baseline length, Table 1 shows the resolution of images of both of these types of emission will be 90 63 SKA at three values of the maximum baseline, possible. We note, however, the difficulty in mak- 91 64 assuming we can taper to the appropriate length ing images with a spatial dynamic range of 92 65 if desired. In subsequent discussion, we will trans- >1000 (take the case of Jupiter, with a diameter 93 66 late these resolutions to physical dimensions at the of 140,000 km, and resolution of 100 km) – this 94 67 distances of solar system bodies. willTED be challenging, not only in the measurements 95 68 The specification calls for A/T of 5000 at 200 (good short spacing coverage – down to spacings 96 69 MHz; 20,000 from 500 MHz to 5 GHz; 15,000 at of order meters – is required), but in the imaging 97 70 15 GHz; and 10,000 at 25 GHz. The specification itself. 98 71 also calls for 75% of the collecting area to be with- 72 in 300 km. Let us assume that 90% of the collect- 3.1. Non-thermal emission 99 73 ing area is within 1000 km. The bandwidth 74 specification is 25% of the center frequency, up Non-thermal emissions from the giant planets 100 75 to a maximum of 4 GHz, with two independently at frequencies between 0.15 and 20 GHz are lim- 101 76 tunable passbands and in each polarization (i.e., ited to synchrotron radiation and atmospheric 102 77 16 GHz total bandwidth at the highest frequen- lightning. Both topics have been discussed before 103 78 cies). Given these numbers, we can then calculate Table 3 SKA linear resolution for giant planets a Table 1 Body Distance (AU) Resolution (km) Resolution in masec for SKA m = 2 GHz m = 20 GHz

m (GHz) h300 h1000 h3000 Jupiter 5 120 10 0.5 410 120 40 Saturn 9 210 20 1.5 140 40 14 Uranus 19 420 40 540124Neptune 30 690 70 25UNCORREC 8 3 1 a Assuming maximum baseline of 1000 km. ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 3

104 in connection to SKA by de Pater (1999). We re- JupiterÕs synchrotron radiation has been imaged 151 105 view and update these discussions here. at frequencies between 74 MHz and 22 GHz (see, 152 e.g., de Pater, 1991; de Pater et al., 1997; Bolton 153 106 3.1.1. Synchrotron radiation et al., 2002; de Pater and Butler, 2003; de Pater 154 107 Synchrotron radiation results from energetic and Dunn, 2003). A VLA image of the planetÕs 155 108 electrons (1–100 MeV) trapped in the magnetic radio emission at k = 20 cm is shown in Fig. 156 109 fields of the giant planets. At present, synchrotron 1(a); the spatial distribution of the synchrotron 157 110 emission has only been detected from Jupiter, radiation is very similar at all frequencies (de Pater 158 111 where radiation at wavelengths longer than about and Dunn, 2003). Because the radio emission is 159 112 6 cm is dominated by this form of emission (Berge optically thin, and Jupiter rotates in 10 h,F one 160 113 and Gulkis, 1976). Saturn has no detectable syn- can use tomographic techniques to recover the 161 114 chrotron radiation because the extensive ring sys- 3D radio emissivity, assuming the emissions are 162 115 tem, which is almost aligned with the magnetic stable over 10 h. An example is shown in Fig. 163 116 equatorial plane, absorbs energetic particles 1(b) (Sault et al., 1997; Leblanc et al., 1997; de Pa- 164 117 (McDonald et al., 1980). Both Uranus and Nep- ter and Sault, 1998). The combination of 2D and 165 118 tune have relatively weak magnetic fields, with sur- 3D images is ideal to deduce the particle distribu- 166 119 face magnetic field strengths 20–30 times weaker tion and magnetic field topology from the data 167 120 than Jupiter. Because the magnetic axes make (Dulk et al., 1997; de Pater and Sault, 1998; Dunn 168 121 large angles (50–60) with the rotational axes of et al., 2003). 169 122 the planets, the orientation of the field of Uranus The shape of JupiterÕs radio spectrum is deter- 170 123 with respect to the solar wind is in fact not too dis- mined by the intrinsic spectrum of the synchrotron 171 124 similar from that of Earth (because its rotational radiating electrons, the spatial distribution of the 172 125 pole is nearly in the ecliptic), while the magnetic electrons and JupiterÕs magnetic field. Spectra 173 126 axis of Neptune is pointed towards the Sun once from two different years (1994 and 1998) are 174 127 each . These profound changes in shown in Fig. 2 (de Pater et al., 2003; de Pater 175 128 magnetic field topology have large effects on the and Dunn, 2003). The spectrum is relatively flat 176 129 motion of the local plasma in the magnetosphere shortwards of 1–2 GHz and drops off more steeply 177 130 of Neptune. It is unclear if there is a trapped pop- at higherTED frequencies. PROOAs shown, there are large 178 131 ulation of high energy electrons in the radiation variations over time in the spectrum shortwards 179 132 belt of either planet, a necessary condition for of 1–2 GHz, and perhaps also at the high frequen- 180 133 the presence of synchrotron radiation. Before the cies, where the only two existing datapoints at 15 181 134 Voyager encounter with the planet, (de Pater and GHz differ by a factor of 3. Changes in the radio 182 135 Goertz, 1989) postulated the presence of synchro- spectrum most likely reflect a change in either the 183 136 tron radiation from Neptune. Based on the calcu- spatial or intrinsic energy distribution of the elec- 184 137 lations in their paper, the measured magnetic field trons. The large change in spectral shape between 185 138 strengths, and 20-cm VLA observations (see, e.g., 1994 and 1998 has been attributed to pitch angle 186 139 de Pater et al., 1991a) we would estimate any syn- scattering by plasma waves, Coulomb scattering 187 140 chrotron radiation from the two planets not to ex- and perhaps energy degradation by dust in Jupi- 188 141 ceed 0.1 mJy. This, or even a contribution one or terÕs inner radiation belts, processes which affect 189 142 two orders of magnitude smaller, is trivial to detect in particular the low energy distribution of the 190 143 with the SKA. It would be worthwhile for the electrons. With SKA, we may begin investigating 191 144 SKA to search for potential synchrotron emissions the cause of such variability through its imaging 192 145 off the disks of Uranus and Neptune (and SKA capabilities at high angular resolution, and simul- 193 146 can easily distinguish the synchrotron emission taneous good UV coverage at short spacings. As 194 147 from that from the disk based on the spatial sepa- shown by de Pater (1999), this is crucial for inter- 195 148 ration), since this information would provide a comparison at different frequencies. With such 196 149 wealth of information on the inner radiation belts images we can determine the spatial distribution 197 150 of these planets.UNCORRECof the energy spectrum of electrons, which is 198 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

4 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx

F

Fig. 1. Radio images of JupiterÕs synchrotron emission: (a) Image made from VLA data taken at a frequency of 1450 MHz. Both the

thermal (confined to JupiterÕs disk) and nonthermal emissions are visible. The resolution is 0.3RJ, roughly the size of the high latitude emission regions. Magnetic field lines from a magnetic field model are superposed, shown every 15 of longitude. After de Pater et al. (1997). (b) Three-dimensional reconstruction of the June 1994 data, as seen from Earth. The planetPROO is added as a white sphere in this visualization. After de Pater and Sault (1998).

199 tightly coupled to the (still unknown) origin and 200 mode of transport (including source/loss terms) 201 of the high energy electrons in JupiterÕs inner radi- 202 ation belts. TED 203 3.1.2. Lightning 204 Lightning appears to be a common phenome- 205 non in planetary atmospheres. It has been ob- 206 served on Earth, Jupiter, and possibly Venus 207 (Desch et al., 2002). Electrostatic discharges on 208 Saturn and Uranus have been detected by space- 209 craft at radio wavelengths and are probably caused 210 by lightning. The basic mechanism for lightning 211 generation in planetary atmospheres is believed 212 to be collisional charging of cloud droplets fol- 213 lowed by gravitational separation of oppositely 214 charged small and large particles, so that a vertical 215 potential gradient develops. The amount of 216 charges that can be separated this way is limited; 217 once the resulting electric field becomes strong to 218 ionize the intervening medium, a rapid Ôlightning Fig. 2. The radio spectrum of JupiterÕs synchrotron emission as 219 strokeÕ or discharge occurs, releasing the energy measured in September 1998 (lower curve) and June 1994 220 stored in the electric field. For this process to (upper curve), with high frequency data points from March 1991 (VLA) and January 2001 (Cassini; Bolton et al., 2002). 221 work, the electric field must be large enough, Superposed are model calculations that match the data 222 roughly of theUNCORREC order of 30 V per electron mean free (Adapted from de Pater and Dunn, 2003). ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 5

223 path in the gas, so that an electron gains sufficient de Pater et al., 1991a; de Pater and Mitchell, 269 224 energy while traversing the medium to cause a col- 1993; DeBoer and Steffes, 1996). 270 225 lisional ionization. When that condition is met, a The thermal emission from all four giant plan- 271 226 free electron will cause an ionization at each colli- ets has been imaged with the VLA. To construct 272 227 sion with a gas molecule, producing an exponen- high signal-to-noise images, the observations need 273 228 tial cascade (Gibbard et al., 1999). to be integrated over several hours, so that the 274 229 In EarthÕs atmosphere, lightning is almost al- maps are smeared in longitude and only reveal 275 230 ways associated with precipitation, although sig- brightness variations in latitude. The observed var- 276 231 nificant large scale electrical discharges also iations have typically been attributed to spatial 277 232 occur occasionally in connection with volcanic variations in ammonia gas, as caused by a combi-F 278 233 eruptions and nuclear explosions. By analogy, nation of atmospheric dynamics and condensation 279 234 lightning on other planets is only expected in at higher altitudes. Recently, Sault et al. (2004) 280 235 atmospheres where both convection and conden- developed an algorithm to construct longitude-re- 281 236 sation take place. Moreover, the condensed spe- solved images; they applied this to Jupiter, and 282 237 cies, such as water droplets, must be able to their maps reveal, for the first time, hot spots at 283 238 undergo collisional charge exchange. It is possi- radio wavelengths which are strikingly similar to 284 239 ble that lightning on other planets is triggered those seen in the infrared (Fig. 3). At radio wave- 285 240 by active volcanism (such as possibly on Venus lengths, the hot spots indicate a relative absence of 286 241 or Io). NH3 gas, whereas in the infrared they suggest a 287 242 We believe that SKA would be an ideal instru- lack of cloud particles. The authors showed that 288 243 ment to search for lightning on other planets; the the NH3 abundance in hot spots was depleted by 289 244 use of multiple beams would facilitate discrimina- a factor of 2 relative to the average NH3 abun- 290 245 tion against lightning in our own atmosphere, and dance in the belt region, or a factor of 4 compared 291 246 simultaneous observations at different frequencies to zones. Ammonia must be depleted down to 292 247 would contribute spectral information. For such pressure levels of 5 bar in the hot spots, the 293 248 experiments, one needs high time resolution (as approximate altitude of the water cloud. The algo- 294 249 for pulsars) and the ability to observe over a wide rithm of Sault et al. (2004) only works on short 295 250 frequency range simultaneously, including in par- wavelengthTED data of Jupiter, PROO where the synchrotron 296 251 ticular the very low frequencies (<300 MHz). radiation is minimal. 297 Even the longitudinally smeared images are 298 252 3.2. Thermal emission important in deducing the state of the deep atmos- 299 pheres of the giant planets, as attested by numer- 300 253 The atmospheres of the giant planets all emit ous publications on the giant planets. Here, we 301 254 thermal (blackbody) radiation. At radio wave- discuss specifically the case of Uranus, where radio 302 255 lengths most of the atmospheric opacity has been images made with the VLA since 1981 at 2 and 6 303 256 attributed to ammonia gas, which has a broad cm have shown changes in the deep atmosphere 304 257 absorption band near 22 GHz. Other sources of which appear to be related to the changing insola- 305 258 opacity are collision induced absorption by hydro- tion as the two poles rotate in and out of sunlight 306 259 gen, H2S, PH3,H2O gases, and possibly clouds. over the 40 uranian year. Since the first 307 260 Since the overall opacity is dominated by ammo- images were made, the south pole has appeared 308 261 nia gas, it decreases approximately with m2 for brighter than equatorial regions. In the last dec- 309 262 m < 22 GHz. One therefore probes deeper warmer ade, however, the contrast between the two regions 310 263 layers in a planetÕs atmosphere at lower frequen- and the latitude at which the transition occurs has 311 264 cies. Spectra of all four giant planets have been changed (Hofstadter and Butler, 2003). Fig. 4 312 265 used to extract abundances of absorbing gases, in shows an image from the VLA made from data ta- 313 266 particular NH3, and for Uranus and Neptune, ken in the summer of 2003, along with an image at 314 267 H2S(H2S has been indirectly inferred for Jupiter near-infrared wavelengths (1.6 lm) taken with the 315 268 and Saturn)UNCORREC (see, e.g., Briggs and Sackett, 1989; adaptive optics system on the Keck telescope in 316 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

6 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx

F

Fig. 3. Longitude-resolved image of Jupiter at 2 cm (Sault et al., 2004).

317 October 2003 (Hammel et al., 2004). The VLA im- along the northern edge of the south polar region 329 318 age clearly shows that the south pole is brightest, and descending over the pole, where the low radio 330 319 but it also shows enhanced brightness in the far- opacity is indicative ofPROO dry air. 331 320 north (to the right on the image). At near-infrared With a sensitivity of SKA 2 orders of magni- 332 321 wavelengths Uranus is visible in reflected sunlight, tude better than that of the VLA, and excellent 333 322 and hence the bright regions are indicative of instantaneous UV coverage, images of a planetÕs 334 323 clouds/hazes at high (upper troposphere) altitudes, thermal emission can be obtained within minutes, 335 324 presumably indicative of rising gas (with methane rather than hours. This would enable direct map- 336 325 condensing out). We note that the bright band ping of hot spots at a variety of frequencies, 337 326 around the south pole is at the lower edge of the including low frequencies where both thermal 338 327 VLA-bright south polar region. It appears as if andTED non-thermal radiation is received. We can 339 328 air is rising (with condensibles forming clouds) thus obtain spectra of hot spots, which allow us 340

Fig. 4. Two panels comparing VLA (left, Hofstadter and Butler, 2004) and Keck (right, Hammel et al., 2004) images of Uranus from the summer of 2003. In the radio image, the south (upper left) and north (lower right) poles are both brighter (hotter) than equatorial regions. Note the edge of the radio bright region in the south corresponds to a prominent band in the infrared. The radio bright region in the north has noUNCORREC corresponding band. The faint line across the planet on the right-hand side of the infrared image is the ring system. ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 7

341 to derive the altitude distribution of absorbing electrons). Several groups have gathered and ana- 387 342 gases, something that hitherto could only be ob- lyzed VLA data of SaturnÕs rings over the past dec- 388 343 tained via in situ probes. Equally exciting is the ades (see, e.g., Grossman et al., 1989; van der Tak 389 344 prospect of constructing complete 3D maps of et al., 1999; Dunn et al., 2002). These maps, at fre- 390 345 the ammonia abundance (or total opacity, to be quencies > a few GHz, are usually integrated over 391 346 precise) at pressure levels between 0.5 and 20– several hours, and reveal the classical A, B, and C 392 347 50 bars (these levels vary some from planet to pla- rings including the Cassini Division. Asymmetries, 393 348 net). Will ammonia, and other sources of opacity, such as wakes, have been detected in several maps; 394 349 be homogeneous in a planetÕs deep atmosphere research is ongoing as to correlations between ob- 395 350 (i.e., at pressure levels J 10 bar)? Could there be served asymmetries with wavelength and ring incli-F 396 351 giant thunderstorms rising up from deep down, nation angle. 397 352 bringing up concentrations of ammonia and other With the high sensitivity, angular resolution 398 353 gases from a planetÕs deep atmosphere, i.e., reflect- and simultaneous coverage of short UV spacings, 399 354 ing the true abundance at deep levels? Such scenar- maps of SaturnÕs rings can be improved considera- 400 355 ios have been theorized for Jupiter, but never bly. This would allow higher angular resolution 401 356 proven (Showman and de Pater, 2004). and less longitudinal smearing, allowing searches 402 357 A cautionary note here: although excellent for longitudinal inhomogeneities. In addition, it 403 358 images at multiple wavelengths yield, in principle, may become feasible to detect the uranian ring 404 359 information on a giant planetÕs deep atmosphere, and perhaps even the main ring of Jupiter during 405 360 detailed modeling will be frustrated in part because ring plane crossings. We note that the detection 406 361 of a lack of accurate laboratory data on gases and of the Jupiter ring is made difficult by being so 407 362 clouds that absorb at microwave frequencies, such faint and close to an extremely bright Jupiter. 408 363 as NH3 and H2O. This severely limits the precision 364 at which one can separate contributions from dif- 365 ferent gases. Planetary scientists are in particular 4. Terrestrial plants 409 366 eager to deduce the water abundance in a planetÕs 367 deep atmosphere (e.g., Jupiter). The potential of Radio wavelength observations of the terrestrial 410 368 deriving the water abundance in the deep atmos- planetsTED (Mercury, Venus, PROO the Moon and Mars) are 411 369 phere of Jupiter from microwave observations important tools for determining atmospheric, sur- 412 370 was reviewed by de Pater et al. (2004), while Jans- face and subsurface properties. For surface and 413 371 sen et al. (2004) investigated the potential of using subsurface studies, such observations can help 414 372 limb darkening measurements on a spinning space- determine temperature, layering, thermal and elec- 415 373 craft. These studies show that it might be feasible trical properties, and texture. For atmospheric 416 374 to extract limits on the water abundance in the studies, such observations can help determine tem- 417 375 deep atmosphere, but only if the absorption profile perature, composition and dynamics. Given the 418 376 of water and ammonia gas is accurately known. sensitivity and resolution of SKA (see Table 4), de- 419 tailed images of both of these types of emission 420 377 3.3. Rings will be possible. We note, however, similarly to 421 the giant planet case above, the difficulty in mak- 422 378 Planetary rings emit thermal radiation, but this ing images with a spatial dynamic range of 423 379 contribution is very small compared to the planetÕs >10,000 (take the case of Venus, with a diameter 424 380 thermal emission reflected from the rings. of 12,000 km, and resolution of 1 km). The 425 381 Although all four giant planets have rings, radio Moon is a special case, where mosaicing will likely 426 382 emissions have only been detected from SaturnÕs be required, the emission is bright and compli- 427 383 rings. Other rings are too tenuous to reflect detect- cated, and it is in the near field of SKA (in fact, 428 384 able amounts of radio emissions (JupiterÕs syn- many of the planets are in the near field formally, 429 385 chrotron radiation, though, does reflect the but the Moon is an extreme case). The VLA has 430 386 presence of itsUNCORREC ring via absorption of energetic been used to image the Moon (Margot et al., 431 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

8 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx

Table 4 SKA linear resolution for terrestrial planets. Body Distance (AU) Resolution (km) a m = 1 GHz m = 20 GHz Moon 0.002 0.015 0.004 Venus 0.3 2 0.7 Mercury, Mars 0.6 4 1.3 a Assuming maximum baseline of 1000 km. F 432 1997), and near-field imaging techniques are being 433 advanced (Cornwell, 2004), but imaging of the Fig. 5. Image of Mercury at 1.3 cm made from data taken at 434 Moon will be a challenge for SKA. the VLA (Mitchell and de Pater, 1994). The left panel shows the image, while the right panel shows this image after subtraction 435 4.1. Surface and subsurface of a detailed model. The solid line is the terminator, the circle is Caloris basin. The model does well except at the terminator and in polar regions, most likely because of unmodelled topography 436 The depth to which temperature variations pen- and surface roughness. 437 etrate in the subsurface is characterized by its ther- 438 mal skin depth, where the magnitude of the the 1-km resolution, mapping of the near-surface 468 439 diurnal temperaturepffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi variation is decreased by PROO temperatures of the polar cold spots (inferred from 469 440 1=e : lt ¼ kP=ðpCpqÞ, where k is the thermal 441 conductivity, P is the rotational period, q is the the presence of odd radar scattering behavior – 470 Harmon et al., 2001) will be possible, a valuable 471 442 density and Cp is the heat capacity. For the 443 terrestrial planets, using thermal properties of lu- constraint on their composition. Finally, given 472 444 nar soils and the proper rotation rates, the skin accurate enough (well calibrated, on an absolute 473 445 depths are of order a few cm (Earth and Mars) scale) measurements, constraints on the presence 474 446 to a few 10Õs of cm (Moon, Mercury, and Venus, or absence of an internal dynamo may be placed. 475 447 because of their slow rotation). The 1/e depth to TheTED question of the long wavelength emission 476 448 which a radio wavelength observation at wave- from Venus could be addressed by SKA observa- 477 tions. Recent observations have verified that the 478 449 length k probesffiffi in the subsurface is given by: p emission from Venus at long wavelengths ( J 6 479 450 lr ¼ k=ð2p r tan DÞ, where r is the real part of 451 the dielectric constant, and tan D is the ‘‘loss tan- cm) are well below predicted – by up to 200 K 480 452 gent’’ of the material – the ratio of the imaginary (Butler and Sault, 2003). Fig. 6 shows this graphi- 481 453 to the real part of the dielectric. For all of the ter- cally. There is currently no explanation for this 482 454 restrial planets, given reasonable regolith dielectric 455 constant, this is roughly 10 wavelengths. So, the 456 wavelengths of SKA are well matched to probing 457 both above and below the thermal skin depths of 458 the terrestrial planets. 459 The thermal emission from Mercury has been 460 mapped with the VLA and BIMA by Mitchell 461 and de Pater (1994), who determined that not only 462 was the subsurface probably layered, but that the 463 regolith is likely relatively basalt free. Fig. 5 shows 464 a VLA observation, compared with the detailed 465 model of Mitchell and de Pater. Observations with Fig. 6. Microwave brightness temperature spectrum of Venus, from Butler and Sault (2003). The depression of the measured 466 SKA will further determine our knowledge of emission compared to models at long wavelengths, up to 200 K, 467 these subsurfaceUNCORREC properties. Furthermore, given is evident. ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 9

483 depression. Resolved images at long wavelengths (including at very small scales), and temporal var- 529 484 (say 500 MHz, where the resolution of SKA is of iation of composition (which could be used as 530 485 order 100 km at the distance of Venus using only proxy to infer active volcanism, since it is thought 531 486 the 300 km baselines and less, and the brightness that significant amounts of sulfur-bearing mole- 532 487 temperature sensitivity is about 3 K in 1 h) will cules would be released in such events) could be 533 488 help in determining whether this is a global depres- sensed and monitored. 534 489 sion, or limited to particular regions on the planet. Observations of the water in the Mars atmos- 535 490 Although NASA has been sending multiple phere with the VLA have provided important con- 536 491 spacecraft to Mars, there are still uses for Earth- straints on atmospheric conditions and the climate 537 492 based radio wavelength observations. To our of the planet (Clancy et al., 1992). The 22-GHzF 538 493 knowledge, there is currently no planned micro- H2O line is measured, and emission is seen along 539 494 wave mapper for a Mars mission, other than the the limb, where pathlengths are long (this fact is 540 495 deep sounding very long wavelength radar map- key-the resolution of the atmosphere along the 541 496 pers (MARSIS, for example). So observations in limb is critical). Fig. 7 shows an image of this. 542 497 the meter-to-cm wavelength range are still impor- For added sensitivity in these kinds of observa- 543 498 tant for deducing the properties of the important tions (needed to improve the deduction of temper- 544 499 near-surface layers of the planet. Observations of ature and water abundance in the atmosphere), 545 500 the seasonal caps as they form and subsequently only the SKA will help. 546 501 recede would provide valuable constraints on their 502 structure. Observations of the odd ‘‘stealth’’ region 503 (Edgett et al., 1997) would help constrain its com- 5. Large icy bodies 547 504 position and structure, and in combination with 505 imagery constrain its emplacement history. In addition to their odd radar scattering proper- 548 ties (see the Radar section below), the Galilean sat- 549 506 4.2. Atmosphere ellites Europa and Ganymede exhibit unusually 550 low microwave emission (de Pater et al., 1984; 551 507 The Moon and Mercury have no atmosphere to Muhleman et al., 1986; Muhleman and Berge, 552 508 speak of, but Venus and Mars will both benefit 1991TED). Observations with PROO SKA will determine the 553 509 from SKA observations of their atmospheres. deeper subsurface properties of the Galilean satel- 554 510 Short wavelength observations of the venusian lites, Titan, the larger uranian satellites, and even 555 511 atmosphere ([3 cm) probe the lower atmosphere, Triton, Pluto, and Charon. For example, given a 556 512 below the cloud layer ([40 km). Given the abun- resolution of 40 km at 20 GHz (appropriate for 557 513 dance of sulfur-bearing molecules in the atmos- the mean distance to Uranus), maps of hundreds 558 514 phere, and their high microwave opacity, such of pixels could be made of the uranian moons Tita- 559 515 observations can be used to determine the abun- nia, Oberon, Umbriel, Ariel, and Miranda. Push- 560 516 dances and spatial distribution of these molecules. ing to 3000 km baselines, maps of tens of pixels 561 517 Jenkins et al. (2002) have mapped Venus with the could even be made of the newly discovered large 562 518 VLA at 1.3 and 2 cm, determining that the below- KBOs Quaoar and Sedna (Quaoar is estimated 563 519 cloud abundance of SO2 is lower than that inferred to be 40 masec in diameter, Sedna about half 564 520 from infrared observations, and that polar regions that, (Brown and Trujillo, 2004; Brown et al., 565 521 have a higher abundance of H2SO4 vapor than 2004)). These bodies are some 10Õs of K in physical 566 522 equatorial regions, supporting the hypothesis of temperature, probably with an emissivity of 0.9 567 523 Hadley cell circulation. VLA observations are (by analog with the icy satellites), so with a bright- 568 524 hampered both by sensitivity and spatial dynamic ness temperature sensitivity of a few K in a few 569 525 range. The EVLA will solve part of the sensitivity masec beam, SKA should have no problem mak- 570 526 problem, but will not solve the instantaneous spa- ing such maps with an SNR of the order of 10Õs 571 527 tial dynamic range problem-only the SKA can do in each pixel. SKA will be unique in its ability to 572 528 both. GivenUNCORREC SKA observations, cloud features make such maps of these bodies-optical images 573 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

10 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx

F

PROO Fig. 7. Map of water vapor in the Mars atmosphere made from data taken at the VLA in 1991. The background is the thermal

emission from the surface. The contours are the H2O emission, seen only along the limb. From Clancy et al. (1992).

574 will come nowhere near this resolution unless state and low internal density (see, e.g., the discus- 594 575 space-based interferometers become a reality. sions in Burns, 1992; Rivkin et al., 2002). The two 595 moons could also have been formed via impact of 596 a large asteroid into Mars, which could also have 597 576 6. Small bodies helpedTED in forming the north-south dichotomy on 598 the planet (Craddock, 1994). Determination of 599 577 Perhaps the most interesting solar system sci- the properties of the surface and near-surface 600 578 ence with SKA will involve the smaller bodies in could help unravel this mystery. These bodies are 601 579 the solar system. Because of their small size, their 10 km in diameter, so at opposition will be 602 580 emission is weak, and they have therefore not been 30 masec in apparent diameter, so SKA will be 603 581 studied very extensively, particularly at longer able to map them with a few 10Õs of pixels on the 604 582 wavelengths. Such bodies include the smaller satel- moons. This will provide some of these important 605 583 lites, asteroids, Kuiper Belt Objects (KBOs), and properties and their variation as a function of loca- 606 584 comets. These bodies are all important probes of tion on the moons (notably regolith depth and 607 585 solar system formation, and will yield clues as to thermal and electrical properties). As another 608 586 the physical and chemical state of the protoplane- example, consider the eight outer small jovian sat- 609 587 tary and early planetary environment, both in the ellites, about which little is known, either physi- 610 588 inner and outer parts of the solar system. cally or chemically. All eight of them, with 611 diameters of from 15 to 180 km (Himalia), could 612 589 6.1. Small satellites be resolved by SKA at 20 GHz, determining their 613 shapes as well as their surface and subsurface 614 590 It is sometimes hypothesized that Phobos and/ properties. 615 591 or Deimos are captured asteroids because of sur- We note, however, that the imaging of these 616 592 face spectral reflectivity properties. This is incon- small satellites can be challenging, as they are of- 617 593 sistent, however,UNCORREC with their current dynamical ten in close proximity to a very bright primary 618 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 11

619 which may have complex brightness structure. As with diameter >20 km just in the IRAS survey 665 620 such, even with the specification that SKA must (Tedesco et al., 2002). 666 621 have a dynamic range of 106, it will not be trivial 622 to make images of these small, relatively weak 6.3. Near earth asteroids 667 623 satellites. In addition to being important remnants of so- 668 624 6.2. Main belt asteroids lar system formation, NEAs are potential hazards 669 to us here on Earth (Morbidelli et al., 2002). As 670 625 The larger of the main belt asteroids are the such, their characterization is important (Cellino 671 626 only remaining rocky protoplanets (bodies of or- et al., 2002). SKA will easily detect and image suchF 672 627 der a few hundred to 1000 km in diameter), the asteroids. As they pass near the Earth, they are 673 628 others having been dispersed or catastrophically typically at a brightness temperature of 300 K, 674 629 disrupted, leaving the comminuted remnants com- and pass at a distance of a few lunar radii 675 630 prising the asteroid belt today (Davis et al., 1979). (0.005 AU). This distance gives diameters of 6, 676 631 They have experienced divergent evolutionary 60, and 600 masec for NEAs of 10, 100, and 677 632 paths, probably as a consequence of forming on 1000 m, with flux densities of 0.005, 0.5, and 50 678 633 either side of an early solar system dew line beyond mJy lJy at k = 1 cm. Again, these will be easily de- 679 634 which water was a significant component of the tected and mapped. ALMA will also be an impor- 680 635 forming bodies. Vesta is thought to have accreted tant instrument for observing these bodies (Butler 681 636 dry, consequently experiencing melting, core for- and Gurwell, 2001), but it is the combination of 682 637 mation, and volcanism covering its surface with the data from ALMA and SKA that allows a com- 683 638 basalt (Drake, 2001). Ceres and Pallas, thermally plete picture of the surface and subsurface proper- 684 639 buffered by water never exceeding 400K, experi- ties to be formed. 685 640 enced aqueous alteration processes evidenced by 641 clay minerals on their surfaces (Rivkin, 1997). 6.4. Kuiper belt objects 686 642 These three large MBAs all reach apparent sizes 643 of nearly 100 at opposition, so maps with hundreds In general KBOs are detected at optical/near-IR 687 644 of pixels across them can be made, with high SNR wavelengthsTED in reflected PROO sunlight. Since the albedo 688 645 (brightness temperature is of order 200 K, while of comet Halley was measured (by spacecraft) to 689 646 brightness temperature sensitivity is of order a be 0.04, comets/KBOs are usually assumed to have 690 647 few K). Such maps will directly probe regolith a similar albedo (which most likely is not true). 691 648 depth and properties across the asteroids, yielding This assumed albedo is then used to derive an esti- 692 649 important constraints on formation hypotheses. mate of the size based on the magnitude of the re- 693 650 SKA will also be able to detect and map the flected sunlight. Resolved images, and hence size 694 651 smaller MBAs. Given the distances of the MB estimates, only exist of the largest KBOs (see, 695 652 As to the Sun, they typically have surface/subsur- e.g., Brown and Trujillo, 2004). The only other 696 653 face brightness temperatures (the brightness tem- possibility (ignoring occultation experiments-Coo- 697 654 perature is just the physical temperature ray, 2003) to determine the size is via the use of 698 655 multiplied by the emissivity) of 200 K. Given a radiometry, where observations of both the re- 699 656 typical distance (at opposition) of 1.5 AU, this flected sunlight and longer wavelength observa- 700 657 gives diameters of 2, 20, and 200 masec for MBAs tions of thermal emission are used to derive both 701 658 of 1, 10, and 100 km radius, with flux densities of the albedo and radius of the object. This technique 702 659 0.3, 30, and 3000 lJy at k = 1 cm. So the larger has been used, for example, for asteroids in the 703 660 MBAs will be trivial to detect and map, but the IRAS sample (Tedesco et al., 2002). Although 704 661 smallest of them will be somewhat more difficult more than 100 KBOs have been found to date, 705 662 to observe (but not beyond the sensitivity of only two have been detected in direct thermal 706 663 SKA-see the discussion above on Instrumental emission, at wavelengths around 1 mm (Jewitt et 707 664 Capabilities).UNCORREC There are more than 1500 MBAs al., 2001; Margot et al., 2002a) – the emission is 708 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

12 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx

709 simply too weak. ALMA will be an extremely ered with rocky material, and for the rocky mate- 754 710 important telescope for observing KBOs (Butler rial whether it was dust (regolith) or solid rock. 755 711 and Gurwell, 2001), but will just barely be able 712 to resolve the largest KBOs (with a resolution of 6.5.2. Ice and dust grain halo 756 713 12 masec at 350 GHz in its most spread out config- Large particles (rocks and ice cubes) are clearly 757 714 uration). SKA, with a resolution of a few masec shed from cometary nuclei as they become active, 758 715 and a brightness temperature sensitivity of a few as shown by radar observations (Harmon et al., 759 716 K, will resolve all of the larger of the KBOs (larger 1999). The properties of these activity byproducts 760 717 than 100 km or so), and will easily detect KBOs are important as they contain information on the 761 718 with radii of 10Õs of km. Combined observations physical structure and composition of the cometsF 762 719 with ALMA and SKA will give a complete picture from which they are ejected. Observations at the 763 720 of the surface, near-surface, and deeper subsurface highest SKA frequencies should be sensitive to 764 721 of these bodies. emission from these large particles (even though 765 they also probe down through them to the nu- 766 722 6.5. Comets cleus), and can thus be used to make images of 767 these particles-telling us what the distribution 768 723 In addition to holding information on solar sys- (both spatially, and the size distribution of the par- 769 724 tem formation, comets are also potentially the ticles) and total mass is, and how it varies with 770 725 bodies which delivered the building blocks of life time. 771 726 (both simple and complex organic molecules) to PROO 727 Earth. As such, they are important astronomical 6.5.3. Coma 772 728 targets, as we would like to understand their cur- Observations of cometary comae will tell us just 773 729 rent properties and how that constrains their what the composition of the comets is-both the gas 774 730 history. to dust ratio, and the relative ratios of the volatile 775 species. Historically, observations of cometary co- 776 731 6.5.1. Nucleus mae at cm wavelengths have been limited to OH, 777 732 Long wavelengths (cm) are nearly unique in but with the sensitivity of SKA, other molecules 778 733 their ability to probe right to the surfaces of active suchTED as formaldehyde (detected in cornet Halley 779 734 comets. Once comets come in to the inner solar with the VLA-Snyder et al., 1989) and CH should 780 735 system, they generally produce so much dust and be observable. The advantage of long wavelength 781 736 gas that the nucleus is obscured to optical, IR, transitions is that we observe rotational transitions 782 737 and even mm wavelengths. At cm wavelengths, of the molecules, which are much easier to under- 783 738 however, one can probe right to (and into) the nu- stand and accurately characterize in the statistical 784 739 cleus of all but the most productive comets. For equilibrium and radiative transfer models (neces- 785 740 example, comet Hale-Bopp was detected with the sary to turn the observed intensities into molecular 786 741 VLA at X-band (Fernandez, 2002). Given nucleus abundances). Millimeter wavelength observations 787 742 sizes of a few to a few 10Õs of km, and distances of of cometary molecules have proven fertile ground 788 743 a few tenths to 1 AU, the flux densities from com- (see, e.g., Biver et al., 2002), but the cm transitions 789 744 etary nuclei should be from about 1lJy to 1 mJy at of molecules are also important as they probe the 790 745 25 GHz-easy to detect with SKA. Multi-wave- most populous energy states, and some unique 791 746 length observations should tell us not only what molecules which do not have observable transi- 792 747 the surface and near-surface density is, but if tions in the mm-submm wavelengths. 793 748 (and how) it varies with depth. These nuclei should The volatile component of comets is 80% 794 749 be roughly 10–100 masec in apparent size, so can water ice, with the bulk of the rest CO2. All other 795 750 be resolved at the high frequencies of SKA. With species are present only in small quantities. As the 796 751 resolved images, in principle it would be possible comet approaches the Sun, the water starts to sub- 797 752 to determine which areas were covered with active limate, and along with the liberated dust forms the 798 753 (volatile) material,UNCORREC i.e., ice, and which were cov- coma and tails. At 1 AU heliocentric distance, the 799 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 13

800 typical escape velocity of the water molecules is 1 likely due to quenching of the population inver- 830 801 km/s, and the lifetime against dissociation is about sion from collisions in the inner coma. Similar to 831 802 80000 sec, which leads to a water coma of radius the case for water above, SKA will resolve out 832 803 80000 km. Although there have been some claims most of this emission. It will certainly provide val- 833 804 of direct detection of the 22 GHz water line in uable observations of the distribution of OH in the 834 805 comets, a very sensitive search for this emission inner coma, but not much better than is possible 835 806 from Hale-Bopp detected no such emission (Gra- with the VLA currently. The real power of the 836 807 ham et al., 2000). With SKA, such observations SKA will be in observations of background 837 808 should be possible and will likely be attempted. sources amplified by the OH in the coma. The 838 809 The problem is that the resolution is too high-with technique is described in Butler et al. (1997) Fand 839 810 such a large coma, most of the emission will be re- was demonstrated successfully for Hale-Bopp 840 811 solved out. (Butler and Palmer, 1997). Fig. 9 shows example 841 812 Most of the water dissociates into H and OH. spectra. As a comet moves relative to a back- 842 813 The hydroxyl has a lifetime of 1.6 · 105 sec at 1 ground source, the OH abundance along a chord 843 814 AU heliocentric distance, implying a large OH through the coma is probed. Since the SKA will 844 815 coma-of order 10Õ apparent size at 1 AU geocentric be sensitive to very weak background sources, 845 816 distance. The OH is pumped into disequilibrium many such sources should be available for tracking 846 817 by solar radiation, and acts as a maser. As such, at any time, providing a nearly full 2-D map of the 847 818 the emission can be quite bright, and is regularly coma at high resolution (each chord is sampled 848 819 observed at cm wavelengths by single dishes as it along a pencil beam through the coma with diam- 849 820 amplifies the galactic or cosmic background eter corresponding to the resolution of the interfer- 850 821 (Schloerb and Gerard, 1985). Since the spatial ometer at the distance of the comet). Combined 851 822 scale is so large, however, VLA observations of with single-dish observations, this should provide 852 823 cometary OH have been limited to observations a very accurate picture of the OH in cometary 853 824 of only a few comets-Halley, Wilson, SL-2, and comae. 854 825 Hale-Bopp (de Pater et al., 1991b; Butler and Pal- Among the five most common elements in com- 855 826 mer, 1997). Fig. 8 shows a VLA image of the OH etary comae, the chemistry involving nitrogen is 856 827 emission from comet Halley. Though scant, these oneTED of the least well understood PROO (along with sul- 857 828 observations have helped demonstrate that the fur). In addition, ammonia is particularly impor- 858 829 OH in cometary comae is irregularly distributed, tant in terms of organic precursor molecules, and 859

Fig. 8. Images of the OH emission from comet Halley made with the VLA at low (left) and high (right) resolution. From de Pater et al. (1991b). UNCORREC ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

14 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx

F

PROO TED Fig. 9. Spectra of the OH emission from comet Hale-Bopp made as the comet occulted background sources. From Butler and Palmer (1997).

860 can also be used as a good thermometer for the 10 smaller than the water coma). However, if the 876 861 location where comets formed-whether the nitro- individual elements are relatively small, and have 877 862 gen is in N2 or NH3 depends on the temperature any reasonable single dish capability, the NH3 878 863 of the local medium, among other things (Charn- may still be detected. 879 864 ley and Rodgers, 2002). Ammonia has a rich 865 microwave spectrum which has been extensively 866 observed in interstellar molecular clouds (see, 7. Radar 880 867 e.g., Ho and Townes, 1983). Observations of 868 ammonia in cometary comae are therefore poten- Radar observations of solar system bodies con- 881 869 tially very valuable in terms of determining current tribute significantly to our understanding of the 882 870 chemistry and formation history. Recently, comets solar system. Radar has the potential to deliver 883 871 Hyakutake and Hale-Bopp were observed in NH3 information on the spin and orbit state, and the 884 872 (Bird et al., 1997; Hirota et al., 1999; Butler et al., surface and subsurface electrical properties and 885 873 2002). Observations of cometary NH3 will suffer texture of these bodies. The two most powerful 886 874 from the same problem as the H2O and OH - the current planetary radars are the 13 cm wavelength 887 875 NH3 coma isUNCORREC large (although about a factor of system on the 305 m Arecibo telescope and the 3.5 888 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 15

889 cm system on the 70 m Goldstone antenna. A ra- Imaging with the current planetary radar sys- 914 890 dar that made use of the SKA for both transmit- tems is achieved by either measuring echo power 915 891 ting and receiving the echo would have a as a function of the target bodyÕs delay dispersion 916 892 sensitivity many hundreds of times greater than and rotationally induced Doppler shift-delay-Dop- 917 893 the Arecibo system, the most sensitive of the two pler mapping-or by using a radio astronomy syn- 918 894 current systems. However, while, in theory, it thesis interferometer system to spatially resolve a 919 895 would be possible to transmit with all, or a sub- radar illuminated target body. Delay-Doppler 920 896 stantial fraction of, the SKA antennas, the addi- mapping of nearby objects such a Near Earth 921 897 tional complexity of controlling transmitters at asteroids (NEAs) can achieve resolutions as high 922 898 each antenna, providing adequate power and solv- as 15 m (Fig. 10) but such images suffer fromF 923 899 ing atmospheric phase problems makes this option ambiguity (aliasing) problems due to two or more 924 900 potentially prohibitively expensive. Used with the locations on the body having the same distance 925 901 Arecibo antenna as a transmitting site, an Are- and velocity relative to the radar system. Synthesis 926 902 cibo/SKA radar system would have 30 to 40 times imaging of radar illuminated targets provides 927 903 the sensitivity of the current Arecibo planetary ra- unambiguous plane-of-sky images but, to date, 928 904 dar accounting for integration time and possible the spatial resolution has been considerably less 929 905 use of a shorter wavelength than 13 cm. If it were than can be achieved by delay-Doppler imaging. 930 906 combined with a specially built transmitting sta- A noted example of the synthesis imaging tech- 931 907 tion (100 m antenna equivalent size, 5 MW of nique was the discovery of water ice at the poles 932 908 transmitted power, 3 cm wavelength) the SKA of Mercury by using the Goldstone transmitter in 933 909 would have 150 to 200 times the sensitivity of the combination with the Very Large Array (VLA), 934 910 current Arecibo system. This sensitivity would another is the discovery of the so-called ‘‘Stealth’’ 935 911 open up new areas of solar system studies espe- region on Mars by that same combined radar (Fig. 936 912 cially those related to small bodies and the satel- 11). As discussed below, using the SKA as a syn- 937 913 lites of the outer planets. thesisTED instrument will not PROO provide adequate spatial 938

Fig. 10. A shape model for the 0.5 km NEA 6489 Golevka derived from Arecibo delay-Doppler images (Hudson et al., 2000; Chesley et al., 2003). UNCORREC ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

16 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx

F

Fig. 11. Images made with the combined Goldstone + VLA radar instrument. Mars (left) observations done in October 1988. Mercury (right) observations done in August 1991. After (Muhleman et al., 1991; Butler et al., 1993; Muhleman et al., 1995).

939 resolution for studies of Near Earth Objects of the surface of Mars. For the polar ice deposits 968 940 (NEOs) but it will resolve them, mitigating the ef- on Mercury the sensitivity would allow sub-km 969 941 fects of ambiguities in delay-Doppler imaging. resolution, significantlyPROO better than the 2 km Are- 970 cibo delay-Doppler imagery of Harmon et al. 971 942 7.1. Terrestrial planets (2001). However, this will require the capability 972 to perform delay-Doppler imaging within the 973 943 At the distances of the closest approaches of SKAÕs synthesized beam areas. 974 944 Mercury, Venus and Mars to the Earth, the spatial 945 resolution of a 3,000 km baseline SKA at 10 GHz 7.2. Icy satellites 975 946 will be approximately 1 km, 0.5 km and 0.7 km, 947 respectively. The SKA-based radar system would RadarTED is uniquely suited to the study of icy sur- 976 948 be capable of imaging the surface of Mercury at faces in the solar system and a SKA based system 977 949 1 km resolution with a 1.0-sigma sensitivity limit would provide images (or at least detections) of 978 950 corresponding to a radar cross-section per unit these bodies in the parameters responsible for their 979 951 area of about 30 db, good enough to map to very unusual radar scattering properties. As shown by 980 952 high incidence angles. For Mars, the equivalent recent Arecibo radar observations of Ipetus, the 981 953 spatial resolution for the same sensitivity limit third largest moon of Saturn, the radar reflection 982 954 would be <1 km. The very high absorption in properties of icy bodies can be used to infer surface 983 955 the Venus atmosphere at 10 GHz would reduce chemistry in that pure ice surfaces can be distin- 984 956 the echo strength and, hence, limit the achievable guished from ones which in corporate impurities 985 957 resolution. However, short wavelength observa- such as ammonia that suppresses the low loss vol- 986 958 tions would complement the longer 13 cm imagery ume scattering properties of the ice (Black et al., 987 959 from Magellan, provide additional information 2004) The unusual radar scattering properties of 988 960 about the electrical properties of the surface via the Galilean satellites have been known for some 989 961 studies of the polarization properties of the echo time (Campbell et al., 1978; Ostro et al., 1992). 990 962 (Haldemann et al., 1997; Carter et al., 2004), and As such, they are inviting targets for a SKA radar. 991 963 monitor the surface for signs of current volcanic At a distance to the jovian system of 4.2 AU, the 992 964 activity. For both Mercury and Mars, radar smallest spatial size of the SKAs synthesized beam 993 965 images at 1 km resolution would potentially be would be about 6 km while, given the very high 994 966 of great interest for studying regolith properties backscatter cross-sections of the icy Galilean satel- 995 967 on Mercury andUNCORREC probing the dust that covers much lites, signal-to-noise considerations would allow 996 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 17

997 imaging with about 5 km resolution, a good match tem. Internal structure and collisional histories, 1043 998 to the size of the synthesized beam. Depending on important for solar system formation theories, 1044 999 the prospects for NASAs proposed Jupiter Icy can be deduced from measurements of asteroid 1045 1000 Moons Mission (JIMO) and its instrument pay- sizes and shapes and from detailed imagery of their 1046 1001 load, radar images of the icy moons at resolutions surfaces. Variations in the reflection properties of 1047 1002 of a few km would provide unique information main belt asteroids with distance from the sun 1048 1003 about the regoliths/upper surface layers of the icy could pinpoint the transition region from rocky 1049 1004 satellites. Past radar observations of Titan have to icy bodies, again important for theories of solar 1050 1005 been instrumental in shaping our ideas of what re- system formation. There is also considerable 1051 1006 sides on the surface there-the existence of a deep, uncertainty as to the size distribution of cometsF 1052 1007 global methane/ethane ocean was disproved that a SKA based radar system could resolve. 1053 1008 (Muhleman et al., 1990), but recent Arecibo radar Bernstein et al. (2004) have pointed out that there 1054 1009 observations have provided evidence for the possi- is a significant shortage of KBOs at small sizes if 1055 1010 ble presence of small lakes or seas (Campbell et al., comets have nuclei that are in the 10 km range 1056 1011 2003) The Cassini mission, just arriving in the sat- as currently thought. 1057 1012 urnian system, will make radar reflectivity meas- 1013 urements of Titan, but they will not be global, 7.3.2. Near earth asteroids 1058 1014 nor will the resolution be as fine as desired. At a and characterization would be ma- 1059 1015 distance of 8.0 AU, the spatial resolution of a jor objectives of an SKA based radar system. 1060 1016 3000 km baseline SKA at 10 GHz will be approx- NEAs are of great interest due to their potential 1061 1017 imately 12 km-global radar imagery at this scale hazard to the Earth, as objectives for future 1062 1018 would be a powerful tool for studying the surface manned space missions to utilize their resources 1063 1019 and subsurface of this enigmatic body. Given the and as clues to the early history of the solar sys- 1064 1020 extreme sensitivity of the SKA for radar observa- tem. Astrometry and measurements of their sizes 1065 1021 tions, it would even be possible to make detections and spin vectors will greatly reduce the uncertain- 1066 1022 of Triton and Pluto. At the distances of these bod- ties in projecting their future orbits including non- 1067 1023 ies, it will probably not be possible to make re- gravitational influences such as the Yarkovsky ef- 1068 1024 solved images of them (although theoretically it fectTED (Fig. 12). Measurements PROO of the shapes, sizes 1069 1025 is possible, given the SKA resolution) we can still and densities will provide insights as into the inter- 1070 1026 at least measure the bulk properties of their sur- nal structure of NEOs, important both for under- 1071 1027 faces and make crude hemispherical maps. standing their history and also for designing 1072 1028 A SKA system could also be used to investigate mitigation methods should an NEO pose a signif- 1073 1029 the radar scattering properties of some of the smal- icant threat to Earth. Unambiguous surface ima- 1074 1030 ler satellites of Jupiter, Saturn and Uranus. It will gery at resolutions of a few meters will give 1075 1031 be possible to investigate the radio wavelength insights into their collisional histories while the 1076 1032 scattering properties of most of the satellites of polarization properties of the reflected echo can 1077 1033 Jupiter, satellites of Saturn with larger than 50 to be used to detect the presence of regoliths. Shapes, 1078 1034 100 km and the five large satellites of Uranus. sizes and surface structure are currently obtained 1079 from multiple aspect angle delay-Doppler images 1080 1035 7.3. Small bodies (Fig. 10 and Hudson, 1993). A radar equipped 1081 SKA will have the capability to image NEOs out 1082 1036 7.3.1. Primary scientific objectives to about 0.3 AU from Earth allowing large num- 1083 1037 While spacecraft have imaged a small number bers to be imaged at resolutions of less than 20 1084 1038 of asteroids and comets, Earth based planetary ra- m. The current Arecibo 13 cm radar system has 1085 1039 dars will be the dominant means for the foreseea- the capability to image NEOs with about 20 m res- 1086 1040 ble future for obtaining astrometry, and olution to distances of approximately 0.05 AU. 1087 1041 determining the dynamical state and physical With over 100 times AreciboÕs current sensitivity, 1088 1042 properties ofUNCORREC small bodies in the inner solar sys- an SKA based radar system could achieve similar 1089 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

18 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx

F

Fig. 12. Prediction error ellipses for the location in time delay (distance) and Doppler shift (line-of-sight velocity) of the 0.5 km NEA 6489 Golevka for an Arecibo observation in 2003 based on not including (SUM1) and including (SUM2) the non- gravitational force known as the Yarkovsky effect. The actual measurement indicated by ‘‘Arecibo astrometry’’ clearly shows that the Yarkovsky effect is important in modifying the orbits PROO of small bodies (from Chesley et al., 2003). Fig. 13. A composite Arecibo delay-Doppler image of the binary near Earth asteroid 2000 DP107 showing the primary body with the location of the secondary on the dates shown in 1090 resolutions at 0.15 to 0.20 AU and much higher 2000. The diameters of the two bodies are about 800 m and 300 1091 resolutions for closer objects. The synthesized m, the orbital radius and period are 2.6 km and 1.76 days, respectively, giving a density for the primary of approximately 1092 beam of the SKA (assuming 3,000 km baseline 1.7 g cm3 (Margot et al., 2002b). Figure courtesy of J.L. 1093 and 10 GHz frequency) has a spatial resolution Margot. 1094 at 0.2 AU of about 300 m, very much larger than TED 1095 the achievable resolution based on the sensitivity 1096 but small enough to mitigate the effects of delay- representative of the general population. An alter- 1115 1097 Doppler ambiguities allowing improved shape native method of estimating densities for NEAs is 1116 1098 modeling and surface imagery. Doppler discrimi- the measurement of the Yarkovsky effect via long 1117 1099 nation in the synthesis imagery will provide the term astrometric observations (Vokrouhlicky et 1118 1100 plane-of-sky direction of the rotation vector (de al., 2004). The size of the effect is dependent on 1119 1101 Pater et al., 1994) and polarization properties will the spin rate, the thermal inertia of the surface 1120 1102 elucidate regolith properties. and the mass. The first two of these can be meas- 1121 1103 Because of their implications for both the com- ured or estimated allowing the mass to be esti- 1122 1104 position and internal structure of asteroids, meas- mated and, hence, the density if the asteroids 1123 1105 urements of densities would be a major objective volume is known via a shape model. 1124 1106 of SKA observations of NEAs. The discovery of 1107 binary NBAs (Fig. 13; Margot et al., 2002b) pro- 7.3.3. Main belt asteroids 1125 1108 vided the first opportunity for direct measure- A SKA based radar system would have a un- 1126 1109 ments of densities for the 10–20% of NEAs that ique ability to measure the properties of small bod- 1127 1110 are estimated to be in binary configurations. How- ies out to the far edge of the main asteroid belt; 1128 1111 ever, while they provide important information sizes, shapes, albedoes and orbital parameters. 1129 1112 about NEA densities, the primary and secondary The current Arecibo radar system has only been 1130 1113 components of these binaries are a particular class able to obtain a shape model for one MBA, Kleo- 1131 1114 of NEAs (MargotUNCORREC et al., 2002b) and are not fully patra (Fig. 14; Ostro et al., 2000) and measure the 1132 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 19

F

Fig. 14. Shape models for the metallic main belt asteroid 216 Kleopatra derived from Arecibo delay-Doppler radar images. The model shows Kleopatra to be 217 km · 94 km · 81 km. It may be the remains of a collision of two former pieces of an ancient asteroids disrupted core (Ostro et al., 2000).

1133 radio wavelength reflection properties of a rela- ber of additional comets will be studied from 1160 1134 tively small number of asteroids near the inner spacecraft such as the already launched Deep Im- 1161 1135 edge of the belt (Magri et al., 2001) plus those pact and Rosetta missions and from potential 1162 1136 for a few of the very largest MBAs such as Ceres new missions such as a successor to the failed 1163 1137 and Vesta (M. Nolan private communication). Contour mission. Direct measurements of the 1164 1138 Main belt issues that a SKA based radar could ad- sizes of three comets have indicated that comet- 1165 1139 dress are: (1) The size distribution of MBAs would ary nuclei have very low optical albedoes and this 1166 1140 provide valuable constraints on material strength hasTED led to an upward PROO revision of the size esti- 1167 1141 and, hence, on collisional evolution models; (2) mates of comets based on measurements of their 1168 1142 Measurement of proportion of MBAs that are in absolute magnitudes. However, the very small 1169 1143 binary systems would provide information about sample means that the distribution of cometary 1170 1144 the collisional evolution of the main belt and albedoes is very uncertain and, hence, there is still 1171 1145 detection of these systems would also provide considerable uncertainty as to the size distribu- 1172 1146 and densities for a large number of MBAs; tion of comets. An SKA based radar system 1173 1147 (3) Astrometry would also provide masses and could resolve this issue which has ramifications 1174 1148 densities via measurements of the gravitational related to the assumed source of short period 1175 1149 perturbation from nearby passes of two bodies comets in the Kuiper belt. Bernstein et al. 1176 1150 and also, for small bodies, from measurements of (2004) have pointed out that there is a significant 1177 1151 the Yarkovsky effect and (4) From radar albedo shortage of KBOs at small sizes if comets have 1178 1152 measurements determine whether there is a switch nuclei that are in the 10 km range as currently 1179 1153 within the main belt from rocky to icy objects and, thought. A SKA based radar would be able to 1180 1154 if so, whether it is gradual or abrupt. image cometary nuclei out to about 1 AU obtain- 1181 ing sizes, shapes, rotation vectors, and actual nu- 1182 1155 7.3.4. Comets cleus surface images. For objects at larger 1183 1156 Spacecraft flybys have provided reasonable de- distances, size estimates will be obtained from 1184 1157 tailed information about three comets, Halley, range dispersion and also from rotation periods 1185 1158 Borelly and Wild, and over the next 1–2 decades, from radar light curves combined with measure- 1186 1159 prior to the completionUNCORREC of the SKA, a small num- ments of Doppler broadening. Over time these 1187 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

20 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx

1188 measurements would be the major source of com- low we will discuss potential contributions for 1230 1189 etary size estimates. SKA. 1231

1190 7.4. Technical issues 8.1. Indirect detection by astrometry 1232

1191 For many bodies, unambiguous plane-of-sky The orbit of any planet around its central 1233 1192 synthesis imagery is superior to delay-Doppler causes that star to undergo a reflexive circular mo- 1234 1193 imagery. Consequently, for both imaging and astr- tion around the star-planet barycenter. By taking 1235 1194 ometric observations, a SKA based radar would advantage of the incredibly high resolution of 1236 1195 need to have the capability to do both traditional SKA, we may be able to detect this motion. Mak-F 1237 1196 radar delay-Doppler observations and synthesis ing the usual approximation that the planet mass is 1238 1197 imaging of radar illuminated objects. For both small compared to the , the stellar orbit 1239 1198 range-Doppler imaging and astrometric observa- projected on the sky is an ellipse with angular 1240 1199 tions of near earth objects, range resolutions of semi-major axis hr (in arcsec) given by: 1241 1200 20 ns or better will be required. At 10 GHz the mp aAU 1201 angular resolution of even the proposed central hr ¼ ; ð1Þ M D 1202 compact array will be smaller than the angular size pc 1203 of some NEOs requiring that for delay-Doppler where mp is the mass of the planet, M* is the mass 1244 1204 and astrometric observations the SKA will still of the star, aAU is the orbital distance of the planet 1245 1205 need to be used in an imaging mode. While adding (in AU), and Dpc is thePROO distance to the system (in 1246 1206 to the complexity of the observations, the small ). 1247 1207 spatial extent of the synthesized beam will greatly The astrometric resolution of SKA, or the 1248 1208 assist in mitigating the ambiguities inherent in de- angular scale over which changes can be discrimi- 1249 1209 lay-Doppler imaging. Delay-Doppler processing nated (U), is proportional to the intrinsic resolu- 1250 1210 will require access to the complex outputs of the tion of SKA, and inversely proportional to the 1251 1211 correlator at a 20 ns or better sampling rate. This signal to noise with which the stellar flux density 1252 1212 requirement may not be dissimilar from that re- is detected (SNR*): 1253 1213 quired for pulsar observations but it will have TED h 1214 the added complexity that the NEOs will be in U ¼ HPBW : ð2Þ 1215 the near field of the SKA. 2 SNR This relationship provides the key to high preci- 1256 sion astrometry: the astrometric accuracy increases 1257 1216 8. Extrasolar giant planets both as the intrinsic resolution improves and also 1258 as the signal to noise ratio is increased. Astrometry 1259 1217 The detection of extrasolar giant planets is one at radio wavelengths routinely achieves absolute 1260 1218 of the most exciting discoveries of astronomy in astrometric resolutions 100 times finer than the 1261 1219 the past decade. Despite the power of the radial intrinsic resolution, and can achieve up to 1000 1262 1220 velocity technique used to find these planets, it is times the intrinsic resolution with special care. 1263 1221 biased to finding planets which are near their pri- As long as the phase stability specifications for 1264 1222 mary and orbiting edge-on. To augment those SKA will allow such astrometric accuracy to be 1265 1223 planets found by searches, detec- achieved for wide angle astrometry, such accura- 1266 1224 tions using astrometry, which are most sensitive cies can be reached. 1267 1225 to planets orbiting face-on, are needed. Many When the astrometric resolution is less than the 1268 1226 researchers are eagerly searching for ways to di- reflexive orbital motion, that is, when U [ hr SKA 1269 1227 rectly detect these planets, so they can be properly will detect that motion. We use the approximation 1270 1228 characterized (only the orbit and a lower limit to that, hHPBW k/Bmax, so that detection will occur 1271 1229 the mass is knownUNCORREC for most extrasolar planets), Be- when: 1272 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 21  1 bursts at long wavelengths. If extrasolar giant 1317 J 5 k mp aAU SNR 10 : ð3Þ planets exhibit the same bursting behavior, SKA 1318 Bmax M Dpc might be used to detect this emission. If such a 1319 5 1275 The factor of 2 · 10 enters in to convert from detection occurred, it would provide information 1320 1276 radians to arcseconds. on the rotation period, strength of the magnetic 1321 1277 Note, however, that astrometric detection of a field, an estimate of the plasma density in the mag- 1322 1278 planet requires that curvature in the apparent stel- netosphere, and possibly the existence of satellites. 1323 1279 lar motion be measured, since linear terms in the The presence of a magnetic field is also potentially 1324 1280 reflex motion are indistinguishable from ordinary interesting for astrobiology, since such a field 1325 1281 stellar . This implies that at the very could shield the planet from the harsh stellar envi-F 1326 1282 minimum, one needs three observations spaced in ronment. Some experiments have already been 1327 1283 time over roughly half of the of done to try to detect this emission (Bastian et al., 1328 1284 the observed system. A detection of a planetary 2000). 1329 1285 system with astrometry would thus require some These bursts come from keV electrons in the 1330 1286 type of periodic monitoring. magnetosphere of the planet. The solar wind 1331 1287 We use the technique described in Butler et al. deposits these electrons, which can subsequently 1332 1288 (2003) to calculate the expected flux density from develop an anisotropy in their energy distribution, 1333 1289 , and whether we can detect their wobble from becoming unstable. When deposited in the auroral 1334 1290 the presence of giant planets. If all of the detecta- zones of the planet, emission results at the gyrofre- 1335 1291 ble stars for SKA (roughly 4300), had planetary quency of the magnetic field at the location of the 1336 1292 companions, how many of them could be detected electron (fg = 2.8Bgauss MHz, for magnetic field 1337 1293 (via astrometry) with SKA? strength Bgauss in G). This kind of emission occurs 1338 1294 We assume that the planets are in orbits with on Earth, Saturn, Jupiter, Uranus, and Neptune in 1339 1295 semimajor axis of 5 AU. We consider 3 masses our solar system. The emission can be initiated or 1340 1296 of planetary companions: 5 times jovian, jovian, modulated by the presence of a satellite (Io, in the 1341 1297 and neptunian. We assume integration times of 5 case of Jupiter). 1342 1298 minutes, at 22 GHz. From the Hipparcos catalog If we took the mean flux density of Jupiter at 30 1343 1299 (Perryman et al., 1997), there are 1000 stars MHzTED (50,000 Jy at 4.5 PROO AU) to 10 pc, the result- 1344 1300 around which a 5*jovian companion could be de- ant emission would only be 0.2 lJy. This is very 1345 1301 tected, 620 stars around which a jovian compan- difficult to detect, given the expected sensitivity 1346 1302 ion could be detected, and 40 stars around which of SKA at the lowest frequencies. However, the 1347 1303 a neptunian companion could be detected. Virtu- emission is variable (over two orders of magni- 1348 1304 ally none of these stars are solar-type. From the tude), some EGPs may have intrinsically more 1349 1305 Gliese catalog (Gliese and Jahreiss, 1988), there radiated power, and if the emission is beamed, 1350 1306 are 1430 stars around which a 5*jovian compan- there is a significant increase in the expected flux 1351 1307 ion could be detected, 400 stars around which a density. 1352 1308 jovian companion could be detected, and 60 The details of the expected radiated power from 1353 1309 stars around which a neptunian companion could this emission mechanism are outlined in Farrell 1354 1310 be detected. Of these, 130 are solar analogs. et al. (1999) and Zarka et al. (2001). We summa- 1355 rize the discussion here. There exists a very good 1356 1311 8.2. Direct detection of gyro-cyclotron emission correlation amongst those planets that emit long 1357 wavelength radio waves between radiated power 1358 1312 Detection of the thermal and synchrotron emis- and input kinetic power from the solar wind. Gi- 1359 1313 sion from Jupiter, taken to distances of the stars, is ven expressions for the solar wind input power 1360 1314 beyond even the sensitivity of SKA unless prohib- and conversion factor, and a prediction of the 1361 1315 itively large amounts of integration time are spent. magnetic moment of a giant planet, we can write 1362 1316 However, JupiterUNCORREC experiences extremely energetic the expected radiated power as: 1363 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

22 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx  x 4=5 M 4=3 d 5=8 9. The sun 1402 P rad 400 ½GW; ð4Þ xj M j dj The Sun is a challenging object for aperture 1403 1366 where x, M, and d are the rotational rate, mass, synthesis, especially over a wide frequency range, 1404 1367 and distance to primary of the planet, and the sub- due to its very wide range of spatial scales (of or- 1405 1368 scripted j quantities are those values for Jupiter. der 1 degree down to 100), its lack of fine spatial 1406 1369 The expected received flux density can then be eas- structure below about 100, its great brightness 1407 1370 ily calculated, assuming isotropic radiation. (quiet Sun flux density can be 106-107 Jy) and var- 1408 1371 The frequency at which the power is emitted is iability (flux density may change by 4–5 orders of 1409 1372 limited at the high end by the maximum gyrofre- magnitude in seconds), and its variety of relevantF 1410 1373 quency of the plasma: fg 2.8Bgauss [MHz], for emission mechanisms (at least three—bremsstrah- 1411 1374 magnetic field strength Bgauss in G. This usually lung, gyroemission, and plasma emission—occur 1412 1375 limits such emissions to the 10Õs of MHz (JupiterÕs regularly, and others may occur during bursts). 1413 1376 cutoff is 40 MHz), but in some cases (for the lar- The key to physical interpretation of solar radio 1414 1377 ger EGPs), can extend into the 100Õs of MHz. For emission is the analysis of the brightness tempera- 1415 1378 this reason, these kinds of experiments might be ture spectrum, and because of the solar variability 1416 1379 better done with LOFAR, but there is still a possi- this spectrum must be obtained over relatively 1417 1380 bility of seeing some of them at the lower end of short times (less than 1 s for bursts, and of order 1418 1381 the SKA frequencies. 10 min for slowly varying quiescent emission). 1419 1382 If we take the current list of EGPs and use the This means that broadPROO parts of the RF spectrum 1420 1383 above formalism to calculate the expected flux must be observed simultaneously, or else rapid fre- 1421 1384 density, we can determine which are the best can- quency switching must be possible. The Sun pro- 1422 1385 didates to try to observe gyrocyclotron emission duces only circularly polarized emission-any 1423 1386 from. In this exercise, we exclude those planets linear component is destroyed due to extreme Far- 1424 1387 with cutoff frequencies <10 MHz (EarthÕs iono- aday rotation during passage through the corona. 1425 1388 spheric cutoff frequency), and those in the galactic High precision and sensitivity in circular polariza- 1426 1389 plane (because of confusion and higher back- tion measurements will be extremely useful in diag- 1427 1390 ground temperature). Table 5 shows the top four nosticsTED of the magnetic field strength and 1428 1391 candidates, from which it can be seen that the direction. 1429 1392 maximum predicted emission is of order a few Through long experience with the VLA and 1430 1393 mJy (note that Farrell et al. (1999) found similar other instruments, it has been found that only an- 1431 1394 values despite using slightly different scaling laws). tenna spacings less than about 6 km are useful, 1432 1395 But, again, this is the mean emission, so bursts which corresponds to a synthesized beam of 1000/ 1433 1396 would be much stronger, and beaming could im- mGHz. This empirical finding agrees with expecta- 1 1397 prove the situation dramatically. Given the mul- tions for scattering in the solar atmosphere (Bas- 1435 1398 ti-beaming capability of SKA, it would be tian, 1994). 1436 1399 productive to attempt monitoring of some of the Given the specifications of the SKA, some un- 1437 1400 best candidates for these kinds of outbursts in an ique solar science can be addressed in niche areas, 1438 1401 attempt to catch one. but only if the system takes account of the de- 1439 mands placed on the instrument as mentioned 1440 above. For flares, the system should be designed 1441 Table 5 with an ALC/AGC time constant significantly less 1442 Four best candidates for EGP gyrocyclotron emission detection than 1 s, should allow for rapid insertion of atten- 1443 Star fg (MHz) Fr (mJy) uation, and should allow for rapid frequency 1444 s Bootes 42 4.8 switching. There will be little use for the beam- 1445 Gliese 86 44 2.3 forming (phased array) mode, since even very 1446 HD 114762 202 0.28 low sidelobes washing over the Sun will dominate 1447 70 Vir 94 0.13 UNCORRECthe signal, and there is no way to predict where a 1448 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 23

1449 small beam should be placed to catch a flare. In baselines will make imaging at the required preci- 1493 1450 synthesis mode, the main advantage of SKA will sion difficult. If SKA has relatively large antennas 1494 1451 be in its high sensitivity to low surface brightness (20 m) and high polarization precision, it will be 1495 1452 variations. The following solar science could be able to add significantly to this important 1496 1453 addressed: measurement. 1497

1454 9.1. Solar bursts and activity 9.3. Coronal mass ejections 1498

1455 The Frequency Agile Solar Radiotelescope Coronal Mass Ejections (CMEs) are an impor- 1499 1456 (FASR) will be designed to do the best possible tant type of solar activity that dominates condi-F 1500 1457 flare-related science, and it is hard to identify un- tions in the interplanetary median and the SunÕs 1501 1458 ique science to be addressed by SKA in this area. influence on the Earth. Understanding CME initi- 1502 1459 However, if SKA is placed at a significantly differ- ation and development in the low solar atmos- 1503 1460 ent longitude than FASR, it can cover the Sun at phere is critical to efforts to understand and 1504 1461 other times and produce useful results. To cover predict the occurrence of CMEs. It is expected that 1505 1462 the full Sun, small antennas (of order 2 m above CMEs can be imaged through their bremsstrah- 1506 1463 3 GHz, and 6 m below 3 GHz) are required. Lar- lung emission, but such emission will be of low 1507 1464 ger antenna sizes, while restricting the field of contrast with the background solar emission. Bas- 1508 1465 view, can also be useful when pointed at the most tian and Gary (1997) determined that the best con- 1509 1466 flare-likely active region. trast should occur near 1 GHz. Although one of 1510 the FASR goals is to observe CMEs, the nearly 1511 1467 9.2. Quiet sun magnetic fields filled aperture and high sensitivity of SKA to low 1512 contrast surface brightness variations can make it 1513 1468 The magnetic geometry of the low solar atmos- very sensitive to CMEs. In addition to following 1514 1469 phere governs the coupling between the chromo- the temporal development of the CME morphol- 1515 1470 sphere/photosphere and the corona. Hence, it ogy, SKA spectral diagnostics can constrain the 1516 1471 plays an important role in coronal heating, solar temperature, density, and perhaps magnetic field 1517 1472 activity, and the basic structure of the solar atmos- withinTED the CME and surrounding PROO structures. 1518 1473 phere. One can uniquely measure the magnetic 1474 field through bremsstrahlung emission of the chro- 1475 mosphere and corona, which is circularly polarized Acknowledgement 1519 1476 due to the temperature gradient in the solar atmos- 1477 phere. At m [ 10 GHz, bremsstrahlung is often Comments from Jean-Luc Margot, Mike No- 1520 1478 swamped by gyroemission, but it dominates at lan, and Steve Ostro were appreciated. 1521 1479 higher frequencies over much of the Sun. By meas- 1480 uring the percent polarization P% and the local 1481 brightness temperature spectral slope n = olog References 1522 1482 Tb/olog m, one can deduce the longitudinal mag- 1483 netic field B‘ = (107/nk)P%, with B‘ in G (Gel- Bastian, T.S., 1994. ApJ 426, 774. 1523 1484 freikh, 2004). To reach a useful range of field Bastian, T.S., Gary, D.E., 1997. JGR 102, 14031. 1524 1485 strengths, say 10 G, the polarization must be meas- Bastian, T.S., Dulk, G.A., Leblanc, Y., 2000. Apj 545, 1058. 1525 1486 ured to a precision of about 0.1–0.2% (since n is Bernstein, G., et al., 2004. Apj, accepted. 1526 1487 typically between 1 and 2). Berge, G.L., Gulkis, S., 1976. In: Gehrels, T. (Ed.), Jupiter. 1527 1488 Both FASR and EVLA will address this science UofA Press. 1528 Bird, M.K., Huchtmeier, W.K., Gensheimer, P., Wilson, T.L., 1529 1489 area, but FASRÕs small (2 m) antennas mean that Janardhan, P., Lemme, C., 1997. A&A 325, L5. 1530 1490 the complex solar surface will have to be imaged Biver, N., et al., 2002. EM&P 90, 323. 1531 1491 over the entire disk with high polarization preci- Black, G.J., Campbell, D.B., Carter, L.M., Ostro, S.J., 2004. 1532 1492 sion, while EVLAUNCORRECÕs relatively small number of Science 304, 553. 1533 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

24 B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx

1534 Bolton, S.J., et al., 2002. Nature 415, 987. Desch, S.J., Borucki, W.J., Russell, C.T., Bar-Nun, A., 2002. 1591 1535 Briggs, F.H., Sackett, P.D., 1989. Icarus 80, 77. Rep. Prog. Phys. 65, 955. 1592 1536 Brown, M.E., Trujillo, C.A., 2004. AJ 127, 2413. Drake, M., 2001. M&PS 36, 501. 1593 1537 Brown, M.E., Trujillo, C.A., Rabinowitz, D., 2004. Apj, in Dulk, G.A., Leblanc, Y., Sault, R.J., Ladreiter, H.P., J.Con- 1594 1538 press. nerney, E.P., 1997. A&A 319, 282. 1595 1539 Burns, J.A., 1992. In: Kieffer, H.H., Jakosky, B.M., Snyder, Dunn, D.E., de Pater, I., Sault, R.J., 2003. Icarus 165, 121. 1596 1540 M.S., Matthews, M.S. (Eds.), Mars. UofA Press. Dunn, D.E., Molnar, L.A., Fix, J.D., 2002. Icarus 160, 132. 1597 1541 Butler, B.J., Muhleman, D.O., Slade, M.A., 1993. JGR 98, Edgett, K.S., Butler, B.J., Zimbelman, J.R., Hamilton, V.E., 1598 1542 15003. . JGR 102, 21545. 1599 1543 Butler, B.J., Beasley, A.J., Wrobel, J.M., Palmer, P., 1997. AJ Farrell, W.M., Desch, M.D., Zarka, P., 1999. JGR 104, 14025. 1600 1544 113, 1429. Fernandez, Y., 2002. EM&P 89, 3. 1601 1545 Butler, B.J., Palmer, P., 1997. BAAS 29, 1040. Gelfreikh, G.B. 2004. In: Gary, D.E., Keller, C.O., (Eds.),F 1602 1546 Butler, B.J., Gurwell, M.A., 2001. In: Wootten, A. (Eds.), Solar and Space Weather Radiophysics. Kluwer, in 1603 1547 Science with the Atacama Large Millimeter Array (ALMA), preparation. 1604 1548 ASP Conference Series, vol. 235. Gibbard, S.G., Levy, E.H., Lunine, J.I., de Pater, I., 1999. 1605 1549 Butler, B.J., Wootten, A., Palmer, P., Despois, D., Bockelee- Icarus 139, 227. 1606 1550 Morvan, D., Crovisier, J., Yeomans, D., 2002. ACM Gliese, W., Jahreiss, H., 1988. In: Philip, A., Upgren, G.D., 1607 1551 meeting, Berlin. A.R., L. (Eds.), Star Catalogues: a Centennial Tribute to 1608 1552 Butler, B.J., Wootten, A., Brown, B., 2003. ALMA Memo, 475. Vyssotsky, A.N. Davis Press. 1609 1553 Butler, B.J., Sault, R.J., 2003. IAUSS IE, 17B. Graham, A.P., Butler, B.J., Kogan, L., Palmer, P., Strelnitski, 1610 1554 Campbell, D.B., Chandler, J.F., Ostro, S.J., Pettengill, G.H., V., 2000. AJ 119, 2465. 1611 1555 Shapiro, I.I., 1978. Icarus 34, 254. Grossman, A.W., Muhleman, D.O., Berge, G.L., 1989. Science 1612 1556 Campbell, D.B., Black, G.J., Carter, L.M., Ostro, S.J., 2003. 245, 1211. 1613 1557 Science 302, 431. Haldemann, A.F.C., Muhleman,PROO D.O., Butler, B.J., Slade, 1614 1558 Carter, L.M., Campbell, D.B., Campbell, B.A., 2004. JGR 109, M.A., 1997. Icarus 128, 398. 1615 1559 E06009. Hammel, H.B., I. de Pater, Gibbard, S., Lockwood, G.W., 1616 1560 Cellino, A., Zappala, V., Tedesco, E.F., 2002. M&PS 37, 1965. 2004. Icarus, in preparation. 1617 1561 Charnley, S.B., Rodgers, S.D., 2002. ApJ 569, L133. Harmon, J.K., Campbell, D.B., Ostro, S.J., Nolan, M.C., 1999. 1618 1562 Chesley, S.R., et al., 2003. Science 302, 1739. P&SS 47, 1409. 1619 1563 Clancy, R.T., Grossman, A.W., Muhleman, D.O., 1992. Icarus Harmon, J.K., Perillat, P.J., Slade, M.A., 2001. Icarus 149, 1. 1620 1564 100, 48. Hirota, T., Yamamoto, S., Kawaguchi, K., Sakamoto, A., 1621 1565 Cooray, A., 2003. ApJ 589, L97. Ukita, N., 1999. ApJ 520, 895. 1622 1566 Cornwell, T., 2004. EVLA Memo, 75. Ho, P.T.P.,TED Townes, C.H., 1983. ARA&A 21, 239. 1623 1567 Craddock, R.A., 1994. LPSC XXV, 293. Hofstadter, M.D., Butler, B.J., 2003. Icarus 165, 168. 1624 1568 Davis, D., Chapman, C., Greenberg, R., Weidenschilling, S., Hofstadter, M.D., Butler, B.J., 2004. Icarus, in preparation. 1625 1569 Harris, A.W., 1979. In: Gehrels, T. (Ed.), Asteroids. UofA Hudson, R.S., 1993. Rem. Sens. Rev. 8, 195. 1626 1570 Press. Hudson, R.S., et al., 2000. Icarus 48, 37. 1627 1571 de Pater, I., Brown, R.A., Dickel, J.R., 1984. Icarus 57, 93. Janssen, M.A., Hofstadter, M.D., Gulkis, S., Ingersoll, A.P., 1628 1572 de Pater, I., Goertz, C.K., 1989. GRL 16, 97. Allison, M., Bolton, S.J., Kamp, L.W., 2004. Icarus, 1629 1573 de Pater, I., 1991. AJ 102, 795. submitted. 1630 1574 de Pater, I., Romani, P.N., Atreya, S.K., 1991a. Icarus 91, 220. Jenkins, J.M., Kolodner, M.A., Butler, B.J., Suleiman, S.H., 1631 1575 de Pater, I., Palmer, P., Snyder, L.E., 1991b. In: Newburn, R.L. Steffes, P.G., 2002. Icarus 158, 312. 1632 1576 Jr.Jr. et al. (Eds.), Comets in the Post-Halley Era. Kluwer. Jewitt, D., Aussel, H., Evans, A., 2001. Nature 411, 446. 1633 1577 de Pater, I., Mitchell, D.L., 1993. JGR 98, 5471. Jones, D.L., 2004. SKA Memo, 45. 1634 1578 de Pater, I., Palmer, P., Mitchell, D.L., Ostro, S.J., Yeomans, Leblanc, Y., Dulk, G.A., Sault, R.J., Hunstead, R.W., 1997. A 1635 1579 D.K., 1994. Icarus 111, 489. &A 319, 274. 1636 1580 de Pater, I., Schulz, M., Brecht, S.H., 1997. JGR 102, 22043. Magri, C., Consolmagno, G.J., Ostro, S.J., L.Benner, A.M., 1637 1581 de Pater, I., Sault, R.J., 1998. JGR 103, 19973. Beeney, B.R., 2001. M&PS 36, 1697. 1638 1582 de Pater, I., 1999. In: van Haarlem, M.P. (Ed.), Perspectives in Margot, J.L., Campbell, D.B., Campbell, B.A., Butler, B.J., 1639 1583 Radio Astronomy: Science with Large Antenna Arrays. . LPSC 28, 871. 1640 1584 ASTRON Press. Margot, J.-L., Trujillo, C., Brown, M.E., Bertoldi, F., 2002a. 1641 1585 de Pater, I., Butler, B.J., 2003. Icarus 163, 428. BAAS 34, 871. 1642 1586 de Pater, I., Dunn, D.E., 2003. Icarus 163, 449. Margot, J.-L., Nolan, M.C., L.Benner, A.M., Ostro, S.J., 1643 1587 de Pater, I., et al., 2003. Icarus 163, 434. Jurgens, R.F., Giorgini, J.D., Slade, M.A., Campbell, D.B., 1644 1588 de Pater, I., DeBoer, D.R., Marley, M., Freedman, R., Young, . Science 296, 1445. 1645 1589 R., 2004. Icarus, submitted. McDonald, F.B., Schardt, A.W., Trainer, J.H.T., 1980. JGR 1646 1590 DeBoer, D.R., Steffes,UNCORREC P.G., 1996. Icarus 123, 324. 85, 5813. 1647 ASTREV 855 No. of Pages 25, DTD = 5.0.1 ARTICLE IN PRESS 21 October 2004; Disk Used

B.J. Butler et al. / New Astronomy Reviews xxx (2004) xxx–xxx 25

1648 Mitchell, D.L., de Pater, I., 1994. Icarus 110, 2. Rivkin, A.S., Brown, R.H., Trilling, D.E., Bell III, J.F., 1664 1649 Morbidelli, A., Jedicke, R., Bottke, W.F., Michel, P., Tedesco, Plassman, J.H., 2002. Icarus 156, 64. 1665 1650 E.F., 2002. Icarus 158, 329. Sault, R.J., Oosterloo, T., Dulk, G.A., Leblanc, Y., 1997. A&A 1666 1651 Muhleman, D.O., Berge, G.L., Rudy, D., Neill, A.E., 1986. AJ 324, 1190. 1667 1652 92, 1428. Sault, R.J., Engel, C., de Pater, I., 2004. Icarus 168, 336. 1668 1653 Muhleman, D.O., Grossman, A.W., Butler, B.J., Slade, Schloerb, P.P., Gerard, E., 1985. AJ 90, 1117. 1669 1654 M.A., 1990. Science 248, 975. Showman, A., de Pater, I., 2004. Icarus, submitted. 1670 1655 Muhleman, D.O., Butler, B.J., Grossman, A.W., Slade, Snyder, L.E., Palmer, P., de Pater, I., 1989. AJ 97, 246. 1671 1656 M.A., 1991. Science 253, 1508. Tedesco, E.F., Noah, P.V., Noah, M., Price, S.D., 2002. AJ 1672 1657 Muhleman, D.O., Berge, G.L., 1991. Icarus 92, 263. 123, 1055. 1673 1658 Muhleman, D.O., Grossman, A.W., Butler, B.J., 1995. ARE van der Tak, F., dePater, I., Silva, A., Millan, R., 1999. Icarus 1674 1659 &PS 23, 337. 142, 125. F 1675 1660 Ostro, S.J., et al., 1992. JGR 97, 18277. Vokrouhlicky, D., Capek, D., Chesley, S.R., Ostro, S.J., 2004. 1676 1661 Ostro, S.J., et al., 2000. Science 288, 836. Workshop on Asteroid Dynamics, Arecibo Observatory. 1677 1662 Perryman, M.A.C., et al., 1997. A&A 323, L49. Zarka, P., Treumann, R.A., Ryabov, B.P., Ryabov, V.B., 2001. 1678 1663 Rivkin, A. 1997. PhD Thesis, UofA. Ap& SS 277, 293. 1679 1680

TED PROO

UNCORREC