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 Solar system 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 Sun. 16 Finally, it will allow for both indirect and direct observations of extrasolar giant planets. 17 Ó 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 years, 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 Ó 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.