R EPORTS The satellite was seen in the 10-s test exposure at 2001-08-29 14:44 UTC. Imme- A Low-Density M-type Asteroid diate confirmation was obtained during the normal sequence of 1-min exposures. The in the Main Belt discovery image of the companion to Kal- liope is shown in Fig. 1A. Because the J. L. Margot* and M. E. Brown telescope tracks at a nonsidereal rate, it is unlikely that a background object would The orbital parameters of a satellite revolving around 22 Kalliope indicate that maintain a constant relative position with the bulk density of this main-belt asteroid is 2.37 Ϯ 0.4 grams per cubic respect to the primary over the course of centimeter. M-type asteroids such as Kalliope are thought to be the disrupted several minutes. We obtained additional metallic cores of differentiated bodies. The lowdensity indicates that Kalliope confirming observations on 31 August and cannot be predominantly composed of metal and may be composed of chon- 1 September. dritic material with ϳ30% porosity. The satellite orbit is circular, suggesting We estimated the size of the satellite by that Kalliope and its satellite have different internal structures and tidal dis- measuring the primary to secondary photon sipation rates. The satellite may be an aggregate of impact ejecta from an earlier flux ratio. We carefully removed an azimuth- collision with Kalliope. ally averaged profile of the primary before obtaining flux measurements for the second- The main belt of asteroids between Mars and the W. M. Keck II telescope on Mauna Kea, ary. We find an average flux ratio of 25 Ϯ 5 Jupiter is known to exhibit a strong compo- Hawaii, on 29 August 2001 (6, 8). Our ob- (a magnitude difference of 3.5) using images sitional gradient as a function of heliocentric servations were made with the adaptive obtained at multiple epochs and with multiple distance (1). This zoning represents an im- optics system (9), which relies on a Shack- detectors. The error bars represent the spread portant constraint on the formation conditions Hartmann wavefront sensor sensitive to visi- in the measurements and not the actual un- (temperature, pressure, and chemistry) of ble light and a 349-element deformable mir- certainty, which could be much larger owing planetesimals in the primordial solar nebula. ror that corrects the infrared light reaching to the difficulty in separating the flux from Asteroids also preserve the signature of the science instruments. We observed at 1.6- the two components. The primary is resolved processes that affect the formation of solid ␮m wavelength with SCAM, a 256 ϫ 256 and its shape is unknown, such that deconvo- planets, including accretion, chemical and HgCdTe detector that is part of the NIRSPEC lution with the variable point-spread function thermal alteration, and differentiation. Under- instrument (10). In a typical observing run, of the adaptive optics system is not practical. standing the compositional structure and we record a 5- to 10-s test exposure to deter- Assuming that both components have similar Х heating history of the asteroids in the main mine the desired integration time, ensuring albedos, the ratio of radii is therefore Rp/Rs belt provides valuable clues to the formation that the detector does not saturate. We then 5 (within a factor of 2), where subscripts p history of the solar system. nod the telescope to four different offset po- and s refer to primary and secondary, respec- Asteroids are classified into types on the sitions and take an exposure with the science tively. If both components have the same Х basis of their visible and near-infrared reflec- target centered in each one of four quadrants. bulk density, the mass ratio Mp/Ms 125. tance properties (2). Surface compositions This procedure allows for sky subtraction and The position of the secondary with respect can be inferred from those properties, but the minimizes the impact of detector defects. For to the primary was measured on several mineralogical interpretation of the various Kalliope we used six subexposures of 10 s nights over the past few months (Table 1). spectral types is not unique. The M-type class each, resulting in a total integration time of 1 The angular separation and position angle of asteroids are characterized by a moderate min for each one of the quadrants. (measured positive east of north) were esti- albedo (ϳ10 to 25%), a flat or slightly red spectral curve, and the general lack of absorp- tion features (3). The spectra of M-type as- teroids are consistent with metallic iron- nickel; hence, they have been traditionally interpreted as the parent bodies of iron mete- orites. However, a combination of spectrally neutral silicates such as enstatite (MgSiO3) and metal grains yield similar spectra (4, 5), such that enstatite chondrite meteorites are also plausible analogs. Here we examined the dynamic properties of the M-type asteroid Kalliope and its recently discovered satellite (6) to determine the density and infer the composition of Kalliope (7). 22 Kalliope is a ϳ181-km-diameter aster- oid that was discovered on 16 November 1852 by J. R. Hind. The existence of its satellite was unknown until observations at Fig. 1. (A) Discovery image of the companion to 22 Kalliope obtained at the W. M. Keck II telescope on 29 August 2001. [The corresponding modified Julian date (MJD) is 52150.] The image has been rotated and scaled to approximately match the figure at right. (B) The apparent orbit of the Division of Geological and Planetary Sciences, Califor- companion about the primary at the epochs of our observations. Astrometric positions of the nia Institute of Technology, Pasadena, CA 91125, secondary with respect to the primary (origin) and their error bars are shown as symbols. The solid USA. lines represent three revolutions at epochs MJD 52150 to 52161, one revolution at epoch MJD *To whom correspondence should be addressed. E- 52253, and one revolution at epoch MJD 52637. Large variations in the apparent orbit are seen mail: [email protected] because of considerable distance and aspect changes during the ϳ500-day span.

www.sciencemag.org SCIENCE VOL 300 20 JUNE 2003 1939 R EPORTS mated by differencing the positions of the well. Because the data do not conflict with at B1950 ecliptic coordinates (21°, Ϫ23°) centroids of the primary and secondary. Re- the hypothesis e ϭ 0, a circular orbit is used and (191°, Ϫ2°). The former is separated moval of a best-guess model for the wings of in the rest of this paper. As seen by an from our orbit pole by 160° whereas the latter the point-spread function did not affect the observer on Earth, the shape of the orbit is an is within 7° of our orbit pole. In an earlier astrometry appreciably. In addition to the ellipse that varies with time (Fig. 1B). work, Magnusson (16) indicated a preference SCAM detector described above, we used the The best-fit P and a (Table 2) yield a total for the second spin solution and gave error NIRC2 detector on Keck II and the PHARO mass for this system of M ϭ 7.36 ϫ 1018 kg. bars of (Ϯ3°, Ϯ5°). Therefore the preferred camera (11) at the 200-inch telescope in Palo- Assuming for now a perfect knowledge of the lightcurve-derived spin pole and our orbital mar, California. Plate scales for those instru- mass of the primary, neglecting the Ͻ1% pole are consistent. We argue below that on ments are 17, 25, and 10 milliarc sec/pixel, mass contribution due to the secondary, and the basis of tidal evolution scenarios, the respectively, and are known to ϳ1% accura- using the IRAS diameter value of 181 km angular separation between spin and orbital cy. We assigned positional uncertainties to [with 5% uncertainties; i.e., twice the error poles is probably near zero. our measurements equivalent to the plate bars reported by Tedesco et al. (14)], we So far we have assumed that the orbital scale. obtain a density for the primary of 2.37 Ϯ plane maintains a constant orientation in in- The program developed for fitting orbits 0.4gcmϪ3. In the next sections, we place ertial space. This may not be valid if there is to range-Doppler observations of binary near- error bars on P, a, and M by examining the nonnegligible orbital precession due to the Earth asteroids (12) was extended to handle dynamical aspects of the Kalliope binary sys- oblateness of the primary. The orbit of a the case of optical observations. This pro- tem. Analysis of the dynamics also allows us secondary around an oblate primary experi- gram solves the two-body problem and takes to propose a likely formation mechanism for ences a regression of the line of nodes at a account of aspect variations due to geocentric the binary and to place constraints on asteroid rate (17) distance variations and motion of the asteroid internal structure and mechanical properties. d⍀ 3 R 2 across the sky. Corrections are also applied We return to the significance of the low- Х ͩ ͪ – nJ2 (1) for light travel time (13). density measurement at the end of the paper. dt 2 a

We initially modeled the dynamics of the The best-fit orbital plane orientation in where n is the mean motion, J2 is the second system with seven parameters: orbital period J2000 equatorial coordinates is at right ascen- degree coefficient in the spherical harmonic P, semi-major axis a, eccentricity e, three sion and declination (RA, DEC) of (196.2°, expansion of the gravitational potential, and Euler angles defining the orientation of the Ϫ3.4°), which corresponds to ecliptic longi- R is the mean radius of the primary. Based on orbital plane and the pericenter location (⍀, i, tude and latitude of (196.2°, ϩ3.2°). It is a triaxial ellipsoid approximation of the pri- ␻), and the epoch of pericenter passage T. All interesting to compare this orientation with mary (15) with axis ratios a/b ϭ 1.32, b/c ϭ solutions compatible with the data yielded possible orientations of the primary spin vec- 1.20, and the assumption of homogeneous Յ Х e 0.015 with error bars of the same order, tor, which have been derived on the basis of density, we infer J2 0.12 and a precession indicating that the orbit might be circular. lightcurve observations. In a synthesis, Mag- period on the order of 8 years. However, Indeed, an analysis of variance showed that nusson et al. (15) rejected prograde solutions because of the near coincidence of the spin five-parameter models (P, a, ⍀, i, T) repre- (i.e., spin vector pointing north of the eclip- and orbital poles, which is both observed and senting circular orbits fit the data equally tic) and maintained two retrograde solutions expected on the basis of tidal evolution, the changes in orbital plane orientation are small. The maximum excursion in orbital orienta- Table 1. Positions of the secondary with respect to the primary, and postfit residuals. The epochs of tion due to the oblateness of the primary is observations are given as modified Julian dates (MJD), the separations between primary and secondary are given in arc seconds (Љ), and the position angles (PA) of the secondary with respect to twice the angular separation between spin the primary are given in degrees. Postfit residuals (observed minus computed) are normalized to the and orbital poles, which is driven to zero by measurement uncertainties. tides (18).

The nonzero J2 results in an advance of the Љ Ϫ ␴ Ϫ ␴ ␻ Х Ϫ ⍀ Epoch (MJD) Sep. ( ) PA(°) (Ox Cx)/ x (Oy Cy)/ y Detector line of apsides at a rate d /dt 2d /dt. This affects the determination of the orbital period, 52150.615 0.480 157.5 0.057 1.002 SCAM because the observed mean motion is not iden- 52152.606 0.566 348.7 Ϫ0.042 0.921 SCAM 52153.570 0.245 72.9 Ϫ0.345 0.303 SCAM tical to the Keplerian mean motion; i.e., that 52159.522 0.503 333.4 Ϫ1.205 0.448 PHARO which would be observed for an orbit around an 52160.508 0.335 31.6 Ϫ0.054 Ϫ0.658 PHARO idealized (spherical, homogeneous) primary of 52161.457 0.546 164.1 Ϫ0.087 Ϫ0.176 PHARO the same mass (19). Because the axial ratios, 52253.261 0.887 350.2 0.451 Ϫ0.053 SCAM Ϫ Ϫ mass distribution, and therefore J2 are poorly 52637.592 0.522 75.5 0.704 0.626 NIRC2 known, we feel that the Keplerian orbital period 52637.678 0.538 66.4 0.585 0.567 NIRC2 2 is not known to better than 1 part in 3J2 (R/a) (ϳ1%; i.e., P ϭ 3.596 Ϯ 0.04 days). The Table 2. Best-fit (␹2 ϭ 0.5) parameter estimates, their formal errors, and correlation matrix. The orbital Keplerian and observed values of a also differ parameters are period P, semi-major axis a, epoch of pericenter passage T, longitude of perihelion ⍀, and as a result of the primary oblateness. The rela- inclination i. The argument of pericenter is zero. The best-fit solution based on our first six observations tive uncertainty that we adopt for a is three alone yields parameters within 1% of those and a mass within 3% of our current best estimate, albeit times the formal uncertainty or ϳ2%; i.e., a ϭ with larger formal errors. 1063 Ϯ 23 km. With those error bars on P and a, the relative uncertainty on our mass determi- Parameter Estimate PaT⍀ i nation is ϳ6%. Our density measurement and P (days) 3.59567 Ϯ 0.0001 1.00 uncertainties remain unchanged at 2.37 Ϯ 0.4 g a (km) 1063 Ϯ 7.6 Ϫ0.20 1.00 cmϪ3, because the error bars are largely domi- T (MJD) 52152.014 Ϯ 0.01 Ϫ0.86 0.21 1.00 ⍀ Ϯ Ϫ Ϫ nated by uncertainties in the size of the primary. (°) 286.2 1 0.32 0.22 0.25 1.00 With the lightcurve-derived primary spin i (°) 93.4 Ϯ 1 Ϫ0.24 Ϫ0.02 0.31 0.46 1.00 period of ϳ4.15 hours (20), one can show

1940 20 JUNE 2003 VOL 300 SCIENCE www.sciencemag.org R EPORTS that the orbital angular momentum represents have a low Q. There is also evidence that can sustain a volume of void space that is twice only a ϳ15% fraction of the primary spin elastic moduli decrease as a function of dam- the volume of material. For instance, the least angular momentum. Therefore, it is conceiv- age in fragmented or shock-damaged rocks efficient packing of uniformly sized spheres able that the total angular momentum of the (25). The fact that the orbit is circular implies yields a porosity of only 48%. The highest system was exclusively in the form of spin that ␮Q is smaller for the secondary than for porosity reported for an asteroid with accurate angular momentum before formation of the the primary, and that tidal dissipation is more size/mass determination is ϳ50% for 253 binary (21). The secondary may have formed effective in the secondary than in the primary. Mathilde (29), although few asteroid porosities close to the primary and gradually evolved This suggests that the secondary may be a are known (28, 30). It would take a 3␴ error in outward due to tidal interactions. The prima- gravitationally bound aggregate formed by the IRAS size determination (14) to make our ry loses spin angular momentum (the length the reaccumulation of impact ejecta after a density measurement compatible with a dis- of day increases) as the secondary gains or- collision on the primary. rupted metal core with 50% porosity, which we bital angular momentum (the orbit expands), Kaula (26) showed that inclination damp- also consider unlikely (31). Therefore, we con- just as in the Earth-Moon system. The time ing occurs as the orbit expands as a result of clude that Kalliope, an M-type asteroid, is not scale for orbital expansion depends linearly tides raised by the secondary on the primary. predominantly composed of metal and is not a

on Qp, the tidal dissipation factor (number of Because the rate of orbital expansion is pro- progenitor of iron meteorites. (That the asteroid Ϫ cycles to damp), and inversely on k2p, the portional to the (11/2) power of a, the would be composed of a low-density mantle response coefficient to a centrifugal potential. evolution of a and i was most rapid early on. and high-density metallic crust is also extreme- The subscript p indicates quantities related to The binary has been asymptotically ap- ly unlikely.) If Kalliope were instead an undif- the primary, because the relevant tides are proaching the end-state of tidal evolution, ferentiated body composed of chondritic mate- those raised by the secondary on the primary. where the system is fully despun (doubly rial (grain density of ϳ3.5gcmϪ3), the re- For relatively small bodies like Kalliope and synchronous similar to Pluto-Charon) with a quired porosity would be ϳ30%, typical of

its companion, k2p is largely dictated by elas- secondary orbiting in the equatorial plane of fractured or even loosely consolidated asteroids tic forces rather than gravitational forces, and the primary. The alignment of the spin and (28).

hence k2p is inversely proportional to the orbital poles, within observational uncertain- A chondritic composition would be more ␮ rigidity or shear modulus p. Tidal evolution ties, is therefore not surprising. The fact that consistent with recent inferences based on spec- scenarios allow us to place useful constraints the satellite orbits in the same sense of rota- troscopic data (32, 33). Ten out of 27 M-type on asteroid material properties. If the second- tion as that of the spin of Kalliope places asteroids observed by Rivkin et al. (32), includ- ary formed close to the primary ϳ4.6 billion additional constraints on possible formation ing Kalliope, are reported to have an absorption years ago, one can use the standard tidal scenarios. There would be no preference for feature at 3 ␮m that the authors interpret as Х evolution formula (17) to constrain k2p/Qp the sense of rotation in the case of gravita- diagnostic of water of hydration. This finding 4 ϫ 10Ϫ7. For Q ϭ 100, typical of silicate- tional capture of two fragments following a appears to contradict the iron core hypothesis ␮ Х rich bodies (22), one finds a rigidity p fully disruptive collision. But impact ejecta because hydrated minerals would not survive 5 ϫ 1010 Pa, in agreement with values ex- reaccumulating into a satellite after a sub- the elevated temperatures required for differen- pected for rock under overburden pressures catastrophic collision may preferentially tiation. However, explaining water of hydration from 0 to ϳ10 MPa (23). The pressure at the adopt an orbit in the same sense of rotation as on the parent bodies of enstatite chondrites that center of Kalliope is ϳ6.4 MPa. that of the surviving primary. formed in a reducing environment is not entire- The evolution of the eccentricity is a com- It is fruitful to compare the orbital evo- ly straightforward either. Hardersen et al. (33) peting process between tides raised on the lution time scale to that required for the report the detection of a weak 0.9-␮m feature primary, which tend to raise the eccentricity, secondary to reach spin-lock, where the on at least 4 M-type asteroids, which they and tides raised on the secondary, which tend spin and orbital periods are identical. The interpret as diagnostic of orthopyroxene, also to decrease the eccentricity (24): ratio depends on material properties exactly indicative of a reducing environment. The

5 5 as in (3), which is less than unity given that spectroscopic data therefore do not provide a de 57 k2p Ms Rp 21 k2s Mp Rs ϭ ͩ ͪ neϪ ͩ ͪ ne e ϭ 0. We find that the time scale for unique fit to any particular mineralogic com- dt 8 Q M a 2 Q M a p p s s spin-lock is an order of magnitude less than position (34). Radar observations provide an (2) the orbital evolution time scale, regardless obvious way to distinguish metal-rich objects

Using the inverse dependence of the k2 Love of the formation age of the system; hence, (35). The radar data indicate that some M- number on rigidity, the condition for eccen- we expect the secondary to be synchro- type asteroids are largely metallic whereas tricity damping becomes nously rotating. Satellites with rotation pe- others are largely rock (36), consistent with riods that have been synchronized by tidal the dichotomy observed by Rivkin et al. (32). 19 R ␮ Q p s s Ͻ forces are expected to reach a Cassini state. Kalliope has a radar cross section that is ␮ 1 (3) 28 Rs p Qp The synchronous rotation state may be ev- typical of a stony object (37). This is an interesting constraint because it ident in radar or lightcurve data. With a allows us to compare the mechanical proper- flux that is ϳ25 less than that of the pri- References and Notes ties of the primary and secondary, especially mary, the secondary is not expected to have 1. J. Gradie, E. Tedesco, Science 216, 1405 (1982). if we find that most main-belt asteroid satel- affected the lightcurve analysis, but its ro- 2. S. J. Bus, F. Vilas, M. A. Barucci, Asteroids III,W.F. lites are on circular orbits, and especially for tation period may nevertheless appear as a Bottke, A. Cellino, P. Paolicchi, R. Binzel, Eds. (Univ. of Arizona Press, Tucson, AZ, 2002), pp. 169–182. systems with large Rp/Rs ratios. Because self- faint modulation to the lightcurve. 3. The M type is one of the most common asteroid gravity is negligible compared to elasticity in The density of the primary (2.37 Ϯ 0.4 g spectral types after the C types (flat spectra sugges- the tidal response of these bodies, one does cmϪ3) is about a third that of the most com- tive of carbon-rich material) and S types (red spectra not expect a dependence of Q on size (24). monly accepted meteorite analogs (27). Be- with absorption features indicative of silicates). 4. M. J. Gaffey, J. Geophys. Res. 81, 905 (1976). However, the Q of the two components may cause iron meteorites have almost zero micro- 5. E. A. Cloutis, M. J. Gaffey, D. G. W. Smith, R. S. J. Ϫ be different if they are structurally different. porosity and a grain density of 7.4 g cm 3 (28), Lambert, J. Geophys. Res. 95, 281 (1990). For instance, a gravitationally bound aggre- reconciling these two densities would imply a 6. J. L. Margot, M. E. Brown, IAU Circ. 7703 (2001). Ϯ 7. The observations are part of a systematic survey to gate may undergo considerable friction be- macroporosity of 68 5%. We find it unlikely search for and characterize binary systems in the main tween individual fragments and therefore that the remnant core of a differentiated object belt of asteroids. An important goal is to compare the

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binary systems in the near-Earth, main-belt, and Kuiper- 44. W. D. Heintz, Astrophys. J. Suppl. 117, 587 (1998). tute of Technology, the University of California, belt populations, because they provide a wealth of 45. B. Viateau, Astron. Astrophys. 354, 725 (2000). and the National Aeronautics and Space Adminis- information about physical properties, formation pro- 46. E. Bowell, T. Gehrels, B. Zellner, Asteroids, T. Gehrels, Ed. tration. The observatory was made possible by the cesses, and collisional environments (38–40). (Univ. of Arizona Press, Tucson, AZ, 1979), pp. 1108–1129. generous financial support of the W. M. Keck Foun- 8. Kalliope was named for the Muse of heroic poetry 47. We are grateful to M. Britton for follow-up obser- dation. We extend special thanks to those of Ha- (41). The authors propose that the companion be vations at Palomar and to P. Goldreich, T. Ahrens, waiian ancestry on whose sacred mountain we are named Linus, who in various Greek mythological and M. Pritchard for insightful discussions. J.L.M. privileged to be guests. Without their generous accounts is portrayed as the son of Kalliope and the thanks S. Kulkarni for financial support. Some of hospitality, many of the observations presented inventor of melody and rhythm. the data presented here were obtained at the here would not have been possible. 9. P. L. Wizinowich et al., Proc. SPIE 4007, 2 (2000). W. M. Keck Observatory, which is operated as a 10. I. S. McLean et al., Proc. SPIE 3354, 566 (1998). scientific partnership among the California Insti- 17 April 2003; accepted 19 May 2003 11. T. L. Hayward et al., Publ. Astron. Soc. Pac. 113, 105 (2001). 12. J. L. Margot et al., Science 296, 1445 (2002). 13. The software has been thoroughly validated with a wide range of binary systems, including near-Earth and main-belt asteroids, the Pluto-Charon system Episodic Tremor and Slip on the (42), the 1998 WW31 KBO binary (43), and binary stars (44). 14. E. F. Tedesco, P. V. Noah, M. Noah, S. D. Price, Astron. Cascadia Subduction Zone: The J. 123, 1056 (2002). 15. P. Magnusson et al., Asteroids, Comets, Meteors 1993 (1994), pp. 471–476. Chatter of Silent Slip 16. P. Magnusson, Icarus 85, 229 (1990). 17. C. D. Murray, S. F. Dermott, Solar System Dynamics Garry Rogers* and Herb Dragert (Cambridge Univ. Press, 1999). 18. The time scales for orbital plane precession due to the influence of the Sun and Jupiter are on the order We found that repeated slowslip events observed on the deeper interface of the of 2500 years and can be neglected. northern Cascadia subduction zone, which were at first thought to be silent, have 19. R. Greenberg, Astron. J. 86, 912 (1981). unique nonearthquake seismic signatures. Tremorlike seismic signals were found to 20. C.-I. Lagerkvist, A. Claesson, Earth Moon Planets 72, correlate temporally and spatially with slip events identified from crustal motion 219 (1996). 21. This is unlike some binary Kuiper belt objects that data spanning the past 6 years. During the period between slips, tremor activity is have so much angular momentum that formation by minor or nonexistent. We call this associated tremor and slip phenomenon episodic breakup of a single body is not conceivable (38). The tremor and slip (ETS) and propose that ETS activity can be used as a real-time amount of orbital angular momentum in the Kalliope system is not sufficient to invoke formation by spin- indicator of stress loading of the Cascadia megathrust earthquake zone. up of the primary beyond the breakup rate and mass shedding, as in the case of near-Earth asteroid bina- The Cascadia subduction zone is a region that region. They have been observed only in the ries (12). has repeatedly ruptured in great thrust earth- subduction zone region and specifically in the 22. P. Goldreich, S. Soter, Icarus 5, 375 (1966). 23. A. M. Dziewonski, D. L. Anderson, Phys. Earth Planet. quakes of moment magnitude greater than 8 same region as the deep slip events. We refer Int. 25, 297 (1981). (1, 2). Recently, slip events have been detect- to this associated tremor and slip phenome- 24. P. Goldreich, Mon. Not. R. Astron. Soc. 126, 257 (1963). ed on the deeper (25- to 45-km) part of the non as episodic tremor and slip (ETS). 25. H. He, T. J. Ahrens, Int. J. Rock Mech. Min. Sci. Geomech. 31, 525 (1994). northern Cascadia subduction zone interface The seismic tremors described here are 26. W. M. Kaula, Rev. Geophys. 2, 661 (1964). by observation of transient surface deforma- different from small earthquakes. The fre- 27. Using data from a 1937 encounter between 16 tion on a network of continuously recording quency content is mainly between 1 and 5 Hz, Psyche and 94 Aurora, Viateau (45) finds that M-type Psyche also has a low density, although the author Global Positioning System (GPS) sites (3). whereas most of the energy in small earth- indicates that there may be biases in the mass de- The slip events occur down-dip from the quakes is above 10 Hz. A tremor onset is termination. currently locked, seismogenic portion of the usually emergent and the signal consists of 28. D. T. Britt, D. Yeomans, K. Housen, G. Consolmagno, As- subduction zone (4), and, for the geographic pulses of energy, often about a minute in teroids III, W. F. Bottke, A. Cellino, P. Paolicchi, R. Binzel, Eds. (Univ. of Arizona Press, Tucson, AZ, 2002), pp. 485–500. region around Victoria, British Columbia, duration. A continuous signal may last from a 29. D. K. Yeomans et al., Science 278, 2106 (1997). (Fig. 1), repeat at 13- to 16-month intervals few minutes to several days. Tremors are 30. J. L. Hilton, Asteroids III, W. F. Bottke, A. Cellino, P. (5). These slips were not accompanied by strongest on horizontal seismographs and Paolicchi, R. Binzel, Eds. (Univ. of Arizona Press, Tuc- son, AZ, 2002), pp. 103–112. earthquakes and were thought to be seismi- move across the seismic network at shear 31. There are independent measurements of the size of cally silent. However, unique nonearthquake wave velocities. A tremor on an individual Kalliope. E. Bowell, T. Gehrels, and B. Zellner (46) signals that accompany the occurrence of slip seismograph is unremarkable and does not obtain a diameter of 175 km with the highest quality code, which gives a density that is 10% higher than have been identified using data from the re- appear different from transient noise due to the result based on the IRAS value, and a porosity of gional digital seismic network. These pulsat- wind or cultural sources. It is only when a 65% if one assumes a metallic composition. ing, tremorlike seismic signals are similar to number of seismograph signals are viewed 32. A. S. Rivkin, E. S. Howell, L. A. Lebofsky, B. E. Clark, D. T. Britt, Icarus 145, 351 (2000). those reported in the forearc region of Japan together that the similarity in the envelope of 33. P. S. Hardersen, M. J. Gaffey, P. A. Abell, Lunar Planet. (6, 7), but the signals observed in Cascadia the seismic signal at each site identifies the Inst. Conf. Abstr. 33, 1148 (2002). correlate temporally and spatially with six signal as ETS (Fig. 1). 34. M. J. Gaffey, E. A. Cloutis, M. S. Kelley, K. L. Reed, Asteroids III, W. F. Bottke, A. Cellino, P. Paolicchi, R. Binzel, Eds. (Univ. deep slip events that have occurred over the The tremor activity migrates along the strike of Arizona Press, Tucson, AZ, 2002), pp. 183–204. past 7 years. At other times, this tremor ac- of the subduction zone in conjunction with the 35. S. J. Ostro et al., Science 252, 1399 (1991). tivity is minor or nonexistent. These tremors deep slip events at rates ranging from about 5 to 36. C. Magri, G. J. Consolmagno, S. J. Ostro, L. A. M. Benner, have a lower frequency content than nearby 15 km per day. Sometimes there is a gradual B. R. Beeney, Meteorit. Planet. Sci. 36, 1697 (2001). 37. C. Magri, M. C. Nolan, personal communication. earthquakes, and they are uncorrelated with migration, but at other times there is a sudden 38. J. L. Margot, Nature 416, 694 (2002). the deep or shallow earthquake patterns in the jump from one region of the subduction fault to 39. J. A. Burns, Science 297, 942 (2002). another. Tremors vary in amplitude, and the 40. W. J. Merline et al., Asteroids III, W. F. Bottke, A. strongest can be detected as far as 300 km from Cellino, P. Paolicchi, R. Binzel, Eds. (Univ. of Arizona Geological Survey of Canada, Pacific Geoscience Cen- Press, Tucson, AZ, 2002), pp. 289–312. tre, 9860 West Saanich Road, Sidney, British Colum- the source region. During an ETS event, tremor 41. L. D. Schmadel, Dictionary of Minor Planet Names bia, Canada V8L 4B2. activity lasts about 10 to 20 days in any one (Springer, Berlin, ed. 4, 1999). 42. D. J. Tholen, M. W. Buie, Icarus 125, 245 (1997). *To whom correspondence should be addressed. E- region and contains tremor sequences with am- 43. C. Veillet et al., Nature 416, 711 (2002). mail: [email protected] plitudes that are at least a factor of 10 larger

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