Ceres, Vesta and Pallas : protoplanets not just asteroids, Mc Cord T.B., C. Russel, C. Sotin, S. Thomas

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Mc Cord T.B., C. Russel, C. Sotin, S. Thomas. Ceres, Vesta and Pallas : protoplanets not just asteroids,. Eos, Transactions American Geophysical Union, American Geophysical Union (AGU), 2006, in press. ￿hal-00112660￿

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HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Copyright Eos, Vol. 87, No. 10, 7 March 2006

280 nanometers is noticed for the first time Ceres, Vesta, and Pallas: but has not yet been identified, indicating further the exciting nature of Ceres.

Protoplanets, Not Asteroids Vesta: A Dry Protoplanet With an Iron Core

PAGES 105,109 cated that Ceres might be highly evolved Vesta's state and history were discussed by and differentiated, retaining most of its origi­ Keil [2002], who also considers Vesta a pro­ Objects in our solar system are currently nal water, including some still in liquid state toplanet because it is differentiated with an thought to have formed by condensation (Figure 1). intact internal structure. The mystery of Vesta and accumulation from the gas and dust According to the model, even when only was first revealed [McCord et al, 1970] by nebula, out of which the Sun first arose. Dust long-lived radioactive heating on Ceres was telescope spectroscopy, which showed that grains accreted to form objects of approxi­ considered, the ice would have melted and Vesta's surface contained a low-calcium mately one-kilometer in size, which in turn separated from the silicate rock. This process form of the mineral pyroxene and had a sim­ accreted to form objects of about 1000 kilo­ quickly produced a silicate core and a liq­ ilar composition of certain basaltic achon- meters in size. Some of these objects grew to uid mantle, while retaining a thin frozen sur­ drite meteorites. This implies that Vesta become the planets, and formed a core, face. The melting and freezing of water, the melted and differentiated and that there is a mantle, and crust layers through radioactive exothermic mineralization of the silicates connection between Vesta and this type of heating, which melted some or all of the caused by the circulating warm water, and meteorite [McCord et al, 1970].This discov­ material, and subsequent differentiation.The the differentiation itself produced expansion ery and connection started an extensive development of large planets caused gravity and shrinking, making Ceres an active object study of this object, based partly on the perturbations, rearranging the remaining at times during this process. detailed chemistry of basaltic achondrite debris and halting the accretion. As the radionuclide heating waned, Ceres meteorites, to work out the evolution of Inside five astronomical units (AU, where cooled and the liquid water froze inward Vesta. one AU is the mean distance of the Earth from the outer layers, except perhaps for a Vesta apparently either accreted from from the Sun), only the terrestrial planets layer of water near the warm silicate core drier small objects than did Ceres or lost its remain, except for the Asteroid Belt, which is boundary that may exist today. Due to the water early in its formation. Without the shepherded by Jupiter's gravity. The asteroid high water content and its large latent heat moderating effect of the water that Ceres belt mostly contains many small objects that and convection in the liquid and solid water, contained,Vesta's silicates melted and differ­ show signs of heating and melting and are the silicates in Ceres cannot be made to entiated quickly, and it now has an iron core, probably left over from the mutual destruc­ melt even if short-lived radioactive nuclides silicate mantle, and a basaltic surface. Vesta tion of intermediate-sized objects in the (aluminum-26 (26A1) mostly) are considered, could be thought of as the smallest terres­ accretion chain.These fragments are what because the large latent heat of the water trial planet, very different from its largest sis­ are generally thought of when one speaks of absorbs energy and the convection in the ter, Ceres, which seems more like the evolved asteroids. However, a few of the intermediate, water removes heat. icy Galilean satellites of Jupiter. Hubble approximately 1000-kilometer-sized objects This differentiated model predicts a hydro­ Space Telescope imagery (Figure 3) [Zellner in the asteroid belt seem to be intact: Vesta, static shape for the spinning Ceres (9.075- and Thomas, 1997] shows a giant crater near Ceres, and Pallas. This article points out that, hour period) that, if observed, would be fur­ Vesta's south pole that probably excavated although these objects also have been classi­ ther evidence for a differentiated Ceres. After deep into the mantle and perhaps exposed fied as asteroids, they instead appear to be the model was completed, the shape of some of the iron core material. small planets, or protoplanets. Their exis­ Ceres was observed using the Hubble Space McCord and Sotin [2005], in their thermal tence today helps confirm this general the­ Telescope Advanced Camera for Surveys modeling study, pointed out that Ceres' bulk ory of planet formation, and (HST-ACS) [Thomas et al., 2005].The mea­ density implied much water still exists within provides evidence for the study of the sured limb shape indicated Ceres is an Ceres, whereas Vesta has the density of basal­ terrestrial planet formation process. oblate spheroid, with axes of 487.3±1.8 by tic rock and could not contain much, if any, 454.7± 1.6 kilometers. A smooth, oblate water in any form.They noted that if the spheroid is indicative of a hydrostatically smaller objects (~1 kilometer) from which Evidence for Ceres' Differentiation controlled shape.The difference in long and Vesta, Ceres, and Pallas orbit in a similar short axes is inconsistent with a homoge­ Temperature (K) area of the solar system, between 2.36 and neous body and instead requires a central 250 500 750 1000 2.77 AU, and are of similar size, between concentration of mass. This differentiated about 500 kilometers (Vesta, Pallas) and state, and even the observed difference in 1000 kilometers (Ceres) in diameter.Yet they polar and equatorial axes (32.6 kilometers), 400 4- are very different in nature.Their bulk densi­ are predicted in the modeling study of ties are vastly different: Ceres at 2100 kilo­ McCord and Sotin [2005] (32 kilometers for J 300 grams per cubic meter (kg/m3), Pallas at a protoplanet with a silicate core of serpen­ 3 2710 kg/m3, and at Vesta 3440 kg/m3. tine density), a rare agreement between the­ Ceres' potential for rewarding exploration ory and observation. 1200 was not widely recognized until recently Its In addition, the first surface albedo maps relatively low bulk density, requiring signifi­ of Ceres at three wavelengths were calcu­ 1004 cant water content, and telescopic observa­ lated and analyzed [Li et al., 2006] using the same HST-ACS data set. These maps reveal 11 tions showing some hydroxide-bearing sur­ Fig. 1.0 Temperature as a function of depth surface reflectance (albedo) and color fea­ face materials [Lebofsky et al., 1978] should below the surface for Ceres with time after tures ranging in scale from 40 to 350 kilome­ have been clues that Ceres deserved greater accretion, melting of ice, and differentiation, ters. This variety, although of smaller range attention.That greater attention was trig­ according to models by McCord and Sotin gered in part when a recent thermal evolu­ than for some other asteroids and icy satel­ [2005]. Water melts early after accretion and tion model [McCord and Sotin, 2005] indi- lites, could indicate a very active and varied a silicate core forms with a liquid mantle surface in the past during which some of the and solid crust. The liquid water then slowly materials were mixed with the object's man­ freezes from the surface layer, downward, with BY T. B. MCCORD, L. A. MCFADDEN, C.T. RUSSELL, tle. A strong absorption band (30% of the perhaps some liquid water remaining today at C. SOTIN, AND PC.THOMAS continuum reflectance) centered at about the core boundary. Eos, Vol. 87, No. 10, 7 March 2006

Ceres is thought to have formed accreted early enough, they could contain enough short-lived radioactivity (26A1) and create enough heat to boil off their water, leaving • 0 dry material to form the larger objects. These dry objects would then not have the moder­ ating influence of water and would thus grow much hotter, perhaps melting the sili­ cates, as is now seen in Vesta. In this way, they pointed out, the dichotomy between Vesta and Ceres can be explained. Only a i • few hundred thousand years' difference in accretion of the smaller objects that formed Fig. 3. This image of Vesta is derived from a the protoplanets could account for this very shape model by Zellner and Thomas /1997]. different evolution cycle. Considerable topography is evident, especially Fig. 2. Brightness images of Ceres at several the large crater at the southern pole. Pallas and Future Research longitudes from the Hubble Space Telescope Advanced Camera for Surveys /Thomas et Pallas apparently is an intermediate object al., 2005]. From these images, Thomas et al. McCord.T. B., and C.Sotin (2005), Ceres: Evolution between Ceres and Vesta and is still mostly and current state,i Geophys. Res., 110, E05009, calculated the shape of Ceres. Note the several doi:10.1029/2004JE002244. unknown. It is about the same size as Vesta, albedo features suggesting some composi­ McCord,T. B., J. B.Adams, and TV Johnson (1970), but spectroscopy shows its surface composi­ tional variation on the surface. Asteroid Vesta: Spectral reflectivity and composi­ tion to be more similar to that of Ceres. It is tional implications, Science, 168,1445-1447. denser than Ceres and therefore had less Russell, C.T., et al. (2004), Dawn: A journey in space and time,Planet. Space Sci., 52(4),465-489. water when it was formed or at least insights into the formation and evolution of the solar system. Thomas, PC, J. W Parker, L. A. McFadden, C.T. Russell, retained less water during evolution, but it is S. A. Stern, M.V Sykes and E.EYoung (2005), Dif­ much less dense than Vesta and cannot be In 2001, NASA had selected the Dawn mis­ ferentiation of the asteroid Ceres as revealed by its made from pure silicate material. sion, which would have visited and investigated shape, Nature, 437, 1-3, doi: 10.1038. Vesta and Ceres starting in 2010. The mission Zellner, B.H.,and PC.Thomas, (1997), Press Release Pallas's orbit is inclined to the ecliptic 1997-27, Space Telescope Sci. Inst., Baltimore, Md. plane by 35.7 degrees, suggesting a strong was scheduled for launch in June 2006, but gravity interaction and perhaps a major col­ NASA cancelled the project on 2 March. lision after formation. Because of this highly Author Information inclined orbit and the very large change in References Thomas B. McCord, Bear Fight Center, Space velocity needed to reach Pallas by space­ Science Institute,Winthrop,Wash.; E-mail: mccord® craft, it is likely to be unexplored and thus Keil, K. (2002), Geological history of asteroid 4 Vesta: aol.com; Lucy A. McFadden, University of Maryland, poorly understood for a long time. The "smallest terrestrial planet," in Asteroids III, College Park, Md.; Christopher T.Russell, Institute of edited by WEBottke Jr.et al.,pp.573-584,Univ.of Geophysics and Planetary Physics and Department These results are forming the basis for a Ariz. Press, Tucson. of Earth and Space Sciences, University of Califor­ new and much more complete understand­ LebofskyL. A. (1978), Asteroid 1 Ceres: Evidence for ing of this class of objects—which are very water of hydration,Mon. Not. R.Astron. Soc, 182, nia, Los Angeles; Christophe Sotin, Laboratory de different from the fragments normally 17-21. Planetologie et Geodynamique, University of Nantes, Li, J.-Y, L. A. McFadden, J. W Parker, E. E Young, S. A. Nantes, France; and Peter C.Thomas, Cornell Univer­ referred to as asteroids—and indicate that Stern, PC.Thomas, C.T. Russell, and M.V Sykes sity, Ithaca N.Y they have undergone planetary processes. (2006), Photometric analysis of 1 Ceres and surface Data from these objects could reveal new mapping from HST observations, Icarus, in press.

dition on 23 January drilled to a depth of 3029 meters at Japan's Dome Fuji Station in NEWS east Antarctica and recovered ice estimated to be one million years old. If the age is verified, the ice core will be CryoSat-2 is expected to be launched in the oldest ice ever retrieved, 20,000 years In Brief March 2009 for a three-year mission.The sat­ older than the one from East Antarctica's ellite will monitor the thickness of ice on Dome C.The core will be dated using ice land and sea. That information can be used flow law models, cosmic ray analysis, geo­ PAGE 106 for studying how melting polar ice may magnetic time markers, and comparisons to affect sea levels and climate change. The ocean sediment cores. European ice satellite replacement design of its main instrument, the synthetic "We wanted to extract the oldest ice in approved The European Space Agency aperture radar/interferometric radar altime­ the world," said Hideaki Motoyama, an asso­ (ESA) will build and launch a new CryoSat ter, will allow detailed measurements of ciate professor at Japan's National Institute spacecraft to study polar and sea ice, the irregular ice features not possible with ear­ of Polar Research (NIPR), who was chief agency announced on 24 February. The first lier altimeters. driller for the 2005-2006 austral summer sea­ CryoSat was lost on 8 October 2005 when Volker Liebig, ESA director of Earth Obser­ son. "I especially want to know the relation­ the rocket and spacecraft fell into the ocean vation Programs, said,"This decision is very ship between greenhouse gases and air tem­ north of Greenland due to problems with important, as the scientific community in perature between 800,000 and one million the rocket launcher. Europe and elsewhere is eagerly awaiting years ago." To analyze this, oxygen and hydro­ At a 23-24 February meeting of the ESA's resumption of the CryoSat mission." gen isotopes, dust size and distribution, pol­ Earth Observation Programme Board, the len, gas bubbles, and other aspects of the agency received permission from its mem­ —SARAH ZIELINSKI, Staff Writer core will be studied. ber states for CryoSat-2, which will have the In addition, scientists will use the core same mission objectives as the satellite it Ice core could be oldest ever recov­ to research the climate and environmen­ replaces. ered The Japanese Antarctic Research Expe- tal impacts of the Brunhes-Matuyama