Ceres, Vesta and Pallas : protoplanets not just asteroids,

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Ceres, Vesta and Pallas : protoplanets not just asteroids,

Mc Cord T.B., C. Russel, C. Sotin, S. Thomas

To cite this version:

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�

Eos, Vol. 87, No. 10, 7 March 2006

Ceres, Vesta, and Pallas:

Protoplanets, Not Asteroids

PAGES 1 0 5 , 1 0 9

O b j e c t s in our solar system are currently thought to have formed by c o n d e n s a t i o n a n d a c c u m u l a t i o n from the gas and dust n e b u l a , out of which the Sun first arose. Dust grains a c c r e t e d to form o b j e c t s of approxi­

mately one-kilometer in size, w h i c h in turn a c c r e t e d to form o b j e c t s of a b o u t 1000 kilo­

meters in size. S o m e of t h e s e o b j e c t s grew to b e c o m e the planets, and formed a c o r e , mantle, and crust layers through radioactive heating, which melted s o m e or all of the material, a n d s u b s e q u e n t differentiation.The d e v e l o p m e n t of large planets c a u s e d gravity perturbations, rearranging the remaining debris and halting the a c c r e t i o n .

Inside five a s t r o n o m i c a l units (AU, where o n e AU is the m e a n d i s t a n c e of the Earth from the S u n ) , only the terrestrial planets remain, e x c e p t for the Asteroid Belt, which is s h e p h e r d e d by Jupiter's gravity. T h e asteroid belt mostly c o n t a i n s m a n y small o b j e c t s that s h o w signs of heating and melting and are probably left over from the mutual destruc­

tion of intermediate-sized o b j e c t s in the a c c r e t i o n c h a i n . T h e s e fragments are what are generally thought of w h e n o n e s p e a k s of asteroids. However, a few of the intermediate, approximately 1000-kilometer-sized o b j e c t s in the asteroid belt s e e m to b e intact: Vesta, Ceres, and Pallas. This article points out that, although these o b j e c t s also have b e e n classi­

fied as asteroids, they instead a p p e a r to b e small planets, or protoplanets. Their exis­

t e n c e today helps confirm this general the­

ory of planet formation, and

provides e v i d e n c e for the study of the terrestrial planet formation p r o c e s s .

Evidence for Ceres' Differentiation Vesta, Ceres, and Pallas orbit in a similar a r e a of the solar system, b e t w e e n 2.36 and 2.77 AU, and are of similar size, b e t w e e n a b o u t 5 0 0 kilometers (Vesta, Pallas) and 1000 kilometers ( C e r e s ) in diameter.Yet they are very different in nature.Their bulk densi­

ties are vastly different: Ceres at 2 1 0 0 kilo­

grams per c u b i c meter ( k g / m³) , Pallas at 2 7 1 0 k g / m³, and at Vesta 3 4 4 0 k g / m³.

Ceres' potential for rewarding exploration was not widely r e c o g n i z e d until recently Its relatively low bulk density, requiring signifi­

c a n t water c o n t e n t , and t e l e s c o p i c observa­

tions showing s o m e hydroxide-bearing sur­

face materials [Lebofsky et al., 1978] should have b e e n c l u e s that Ceres deserved greater attention.That greater attention was trig­

gered in part when a r e c e n t thermal evolu­

tion m o d e l [McCord and Sotin, 2 0 0 5 ] indi-

B Y T. B . M C C O R D , L. A . MCFADDEN, C . T . RUSSELL, C . SOTIN, AND P C . T H O M A S

c a t e d that Ceres might b e highly evolved and differentiated, retaining most of its origi­

nal water, including s o m e still in liquid state (Figure 1 ) .

A c c o r d i n g to the m o d e l , even when only long-lived radioactive heating on Ceres was c o n s i d e r e d , the i c e would have melted and separated from the silicate rock. This process quickly p r o d u c e d a silicate c o r e and a liq­

uid mantle, while retaining a thin frozen sur­

face. T h e melting and freezing of water, the e x o t h e r m i c mineralization of the silicates c a u s e d by the circulating warm water, and the differentiation itself p r o d u c e d expansion and shrinking, making Ceres an active o b j e c t at times during this p r o c e s s .

As the radionuclide heating waned, Ceres c o o l e d and the liquid water froze inward from the outer layers, e x c e p t perhaps for a layer of water n e a r the warm silicate c o r e b o u n d a r y that may exist today. Due to the high water c o n t e n t a n d its large latent heat and c o n v e c t i o n in the liquid and solid water, the silicates in Ceres c a n n o t b e m a d e to melt even if short-lived radioactive nuclides (aluminum-26 (^{2 6}A1) mostly) are c o n s i d e r e d , b e c a u s e the large latent heat of the water a b s o r b s energy and the c o n v e c t i o n in the water removes heat.

This differentiated m o d e l predicts a hydro­

static s h a p e for the spinning Ceres (9.075- hour p e r i o d ) that, if o b s e r v e d , would b e fur­

ther e v i d e n c e for a differentiated Ceres. After the m o d e l was c o m p l e t e d , the s h a p e of Ceres was o b s e r v e d using the Hubble S p a c e T e l e s c o p e Advanced C a m e r a for Surveys (HST-ACS) [Thomas et al., 2 0 0 5 ] . T h e mea­

sured limb s h a p e indicated Ceres is an oblate spheroid, with a x e s of 4 8 7 . 3 ± 1 . 8 by 4 5 4 . 7 ± 1.6 kilometers. A smooth, oblate spheroid is indicative of a hydrostatically controlled s h a p e . T h e difference in long and short a x e s is inconsistent with a h o m o g e ­ n e o u s b o d y and instead requires a central c o n c e n t r a t i o n of mass. This differentiated state, and even the o b s e r v e d difference in polar and equatorial a x e s ( 3 2 . 6 kilometers), are predicted in the modeling study of McCord and Sotin [2005] ( 3 2 kilometers for a protoplanet with a silicate c o r e of serpen­

tine density), a rare a g r e e m e n t b e t w e e n the­

ory and observation.

In addition, the first surface a l b e d o maps of Ceres at three wavelengths were c a l c u ­ lated and analyzed [Li et al., 2006] using the s a m e HST-ACS data set. T h e s e maps reveal 11 surface r e f l e c t a n c e ( a l b e d o ) and c o l o r fea­

tures ranging in s c a l e from 4 0 to 3 5 0 kilome­

ters. This variety, although of smaller range than for s o m e other asteroids and icy satel­

lites, c o u l d indicate a very active and varied surface in the past during which s o m e of the materials were m i x e d with the object's man­

tle. A strong absorption b a n d ( 3 0 % of the c o n t i n u u m r e f l e c t a n c e ) c e n t e r e d at about

2 8 0 n a n o m e t e r s is n o t i c e d for the first time but has not yet b e e n identified, indicating further the exciting nature of Ceres.

Vesta: A Dry Protoplanet With an Iron Core Vesta's state and history were discussed by Keil [ 2 0 0 2 ] , w h o also considers Vesta a pro­

toplanet b e c a u s e it is differentiated with an intact internal structure. The mystery of Vesta was first revealed [McCord et al, 1970] by t e l e s c o p e spectroscopy, which showed that Vesta's surface c o n t a i n e d a low-calcium form of t h e mineral pyroxene and had a sim­

ilar c o m p o s i t i o n of certain basaltic a c h o n - drite meteorites. This implies that Vesta m e l t e d a n d differentiated and that there is a c o n n e c t i o n b e t w e e n Vesta and this type of m e t e o r i t e [McCord et al, 1970].This discov­

ery and c o n n e c t i o n started an extensive study of this o b j e c t , b a s e d partly on the detailed chemistry of basaltic a c h o n d r i t e meteorites, to work out the evolution of Vesta.

Vesta apparently either a c c r e t e d from drier small o b j e c t s than did Ceres or lost its water early in its formation. Without the moderating effect of the water that Ceres contained,Vesta's silicates melted and differ­

entiated quickly, and it now has an iron c o r e , silicate mantle, and a basaltic surface. Vesta c o u l d b e thought of as the smallest terres­

trial planet, very different from its largest sis­

ter, Ceres, which s e e m s more like the evolved icy Galilean satellites of Jupiter. Hubble S p a c e T e l e s c o p e imagery (Figure 3 ) [Zellner and Thomas, 1997] shows a giant crater n e a r Vesta's south p o l e that probably excavated d e e p into the mantle and perhaps exposed s o m e of the iron c o r e material.

McCord and Sotin [ 2 0 0 5 ] , in their thermal m o d e l i n g study, pointed out that Ceres' bulk density implied much water still exists within Ceres, w h e r e a s Vesta has the density of basal­

tic rock and c o u l d not contain much, if any, water in any form.They noted that if the s m a l l e r o b j e c t s (~1 kilometer) from which

Temperature (K)

250 500 750 ¹⁰⁰⁰

400

4-

J 300

3 1200

1 0 0 4

Fig. 1. Temperature as a function of depth 0 below the surface for Ceres with time after accretion, melting of ice, and differentiation, according to models by McCord and Sotin [2005]. Water melts early after accretion and a silicate core forms with a liquid mantle and solid crust. The liquid water then slowly freezes from the surface layer, downward, with perhaps some liquid water remaining today at the core boundary.

Eos, Vol. 87, No. 10, 7 March 2006

Ceres is thought to have formed a c c r e t e d early e n o u g h , they c o u l d c o n t a i n e n o u g h short-lived radioactivity (^{2 6}A1) a n d c r e a t e e n o u g h heat to boil off their water, leaving dry material to form the larger o b j e c t s . T h e s e dry o b j e c t s would then not have the moder­

ating influence of water and would thus grow m u c h hotter, perhaps melting the sili­

c a t e s , as is now s e e n in Vesta. In this way, they pointed out, the d i c h o t o m y b e t w e e n Vesta a n d Ceres c a n b e e x p l a i n e d . Only a few hundred thousand years' difference in a c c r e t i o n of the smaller o b j e c t s that formed the protoplanets c o u l d a c c o u n t for this very different evolution cycle.

Pallas and Future Research

Pallas apparently is an intermediate o b j e c t b e t w e e n Ceres and Vesta and is still mostly unknown. It is about the s a m e size as Vesta, but s p e c t r o s c o p y s h o w s its surface c o m p o s i ­ tion to b e m o r e similar to that of Ceres. It is d e n s e r than Ceres a n d therefore had less water w h e n it was formed or at least retained less water during evolution, but it is m u c h less d e n s e than Vesta and c a n n o t b e m a d e from pure silicate material.

Pallas's orbit is inclined to the ecliptic p l a n e by 35.7 degrees, suggesting a strong gravity interaction a n d perhaps a major col­

lision after formation. B e c a u s e of this highly inclined orbit and the very large c h a n g e in velocity n e e d e d to r e a c h Pallas by s p a c e ­ craft, it is likely to b e unexplored and thus poorly understood for a long time.

T h e s e results are forming the basis for a new and much m o r e c o m p l e t e understand­

ing of this class of o b j e c t s — w h i c h are very different from the fragments normally referred to as asteroids—and indicate that they have undergone planetary p r o c e s s e s . Data from these o b j e c t s c o u l d reveal new

NEWS

In Brief

PAGE 106

E u r o p e a n i c e s a t e l l i t e r e p l a c e m e n t a p p r o v e d T h e European S p a c e Agency (ESA) will build a n d launch a new CryoSat spacecraft to study polar a n d s e a i c e , the a g e n c y a n n o u n c e d on 24 February. T h e first CryoSat was lost on 8 O c t o b e r 2 0 0 5 when the rocket and spacecraft fell into the o c e a n north of Greenland due to p r o b l e m s with the rocket launcher.

At a 2 3 - 2 4 February meeting of the ESA's Earth Observation Programme B o a r d , the a g e n c y received permission from its m e m ­ b e r states for CryoSat-2, which will have the s a m e mission objectives as the satellite it replaces.

• ⁰

i •

Fig. 2. Brightness images of Ceres at several longitudes from the Hubble Space Telescope Advanced Camera for Surveys / T h o m a s et al., 2005]. From these images, Thomas et al.

calculated the shape of Ceres. Note the several albedo features suggesting some composi­

tional variation on the surface.

insights into the formation and evolution of the solar system.

In 2 0 0 1 , NASA had s e l e c t e d the Dawn mis­

sion, which would have visited and investigated Vesta and Ceres starting in 2010. T h e mission was scheduled for launch in J u n e 2006, but NASA c a n c e l l e d the project on 2 March.

References

Keil, K. ( 2 0 0 2 ) , Geological history of asteroid 4 Vesta:

The "smallest terrestrial planet," in Asteroids III, edited by W E B o t t k e Jr.et a l . , p p . 5 7 3 - 5 8 4 , U n i v . o f Ariz. Press, Tucson.

LebofskyL. A. ( 1 9 7 8 ) , Asteroid 1 Ceres: E v i d e n c e for water of hydration,Mon. Not. R.Astron. Soc, 182, 1 7 - 2 1 .

Li, J.-Y, L. A. McFadden, J. W Parker, E. E Young, S. A.

Stern, PC.Thomas, C.T. Russell, and M.V Sykes ( 2 0 0 6 ) , Photometric analysis of 1 Ceres and surface mapping from HST observations, Icarus, in press.

CryoSat-2 is e x p e c t e d to b e l a u n c h e d in March 2 0 0 9 for a three-year m i s s i o n . T h e sat­

ellite will monitor the thickness of i c e on land and s e a . That information c a n b e used for studying how melting polar i c e may affect s e a levels a n d climate c h a n g e . T h e design of its main instrument, the synthetic aperture radar/interferometric radar altime­

ter, will allow detailed m e a s u r e m e n t s of irregular i c e features not possible with ear­

lier altimeters.

Volker Liebig, ESA director of Earth Obser­

vation Programs, said,"This d e c i s i o n is very important, as the scientific c o m m u n i t y in Europe and elsewhere is eagerly awaiting resumption of the CryoSat mission."

— S A R A H ZIELINSKI, Staff Writer

I c e c o r e c o u l d b e o l d e s t e v e r r e c o v ­ e r e d T h e J a p a n e s e Antarctic R e s e a r c h Expe-

Fig. 3. This image of Vesta is derived from a shape model by Zellner and T h o m a s / 1 9 9 7 ] . Considerable topography is evident, especially the large crater at the southern pole.

McCord.T. B., a n d C.Sotin ( 2 0 0 5 ) , Ceres: Evolution and current s t a t e , i Geophys. Res., 110, E 0 5 0 0 9 , d o i : 1 0 . 1 0 2 9 / 2 0 0 4 J E 0 0 2 2 4 4 .

McCord,T. B., J. B.Adams, a n d T V J o h n s o n ( 1 9 7 0 ) , Asteroid Vesta: Spectral reflectivity a n d c o m p o s i ­ tional implications, Science, 168,1445-1447.

Russell, C.T., et al. ( 2 0 0 4 ) , Dawn: A journey in s p a c e and time,Planet. Space Sci., 5 2 ( 4 ) , 4 6 5 - 4 8 9 . Thomas, P C , J. W Parker, L. A. McFadden, C.T. Russell,

S. A. Stern, M.V Sykes a n d E.EYoung ( 2 0 0 5 ) , Dif­

ferentiation of the asteroid Ceres a s revealed by its shape, Nature, 437, 1 - 3 , doi: 10.1038.

Zellner, B . H . , a n d P C . T h o m a s , ( 1 9 9 7 ) , Press R e l e a s e 1997-27, S p a c e Telescope Sci. Inst., Baltimore, Md.

Author Information

T h o m a s B. McCord, B e a r Fight Center, S p a c e S c i e n c e Institute,Winthrop,Wash.; E-mail: m c c o r d ® a o l . c o m ; Lucy A. McFadden, University of Maryland, College Park, Md.; Christopher T.Russell, Institute of Geophysics a n d Planetary Physics a n d Department of Earth a n d S p a c e S c i e n c e s , University of Califor­

nia, Los Angeles; Christophe Sotin, Laboratory d e Planetologie et Geodynamique, University of Nantes, Nantes, France; a n d Peter C.Thomas, Cornell Univer­

sity, Ithaca N.Y

dition on 23 J a n u a r y drilled to a depth of 3 0 2 9 meters at J a p a n ' s D o m e Fuji Station in east Antarctica a n d r e c o v e r e d i c e estimated to b e o n e million years old.

If the age is verified, the i c e c o r e will b e the oldest i c e ever retrieved, 2 0 , 0 0 0 years older than the o n e from East Antarctica's D o m e C.The c o r e will b e dated using i c e flow law models, c o s m i c ray analysis, geo­

m a g n e t i c time markers, a n d c o m p a r i s o n s to o c e a n s e d i m e n t c o r e s .

"We wanted to extract the oldest i c e in the world," said Hideaki Motoyama, an asso­

c i a t e professor at J a p a n ' s National Institute of Polar R e s e a r c h (NIPR), w h o was c h i e f driller for the 2 0 0 5 - 2 0 0 6 austral s u m m e r s e a ­ son. "I e s p e c i a l l y want to k n o w the relation­

ship b e t w e e n g r e e n h o u s e gases a n d air tem­

perature b e t w e e n 8 0 0 , 0 0 0 a n d o n e million years ago." To analyze this, oxygen a n d hydro­

gen isotopes, dust size a n d distribution, pol­

len, gas bubbles, a n d o t h e r a s p e c t s of the c o r e will b e studied.

In a d d i t i o n , s c i e n t i s t s will u s e t h e c o r e to r e s e a r c h t h e c l i m a t e a n d e n v i r o n m e n ­ tal i m p a c t s of t h e B r u n h e s - M a t u y a m a