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DUST from KUIPER BELT COMETS on the EARLY EARTH. M.Maurette1 and A.Morbidelli2, 1CSNSM, Bat

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DUST from KUIPER BELT COMETS on the EARLY EARTH. M.Maurette1 and A.Morbidelli2, 1CSNSM, Bat 64th Annual Meteoritical Society Meeting (2001) 5317.pdf DUST FROM KUIPER BELT COMETS ON THE EARLY EARTH. M.Maurette1 and A.Morbidelli2, 1CSNSM, Bat. 104, 91405 Orsay-Campus, France ([email protected]), 2Observatoire de la Côte d’Azur, BP4229, 06304 Nice Cedex, France. One of the possible interpretation of the lunar This fit between the predicted and measured cratering record argues for a period of heavy bom- composition of the Earth's hydrosphere shows that the bardment (PHBomb), prior to 3.9 Ga, during which the K(t) curve conjectured by Hartmann is validated within cratering rate, K(t), decreased exponentially with time, a factor of 2 to 3, and this is supported by two other approximately by a factor of 2 every 50 Ma (see fig.6.6 additional tests [6]. The resulting huge amplification in ref.1). This decay rate is consistent with a source of (x106) of the impactor flux during the PHBomb peak projectiles in the Kuiper belt. If Kuiper belt objects poses a problem because with the present day (KBOs) dominated the impact rate on the Earth, it is configuration of solar system bodies and nearby stars then plausible that early micrometeorites accreted by the maximum expected value is about 1,000. We tried the young Earth were KBOs interplanetary dust. The to move solar system bodies and nearby stars and/or relative variation of their mass flux with time can be change the formation time intervals of these bodies as approximated by K(t)•Φ0 (see ref. 2) where Φ0 is the to gain an additional factor of 1,000. We did not find present day micrometeorite flux, of about 40,000 any solution as yet and conclude that most models tons/y [3]. This scaling predicts that during the "peak" produce a flux of comets to the Earth during the of the PHBomb (i.e., the first ≈100 Ma of the initial PHBomb peak that is orders of magnitude smaller. phase) the huge mass of KBOs dust deposited on the This might give a hint that the early configuration of 6 8 young Earth (about 10 •Φ0•10 tons) was roughly the solar system was very different from what we can equivalent to a ≈5 km-thick layer, while a similar imagine so far. amount of this dust was volatilised upon atmospheric entry. References: [1] Hartmann W.K. (1999) Moons and There is a good agreement between the isotopic planets, Walsworth, pp.1-428. [2] Maurette M. et al (2000) Planet. Space Scien., 48, 1117-1137. [3] Love (water) and chemical (Ne/N2 ratio) composition of the present day hydrosphere with the corresponding values S.G. et Brownlee D.E. (1993) Science, 262, 550-553. measured in Antarctic micrometeorites (AMMs); fur- [4] Honda H. et al (1991) Nature, 349, 149-151. [5] thermore the solar composition of neon in AMMs fits Gounelle et al (2001) LPS, XXXII, #1626, CD-ROM. the generally held view that the initial neon on the [6] Matrajt et al (2001) LPS, XXXII, #1540 (CD- early Earth has also a solar composition [4]. This al- ROM). ready strongly suggests both that early micrometeorites (i.e., KBOs dust) were similar to AMMs and that they did play a major role in the formation of the hydro- sphere. This is further confirmed using the K(t)•Φ0 scaling to estimate the total amount of volatile species (Ne, N2, H2O and CO2) injected in the early atmos- phere by KBOs dust. With the exception of water which seems to be depleted by a factor of ≈3 the com- putations yield the right amounts of species. The invariance of the micrometeorite composition over a time scale of ≈ 4.5 Ga would imply either that the parent bodies of AMMs are KBOs objects, or that all comets including those found in the Oort cloud have a strikingly similar dust component, which should bear clues about the origin of the solar system [5]. In- deed, the average composition of about 1030 early KBOs micrometeorites which is imprinted in the com- position of the hydrosphere corresponds to the average composition of about 100 individual AMMs with sizes of about 100-200µm, recovered from 50,000 years old Antarctica ices!.
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