sign in sign up
<<
Home , Micrometeorite

Lunar and Planetary Science XXXIII (2002) 1519.pdf

THE CRUCIAL ROLE OF TO IDENTIFY COMETARY MICROMETEORITES FROM HISTORICAL AND FUTURE LEONIDS SHOWERS TRAPPED IN AND GREENLAND SNOWS. R.Cuillierier1, J.Duprat2, M.Maurette2, C.Hammer3. 158 av. S. Bolivar, 75019 Paris, France, 2CSNSM, Bâtiment 104, 91405 Orsay-Campus, France, 3Department of Geophysics, Niels Bohr Institute, Copenhagen, DK- 2100 Denmark. Contact : [email protected]

particles were initially trapped in the same chunk of Rationale for the recovery of dust particles from cometary ices. Leonids showers. Last year we described plans to The testing of EMMA with particles recovered recover ≈ 100µm-size micrometeorites from past and from annual snow layers faces the challenge that the future Leonids showers with a well certified cometary LEOs, which did fall for a few hours at peak intensity, origin ( Temple-Tuttle), from the corresponding will be dispersed in a high background of SPOs annual snow layers deposited in central Greenland and accumulated over a one year period. For example, for Antarctica. Finding such a cometary material the 1966 shower, the peak intensity was about 100,000 "returned" to the would be a corner stone in per hour and lasted for about 6 hours. In this specific planetary sciences, for example because it might case the ratio of LEOs to SPOs in the 1966 snow layer decisively help in identifying the best scenario of solar is expected to be about 5 and 0.5 in central Greenland system formation. Furthermore, we stressed its and Antarctica, respectively (this marked decrease in importance to test the validity of our scenario Antarctica is due to the shallow incidence of the LEOs "EMMA" (for Early MicroMeteorite ) on the ecliptic plane). But the total number of LEOs proposed to explain the formation of the atmosphere recovered per ton of snow would be roughly similar in and oceans of the early Earth during the accretion of a Antarctica and Greenland due to the much smaller gigantic storm of shootings stars, that lasted at least precipitation rates in Antarctica. 100 Ma after the giant -sized impact on the Earth In Antarctica, where the 1966 snow layer appears that did form the while simultaneously blowing at a depth of ≈ 4m along a trench, the testing of away any previously acquired atmosphere [1]. This EMMA would be quite straightforward if our scenario scenario would also help defining the prebiotic is invalid. In this case, SPOs would not be cometary chemistry that led to the birth of life on the early Earth dust grains, and the SPOs micrometeorites collected in and possibly on Mars, and which likely involved the 1966 layer would be very different from the interactions of organics with minerals surface acting as remaining cometary LEOs micrometeorites. But even both adsorbents of organics and catalysts of their in this case, SPOs , which are made of an hydrous- reactions. It also allows to extrapolate the predictions carbonaceous material, cannot originate from the made for the Earth to the other terrestrial . families of still existing to day, as sampled by Over the last few years we accumulated strong carbonaceous collected on the Earth over evidence supporting the validity of EMMA, such as the last century or so [1]. For example, SPOs show right predictions about the total amounts of volatile much higher Ne/N2 ratio, and higher pyroxene to species now found in the hydrosphere and atmosphere olivine ratios (except for the very rare CR2 chondrites) of the Earth. But we decided to conduct a last and than the parent asteroids of the hydrous-carbonaceous definitive test of EMMA, which deals with the chondrites. But if LEOs look similar to SPOs, the prediction that Antarctic micrometeorites originating testing of EMMA would be a hard task because from the sporadic micrometeorite flux (SPOs), and that additional and difficult microanalyses should be we collect since 1987, are indeed cometary dust grains, conducted as to prove that the two populations of which should be very similar to Leonids particles micrometeorites are indeed undistinguishable, before (LEOs). We had to conduct this test because various concluding that EMMA is valid. Fortunately, a new groups, such as Dermott et al and Flynn et al, still simple idea was recently discovered which rendered argue that SPOs originate from asteroids and not from this testing much easier even in this difficult case ! . For this test we have to analyse large cometary dust particles with sizes ≥100µm, quite comparable to A unique neon "genetic" signature of Leonids that of SPOs. We cannot await for the analysis of the particles. LEOs should be drastically different from much smaller micron-sized dust grains to be collected SPOs with regard to effects associated with their much soon during the "Star Dust" mission. Indeed, these tiny shorter exposure to solar radiation in the interplanetary particles might originate from a quite different medium, after their ejection from comet Temple- "fabrics" in the early solar system, and consequently Tuttle. Indeed, the most reliable models to describe could have a composition very different from that of these showers could be identified for the first time in the much more massive SPOs, even if such types of November 1998 through right predictions of the time of occurrence of the 1998 shower. All these models, Lunar and Planetary Science XXXIII (2002) 1519.pdf

SEARCH FOR PAST AND FUTURE "FROZEN" LEONID SHOWERS: J. Duprat, C. Hammer et al.

such as those proposed by Asher, and Lyytinen and mixture of pristine organics. We also note that the high Van Flandern, predict that these showers are produced contents of solar neon observed in the small IDPs when the upper Earth's atmosphere intersects clusters collected by NASA in the stratosphere, implies and/or filaments of dust released by comet Temple- exposure times in space quite similar to those of SPOs. Tuttle along its orbit, at most a few centuries before This indicates that the constituent organics of IDPs their impact with the Earth. On the other hand, have also been subjected to radiation reprocessing in measurements of 26Al/10Be ratios in SPOs, indicate space. All observations converge as to show that much longer exposure times, ranging from a few before , the most pristine mixture of hundreds centuries to ≥ 10,000 centuries. organics should be found only in LEOs. This implies that the isotopic composition of the It is generally considered that the bulk of the constituent neon of LEOs, when displayed on the usual carbonaceous material trapped in object as different as 3 isotopes plot (20Ne/22Ne vs 21Ne/22Ne) would cluster CCs, SPOs and IDPs is a kind of insoluble rather inert around a unique point showing the highest 20Ne/22Ne carbonaceous material related to “kerogens”. This ratio (13.8), and corresponding to a pure SW material, which might be the end product of some component uncontaminated with both SEPs Neon radiation reprocessing of pristine organics would be of (20Ne/22Ne=11.2), and 21Ne, a typical cosmogenic little help in the prebiotic chemistry of life. So, trying nuclide (unless LEOs did acquire measurable amount to learn about the major families of pristine organics in of this isotope during their very early history in the LEOs, and how they were affected by frictional solar nebula). Consequently, LEOs should be found heating during atmospheric entry, might open new right on the ordinate axis, at a value of 13.8, a quite ways to tackle the prebiotic chemistry of life. To day, unique signature indeed. This is related to their small it is generally assumed that the proportion of shower exposure time to solar wind nuclei (SW) and solar particles (with pristine organics) in the contemporary energetic particles (SEPs) with low penetration in sporadic flux of meteors is about a few percents. But solids. From our previous ion implantation these shower particles should have been very abundant experiments intended to predict the effects of SW and on the early Earth at the time of the late heavy SEPs nuclei in grains in space (2) we bombardment, when the flux of juvenile deduced that measurable amount of SW neon should micrometeorites was amplified by a factor of up to one be observed after a few centuries of exposure of LEOs million. They could have delivered to the early Earth a in the . But the amount of SEPs mixture of unknown organics that might have played neon should be undetectable because their flux is about some important role in prebiotic chemistry. With the one million time smaller than the SW flux. The now prospect of getting a better understanding of this "simple" critical test of EMMA would be to make sure problem, we start collaborating with : J. Bada ; that the particles found in this unique cluster, are K. Kobayas (who did extensive works about the effects undistinguishable from SPOs collected outside the of various ionising radiations on organics), and; 1966 “Leonids” snow layer, in particular with regard to G. Matrajt, who developed (in collaboration with their pyroxene/olivine ratio and D/H ratio of their G. Flynn and G.P. Gallien) a powerful protocol to constituent water which should be ≥ 10 times higher analyse organics on a microscale with a variety of than in carbonaceous chondrites, and similar to the instruments including: a TOF-SIMS, yielding the terrestrial SMOW value, respectively. masses of the numerous fragments of the constituent organics of extraterrestrial materials; a nuclear Preliminary implications: a source of new microprobe, giving the distribution of C/N ratios on a organics for the prebiotic chemistry of life ? SEPs microscale; and X-rays and IR Fourier-transform protons are much more penetrating than SW ions, and microscopes, both on line with a beam of synchrotron they can fully irradiate the volume of a rotating radiation at the Brookhaven National Laboratory. ≈ 100µm sized interplanetary dust particles. For SPOs, the fluence of SEPs protons with energies near that of References: [1] Maurette M (2002) Pour la the maximum rate of ionisation energy loss, are Science, January 2002 issue, in press; [2] Maurette M. bracketed between ≈ 3.1013 and 3.1015 p / cm2. They (1976) Nucl. Inst. Methods 132, 579-585; [3] Bada J. should produce a marked degradation of complex (2001), personal communication; [4] Matrajt et al organics, while the much lower fluences reaching (2001) . Sci. 36, Suppl. 9, A127. LEOs would produce undetectable damage. Jeffrey Bada [3] made the conjecture that carbonaceous Acknowledgments. We thanks IFRTP and the chondrites (CCs) during their flight time to the Earth University of Copenhagen for funding these attempts did receive a sufficient dose of ionising radiation to collect particles from Leonids showers, and CNES originating from their exposure to galactic cosmic rays, and IN2P3 for financial supports. as to severely alter the composition of their initial