[meteorite-list] Mercurian Meteorites, Chances of: REPOST
From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:26 2004 Message-ID: <3CB77F03.7B1BBDCD_at_bhil.com> <!doctype html public "-//w3c//dtd html 4.0 transitional//en"> <html> Hi, <p> Originally posted last June (2001), the statistics of transfer of impact ejecta from planet to planet suggested that there should be 1 or 2 Mercurian meteorites that have been collected but not (then) identified. The ratio of Martian to Mercurian meteorites should be about 15:1. Since basalt requires a differentiated world, Vesta is about as small as a basalt producer is likely to be, so the candidates for the origin of these rocks have to be bigger. <br> If the Mercurian origin of the two current candidates is confirmed, it would suggest that the statistical simulations of the study cited below were reasonably in line with the real world evidence. Interestingly, you can see that the authors include several paragraphs attempting to explain why there aren't any Mercurian meteorites (Science Professional Rule 37B: always cover your...), instead of predicting their discovery. As Gallileo could have told you, boys, sometimes, you've got to stick your neck out <i>just a little bit.</i> <br> Another conclusion of the Gladman study is that there should be some Venusian meteorites, a greater number than Mercurian meteorites. So, where are <b><i>they </i></b>hiding? (Following Rule 37B, maybe the atmosphere of Venus is too thick for anything to escape, etc. etc. Following the Gallileo rule, keep looking!) <p> Here's the original post: <p>Subject: Mercurian Meteorites, chances of <br>Date: Sat, 30 Jun 2001 23:23:28 -0500 <br> <p>Hi, List, <p> On the question of how much ejecta from Mercury falls into the Sun, <br>here is the results of the largest-scale, most complete computer <br>simulation of knocking rocks off planets. <br> As you can see below, only 4% of rocks knocked off Mercury fall into <br>the Sun. The statistics suggest that there should have been 1-2 <br>Mercurian meteorites collected by now, but that number is so small, it <br>could just as well be zero. <br> On the other hand, would we know (or suspect) a Mercurian meteorite <br>if we had one? At least one theory of Mercury's formation says that it <br>was whacked with a more-than-Mars-sized impactor (like the one that <br>became the Earth's Moon) which blew off much of its original (silicate) <br>mantle. <br> Mercury's surface shows a great lack of contrast in reflectivity <br>(unlike the Moon which it superficially resembles). It has been <br>suggested that the intercrater and smooth plains of Mercury are all <br>basin ejecta rather than volcanic in origin. If the smoother areas are <br>basaltic vulcanism, it is unique and unlike that of the other planets. <br> The exact nature of the Mercurian mantle (and core) is largely a <br>matter of assumption and supposition. It is easy to fit Mercury into <br>current theories of early solar system history because of its high <br>density. But we ought to remember that the density "match" between the <br>Moon and the Earth's mantle misled and bamboozled theorists into an <br>incorrect theory of the Moon's composition for more than a century. <br> What we need is... MORE MISSIONS. <p>Sterling K. Webb <br>--------------------------------- <p>FROM: <br> Brett J. Gladman, Joseph A. Burns, Martin Duncan, Pascal Lee, and <br>Harold F. Levison; The exchange of impact ejecta between terrestrial <br>planets. Science, March 8, 1996 v271 n5254 p1387(6). <p>TEXT: <p> Table 2. The fates of meteoroids after a [v.sub.infinity] = 1 km/s <br>launch from Mars and Mercury. The simulation for Mars included 900 <br>particles and ran for 100 Myr; the simulation for Mercury included 200 <br>particles over 30 Myr. No collisional effects were included. The <br>position of Mercury was not tracked in the martian simulation, so <br>collisions with it were not possible. <p>Particles (% of total) from parent body <p>Meteoroid fate Mars Mercury <p><tt>Impact Mercury N.A. 76</tt> <br><tt>Impact Venus 7.5 6.5</tt> <br><tt>Impact Earth 7.5 0.5</tt> <br><tt>Impact Mars 9.0 0</tt> <br><tt>Sun-grazing 38 4</tt> <br><tt>Reach Jupiter 15 2</tt> <br><tt>Survivors 23 11</tt> <p> Just one of the 200 particles was found to hit the Earth, after 23 <br>Myr. This 0.5% delivery efficiency is 50 times higher than previously <br>suggested but is based on poor statistics. It is about an order of <br>magnitude smaller than the efficiency for Mars. <b><i><font size=+1>If we accept this</font></i></b> <br><b><i><font size=+1>efficiency and if the mercurian impactor flux is comparable to that of</font></i></b> <br><b><i><font size=+1>Mars, the existence of 12 martian meteorites should lead us to expect a</font></i></b> <br><b><i><font size=+1>few mercurian meteorites.</font></i></b> <br> However, a purely gravitational model may not be sufficient to <br>accurately simulate the transfer of material from Mercury to Earth. <br>Radiation forces in the inner solar system cause significant orbital <br>evolution over tens of millions of years, times like that required for <br>our single meteoroid to reach Earth. Orbital collapse as a result of <br>Poynting-Robertson (P-R) drag at Mercury's heliocentric distance takes <br>only 5 Myr for a meteoroid 1 cm in radius with a density of 5 <br>g/[cm.sub.3]. On the other hand, the Yarkovsky effect, which dominates <br>P-R effects for particles of this size with spin periods longer than 1 <br>second, may induce some mercurian meteoroids to spiral outward to Earth. <p> However, mercurian meteoroids may be catastrophically fragmented by <br>dust-sized impactors, which, because of gravitational focusing, increase <br>significantly as the sun is approached. Collisional lifetimes of 100-g <br>bodies at Mercury's distance are estimated to be less than [10.sup.5] <br>years. Because of these complications, the likelihood of finding <br>mercurian meteorites is difficult to quantify. <br> <br> <br> <br> </html> Received on Fri 12 Apr 2002 08:42:43 PM PDT |
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