The Exchange of Impact Ejecta Between Terrestrial Planets

Abstract

models that assume stepwse mutaton of the STRP ~m p y that the varlaton of allele sizes Increases Inearly w~th tme (48) [D. 5. Godstein, A. Ruz Lnares, L. L. Cavali-Sforza, M. W. Feldman, Genetics 139, 463 (1995jl. Thus, we can est~mate R as the ratio of varances of allele szes (in base pa~rs) for alleles less than 110 bp (22 5/0.75 = 30.0), glvng a maxlmum age of 167,000 Y.B.P. Because we have considered only a slnge locus, the standard error on this varance-ratlo estmate IS high. However, the apparent lower boundary of 85 bp to the STRP allele slze results In an underestmate of the t~me of orlgn of the Au(-) chromosome in Afrca, so agaln ths estlmate of R is conservatve. 42. Despte a smaller effect~ve populat~on sze of A h -) chromosomes (because of a lower frequency), the mean varlance of STRP alleles on Alu(-j chromsomes among the 10 sub-Saharan Afr~can popua-t~ons is still substantial [64% of the mean varance of STRP alleles on Alu(+) chromosomes], suggest~ng that whle the orig~n of the Alu(-) chromosome IS more recent than the orig~n of the Alu(+j chromosome, it is st11 quite ancent. 43 C B. Str~nger, In The Origin and Evolution ofHumans and Humanness, D. T. Rasmussen Ed. (Jones and Bartett, Boston, 19931, pp. 75-94. 44 M. Slatk~n, Mol Blol. El/ol 12, 473 (1995). 45 Both the STRP and the Alu deletion polymorphism are located In noncodng regions of the CD4 gene and are unlkey to have any functional s~gn~f~cance. Except In the unlikely case of strong posltlve CISactng epstass of functorial varants flankng exons 1 and 2 of the 90-bp Au(-) chromosomes, seecton cannot expla~n the mantenance of the l~nkage d~sequlhbrum seen In non-Afr~can populatons because there would be nothing to prevent format~on of "non-90 bp" Alu(--) chromosomes by recombna-t~on between the markers or mutaton at the STRP, In addtion, the very low frequency of the Alu(-) chromosomes In Asa, the Pacifc islands, and the New World, as well as the very h~gh frequency of 85-bp Alu(+j and 11 0-bp Alu(+) chromosomes In all non-African populat~ons, argues aga~nst strong positive selecton for the Alu(-) chromosome. S A.Orbital histories of ejecta from the terrestrial planets were numerically integrated to study their transfer to Earth. The properties of the lunar and martian meteorites are consistent with a recurrent ejection of small meteoroids as a result of impacts on their parent bodies. Long-range gravitational effects, especially secular resonances, strongly influence the orbits of many meteoroids, increasing their collision rates with other planets and the sun. These effects and collisional destruction in the asteroid belt result in shortened time scales and higher fluxes than previously believed, especially for martian meteorites. A small flux of mercurian ejecta appears possible; recovery of meteorites from the Earth and Venus is less likely.

Dynamical Considerations for Life in Multi-Habitable Planetary Systems

“…This difference is due in part to the more densely occupied phase space of the particles, there being far less spread in the velocities. However, Gladman et al (1996) showed that the Martian meteorites on Earth also showed evidence for initial velocities near the escape velocity from Mars-a result consistent with our statement.…”

“…Panspermia in the solar system has been studied at some length (e.g., Melosh & Tonks 1993;Gladman et al 1996;Worth et al 2013; see this last reference for a good historical review). And from observations of meteorites, some hypothesize that a fraction of life on Earth may have originated on Mars (which may have supported life sooner than the Earth (e.g., McKay et al 1996)).…”

Planetary targets in the search for extrasolar oxygenic photosynthesis

“…the zone within which oxygenic photosynthetic organisms would live, would be sterilised within 100,000 years (Clark 2001). Although computer models suggest that rocks can potentially be transferred between Earth and Mars, for instance, in 10 years, more typical times are likely to be 10 5 to 10 7 years (Gladman et al 1996).…”

Evolution of Mercury-like orbits: A numerical study

“…Leake et al (1987) find that a considerable fraction of the total mass that impacted the terrestrial planets during the late heavy bombardment could have additionally impacted Mercury by a Vulcanoid population. In the case of scenario (ii) we would not expect the presence of such objects unless they are the result of a very recent and major impact or collision in the region, since the survival time scale near Mercury is of the order of a few Myr (Gladman et al 1996;Evans & Tabachnik 1999).…”