Have Earth rocks gone to Mars?

Have Earth rocks gone to Mars?

The possibility that three meteorites named Shergotty, Nakhla and Chassigny came from Mars has intrigued planetary scientists for a decade. Geochemically they seem reasonable candidates as samples of the plante, but only in the last few years have some researchers concluded that the rocks might actually have been blasted free of Mars' gravity by other meteorites. A different question, however, is whether meteorite impacts on Earth have similarly driven rocks from this planet to Mars.

According to S.A. Phinney, now at Wright-Patterson Air Force Base in Dayton, Ohio and colleagues from the University of Arizona in Tucson, kicking a rock hard enough to free it from Earth's gravity would require a meteorite capable of making a crater more than 60 miles across. In addition, Mars' orbit is much larger than Earth's, so the chance of an Earth rock hitting Mars is about 10 percent that of the same thing in reverse.

Phinney's group used a computer to calculate where 1,000 particles would go if ejected from Earth in random directions, moving about 2.5 kilometers per second faster than the minimum speed necessary to escape. Of the 1,000 hypothetical particles, 291 hit Venus and 165 returned to Earth; 20 went Mercury, 17 to Mars, 14 to Jupiter and 1 to Saturn. Another 492 left the solar system completely, primarily due to gravitational close encounters with either Jupiter or Mercury that "slingshot" them on their way. Other computer runs showed the same general trend.

The group found fewer particles seemed likely to get to Mars than suggested by some previous analyses. Even so, the authors point out, "it could be further argued that if Mars material reached Earth, Earth material might have been reached Mars and other planets, perhaps carrying with it viable microorganisms and spores residing in the near-surface rock and soil. Thus Mars may in fact have already been contaminated with Earth life." On the other hand, Phinney and his co-workers say, "the conditions on Mars are sufficiently extreme that any microbes that survived the trip in space would probably be killed on its desolate surface. Thus we are still unlikely to find life on Mars."