How Do We Know that the Meteorites Come from Mars? The SNC meteorites were recognised as different from asteroidal meteorites long before their martian origin was accepted. They are distinguished from other meteorites by their young crystallisation ages, down to 165 Myr. A young age implies formation on a body that was still active (i.e., not totally cooled and solidified) well after the initial accretion and aggregation of the Solar System ~ 4560 Myr ago. In other words, the SNCs must originate from a planet-sized body, not an asteroid. The mechanism by which the meteorites reach the Earth is by impact ejection: as asteroids impact the martian surface, craters are formed. If the impactor enters the martian atmosphere at a sufficiently shallow angle and with a high enough velocity, then ejecta thrown from the surface can escape to orbit the Sun as small bodies in space, prior to landing on the Earth as meteorites. The martian origin can be deduced through a process of elimination, by considering all the bodies in the Solar System in turn. Several of the planets can be rejected almost immediately. Mercury is too close to the Sun to allow ejecta to escape outwards to Earth. Jupiter, Saturn, Uranus and Neptune are gas planets, and not rocky. The satellites of the giant planets, although rocky in nature, are unlikely source objects, since any ejecta blasted from their surfaces will not escape from the gravitational attraction of their parent planet. Pluto is a mixture of rock and ice, as are comets, and are not thought to have been molten. So we are left with Venus, Earth and its Moon, and Mars. Venus? There is a low, but finite, possibility of Venus being the parent planet. Venus is about the same size and mass as the Earth, and so has a similar escape velocity. But Venus has a very thick atmosphere with a high surface pressure (~ 96 atms). In order for ejecta to escape the planet, they must have very high energies when they are removed from the surface, such that by the time they have traversed the atmosphere, the ejecta still have sufficient velocity to escape. Similarly, the incoming impactor must be travelling at great speed, so that it is not decelerated so much that it is unable to impart sufficient energy to the ejecta to enable debris to escape. And on top of all this, once material has escaped from Venus' surface, it must have the correct orbital vectors to thrust against the gravitational tug of the Sun, and travel outwards to the Earth. Venus, therefore, is dynamically unfavourable as the parent of the SNCs. Earth? Impactors frequently hit the Earth, so it could be argued that the SNC meteorites were broken from the Earth's surface with insufficient energy to escape totally. When the rocks fell back to Earth, they acquired the fusion crust characteristic of meteorites. The compelling counter-argument to this is that all materials from the Earth have a characteristic composition on a plot of 18O/16O against 17O/16O. As the diagram shows, the oxygen isotopic composition of the SNCs falls on a single line (indicating that they all come from the same planet), but because this line is displaced from the Earth line, the SNCs cannot come from the Earth. Moon? The Moon is heavily cratered, and none of the dynamic arguments applied to Venus apply to the Moon. But as discussed in the previous chapter, material does come from the Moon, and has been identified by its close similarities to materials collected during the Apollo and Luna missions. This composition is very different from that of the SNC meteorites. Also, the oxygen in lunar rocks follows the same pattern as that for terrestrial rocks. So the SNC meteorites cannot come from the Moon. So it must be Mars! By default, then, the SNC meteorites most probably come from Mars! But there is additional, positive, evidence that links them to the planet, which comes from gases trapped within the Elephant Moraine (EET) A79001 meteorite. This meteorite contains inclusions of black glass scattered throughout its mass. The glass was formed by shock melting of mineral grains, presumably during the impact event that lofted the meteorite from its parental surface. Analysis of gas trapped within the glass during the impact shock shows that it is identical in composition to that of Mars' atmosphere (measured by the Viking landers in 1976. The only way that Mars' atmosphere could have become trapped in EET A79001 is if it came from Mars. Thus EET A79001 comes from Mars, and since (on the basis of their oxygen isotopic composition) all the other SNC meteorites come from the same parent as EET A79001, then they too must come from Mars.

Of the martian meteorites, several have been found in Antarctica, not because there is a higher incidence of their falling at the Earth's poles, but because there is a greater chance of them being collected there. This is because the icy desert preserves the stones for thousands (sometimes millions) of years - elsewhere on Earth the martian rocks, which after all are of a planetary nature, are more difficult to distinguish from equivalent terrestrial materials.