SNC meteorites

SNC meteorites are a group of achondrites thought to have come from the surface of Mars and named after the initials of the places where the first three were found: Shergotty, India in 1865, Chassigny, France in 1815, and Nahkla, Egypt in 1911. The SNC subgroups are the shergottites, nakhlites, and chassignites. For some mineralogical details of the SNC meteorites, see also the entry on Mars meteorites.


Establishing the origin of the SNC group

The ages of the SNC meteorites gave the first hint of an unusual origin. The vast majority of meteorites found on Earth are thought to be bits that broke off when asteroids, chiefly in the main asteroid belt between Mars and Jupiter, collided with each other at various times in the past. Like the planets, asteroids formed about 4.5 billion years ago, not long after the Sun itself began to shine. However, being much smaller than planets, they quickly lost their original supply of internal heat. Except for the few largest ones, asteroids have been cold and completely solid, inside and out, virtually since the birth of the solar system. This is reflected in the fact that most meteoric rock seems to have been crystallized for all of the last 4.5 billion years.


But the SNC meteorites are different. They appear to have solidified from molten rock between about 1.35 billion years and 150 million years ago. The only place that molten rock has existed in the solar system in such relatively recent times is inside planets. So, presumably, this is where the SNC meteorites have come from. Because of their proximity to Earth, Mars and Venus are the obvious candidates. And, of these two, Mars is much the more likely prospect. Its lower gravity and much thinner atmosphere would make it a far easier place from which to eject rocks from the surface into space. But the most compelling evidence came when scientists examined tiny samples of gas that had been trapped within EETA 79001, a 7.9-kilogram (17-pound) SNC meteorite found in Antarctica's Elephant Moraine in 1980. The composition of this gas matched exactly measurements of the martian atmosphere made by the Viking landers.



1. O'Keefe, J., and Ahrens, T. "Oblique Impact: A Process for Obtaining Meteorite Samples from Other Planets," Science, 234, 346 (1986).