Martian meteorites include three rare groups of achondritic (stony) meteorite (16 objects total) with isotope ratios that are said to be consistent with each other and inconsistent with the earth. It should be pointed out, however, that the isotope ratios do not actually match Mars ratios especially well, to the extent that Mars ratios are known, although they do differ substantially from Earth isotope ratios and from what is known of Lunar ratios.

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. Only 14 SNC meteorites have been found to date, including several from Antarctica.

Their ages give 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. With one exception, they appear to have solidified from molten rock between about 1.3 billion years and 200 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-kg (17-lb) 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.

Shergottites

The most abundant type of the SNC meteorites believed to have come from Mars, with 17 known examples by mid-2002; the type member is the Shergotty meteorite, which fell in India in 1865. Shergottites are igneous rocks of volcanic or plutonic (formed deep under the surface) origin, and they resemble terrestrial rocks more closely than do any other achondrite group. They all have exceptionally young crystallization ages of 150 to 200 million years, and usually show signs of severe shock metamorphism. Typically, the plagioclase in shergottites has been converted to maskelynite, a glass that is produced when plagioclase is subjected to high shock pressures. The maskelynite was probably formed by the impact forces that blasted the shergottites away from the Martian surface.

Nakhlites

One of the types of SNC meteorites believed to have come from Mars, nakhlites probably formed as lava flows with unusual compositions. They are named for the first member of the group to be found, which fell in pieces in El Nakhla, Egypt, in 1911; local legend has it that one of the fragments hit and killed a dog, though this story may be apocryphal (see Nakhla meteorite).

The nakhlites consist mainly of green augite crystals with some olivine in a very fine-grained blend of plagioclase, feldspar, pyroxenes, iron-titanium oxides, sulfides, and phosphates. Most intriguingly, there are traces of pre-terrestrial aqueous alteration products in the form of hydrated minerals, including clay minerals and carbonates. Some researchers think that the presence of these hydrated minerals in the nakhlites, in addition to concentrations of water-soluble ions such as those of chlorine, potassium, sodium, and calcium, suggests that they were once in an environment in which liquid seawater was present for some time, perhaps under an ancient Martian ocean. A problem with this idea is the comparatively young age of the nakhlites: they seem to have crystallized only 1.3 to 1.4 billion years ago and to have been altered by water a mere 700 million years ago, long after Mars supposedly lost its ancient lakes, rivers, and seas.

Chassignites

One of the types of SNC meteorites believed to have come from Mars. The group is named for its only known member, a meteorite that was seen to fall in Chassigny, France, in 1815; its subsequent recovery led to it being one of the first meteorites to be recognized as a genuine rock from space. Chassigny resembles a terrestrial dunite - a coarse-grained, deep-seated igneous rock - and consists of about 91% iron-rich olivine, 5% clinopyroxene, 1.7% plagioclase, 1.4% chromite, 0.3% melt inclusions, and other minerals. Cracks within Chassigny are filled with carbonate and sulfate salts that point to chemical alteration by water before its arrival on Earth. Its crystallization age of 1.36 billion years and its composition, suggest a close relationship with the nakhlites and an origin in the same parent magma on Mars. However, Chassigny contains noble gas values that are entirely different from those found in other Mars meteorites or in the Martian atmosphere. If these gases came from the Martian mantle, as suspected, Chassigny must have originated within a magma pluton deep inside the Martian crust.