We hear a lot about meteorites from Mars. How do we know they come from Mars? And how do they get to Earth?

A few dozen meteorites found on Earth, chiefly on the Antarctic ice cap, are now believed to have come from Mars. The first evidence for a Martian origin was discovered in the early 1980s, when some of the SNC meteorites were found to have radiometric ages of only about one and a half billion years, some even lower. These meteorites - achondrites - are actually igneous rocks, essentially similar to terrestrial basalts. So young basaltic meteorites must have come from somewhere where there was igneous activity rather recently in terms of the age of the solar system (about 4.5 billion years). Volcanism on the Moon stopped roughly 2.5 billion years ago, and in the asteroids 4.5 billion years ago. Mars, on the other hand, has many young-looking large volcanos, so it is a likely source for extraterrestrial volcanism perhaps continuing today. Furthermore, some of the SNC meteorites have trapped gases just like those analyzed on Mars by the Viking spacecraft in the 1970s.

How do meteorites from Mars get to Earth? The answer is that they are blasted off the planet by large meteorite impacts. Mars has a thin atmosphere and only 1/3 the gravity of Earth, so such an event is physically plausible. This incidentally is probably why we think the SNC meteorites are not from Venus. Venus may still be volcanically active, but it has an extremely thick atmosphere, which stops most meteorites and would prevent those that do hit from knocking pieces into space, especially in light of the strong gravity of Venus, almost as strong as that of the Earth.

This week's question comes from Dr. Paul Lowman. Paul Lowman is a geophysicist, originally a geologist, in the Geodynamics Branch of the Laboratory for Terrestrial Physics. He joined GSFC in December, 1959, in the Theoretical Division led by Dr. Robert Jastrow. He initially worked on the origin of tektites, argued by Dr. John O'Keefe to come from the Moon. He helped plan the Apollo geophysical experiments, and trained Mercury, Gemini, Apollo, and Skylab astronauts in geologic terrain photography. His most recent work has been compilation of a global tectonic activity map, appearing in his 2002 book "Exploring Space, Exploring Earth." AND he fearlessly (or foolishingly, if you ask him) rides his bike to work along Greenbelt Road (a very high trafficked road) every day!! WOW!