A. What is ALH 84001, the Mars meteorite with the possible fossils?ALH 84001 is a meteorite, a rock that fell to Earth from space. It was found in December 1984 in Antarctica by a U.S. meteorite-hunting expedition. When it was found, ALH 84001 weighed about 4 and 3/4 pounds (1.93 kilograms). It was shaped like a rounded brick or a large potato, about 6 inches long by 4 inches by 3 inches, and was partly covered with black glass (like it was dipped in tar). The glass, called fusion crust, forms on all meteorites when they burn through the Earths atmosphere. ALH 84001 looked green inside, which really excited the expedition. But back at civilization, ALH 84001 looks much grayer inside than green.
ALH 84001 formed originally from molten lava, about 4.5 billion years ago, which is called its igneous age, possibly from an ancient martian volcano. To a geologist, ALH 84001 is an igneous rock, similar to many that crystallized from lava inside the Earth. ALH 84001 is also similar to an important group of igneous meteorites (the diogenites), and was classified as one of them until 1994 when its martian origin recognized by Mittlefehldt (1994).
Long after ALH 84001 crystallized from molten lava and cooled, about 4.0 billion years ago, it was heated again and deformed by a strong shock. This heating and shock were probably from the nearby impact of an asteroid or meteorite. Some time after this impact, possibly about 3.6 billion years ago, some kind of liquid flowed through ALH 84001 and deposited rounded globules of carbonate minerals. The possible martian fossils are in these carbonate globules. The only more recent event that we can see in ALH 84001 is another shock event. This shock may have come from the meteorite impact that lofted ALH 84001 off Mars. For a blow-by-blow description of ALH 84001s geological history, see Treiman (1995).
B. How can you possibly claim that ALH 84001 came from Mars?
Its nearly certain that ALH 84001 is from Mars, even though people have never been to Mars and no rocks have ever been collected on Mars. In fact, there are 11 other meteorites, called the SNCs, that are also almost certain to be from Mars. The strongest evidence for their martian origin is that they, including ALH 84001, contain traces of gas that is just like the martian atmosphere. We know the composition of the martian atmosphere, because the Viking Lander spacecraft analyzed it, on Mars, in 1976. The martian atmosphere is really different from the Earths atmosphere, or Venus, or any other source of gas thats ever been found.
C. How could ALH84001 get from Mars to Earth?
To get off Mars, ALH84001 must have left its surface going faster than Mars escape velocity, about 5 kilometers per second (about 11,000 miles per hour). The only known natural process that can get rocks moving so fast is meteorite impact--volcanoes cant throw rocks fast enough. If a large enough meteorite or asteroid hit Mars, some rocks nearby on Mars surface would be blasted up faster than the escape velocity and could leave Mars completely. The best estimates are that an asteroid bigger than about 1/2-2 kilometers could launch rocks like ALH 84001 off Mars and into space.
After it left Mars, ALH 84001 orbited the Sun on its own, like a small asteroid. It started out with an orbit nearly like Mars. But its orbit changed each time it passed close to Mars or collided with an asteroid. Also, gravity from the planets (especially from enormous Jupiter) slowly nudged ALH 84001 farther and farther from Mars. By chance, the orbit of ALH 84001 changed enough so that it came near the Earth's orbit. 13,000 years ago, the Earth and ALH 84001 collided.
D. When did ALH 84001 leave Mars?
ALH 84001 was probably blasted off Mars about 16 million years ago; it could have been longer ago, but not much more recently. This age is actually how long ALH 84001 was in space, exposed to cosmic rays, as it traveled between Mars and the Earth. ALH 84001 might have been in space longer if were shielded from cosmic rays, for instance if it were from the inside of a larger boulder. In this case, the boulder would have broken up 16 million years ago, exposing ALH 84001 to cosmic rays.
E. When did ALH 84001 land on the Earth?
The best estimate is that ALH 84001 landed on Earth, in Antarctica, about 13,000 years ago.
F. How and when was ALH 84001 found?
ALH 84001 was found in Antarctica during the 1984-1985 Antarctic summer. It was found by a team of meteorite hunters from the ANSMET program, which is sponsored by the Polar Programs Office of the U.S. National Science Foundation. ANSMET stands for ANtarctic Search for METeorites, and has been funded since 1977 by the NSF. In that time, ANSMET meteorite hunters have found more than 7000 meteorites, almost triple the number known from outside Antarctica. The Antarctic Meteorite Location and Mapping Project (AMLAMP) provides locations, maps, and databases of meteorite finds.