Most ages in meteorite studies come from measurements on the naturally occurring radioactive isotopes of chemical elements, how they formed, and what other isotopes they have decayed to. The time when ALH 84001 crystallized from molten rock comes from two radioactive isotope systems: samarium-neodymium (Sm-Nd), and rubidium-strontium (Rb-Sr).
The Sm-Nd isotope system is among the most reliable for determining when an igneous rock cooled and crystallized. For ALH 84001, it yields a crystallization age of 4.5 billion years ago (Jagoutz, 1994; Nyquist et al., 1995). The Rb-Sr system often gives crystallization ages, but can be reset to younger ages by later events in a rocks history. Nyquist et al. (1995) measured an Rb-Sr age for ALH 84001 of 4.5 billion years ago, the same from the Sm-Nd radio-isotope system. Jagoutzs (1994) data suggest a younger Rb-Sr age for ALH 84001, and Wadhwa and Lugmair (1996) measured a Rb-Sr age of 3.8 billion years ago. This younger age may be the shock event that heated and deformed ALH 84001.
Most ages in meteorite studies come from measurements on the naturally occurring radioactive isotopes of chemical elements, how they formed, and what other isotopes they have decayed to. The age of the first shock event that heated and crushed ALH 84001 comes principally from a radioactive isotope system potassium-argon, or K-Ar.
Argon is a gaseous element, and its isotope 40Ar comes mostly from the decay of the radioactive isotope of potassium, 40K. Argon escapes (leaks out) of hot rocks, but cant escape easily from cold rocks. So K-Ar ages represent the last time the rock cooled down. For ALH 84001, Ash et al. (1996) measured a K-Ar age of about 4.0 billion years ago, and inferred that it represented cooling after a meteorite impact, perhaps the impact that deformed ALH 84001.
The Rb-Sr radioactive isotope chronometer may also record the same event as K-Ar. The Rb-Sr data of Jagoutzs (1994) suggest an age younger than 4.5 billion years ago, and Wadhwa and Lugmair (1996) measured a Rb-Sr age of 3.8 billion years ago.
Age of Carbonate Formation
The age of the rounded globules of carbonate minerals in ALH 84001 is not at all certain, but is important for evaluating McKay et al.s (1996) claim that the globules contain traces of ancient martian life. McKay et al. cited Knott et al. (1995) for their preliminary K-Ar age (see also Shock Age) for the carbonate globules of 3.6 billion years ago. On the other hand, Wadhwa and Lugmair (1996) reported that the carbonate globules formed 1.39 billion years ago, based on their data in the Rb-Sr radioisotope system (see also Igneous Age).
Cosmic Ray Exposure Age
Most ages in meteorite studies come from measurements on the naturally occurring radioactive isotopes of chemical elements, how they formed, and what other isotopes they have decayed to. The cosmic ray exposure age is how long a meteorite orbited in interplanetary space, exposed to cosmic rays from the Sun and the galaxy. As these cosmic rays (high-energy elementary particles) hit a meteorite, they produce some characteristic new isotopes (by transmutation) of chemical elements, both radioactive and stable. The longer a meteorite is exposed to cosmic rays, the more of these new isotopes are present.
For ALH 84001, isotopes of the elements helium (3He), neon (21Ne), and argon (38Ar) have been used to calculate a cosmic ray exposure age. Most of these ages are between 16 and 17 million years, with a few measurements as young as 12 million years (Eugster, 1994; Nishiizumi et al., 1994; Miura et al., 1995; Swindle et al., 1995).
The terrestrial age of a meteorite is how long ago the meteorite fell to Earth. If people witnessed the meteorites fall, then its terrestrial age is obvious. But most meteorites, and all Antarctic meteorites like ALH 84001, fell without being noticed by people. Fortunately, terrestrial ages of meteorites can also be calculated from the abundances of some naturally occurring radioactive isotopes of chemical elements. These radioactive isotopes are formed in a meteorite, in space, as it is bombarded by cosmic rays (high-energy elementary particles). After a meteorite lands on Earth, it isnt hit by cosmic rays any more, no more of the radioactive isotopes form, and the ones already in the rock continue decaying away. So, the more of these isotopes in a meteorite, the less time it has spent on Earth. Terrestrial ages are usually determined from isotopes of the elements carbon (14C), beryllium (10Be), and chlorine (36Cl).
The best measure of ALH 84001s terrestrial age comes from its abundance of carbon-14 (14C). Carbon-14 dating is most commonly used to get ages for things like archaeological artifacts and the Shroud of Turin. The abundance of carbon-14 in ALH 84001 gives a terrestrial age of about 13,000 years (Jull et al., 1995).
The strongest link between Mars and the martian meteorites is the discovery of martian atmosphere gas inside the meteorites. But even before martian atmosphere gas was discovered in the meteorites by Bogard and Johnson (1983), many scientists thought that the meteorites were from Mars because of their young crystallization ages and their complex chemical compositions. Even then, it was certain that the meteorites were not from the Earth because their oxygen isotope compositions are utterly distinct from those of Earth rocks (Clayton and Mayeda, 1996).
A. So how different is the martian atmosphere?
The chemical make-up of Mars atmosphere was measured, on Mars, by the Viking lander spacecraft. The landers analyzed the composition of the atmosphere (mostly of carbon dioxide, with a little nitrogen and argon and other gases), and also the isotopic composition of many elements in the atmosphere. (Remember that isotopes are the varieties of chemical elements, each isotope with a different weight.)
From the Viking analyses, Mars atmosphere is unique in the solar system, at least unique among the planets, atmospheres, and asteroids that have been sampled. In bulk elemental composition, Mars atmosphere has unusual abundances of the elements nitrogen (N), argon (Ar), krypton (Kr), and xenon (Xe) relative to each other. The martian atmosphere bulk composition is not like any other known source of gas. Also, the Viking landers found that Mars atmosphere has unusual isotopic abundances; it is rich in nitrogen with weight 15 (15N) compared to nitrogen with weight 14 (14N), and relatively rich in two isotopes that form during radioactive decay of other elements, 40Ar and 129Xe. Telescopic observation from Earth also showed that the martian atmosphere is very rich in heavy hydrogen (also called deuterium, D) compared to the Earth. Again, this mix of isotopic abundances is not like any other known source of gas.
This martian atmosphere gas was first discovered in the Antarctic meteorite EETA79001 (Bogard and Johnson, 1983), and traces of it are present in almost all of the martian (SNC) meteorites.
B. Is this martian atmosphere in ALH 84001?
Yes. This same martian atmosphere gas is also present in ALH 84001, which has hydrogen rich in deuterium, nitrogen rich in 15N, argon rich in 40Ar, and xenon rich in 129Xe (Miura and Sugiura, 1995; Gilmour et al., 1995, 1996; Grady et al., 1996; Leshin et al., 1996; Miura et al., 1995; Swindle et al., 1995).
C. Is there other evidence that the martian meteorites are from Mars?
All circumstantial. The martian meteorites except ALH 84001, the SNCs, all crystallized from lava less than 1300 million years ago. The only objects in the solar system with volcanoes active so recently are Venus, Earth, Mars, and Io. The SNCs arent from the Earth because their oxygen isotope compositions are utterly distinct from anything on Earth. Theyre probably not from Venus, because most of the SNC meteorites contain extraterrestrial clays or water-bearing minerals, and they could not have formed on Venuss hot surface. Io remains possible, but pretty unlikely as the escape velocity for a rock near Jupiter is so high.
Although ALH 84001 is not young like the other SNCs, it is linked to them in other ways. ALH 84001 has the same oxygen isotope compositions as the SNCs, the same carbon isotope compositions, the same relative abundances of manganese and iron, and many other obscure geochemical clues. And, of course, ALH 84001 also contains martian atmosphere gas.
D. Could ALH 84001 be from an asteroid?
Sure, but it seems unlikely. We dont have samples of every asteroid out there, so its possible that ALH 84001 is from an asteroid that happens to contain gases identical to the martian atmosphere. But no known meteorite from an asteroid contains gases anything like the martian atmosphere. Paraphrasing Robert Heinlein, The race may not be to the swift, but thats where to put your money.
E. Arent the asteroids broken fragments of an Earth-like planet?
No, almost certainly not. This idea originated from Bodes law about the regular spacing of planets in the solar system, which predicted that there should be a planet between Mars and Jupiter. A search was started in the early 1800s, and quickly found a number of small bodies, the asteroids, in the right place. But the total mass of all the asteroids is only about 3% of the Moons mass or 0.04% of the Earths--hardly much of a planet. And meteorites from the asteroids have such varied compositions that they could not really have come from an Earth-like planet. Also, the orbits of the asteroids do not seem to have ever all intersected at a single place; if they were from a broken planet, the asteroids orbits must have intersected each other at the place and time the planet broke up.