History of ALH84001
Most of the arguments establishing the times at which various things
happened to ALH84001 are not given in the Science article, but rather in
earlier articles which are cited there. Unfortunately, I have not been
able to track down these articles. Instead, I will give some educated
guesses as to how the various numbers could be estimated, and references
to the cited articles.
It formed from volcanic magma 4.5 billion years ago
This is probably gotten from the isotopic composition of the rock.
Most elements occur as several different isotopes (same number of protons
in the nucleus, but different numbers of neutrons). Some of the
isotopes are unstable, decaying to produce other nuclei, and each decay
happens at its own rate, with its own half-life. When the rock is molten,
it is exchanging material with its surroundings and so has pretty much
the same abundance of each isotope as the planet at large. When it solidifies,
its material is locked in place, so as the unstable isotopes are depleted by
natural radioactive decay, they cannot be replenished from the environment.
By measuring the relative abundances of various isotopes in the rock, and
knowing the half-lives of the radioactive decays that produce them, you can
work backward to figure out how long the rock would have to have been solid
in order for the isotope abundances to get to be what they are today.
For dating of ALH84001, the references are
E. Jagoutz et al., Meteoritics 29, 479 (1994)
and L.E. Nyquist et al., Lunar Planet. Sci. 26, 1065 (1995).
It is unusual for a Martian meteorite to be this old; all the others are
at most 1.3 billion years old. To be 4.5 billion years old -- the same age
as the meteorites which come from the asteroids, as well as of the oldest
rocks on the moon -- ALH84001 would have to be one of the original rocks which
formed when Mars (and the other inner planets) first cooled enough to form a
solid surface.
It was smashed by asteroid strikes 4.0 and 3.6 billion years ago
I have heard that these are fairly uncertain numbers, but the precise numbers
are probably not as important as the order of magnitude. Presumably, the
number 3.6 billion comes from isotope dating of the carbonate globules.
It is likely (but again, probably not important) that the shocks were delivered
by asteroid impacts, because asteroid impacts were very common in the solar
system at that time. For example, that is the time that the major craters on
the Moon were formed.
For the estimate of the time of the first shock, the reference is
R.D. Ash et al., Nature 380, 57 (1996).
For the estimate of the time the carbonates formed, the reference is
S.K. Knott et al., Lunar Planet. Sci. 26, 765 (1995).
Between these times, it was immersed in water charged with carbon
dioxide
The evidence that ALH84001 was immersed in carbonated water is that this
is how carbonate minerals form. On Earth, this is the process of "weathering"
-- carbon dioxide from the atmosphere dissolves in rain water, and reacts
(slowly) with the silicate rocks it falls on to form silicon dioxide and
carbonate rocks. There is plenty of evidence that liquid water once flowed
on Mars, although not later than perhaps 3 billion years ago. The Martian
atmosphere is still mostly carbon dioxide; during ALH84001's youth the water
would have had carbon dioxide dissolved in it. In order to produce the
carbonate globules along the faults in the rock, the faults would have to
exist first, and then the water would have to seep into the rock along them.
Thus there must have been some shock that faulted the rock, and afterwards it
must have been immersed in carbonated water. The fact that some of the
carbonate globules are themselves broken along fault lines in the rock then
means that there must have been at least one more major shock that happened
after the globules formed.
The reference is C.S. Romanek et al., Nature 372, 655 (1994).
It was blasted into space by a meteorite strike 16 million years ago
It had to be a meteorite impact that blasted ALH84001 off the surface of Mars,
because no other natural event is known that can get a rock moving fast enough
(5.4 km/s) to escape Mars's gravity. The fact that this happened 16 million
years ago, or that the rock spent 16 million years in space, is based on cosmic
ray exposure. Cosmic rays produce characteristic sorts of damage in the rock.
By looking at how much of this damage there has been, you work out how long the
rock was exposed to cosmic rays, i.e., how long it was wandering in space.
The reference is D.D. Bogard, Lunar Planet. Si. 26, 143 (1995).
It landed in Antarctica 13,000 years ago
I suspect that this has to do with dating the ice under the meteorite, which
you can do by examining the air that was trapped when it fell on Antarctica
as snow.
The reference is A.J.T Jull et al., Meteoritics 30, 311 (1995)