Re: how do we know the meteorite was from Mars that showed life signs?

Ron:
Sorry about the delay, but I have an answer to your question. First of all, as of now, there is significant debate on whether the meteorite in question, named ALH840001, actually contained "signs of life" or not. The issue is highly volatile, and several papers have been published that examined amino acids in the meteor or that the rock was contaminated with terrestial substances. Having said that, I will continue.

The original paper to which you are referring was published in Science in August 1996 by David McKay, Richard Zare, et al. They relied on previously published data that this particular meteorite was in fact from Mars. This determination was made on the basis of data collected from the Viking probes sent to Mars in 1976. By analyzing the abundance of isotopes of certain elements found within the meteor and in silica inclusions within the meteorite and comparing these abundances to the same data collected on Mars and on Earth, the origin of the meteorite can be determined. In this case, a match to neither set of data would mean the meteorite came from somewhere else.

The distributions of isotopes of various substances vary from place to place in the Universe, so once you know the distribution from a location, you can ascertain whether material obtained is from the same location or not.

The origin of ALH840001 was determined by D. W. Mittlefehldt, in Meteoritics, 29, 214, in 1994. To quote an abstract/title from a similar method applied to another meteorite:

H. P. McSween Jr., Meteoritics, 29, 757 (1994).
Analyses of the gases in glassy inclusions in the SNC meteorites EET79001 by Bogard and Johnson [Science, 221, 651 (1983)], Becker and Pepin [Earth Planet. Sci. Lett., 69, 225 (1983)], and Marti et al. [Science, 267, 1981 (1995)] have shown that the abundance and isotopic compositions of the trapped gases in the SNC meteorites and the measured atmospheric compositions on Mars, measured in situ by the Viking landers, have a direct one-to-one correlation (more than nine orders of magnitude in concentrations). This remarkable agreement is one of the strongest arguments that the SNC meteorites represent samples from Mars.

I hope this answers your question.

-Matt-

[Moderator's note: Considerable technical detail can be found at ALH840001 : Technical Discussion.]