A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

How did rocks from Mars end up here on Earth?

September 16, 2005

Dear Cecil:

I've just finished reading about how researchers have concluded that Mars was never warm based on analysis of a meteor--found on earth. More distantly you'll remember that they were debating whether a fossil of a living organism had been found in a martian meteor found in Antarctica. My question: How do they know these meteors came from Mars? Are they labeled? Have they found meteors from other planets that they recognize? Why waste all this money and time on the space program if there are rocks from these planets just lying around earth?

The rocks were found on earth, Kent, so they're meteorites, not meteors. (Meteors are space rocks that fry in the atmosphere; meteorites are the remnants that make it all the way down.) Once you get that straight, try chewing on this: Scientists think the martian meteorites came from Mars because--I realize the nontechnical mind won't find this entirely persuasive--they couldn't plausibly have come from anywhere else.

The Martian meteorites--34 have been identified to date--are collectively known as the SNC (pronounced "snick") group. The letters stand for three related categories of geologically unusual meteorite: shergottites, the prototype of which fell in 1865 near the town of Shergotty in India; nakhlites, named for a rock that dropped on Nakhla, Egypt, in 1911; and chassignites, so called because of the specimen thatlanded in Chassigny, France, in 1815.

For years it was assumed that the SNC meteorites were hunks of asteroid like pretty much every other space rock. Then in 1979 geologists Hap McSween and Ed Stolper advanced the seemingly barmy thesis that the meteorites had come from Mars based on the following logic: (a) the SNC stones were igneous, i.e., volcanic; (b) all examples then known had solidified 1.3 billion years ago or less; (c) only planet-sized bodies still had volcanoes going that late in the game--earth's moon cooled off roughly 3 billion years ago, and asteroids chilled shortly after their creation roughly 4.5 billion years ago; (d) of the two planets that might reasonably be said to be within a stone's throw of us, Venus's strong gravitational pull and thick atmosphere make it an unlikely launching padfor meteors; so (e) that pretty much leaves Mars.

Persuaded? McSween's fellow rock jocks initially didn't buy it either. But then other researchers analyzed the minute bubbles trapped in a shergottite meteorite, which presumably were a trace of the atmosphere of the rock's home planet. The noble gases in the bubbles (argon and such, which being noble don't change over time and so are a sort of planetary fingerprint) precisely matched those in the Martian atmosphere as sampled not long before by the Viking landers.

A pivotal question remained: What launched the pieces of Martian rock into space? A monster meteorite strike was the obvious answer, but some objected that the energy of such an impact should have turned the future SNC meteorites into glass or vaporized them altogether. Two developments helped blunt that argument. The first was theoretical work demonstrating that a percentage of impact debris would (OK, could) survive ejection into space relatively unscathed, and the second was the discovery of a lunar meteorite in Antarctica in 1982. (That find, incidentally, addresses your question about whether meteorites from other planets have been discovered. Answer: yes, if you count the moon. We know the Antarctic meteorite came from the moon because it matched samples brought back by the Apollo astronauts.) If a celestial chip shot can blast rocks off the moon, the thinking went, they can get blasted off Mars, too.

At this point you may be saying: This is not a story I'd care to run past a judge. Maybe not, but it seems to have carried the day in the scientific community. (You understand I'm omitting a lot of corroborative technical detail.) That's more than can be said of the claim that one Martian rock contains fossils of stuff that once was alive. According to McSween, nobody believes this now except its original proponents.

That brings us to your last question: If we've got the rocks, why go intospace for more? Couple reasons: (1) With all respect to the geniuses who thought it up, some of us would like a little proof of this cockamamie theory, e.g., an actual Mars rock for comparison. (Conceivably a sophisticated probe could do the analysis on Mars and radio back the results.) (2) Because most Martian meteorites are relatively young, and the interesting stuff on Mars happened earlier. OK, maybe the odds of there having been Martian life way back when are slim. If it happened, though, wouldn't you want to know?

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