First published in The Tablet in February, 2005… and on The Catholic Astronomer several years ago. Note that Spirit and Opportunity, originally designed to be a 90 day mission, have now been in operation for more than 5000 days!
In January , the Opportunity rover that has been trundling across Mars came upon a pitted lump of iron and nickel, about the size of a basketball. The rover’s chemical tests confirmed that it had found an iron/nickel meteorite, a stray bit of a broken-up asteroid fallen from the sky, with a composition like those that have fallen onto the Earth.
A lump of metallic iron is not what one would expect to find on Mars. The Martian atmosphere is rich in carbon dioxide; the oxygen from that carbon dioxide, and from the water we now know once flowed on the surface, should be enough to turn metallic iron into a rusty pile of iron oxide. And in fact, oxidized iron is what we normally see on the surface of Mars. That’s where it gets its red color. The high nickel content is another clue: when planets are formed, that rusting process separates the nickel from the iron, with most of the nickel winding up deep in the planet’s core.
Indeed, since the Mars iron was found on a flat desert plain as bleak as the South Dakota badlands, far from any Martian volcano or other source of rock, it’s pretty easy to conclude that outer space is the only place it could have come from.
It’s no surprise that meteorites would be found on Mars. [In 2000], Phil Bland and Tom Smith at the Open University calculated that Mars should have many more meteorites on its surface than Earth does. Given the thin air and cold temperatures, meteorites rust away more slowly on Mars than on Earth. And since Mars lies halfway to the asteroid belt from us, those space rocks don’t have to travel so far to reach its surface.
Indeed, Bland and Smith warned that probes to the Martian surface will have to pay attention to be sure that any rock they analyze, thinking it came out of the interior of Mars, is not in fact a stray bit of asteroid.
How could you tell? It’s not a trivial problem to do remotely. If you have a sample in the lab, you can measure things like oxygen isotope abundances which vary distinctively from planet to planet. Or you could look for the decay products of radioactive elements made inside the meteorite when it was exposed to cosmic rays while traveling in space. But the only obvious test that the Mars robots can do is to measure the iron and nickel. Mars rocks are rusty, while most meteorites are not.
Most meteorites; but not all of them. We have found about two dozen rocks on Earth with exactly the same chemical composition as Moon rocks; we can tell, because we have Moon rocks in hand that the Apollo astronauts returned. (In fact, one of those Apollo rocks contained a bit of meteoritic material; meteorites can be found on the Moon, too.)
But there are also another couple of dozen different meteorites whose compositions indicate they are relatively young, some a mere hundred million years old, unlike the 4.5 billion year age of most other meteorites. And they come from a place with air rich in carbon dioxide, and perhaps some liquid water. In other words, we think they come from Mars itself. One of them even has bubbles of gas trapped in the rock with exactly the composition of Mars’ atmosphere.
The only problem here is that, judging from what we’ve seen from our orbiters and landers, most of Mars’ surface is covered with a kind of rock different from most of the purportedly Martian meteorites. So it was with some relief when in April 2004, the Opportunity rover did come across a rock on the surface of Mars with the same composition as our “Martian” meteorites. Proof, it was thought then, that those meteorites did indeed come from Mars.
But it’s always possible that there is some other source for those meteorites. Maybe what we found on Mars last April had also fallen there from space – just like our lump of iron.
Most scientists, including me, are pretty confident that’s not the case. Mars is the only place we know that’s big enough to have stayed molten inside, up to a hundred million years ago when the meteorites crystallized out of their lava. (All the asteroids, and even the Moon, have been cold and solid for billions of years.) And the Mars atmosphere has evolved in a strange way; it’s hard to imagine that another place could have reproduced the composition of the air seen in the Martian meteorites.
But confidence is not proof.
And that’s all right. Science does not deal with proofs; it describes, it does not prove. When more evidence lets us improve our descriptions, we’ll happily change our textbooks. A real scientist would never insist that any one given description is absolutely correct.
Indeed, all of life consists of making decisions, and carrying on, without any surety of absolute proof. Whether it is belief in Mars rocks, or belief in God, we ultimately can only decide which description makes the best sense of the things we do know, in the light of our own experience of how we’ve seen the universe work.
[This is by no means the only iron meteorite found on Mars, as outlined in this Sky and Telescope article…]