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The odds of a rock being forcefully ejected from Mars, making its way through the solar system, and landing on Earth are low. But after analyzing the color and composition of a few meteorites, including the Martian meteorite, which has rekindled the search for past life on Mars, scientists are realizing that sometimes the unlikely is the only possibility.
by Allan Treiman, Lunar and Planetary Institute, Houston, Tex.
"Psst! Hey, buddy, come here! You look like a smart sort of person. Want a Mars rock?"
"Well, maybe, if the price were right. But that is just an ugly, old black rock! How can I be sure it really is from Mars?"
It seems crazy to claim that we have rocks from Mars. No human has ever set foot on Mars, no spacecraft has ever returned from Mars, and only two spacecraft ever landed safely on Mars. But the idea that some meteorites come from Mars, first suggested in 1979, is now accepted by most planetary scientists.
Meteorites fall to the Earth from space without signs or name tags like "Product of the People's Republic of Mercury." How can we know where any meteorite comes from? Most likely, they come from asteroids, which circle the Sun between the orbits of Mars and Jupiter. Some meteorites are almost certain to come from the asteroid Vesta. A few are from the Moon, and a few are from Mars!
Meteorites come in three basic varieties: iron, stony irons (mixed iron and silicates), and stones or silicate rock meteorites. Most of the stony meteorites are chondrites, collections of the dust, sand, and gravel that orbited the Sun before the planets and asteroids formed. A small percentage of meteorites are achondrites—igneous rocks that formed from lava on planets or from large asteroids. Most achondrites are basalts or breccias of basalt fragments and look very much like Earth basalts.
A dozen of the meteorites that have been analyzed are from the Earth's Moon. They were compared with Moon rock samples brought to Earth by the Apollo astronauts and the Luna robots, and they were found to be essentially identical. Without the Apollo and Luna samples from the Moon, we might still be arguing about these meteorites.
Since most meteorites are not similar to Moon rocks, scientists looked to other sources of rocks in the solar system. Asteroids, which are made of rock and come in different sizes, are the most likely sources of meteorites. In size, asteroids can range from 910 km across (Ceres, for example) to the smallest rocks that can be detected by telescope. We know the orbits that some meteorites were in before they hit the Earth, and most of them have been tied to the asteroid belt, which suggests that they are formed of asteroids. The orbit of a meteorite can be figured by photographing their meteor streaks in the sky from two (or more) different places. These photographs can be combined to make a three-dimensional view of the meteor streak, which is the meteorite's track through the air.
Another clue to a meteorite's origin is its color: is any asteroid or planet the same color as the meteorite? The most useful colors for this test are not those that can be seen in visible light, but colors that appear in the infrared light spectrum. Rocks and asteroids appear to be very different in color when viewed in infrared light. Most asteroids don't look similar in color to most meteorites, but some do. One asteroid, Vesta, has the same infrared color as most of the basalt meteorites. With this color match, it is almost certain that these basalt meteorites come from Vesta or other asteroids like it.
But what about meteorites from Mars? A dozen of the basalt meteorites are quite different from the Vesta basalt meteorites. They contain some water, while the Vesta basalts are bone-dry, and they crystallized from lava between 1,300 and 180 million years ago. The Vesta basalts all crystallized about 4,500 million years ago, which means they are almost as old as the solar system. In many other ways, the chemical and isotopic compositions of the oddball meteorites are quite different from the Vesta basalts and from Earth basalts.
The oddball meteorites were grouped together as `SNC' meteorites, but where did they come from? Their young crystallization ages suggest that they came from a place with recent volcanic activity. And their chemical compositions are similar to Mars soil, which was analyzed on Mars by the Viking Lander spacecraft. Where else could they come from except Mars? The cratered surfaces of Mercury and the Moon are much too old and dry; Venus is much too dry; the outer planets are all gas, and their moons are mostly ice. Mars is the only possible source!
The best evidence for the SNC meteorite originating at Mars was provided by measurements of the composition of the planet's atmosphere made by the Viking Lander spacecraft. In a different study, D. Bogard of the Johnson Space Center heated black glass from inside a SNC meteorites and analyzed the gas that bubbled out. To his surprise, the argon gas from the meteorite had the same amount of isotopes as the Martian atmosphere. Since then, Bogard's work has been extended to element and isotope abundances of nitrogen, neon, krypton, and xenon; within error, the Martian atmosphere (measured by the Viking spacecraft on Mars) and the gas from the SNC meteorites are identical.
The identity of the meteorite gas and the Martian atmosphere would mean nothing if this gas composition were commonly found in the solar system. So far, however, the gas is unique to Mars. Gas like it has not been found anywhere else: not on Earth, not in meteorites, not in other planet atmospheres, and not in the Sun. The similarity in gas is not absolute proof that the Martian meteorites are from Mars. There could be some planet or asteroid, so far undetected, that has young volcanoes, chemical compositions like Martian soil, and gas like the Martian atmosphere. But it seems simplest to accept that the SNC meteorites really are from Mars.
"See, buddy, they really are Mars rocks! Still interested? It's a one-time only offer!"
"Maybe. But how did you get them?"
How could a rock from Mars get to Earth? It would have to get off Mars somehow to escape Mars' gravity. It would have to escape and wander through the solar system, closer to the Sun, until it got into the Earth's orbit.
Mars is a large place, about half the diameter of Earth, and it has about one-third of the Earth's surface gravity. How could a rock be propelled off the Mars' surface faster than 5 kilometers per second—Mars' escape velocity? Volcanic explosions aren't strong enough to propel rocks off Mars. The only process with enough energy appears to be a meteorite impact. If a small asteroid hit Mars, it would make an impact crater, and a bit of its energy could go to blast rocks up from Mars' surface, and some of these rocks would fly out of Mars' atmosphere. Once out of Mars' atmosphere, most of the rocks would crash back down onto Mars, but a few would would leave Mars completely to orbit the Sun as little asteroids. The shock from an asteroid impact also can force Martian atmosphere gas into rocks. In experiments, gas in fractures in a rock becomes trapped as an impact slams the fractures shut.
The passage from Mars to Earth is relatively easy. Most of the ejected Mars rocks come right back to Mars, making meteors and meteorites on Mars. Some of the rocks do not hit Mars, and their orbits get bent into long ellipses that extend outward into the asteroid belt and inward toward Earth. Repeated close encounters with Mars, over about 10 million years, can nudge the ejecta orbits to cross the Earth's orbit. That is how we get new Martian meteorites!
I am a scientist at the Lunar and Planetary Institute. As a child, I loved astronomy, rocks and fossils, and word play. I finished high school, got a B.A.
in chemistry from Pomona College, and a Ph.D. in geology from the University of Michigan. Unable to get a real job in oil or mining, I found a research position where I could work on meteorites.
It was so much fun that I kept doing meteorite research, and I am constantly amazed that I get paid for it. I find that 90% of my work time is spent writing about the research that fills the remaining 10%.
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