How Did a Piece of Mars Get to Earth?

Scientists think that martian meteorites found on Earth have been ejected from the red planet by asteroid impacts. Calculations suggest that there are two ways this can happen. First, an asteroid at least ten kilometers (six miles) wide could fall straight down to the martian surface. The impact crater left behind in this event would be round and at least a hundred kilometers wide.

It’s equally possible that a smaller asteroid, perhaps one kilometer in diameter, might be able to knock material free from Mars, if it approached the martian surface at a very low angle. In this case, the asteroid would swoop in more like an airplane landing than a rock falling straight down. Its impact crater would be elliptical and smaller, as small as ten kilometers wide.

No one can say exactly what trajectory ALH84001 took when it spalled off the planet. Scientists will only say that it drifted among the planets of the inner solar system until it encountered Earth some 13,000 years ago. But, in light of the two possibilities suggested above, scientists now think they may know the place on Mars where ALH84001 originated. This is important because identification of the meteorite’s site of origin may have a bearing on future Mars missions.

Planetary scientist Nadine Barlow of the University of Central Florida led the search for the source crater. With the help of a comprehensive crater catalog that she had compiled while doing her graduate work at the University of Arizona in the mid-1980s, and taking into account a number of distinctive characteristics of the meteorite itself, Barlow was able to narrow the search to two possible source craters on Mars.

Because of its great age, the meteorite must have come from the most ancient of martian terrain. Because the meteorite was chipped off Mars only 16 million years ago – very recently on a geologic timescale – Barlow reasoned that the source crater must still exhibit young features, such as a sharp crater rim and an unweathered blanket of ejected material surrounding the crater.

Also, ALH84001 shows evidence of a shock event that preceded its ejection from Mars. In other words, while it was still a part of the martian crust, an asteroid or comet impact may have occurred in the same vicinity. Thus older craters may be found near the source crater. And there is one more clue to the meteorite’s origin. The presence of carbonates in the meteorite suggests that the impact site was located in a watery region.

Barlow’s study used her own crater catalog to identify two possible sites of origin for ALH84001. Both are small, elliptical types located in the heavily cratered southern highlands of Mars. The first crater is located in the Sabaeus Sinus region of Mars just south of the planet’s equator. It is located less than 10 kilometers from the second candidate, an older crater in an area that also shows some possible evidence of ancient river and flooding activity.

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