Ohio Schools Evolve

7 November 2002

Liftoff for Mars Meteorites

A trip from Mars to Earth would be a blast for an astronaut, but it's no big deal for a rock. That's the conclusion of a new study, which finds that small asteroids striking the newest, most pristine parts of Mars are powerful enough to launch millions of chunks into space. The results may explain why nearly all Mars rocks discovered on Earth are relatively young objects, rather than relics of the red planet's ancient past.

Meteorite hunters have recovered 26 rocks that bear chemical traces of the martian surface and atmosphere. These meteorites--preserved mostly in Antarctic ice fields or Saharan dunes--must have been ejected from Mars by impacts big enough to fling them out of the planet's gravitational grasp and send some of them toward Earth. However, the rocks' ages are odd. All but one are between 200 million and 1.3 billion years old, even though about half of Mars' surface is 4 billion years old. Because asteroids must strike all areas on Mars equally over time, planetary scientists wondered why Earth hasn't gathered more samples of ancient Mars.

The answer lies in the varied terrain, according to an online paper in the 7 November Science Express. Planetary scientist James N. Head of Raytheon Missile Systems in Tucson, Arizona, and his colleagues used a computer model to investigate how shock waves from impacts hurtle martian rocks into space. When the terrain is young--smooth basalt from recent eruptions, for example--an asteroid that creates a crater just 3 kilometers wide can propel millions of fist-size and basketball-size rocks into space. But older terrain, battered by eons of impacts and laced with fractures, absorbs much of the impact's energy. As a result, it takes a 20-kilometer crater--a far more rare event--to expel swarms of rocks into space. "It's so much easier to launch the young material that it dominates what we receive on Earth," Head says.

"This is important work, because it explains an observation that has puzzled us," says planetary geologist Allan Treiman of the Lunar and Planetary Institute in Houston. The claim that a 3-kilometer crater is big enough to blast Mars rocks to Earth is "a very welcome surprise," Treiman says. Ejections in the Earth-to-Mars direction are harder, he notes, because Earth's higher gravity and thicker atmosphere keep many impact-launched rocks from escaping.

--ROBERT IRION