Jason Shilling Kendall: Citizen Astronomer

William Paterson University
Amateur Astronomers Association of New York

How does a rock from Mars become an asteroid?


2012-09-18:
Hello Jason, my wife and I really enjoyed your spirited presentation at the Explorers' Club. Over the course of the Q&A, I found myself with a question, which there was no time to pose: Unless I misheard, I think you mentioned an asteroid that came from Mars, and discussed what its composition might mean for the origins of life on earth. My question is this: How does a rock from Mars become an asteroid? Does it get ejected from the surface by a different asteroid smacking into Mars? If so, can it really be ejected with such force that it escapes Mars' gravitational pull? Or were you referring to an asteroid that's been whizzing around our solar system since Mars was first formed? Thank you. And thanks for the stellar (ha!) talk/slideshow/video/3D experience last night. It was a great deal of fun for us.
--J. M.

Thanks for the question. Yes, there are numerous known meteorites from Mars for which the evidence is quite compelling. The meteorites are thought to be from Mars for two basic reasons: what makes up the rocks, and the gases that are trapped inside. The particular isotope ratios of the elements in the 99 know Martian meteorites are not in ratios found on Earth. Also, if you release the gases trapped in the meteorite samples in a very controlled way, you can actually tell what gases got trapped in there. That seems to say that it's like Martian air, rather than Earth air. This air composition was measured by the Viking landers back in the 1970's. So, how did they get here? Not from a huge volcano, even though the samples are volcanic in nature. It is thought that an asteroid impact on Mars threw some material off Mars to orbit the Sun and eventually collide with Earth. This is an OK picture, because Mars' surface gravitys only 38% that of Earth. So, it doesn't take as much energy for something to be thrown into space and not fall back down to Mars as it does on Earth. Meteorites are the poor man's sample return mission. To give a feel of gravity is different on Mars, if you can do a good 3 foot jump straight up in the air, then on Mars, you'd be able to jump almost nine feet up. That's why Curiosity's long arm, which weighs over 200 pounds on Earth, only weighs 70 or so pounds on Mars. Curiosity is of course not coming back or returing a sample. But with its onboard geochemistry laboratory, it'll be able to know the composition of the Martian soil and rocks and determine if Mars was habitable in the past.

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William Paterson University Department of Physics American Astronomical Society Amateur Astronomers Association of New York Astronomical Society of the Pacific