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A whole lotta shakin' goin' on thanks to Mars

Paul Sutherland, News Editor
21 November 2013, 12:11

Sen— Mars is shaking up passing asteroids, planetary scientists have discovered, giving them a fresh new appearance.

The finding is the latest bit of evidence that helps solve a long-standing mystery of why the surfaces of most asteroids seem to be redder than the meteorites they produce that have hit the Earth.

An answer to this riddle was proposed in 2010 by Richard Binzel, a professor of planetary sciences at Massachussetts Institute of Technology. He suggested that asteroids orbiting the Sun in the main belt far beyond Mars but before Jupiter were reddened by cosmic radiation that altered the chemical nature of their surfaces.

Binzel found that asteroids that had left the main belt and came close to Earth, however, were affected by our gravity which caused them to suffer quakes that shook up their surfaces and exposed fresh material underneath. The less-red meteorites falling to Earth came from these close-passing asteroids.

The latest discovery is that Mars also “refreshes” asteroids in this way when they pass close enough by, despite the fact that it is a smaller planet, a tenth as massive as Earth, and has a weaker gravitational pull that is only a third of our world’s.

Binzel and colleague Francesca DeMeo calculated the orbits of 60 refreshed asteroids, and found that 10 per cent of these never cross Earth’s orbit. Instead, these asteroids only come close to Mars, indicating that the Red Planet can affect their surfaces too.

They suggest that though Mars has less influence as a Solar System body, the fact it orbits closer to the asteroid belt means that there is a greater chance of close encounters with these rocky bodies. Incidentally, its two small moons, Phobos and Deimos, are believed to be captured asteroids.

Binzel said: “Mars is right next to the asteroid belt, and in a way it gets more opportunity than the Earth does to refresh asteroids. So that may be a balancing factor.”

Mars

A Hubble Space Telescope image of Mars, a force to be reckoned with. Credit: NASA/ STScI

DeMeo looked through an asteroid database created by the International Astronomical Union’s Minor Planet Center after she suspected that Mars might have a hand in altering asteroid surfaces. The database lists observations of 300,000 asteroids and their orbits, with 10,000 of them considered near-Earth asteroids.

Over the past decade, Binzel’s group has tracked the brightest of these asteroids, measuring their colours to find out which may recently have been refreshed. For this most recent paper, the researchers looked at 60 such asteroids, mapping out the orbit of each and determining which orbits had intersected with those of Earth or Mars.

DeMeo then calculated the probability, over the last 500,000 years, that an asteroid and either planet would have intersected, creating a close encounter that could potentially generate asteroid quakes.

Binzel added: “Picture Mars and an asteroid going through an intersection, and sometimes they’ll both come through at very nearly the same time. If they just barely miss each other, that’s close enough for Mars’ gravity to tug on the asteroid and shake it up. It ends up being this random process as to how these things happen, and how often.”

From their calculations, the researchers found that 10 per cent of their sample of asteroids only cross the orbit of Mars and not Earth’s. DeMeo explored other potential causes of asteroid refreshing, calculating the probability of asteroids colliding with each other, as well as the possibility for a phenomenon called “spin-up,” in which energy from the sun causes the asteroid to rotate faster and faster, possibly disrupting its surface.

From her calculations, DeMeo found no conclusive evidence that either event would significantly refresh asteroids. Binzel said: “Mars is the only game in town.”

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