Newspaper article The Christian Science Monitor

Could a Missing Moon Explain Mars's Mysterious Little Satellites?

Newspaper article The Christian Science Monitor

Could a Missing Moon Explain Mars's Mysterious Little Satellites?

Article excerpt

The Martian moons, Phobos and Deimos, have long intrigued scientists. The two small, potato-shaped space rocks that whirl around Mars aren't anything like our own moon. They're much smaller, oddly shaped, and orbit the Red Planet farther away than scientists would expect to see in a system with just two little moons. So how did they get there?

Some scientists argue Mars's moons are asteroids, stolen from the Asteroid Belt eons ago. Others say they formed from the debris shot into space after a large celestial body slammed into the planet billions of years ago, in a scenario similar to how our own moon formed. But neither theory fully explained their unique orbits.

To sort out the complexities, an international team of researchers created a computer simulation of the impact theory. Their model suggests another, larger moon facilitated the formation of Phobos and Deimos. Their findings are reported in a paper published Monday in the journal Nature Geoscience.

When scientists first began puzzling over the two moons, most assumed that the lumpy satellites, just 14 and 8 miles across, must have been asteroids captured by Mars's gravitational field.

But "you can't, in any reasonable way, make the moons of Mars by capturing them as asteroids," says David Stevenson, a planetary scientist at the California Institute of Technology who was not part of the new study, in an interview with The Christian Science Monitor. "The dynamic challenges in capturing asteroids and putting them in orbits like Phobos and Deimos are so huge that people, for the most part, decided long ago that that was not a good idea."

So then the question was, can an impact scenario explain it? Yes, says the lead author of the new paper, Pascal Rosenblatt of the Royal Observatory of Belgium. But it was probably a complex process, he acknowledges.

The story goes something like this:

Billions of years ago, a celestial body about one third the size of Mars slammed into the Red Planet. Debris from the impact shot out into space but didn't escape the Mars system. A ring of dust and debris, similar to the one around Saturn, formed around the planet.

Over time, the close-in debris spiraled back into Mars, while the outer rings combined to form the planet's satellites.

That story sounds simple enough, but there's a catch. The current system doesn't match what scientists would expect based on calculations of their formation.

One such challenge of this story is the "synchronous radius," which is the "dead zone" within which satellites will fall back into their host planet. In the Mars system, that distance is about 18 Mars radii.

Mars rotates slowly enough that satellites or debris orbiting within the dead zone have to move faster than than the planet spins. The problem is that the satellites outpace the tidal bulge they pull up, which creates enough torque to steal momentum from the satellite, ultimately causing it to spiral down into Mars. …

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