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Thomas J. Wdowiak is an Athena scientist who takes time out of his busy schedule to show kids how to do fun science experiments at home. When he isn't teaching at the University of Alabama at Birmingham or training at the Jet Propulsion Lab in California, he writes for the local newspaper in Birmingham in the "Just for Kids" section. He goes by "Tommy Test Tubes," a nickname given to him when he was a kid. Now "Tommy Test Tubes" contributes his experience and knowledge to the Athena web page with a new column. If you have a question for Tom, click here.

What are the two moons of Mars named and how did they get into orbit around Mars?

The first part of this question is very easy to answer because the discoverer of the martian satellites wrote about how he named them. The second part is still very much a mystery, the answer to which remains in the future!

Asaph Hall, an astronomer at the U.S. Naval Observatory in Washington DC, made the discovery of the martian moons in 1877. He chose the names from the ancient book of the Greek writer Homer called the Iliad. In that writing Mars, the god of war, was called Aries, and had two helpers Phobos, or Fear, and Deimos, or Flight, to handle his horses. Of course the word phobia or fear of things is a familiar one.

Why does Mars have moons? Well, in the first place, Phobos and Deimos are really small compared to Earth's moon. They are very small compared to most other moons such as those of Jupiter. Saturn has some small satellites that orbit with parts of the ring structure of that humongous body. Color measurements made from robotic missions in orbit around Mars, and at times in position to see the moons, shows them to be very black. This is probably why they were not found until 1877 even though astronomers had searched. On the other hand Jonathan Swift, during the 1700’s wrote in his story “Gulliver’s Travels” that Mars had two little moons!

Not only are they small, they orbit pretty close to the planet. Phobos has an orbit that is only 2.76 times as large as Mars. For Deimos the distance is about 7 times the radius of Mars. In comparison the Earth’s Moon orbits at a distance that is around 60 times the radius of the Earth! Both of the moons of Mars have orbits that are elliptical in shape, but not by much. Deimos’ orbit is pretty close to being a circle. In fact the Earth’s Moon ‘s orbit is more elliptical than either of Mars’ satellites. Another thing about the orbits of Phobos and Deimos is that they are pretty much level with each other.

Given that the Earth’s Moon is much larger, and Venus and Mercury don’t appear to have satellites, the moons of Mars are pretty unique as far as the inner Solar System is concerned. Why are they there? Either they were formed along with Mars, about 4.5 billion years ago, or else Mars has captured them. A lot of people (including myself) favor the idea that Phobos and Deimos are captured asteroids. These are small bodies concentrated between the orbits of Mars and Jupiter, what has been called the “asteroid belt”. Asteroids do orbit outside the asteroid belt including within the Earth’s orbit, so it is possible for asteroids passing near Mars to be captured if they come within a distance equal to several hundred times of the martian radius. This places them within the grip of Mars’ gravity. Obviously, both Phobos and Deimos are within that distance. The catch, a very appropriate word, is that energy in the motion of the asteroid would have to be reduced, otherwise it would just fly by in an altered orbit (unless, of course, it hit Mars and all those craters there tell us that has happened over the last 4.5 billion years.)

Flying by or crashing into a planet are easy things to happen. Losing energy to go into and orbit around a planet is another matter. A simple experiment shows why. One marble or billiard ball hitting another does not result in both traveling together, Try to do it! Unless the impact is severe enough to smash (as in making a crater) the balls or marbles just bounce off each other.

On the other hand, if three balls are involved, two can stay together with the third going off on its own. A possible scenario for getting a martian satellite is for an asteroid passing in the vicinity of Mars to be hit by a smaller object. Some energy in the motion of the asteroid goes into the blast of the impact, shooting stuff into space, and it slows down into an orbit around Mars. Spacecraft flying by asteroids show they are heavily cratered, so impacts do happen. The trick is that it would have to occur within the gravitational influence of Mars and be just right. That’s pretty tricky. Also, remember how “neatly put together” the orbits of Phobos and Deimos are. On the other hand, Mars just has two satellites and there have been billions of years of time available.

The reality is that we probably need to know much more about the moons of Mars, including what can be learned by a visit to them. Also, as we become more experienced with all the kinds of things that go on throughout the Solar System, new ideas are sure to emerge in the minds of imaginative people. Then a good explanation will emerge, and that is what scientific exploration is all about.

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