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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