Last time, we adopted a different format to take a look at NASA’s aspirations in relation to our mysterious neighbor, Mars. This time around, we’re back to normal, and will be dipping into some interesting facts about the many moons that fill our solar system.
As we contemplated future missions to Mars in the last blog, I thought that would be a great place to start. So, today, we’ll look at 10 facts you might not have known about the two moons of Mars – Phobos and Deimos.
1. Why are Phobos and Deimos named that way?
Phobos is named after the Greek god Phobos, a son of Ares (Mars) and Aphrodite (Venus) which was the personification of Horror. Deimos was named thus, as he was the twin brother of Phobos, and is meant to personify terror.
2. How were they discovered?
The moons were discovered by astronomer Asaph Hall on 18 August 1877, at the United States Naval Observatory in Washington, D.C. He was specifically searching for Martian moons at that time, as he thought he’s spotted something on August 10th. However, but bad weather prevented his confirmation, and he had to wait over a week to confirm his discovery.
3. How big are Phobos & Deimos?
Phobos is an irregularly shaped object, measuring some 27 x27 x 18 km, (16.7 x 13.6 x 11 miles).
Deimos is much smaller, measuring some 15 x 12.2 x 11km, (9 x 7.5 x 6.8 miles)
4. How far out is Phobos from the surface?
To begin with, Phobos has been described as “the best studied natural satellite in the Solar System“. Its close orbit around its parent planet produces some unusual effects. With an altitude of only 5,989 kilometers (3,721 mi), Phobos orbits Mars below the synchronous orbit radius. What that means is that it moves around Mars faster than Mars itself rotates.
So, from the point of view of an observer on the surface it rises in the west, moves rapidly across the sky, and sets in the east, approximately twice each Martian day. Since it is close to the surface and in an equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4°. Its orbit is so low that its angular diameter, as seen by an observer on Mars, varies visibly with its position in the sky. In fact, Phobos can appear as large as one-third as wide as the full Moon as seen from Earth.
5. What about Deimos?
Deimos orbits at 23,400km, (14,500 miles), and has a very circular orbit close to Mars’ equatorial plane. Because of this, Deimos regularly passes in front of the Sun – as seen from the surface of Mars – but is far too small to cause a total eclipse.
6. Where are they thought to have originated?
Scientists are still not sure. One Hypothesis regarding both moons, is that they might be second-generation Solar System objects that coalesced in orbit after Mars formed. Another hypothesis is that Mars was once surrounded by many small-sized bodies, perhaps ejected into orbit around it by a collision with a large planetsimal.
Of note is the fact that the high porosity of the interior of Phobos in particular – and the surface scarring – is inconsistent with an asteroidal origin.
7. What interesting facts do we know about Phobos?
Tidal deceleration is gradually decreasing the orbital radius of Phobos. It is thought that Phobos will be destroyed in less than 30–50 million years. Because Phobos has an irregular shape and assuming it is indeed porous, it has been calculated that there is little danger of it smashing into the Martian surface, as it will break up when it reaches approximately 2.1 Mars Radii.
8. And Deimos?
This little moon has a density very similar to that of a C- or D-type asteroid. Like most bodies of its size, Deimos is highly non-spherical and is composed of rock rich in carbonaceous material.
Of interest is the fact that it is cratered, but the surface is noticeably smoother than that of Phobos, which might be caused by the partial filling of craters with regolith. That means it is even more porous than Phobus and tends to suggest it might have originated from a different source and at a different time
9. Does the Martian family have a ring system?
Interesting point. Faint dust rings produced by Phobos and Deimos have long been though to exist, but so far, attempts to observe them clearly have failed. Whether these are actual rings, or clouds created by impacts from other bodies, is not known, but it raises another interesting point. How would such material stick to an object with almost no gravity? We shall see…
10. And finally, is there anything else we need to know?
Actually, yes there is. The unique Kaidun meteorite which fell on a soviet military base in the Yemen in 1980 is thought to be a piece of Phobos, but this has been difficult to verify since little is known about the detailed composition of the moon.
So there you go. For such tiny, insignificant moons, we’ve learned quite a lot. Next time, we’ll look at 10 interesting things about the Moon’s of the next planet out in our Solar System, Jupiter.
An astronomy and law graduate, he is the creator of the international number one bestseller, The IX, and also has the privilege of being a member of the Science Fiction and Fantasy Writers of America and the British Fantasy Society.
When not writing, Andrew devotes some of his spare time to assisting NASA with one of their remote research projects, and writes educational articles for Astronaut.com and Amazing Stories.
He also enjoys Greek dancing and language lessons, being told what to do by his wife, and drinking Earl Grey Tea.
If you would like to find out more, visit his blog or website at:
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