The moons of Mars are not only potential stepping stones to the Red Planet itself, but interesting objects in their own right: Two relatively large, irregularly-shaped, asteroid-like bodies in close proximity to each other and in orbit around an entire world of possibilities. Given the difficulties of landing on the Martian surface, the immense value to be gained studying asteroids and objects similar to them, and the utility of being in Mars orbit, the attendants of the God of War - named in the Greek for Fear and Dread - may yet prove to be more auspicious than their names imply.
The progress of our adventure so far (current in bold):
1. The SunPhobos:
4. Earth (Vol. 1)
5. Earth (Vol. 2)
6. Earth (Vol. 3)
7. Earth (Vol. 4)
8. Earth (Vol. 5)
9. Earth (Vol. 6)
11. Mars (Vol. 1)
12. Mars (Vol. 2)
13. Mars (Vol. 3)
14. Phobos & Deimos
25. Tethys, Dione, and Rhea
28. Rings & Minor Moons of Saturn
30. Moons of Uranus
33. The Kuiper Belt & Scattered Disk
35. The Interstellar Neighborhood
Phobos, the inner moon, orbits Mars at a distance of about 9,400 km in a highly regular, circular orbit that only varies in distance by 1.5% over its course. By contrast, our Moon is about forty times further away from Earth, but because Phobos is much smaller and less massive than the Moon, it has little appreciable gravitational influence on Mars and is much smaller in the Martian sky. Deimos, the outer moon, orbits Mars at a distance of about 23,000 km, and because it's significantly smaller than Phobos appears only as a bright star in the Martian sky. See Vol. 1 of the Mars sub-series for a wider context within the solar system. Orbital diagram, to scale:
Both moons orbit close to the Martian equator, which means their orbital planes are both within a couple of degrees of Mars' rotational inclination relative to the ecliptic plane (26° and 27°, respectively). This fact, and the circularity of the orbits, has important implications for determining how the moons formed, which I discuss below. Some contextual images - Phobos in front of Mars, taken by the Mars Express orbiter:
Phobos passing near alignment with Deimos:
Phobos passing in front of Jupiter:
Relationship in the Martian sky:
There are two competing origin hypotheses for Phobos and Deimos, although it is generally agreed that both moons should be explained by the same process: The first hypothesis, based on the obvious similarities between the two objects and asteroids, is that they were originally part of the Main Belt and were perturbed toward Mars by Jupiter at different times. The second hypothesis is that they are remnant pieces of ejecta from Mars itself thrown into orbit by a major impact. No clear consensus has yet developed on the question, but to my semi-layman's eyes the balance of evidence appears to lean toward a Martian origin.
Although it is physically possible for the moons to have originally been asteroids, and a number of physical characteristics resemble those of C-type or "carbonaceous" asteroids found in abundance in the Main Belt, there is still a big question about how captured objects would have both ended up in highly circularized orbits near to the Martian equatorial plane. Recall that this plane is inclined 25° from the ecliptic - it seems intuitively unlikely that two different objects from another part of the solar system would have both been perturbed into orbit around Mars, and both ended up with circular orbits close to an equator drastically inclined from the ecliptic. Of course, this intuition is not sufficient, because there are tidal and other processes that can progressively move objects in varying orbits toward circular orbits close to a single plane.
However, the moons also possess a great deal of compositional similarity to Mars that is not consistent with an asteroid origin, Phobos has a high degree porosity (i.e., sponginess) that is not like an asteroid, and the orbits of the moons are more simply explained by their having been ejected from Mars itself. Features on Phobos in particular favor a history of being repeatedly sand-blasted by subsequent ejecta from the planet, which further increases the credibility of the hypothesis that the moons formed from just such material. In addition, both moons are very dusty, covered in dozens to hundreds of meters in fine-grained regolith, which also supports the impact-accretion scenario: Asteroids, by contrast, tend to be much chunkier and more heterogeneous because they are usually fragments or loose agglomerations of fragments from other bodies. This does not rule out an asteroid hypothesis though - captured asteroids could just as well be pelted with ejecta and build up thick coats of dust once in Mars orbit.
The fact that Deimos appears dustier and smoother while Phobos looks blasted may be a result of their respective orbits, with Phobos closer to Mars and thus having to move through thicker ejecta more often. Deimos, meanwhile, could just passively absorb dust further out over time without being subjected to a lot of hypervelocity impacts that may remove more loose material than they deposit. This would be consistent with the fact that Phobos is considerably larger and more massive than its peer while having starker features.
Knowing how old the two moons are will have to await probe landings and perhaps sample return missions - something that is easier than it sounds due to the very low gravity involved. A Russian probe called Phobos-Grunt had been planned to do exactly that recently, but it failed to leave Earth orbit and subsequently burned up in the atmosphere this January, so we will have to wait a few years more to get these answers.
1. Orbital and Rotational Features:
Phobos moves at a little less than twice the speed of Deimos along its orbit - 2.1 km/s as opposed to 1.3 km/s - which further contributes to the higher kinetic energy of the ejecta material it periodically slams into. A video of Phobos passing in front of Deimos, giving a sense of their relative apparent speeds:
There are periodic partial solar eclipses when Phobos transits the Sun:
Such eclipses cast a shadow on Mars visible from orbit - literally the "shadow of Fear":
Deimos, being both much smaller and much farther away than Phobos, does not produce such dramatic imagery. Both moons, however, are close enough to Mars that they are always below the horizon at latitudes greater than certain limits - 70.4° in the case of Phobos and 82.7° for Deimos. One pecularity that differentiates them, however, is that Phobos is so close and moves so quickly along its orbit that it out-races the rotation of Mars, rising in the West and setting in the East twice per Martian day. Deimos, however, does not advance very quickly at all relative to the surface of Mars - it takes a full 2.7 days to rise and set, and does so from East to West because the planet rotates faster than the moon orbits. What this means is that the moons appear to move in opposite directions from the Martian surface even though they in fact orbit in the same direction.
Both moons are tidally locked to Mars, so their rotational periods are the same as their orbital periods - i.e., they only complete a rotation via completing an orbit, and thus always present the same face to Mars. For Phobos this period is 7 hours and 39 minutes and Deimos 30 hours and 18 minutes, but their respective positions relative to the surface are complicated by the 25-hour rotation of the planet itself. Animations of each moon's rotation:
2. Size and Mass Characteristics
Phobos has a mass of about 10 quadrillion kg or about 1.8 billionths the mass of Earth with a mean radius of 11.1 km. Mean radius is a somewhat complicated figure to work with in dealing with irregularly-shaped bodies like Phobos, because there may be parts of the object extending considerably farther away from (or closer to) the center of mass than the mean radius. Phobos is not that irregular as far as such objects go, but it will nonetheless present challenges to future human crews attempting to operate in its vicinity or "on" its surface (in such low gravity, you are never quite "on" anything). Surface gravity along its equator varies widely depending on where you are relative to mass concentrations, from 190 to 860 millionths of g, so at maximum an Earth-weight 150-lb person would weigh 0.13 lbs or about 2 ounces / 57 grams - the weight of a papercup of water on Earth. Working in such low gravity will present interesting dangers when astronauts are exploring under Phobos's surface, since slow motion cave-ins could doom them without their even noticing it was happening until it was too late.
Deimos has a mass of about 1.5 quadrillion kg or about 2.5 ten-billionths of an Earth mass with a mean radius of 6.2 km. Its surface gravity also varies depending on location, but along its equator is roughly 400 millionths of g - so a 150 lb Earth-weight person would weigh about 0.96 ounces or 27 grams on the outer moon. However, as noted, this could vary by a substantial margin depending on where on Deimos one were located.
If we class them with asteroids, the moons of Mars are middling on the scale of size - dozens of asteroids are in the hundreds of kilometers - but in terms of statistical distribution, they would be very close to the narrowest peak due to the far greater abundance of much smaller objects. Some rough size comparisons illustrating the scale of Phobos and Deimos against other things less than 100 km in diameter - mouse over to see the names of the objects:
Both moons are in vacuum, so temperature swings radically between light and shadow. However, their greater distance from the Sun than our Moon means the high temperature is not nearly as great, and thus the total range is constrained on the high end to just below freezing. The fact that the moons rotate a little less than once (Deimos) or twice (Phobos) per day means the shadowed sides have less time to stay chilled than on the Moon, where the nights are two weeks long, so they don't become quite as cold either. This image shows temperatures on some sunlit and shadowed parts of Phobos, near the moon's largest and most prominent feature, Stickney crater:
Surface temperatures would be very similar for Deimos since it experiences the same solar environment as its companion, although the range might be slightly wider since its days and nights are longer. If either moon has any permanently shadowed or permanently sunlit areas near their poles (I don't know), these would likely have anomalously high or low temperatures compared to the rest of the body.
4. Internal Structure
Not a lot is known about the internal structure of either moon, beyond the fact that both have highly porous regolith and Phobos at least is thought to have substantially-sized internal voids. This is likely a result of the moons being what are called "rubble pile" bodies - their gravity was never sufficient to heat up and fuse material strongly together, so instead it adheres weakly and is bound together both by surface melting from initial impact, the small gravity field, and perhaps electrostatic attraction in the surface dust. What this means for the stability of the bodies cannot be known with any precision until direct observations are made, but it does raise interesting challenges if humans were ever to drill deeply into them for whatever reason - apropos of the possible slow-motion cave-ins I mentioned before.
5. Surface Features
There are 20 features of Phobos given official names by the (IAU) - the global scientific body that names astronomical bodies and features thereon, among other functions. Nomenclature for Phobos is based on themes related to Gulliver's Travels, and also on historical astronomers and their wives. A full list of approved names of Phobos features can be found here via the US Geological Survey, which also provides a comprehensive cylindrical map of the moon, and the following partial map superimposed on photographic images:
An approximate true-color video of Phobos as the Mars Express probe passes by, showing a substantial part of the surface:
Enhanced-color views of Phobos:
What you notice immediately from the enhanced images above are the bright streaks appearing to come out of Stickney crater. At first it was assumed that these streaks were ejecta rays from the impact that created the crater, but close study revealed that the steaks do not in fact converge on the crater - they are close to parallel, and actually result from the moon passing through ejecta from Mars along its orbit. It only appears related to Stickney because the crater is close to the leading edge of Phobos. Additional views of Stickney and the linear tracks - close-up color photos are enhanced color:
The smaller crater inside of Stickney, called Limtoc:
There's a rather large, bright-colored boulder on the inner moon about 85 meters wide called the Phobos monolith that has inspired nutty conspiracy theories, and pollutes any Youtube search results related to the moon with breathless crackpottery. "Monolith," BTW, just means it appears to be a single object - the term has no special significance despite its use in the film and novel 2001: A Space Odyssey to describe an alien artifact. Here is a photo of the monolith:
Deimos, in contrast, has only two officially-named features: Craters Swift and Voltaire, whose namesakes are obvious. This is partly due to the fact that Deimos is relatively smooth and featureless, but there are more than two easily visible craters on the moon, so most likely the IAU just hasn't gotten around to naming the rest yet - they may be waiting for more detailed exploration before taking up the matter, which will probably have to wait a while because there isn't a lot of motivation for it. Phobos is far more attractive due to its proximity to Mars. A "map" of Deimos:
Various views of Deimos, starting with a set of computer-generated orthographic perspectives and then some more photographs:
IV. Modern Relevance to Humanity
The moons of Mars only became relevant to humanity in modern times with their discovery in 1877 by American astronomer Asaph Hall. He had been purposefully looking for moons of the Red Planet, and his efforts were rewarded. Not much happened on the subject for another eight decades or so until the 1950s, when faulty observations caused some scientists to briefly think the density of Phobos suggested a hollow object made out of thin sheets of metal, causing President Eisenhower's science advisor to take the possibility seriously - remember, telescopic observations at the time could not show anything more detailed than a dot of light, so it might very well have been artificial if the density observations proved true.
Of course, they did not prove true - the real figures, although they indicate high porosity, are consistent with a pile of rocky / dusty rubble. That didn't stop one wiseacre from perpetrating a famous April Fool's hoax in 1959 where he cited a fake university as declaring that the moons of Mars were found to be artificial satellites. The "news" was taken quite seriously, since it was plausible under existing information.
Subsequently, the moons - largely Phobos - have been tangential subjects of exploration by a number of Mars flyby missions and orbiters. The Soviet Union had attempted and failed to land probes on Phobos in 1988 in two separate, tandem missions - the aptly-named Phobos 1 and Phobos 2. Most recently, Russia attempted and failed again in the past year with the aforementioned Phobos-Grunt mission.
V. Future Relevance to Humanity
It is much easier to "land" on Phobos than on Mars, and also much easier to leave, yet it's so close that the Red Planet fills an entire quarter of the sky on the Mars-facing side: Thousands of times larger than Earth from the Moon. What this means is that Phobos will very likely play a major role in enabling human exploration and colonization of Mars. A cautious approach would be to establish a continuous human presence on the inner moon well before mounting the first Mars landings, and build up a "base camp" infrastructure to support subsequent ground operations. The significant light-speed delay in communications from Earth (up to about 20 minutes) means that it would be safer and more efficient to have communications infrastructure and supply depots on-location in Mars orbit. These could just as easily be orbited independently, but Phobos seems like it might be a convenient location to concentrate operations and have human crews available to deal with any issues.
In fact, although what passes for a Congress at the moment refuses to fund anything, NASA since 2011 has been looking in greater detail at sending humans to Phobos as a Mars landing precursor that could also meet some of the same objectives as planned human missions to a near-Earth asteroid. The imagery would be breathtaking, there would be real exploration occurring, and it would help build toward human exploration of Mars, but it would retire some of the risks of human Mars missions without having to take them all on at once. For one thing, it would take the same amount of time to get to Phobos as to Mars because they're basically in the same location relative to the vast distances between planets, so mission duration challenges would be addressed. Not much energy is needed to land on or escape from Phobos, so the crew would not have to bring as much fuel, nor would their craft have to be specially designed, robust "landers" - the gravity is so weak that even delicate spacecraft designed to work in weightlessness would be fine.
A lot of things that we need to learn about asteroids could also be learned on Phobos - how to operate in an irregular, very-low-but-not-zero gravity environment; how rubble-pile objects form and how stable they are; how to drill into them without shattering them or causing slow-motion avalanches or cave-ins that are just as dangerous for their momentum even if the astronauts see them coming for a long time; and so on. And meanwhile an entire planet other than Earth would fill the sky over their heads. As a tentative research base, an asteroid-equivalent manned mission, a staging area for manned Mars landings, and a logistics and communications center to enable ongoing human activity in the Mars system, Phobos seems like it will be very useful.
Some day, perhaps even more useful than that:
As envisioned in Kim Stanley Robinson's award-winning Mars trilogy, Phobos could serve as the counterweight and terminal station to a space elevator for Mars - a far more practical prospect for a world with 1/3 the gravity of Earth. Whether this can be done will depend on what is found by earlier missions to the potato-shaped moon - it may be too unstable to support such uses, or its porous structure may be ideal once artificially reinforced in some way. [I completely mis-remembered the Mars trilogy's treatment of a space elevator - they did not, as I thought, move Phobos to a higher orbit for use as a counterweight, but imported a different asteroid.]
The relevance of Deimos to humanity is less clear, except as another asteroid-like body in the Mars system that might be useful to land on and study. It would be less helpful as a staging area for Mars landings or planetary observation, but there might still be reasons to go there after initial exploration. It's even easier to get to than Phobos, and even easier to leave because it's a little farther outside of the Martian gravity well. Still, most likely early efforts would be focused on Phobos.
VI. Future of Phobos & Deimos
The scenarios outlined above are highly likely if humanity expands to Mars, so I see it as likely the mass of Phobos will, over time, end up converted to functional spaces and habitats until it's basically a 3D city rather than a moon.
This process would be greatly accelerated if it did become the counterweight of a space elevator, and would thus end up serving the purpose of a city along a heavily-traveled river, railroad, or highway. At that point Phobos no longer actually exists, but is simply a material, economic, and cultural extension of Mars.
However, if this fails to happen; if humanity ends up ignoring Phobos for whatever reason, or if it fails to achieve its objectives in space; or if there is some future decision on whatever basis to revert the moon back to a natural state; its orbit will ultimately decay toward Mars within the planet's Roche limit, where it will be tidally broken apart into smaller and smaller fragments that ultimately reenter the Martian atmosphere. This would happen 7-10 million years from now: A blink of the eye in solar system time. An animation of Roche limit processes breaking up a mass:
The ambiguity of Deimos' usefulness in some ways makes its future even clearer, because unless some specific use is found for it as a single location, most likely it would just be strip-mined to support the growth of general activity in Mars orbit. Since its gravity is trivial, removing material from its surface and sending it elsewhere - e.g., to Phobos - is also trivially easy. So it either becomes, like Phobos, a city unto itself, or its material is broken up to support other locations. I think we will know within a few centuries which fate will likely apply.
VI. Catalog of Exploration
Every probe listed at the end of Vol. 3 of the Mars sub-series either conducted limited observation of Phobos and Deimos, was intended to do so, or could have done so if it had succeeded. However, only three missions have ever been launched with dedicated probes to moons of Mars, and all three concerned only Phobos, so Deimos has never been independently explored. Moreover, all three failed, so in fact Phobos has also never been independently explored. Russia claims it will launch another Phobos-Grunt in 2016.
1. Past missions:
2. Future missions:
Second Phobos-Grunt attempt, Russia 2016.