A few weeks ago, Mars Curiosity looked up from her many duties to glimpse Panic sliently overtaking Terror above the dusty plains of Barsoom. Even stripped of pulp sci-fi drama, the resulting video is an eerie sight to behold. Imagine the arid, frozen tableau, instruments at the end of a long mast designed to look at nearby rocks craned up at the sky in the pitch-black Martian night It would have been colder than the South Pole in winter, with stars and planets piercing the near vacuum like points of lased diamond.
What you're seeing is the little Martian moon Phobos passing between the surface of the planet and the even smaller outer moon, Deimos, as seen from Gale Crater just a few degrees south of the equator. That's quite an eyeball: They are among the smallest moons in the solar system, but Curiosity's mastcam picks them up in surprising detail.
They may look small, distant, and downright harmless in the clip. But Phobos and Deimos are well named after the Greek gods of panic and terror respectively, for more reasons than mythological association with Mars, the Roman god of war. Their fate is sealed by the Laws of Newton, one will meet fire, the other ice. If you want to learn more about this epic arrangement between an ancient God and its minions, join me below the fold.
There are cryptic references to Martian moons found in literature long ago, most notably in Gulliver's Travels. But two tiny moons were officially entered into the astronomical record in August, 1877, by the American astronomer Asaph Hall, while using the mighty 26-inch lens in the US Naval Observatory facility in Washington, D.C. It was the largest refracting telescope on earth at the time and it is still in use today. Hall named them Phobus and Deimus, taken from Book XV of the Iliad, where Ares summoned two fearful minions. For decades they remained tiny, mysterious points of light. Thanks to bigger telescopes and recent spacecraft we now know them as small worlds in their own right. But they still hold the key to unlocking many mysteries.
Deimos on the left and Phobos to the right shown to relative scale as seen by the Mars Reconnaissance Orbiter, February 2009 and March 2008 respectively
Phobos is only 10 - 15 lumpy miles across, but it would subtend about nine to 12 minutes of arc depending on where the observer happened to be standing on the Martian surface. Twelve arc-minutes is almost half the size of our full moon as seen from earth! The large apparent size for barely an afterthought of a moon and its wide variance is because Phobos is so close, less than 4000 miles above the Martian equator, that the latitude of the observer substantially changes its apparent size in the sky. Its so close it would be permanently below the horizon near the Martian poles.
Here it is in ominous relief as seen by Mars Express. If Phobos or something like it were suddenly hanging like that above earth, we'd probably have an appropriately named global panic attack and rightfully so. Any space-faring nuclear nation would be able to send giant shards of such an object, maybe even the whole thing, down on us if they had a cabal of properly dedicated mad scientists.
Both diminutive moons are thought to be captured asteroids and look the part. Although how exactly that would happen is a subject of debate among orbital mechanics. Phobos is the dimmer of the two, not much brighter than light charcoal. It completes an orbit every seven and a half hours. Because Mars rotates once every 24 hours and change, Phobos would rise in the west and set in the east every 11 hours.
This ass-backward dynamic means it is being robbed of orbital velocity. Phobos is spiraling in! It is thought to be a self-gravitating pile of rubble and will likely disintegrate long before hitting the tippy-top of the rarefied Martian stratosphere, but without knowing more about its internal structure and composition it's hard to say precisely when that might happen. Best estimates are that sometime, in the next 10 million and 100 million years, with highest probability being centered between 35 and 45 million years, Phobos will be torn into ribbons of debris, perhaps much brighter than the dull surface. Which would endow Mars with a spectacular set of tightly fitting, constricting rings. The material will then burn in over the subsequent millennia like a thousand chunks of Shoemaker-Levy 9 in miniature slo-mo.
With modern instruments, we'd be able to watch that debris light up the surface like an arc welder and leave angry glowing marks where the surviving pieces hit. It would be quite a sight to see even through a telescope, and it may not be the first time its happened. There are features on the surface consistent with a similar fate for other bodies roughly the same size in the distant past. Which kind of makes one wonder exactly what might have formed or been captured originally, if that object or objects broke up under tidal forces and reformed multiple bodies of which these two are merely the last survivors, or if something similar hasn't happened to earth over the eons too ...
Deimos is about half the size of its sibling and orbits about 14,000 miles away, making it barely discernible as a misshapen disk by a hypothetical, sharp-eyed Martian observer standing next to Curiosity. It would subtend about 2 arc minutes, hardly much larger than Venus' disk at its closest approach to earth. But Curiosity's mastcam picked it up well in the video, too.
Because of its 30 hour orbit, future explorers will see Deimos rise in the east and set in the west about once every three days. Much more like a decent astronomical object! Thus, much like our own moon, it is being accelerated to a higher, longer orbit. Given enough time, it would theoretically leave the immediate Martian sphere of influence, and probably become a serious threat as a Near Mars Object.
Future interactions with Mars might result in impact, but more likely it would be kicked further into the inner solar system, or flung out toward the asteroids and the icy Kuiper Belt beyond. This wouldn't happen for a long time; Deimos fate might remain unsettled until after the sun sheds its outer layers as a planetary nebula and becomes a white dwarf, so perhaps it's a moot point.
Both Phobos and Deimos have such miniscule gravity they can't flow into a more familiar spherical shape. The gravity is so weak a human wearing the heaviest spacesuit would practically float like ghost, be able to jump hundreds of feet high, or fly over the moons' respective surfaces like Buck Rogers with a sporty little jetpack on just a few kilos of fuel. And to make them weirder still, each is thought to be so porous they would have to contain substantial voids, perhaps thousands of bubbles up to the size of a cozy room, or a few giant cracks that could hide a fleet of spaceships. They may both harbor large reservoirs of low density material like frothy frozen water or other ices. Or all of the above.
You'd have to go all the way to Pluto to find equally strange moons in orbit around a solid planet. They are a paradox: like old, familiar friends to legions of stargazers, and still delightfully weird, mysterious objects to this day.
No matter what they are made of or how they got where they are, Phobos and Deimos are both damn handy, natural space stations and supply depots just begging for a suite of scientific instruments as soon as possible. And one day, hopefully sooner rather than later, they can act as sanctuaries and homes for future explorers and permanent residents alike.