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The strange tale of two moons

The view of the Martian sky does not just include one moon, but two tiny moons that were likely asteroids that were captured by Mars' gravitation.
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Viewer's of the Martian sky would be treated to the unusual sight of not one, but two tiny moons, likely asteroids that were captured in the distant past by Mars’ gravitation.

Both were discovered in August 1877 as a result of a systematic search by Asaph Hall (1829-1907) of the U.S. Naval Observatory. Hall actually became so disconsolate after not finding anything that he considered giving up the search, but after some encouragement from his wife, he persisted and found two satellites within several nights of each other.

Hall aptly named them Phobos (fear) and Deimos (panic) after the two sons of Mars who served as his chariot attendants as well as his constant companions.

It is interesting to note that almost three centuries earlier, Johannes Kepler had thought it possible that Mars possessed two moons. Even more remarkable was a description in 1726 by Jonathan Swift in his book "Gulliver’s Travels" of astronomers in the land of Laputa who discovered " ... two lesser stars, or satellites, which revolve around Mars."

Both satellites revolve around Mars in nearly circular orbits and also very nearly in the plane of the planet’s equator. Phobos orbits a mere 3,700 miles (6,000 kilometers) above the Martian surface. And it's getting closer.

Astronomers have deduced that Phobos is drawing closer to Mars at the rate of 0.7-inches (1.8 centimeters) per year and conceivably could crash into Mars in 40 to 50 million years. Before that happens, however, strong tidal forces induced by Mars should break Phobos into a myriad of particles that would encircle Mars in a series of thin rings.

Deimos is a bit farther out at a distance of 12,400 miles (20,000 kilometers).

Earth's Moon is, on average, 238,900 miles (384,402 kilometers) from the planet.

For any place beyond 83 degrees north or south of the equator, for example, Deimos can never be seen. Phobos moves in a lower orbit and can never been seen beyond 70º north or south of the Martian equator.

Because they both move almost exactly parallel to it, the best views of Deimos and Phobos would be from the red planet's equator -- not far from where NASA's Spirit rover is and where it's twin, Opportunity, is due to land later this month.

An astronaut, standing at the equator would see these two moons move across the sky in quite different ways.

Like our own Moon, both Deimos and Phobos move from west to east.

Since the Earth rotates from west to east on its axis more than 27 times faster than it takes for the Moon to revolve once around the Earth, we’re accustomed to seeing it rise in the east, cross the sky and set in the west. This happens because on the Earth’s surface we’re all carried along by the Earth’s rotation toward the east, so about every 25 hours we are rotated first toward the Moon, before eventually overtaking it and ultimately leaving it behind (in the west).

Deimos takes 30 hours and 18 minutes to make one swing around Mars. But since it takes Mars 24 hours and 37 minutes to make one full turn on its axis, its rotation period comes quite close to matching the time it takes Deimos to make one full circuit around Mars.

In fact, if Deimos’ orbit were just 1,800 miles (2,900 kilometers) lower, its orbital period would indeed match Mars’ rotation exactly; Deimos would then appear to move around Mars in a synchronous orbit and would hover eternally over one particular spot on the Martian surface.

Similarly, here on the Earth, we have launched many "geosynchronous" satellites that perch at an altitude of 22,300 miles (35,800 kilometers), where they appear to hover over specific regions of our globe.

But since Mars rotates a bit faster than the revolution period of Deimos, we would indeed see it rise in the east, but it would then appear to move across the Martian sky at a very slow pace. In fact, it would take about 33 hours to reach that point directly overhead (or very nearly so). It would then take yet another 33 hours to descend the sky before we would see it finally set in the west.

And then, we would have to wait another 66 hours before it again reappears above the eastern horizon.

In contrast, Phobos, takes only 7 hours and 39 minutes to rotate around Mars. So it has the distinction of being the only natural satellite in the solar system revolving about its planet in a time shorter than the planetary "day," running three laps around Mars each day.

As seen from the Martian equator, Phobos appears to move far more rapidly than the sluggish Deimos. In fact, just 2 hours and 48 minutes after Phobos has risen, it is already overhead. And after another 2 hours and 48 minutes it is setting; an astronaut on Mars could witness it rising twice during a single Martian night.

And since Phobos’ west-to-east motion is much faster than Mars’ rotation period, it would appear to rise in the west and set in the east.

Furthermore, about every 10 hours and 18 minutes, Phobos appears to rapidly race closely past Deimos as they trek in opposite directions. Phobos, in fact, probably even appears to briefly eclipse Deimos for some parts of Mars on each pass.

Try picturing this: during the 66-hours that Deimos moves ponderously in the sky toward the west, Phobos appears to whiz rapidly in the opposite direction more than six times!

You might also wonder whether Deimos and Phobos can display phases like our own moon. Phases, of course, would be dependent on the angle of illumination either moon makes relative to the Sun as seen from Mars.

If, for example, it were rising in the west just as the Sun were setting, it would be at its "New" phase. A little over four hours later, it will already have moved well past the overhead point to a position roughly halfway up in the east and would appear "Full." When it sets in the east about an hour and half later, it will have waned to its Last Quarter phase.

As for Deimos, because the Sun appears to move across the sky more than twice as fast, this moon would appear to go through a full set of phases more than twice during the 66 hours that it is continuously above the horizon.

Unfortunately, because of the very small size of both satellites, we should not expect to see the same kind of sight that we're accustomed to seeing with our own Moon. Deimos, for example, would appear only about 1/19 the apparent width of our Moon. It would shine at its very best when at its "Full" phase, but because of its very small size it would probably look more like an oversized version of Venus to the unaided eye.

Phobos, being the closer and larger of the two moons would appear noticeably bigger and brighter. In apparent size, it would appear about one-third as large as our Moon; at its peak brightness it would shine perhaps 20 times brighter than Deimos.

Of course, we know today from photographs taken by spacecraft such as Mariner IX, the Vikings and the Mars Global Surveyor that both are not spheres like our Moon, but are rather irregularly shaped lumps, pitted (especially in the case of Phobos) with a variety of craters.

One crater on Phobos measures roughly 6 miles (10 kilometers) across and especially stands out in photographs. It has been named "Stickney" in honor of Asaph Hall’s wife.

Some have suggested that as seen from Mars, Phobos would resemble a shiny potato in the sky. But perhaps Isaac Asimov (1920-1992) said it best in his book, "Science, Numbers and I" (Doubleday, 1968) that " . . . the interplay of light and shadow (on Phobos) will produce a fascinating display of kaleidoscopic change that will never exhaust the fancy."