Mars may have the most similar environment to Earth, even though it is frigid and dry like Antarctica, but if someone decides to make Mars their home someday, they may want to keep a wary eye to the sky. They’ll know it is definitely not Kansas when looking up into the sky as two moons are often visible making for a truly alien sight. One of these moons is a huge threat to Mars and anything that might be on its surface, including people.
On August 10, 1877 Asaph Hall discovered Deimos, shining dimly close to Mars at magnitude 12.8. Hall continued his search, and six days later he discovered Phobos, shining at magnitude 11.6 even closer to Mars. In Roman mythology, Phobos (meaning “fear”) and Deimos (meaning “terror”) were both attendants of Mars, the god of war. It is interesting to note that these moons were predicted to exist long before they were discovered. Jonathan Swift mentioned them in Gulliver’s Voyage to Laputa (1727) with nearly precise orbital characteristics, but Phobos and Deimos were not discovered for another 150 years. They were also mentioned in Voltaire’s Micrometers (1750). The logic was that if the Earth had one moon and Jupiter at that time was known to have four, then Mars could not possibly have less than two.
Phobos is a tiny moon only 16.7 by 13.4 by 11.7 miles and orbiting only 3700 miles above the surface of Mars. This is so close that Phobos would never be visible above the horizon for anyone on Mars at latitudes greater than 69 degrees. Elsewhere, Phobos would be barely one-third the size of Earth’s Moon in our sky and no brighter than magnitude –3.9. It is so close that Phobos would appear noticeably larger overhead than on the horizon. It would grow from 8 arc minutes across to 12.3 arc minutes across before shrinking again. It would appear more like a battered potato instead of a globe. Mars rotates once on its axis in 24 hours and 37 minutes while Phobos orbits Mars in only 7 hours and 39 minutes. This makes Phobos appear to rise in the west and set in the east around three times per day! Such a close proximity to Mars causes Phobos to occasionally and suddenly appear or disappear in the Martian night as it is eclipsed by Mars’s shadow. It is a fascinating sight to see Phobos go through a complete cycle of phases as it rises and sets in a speedy 4.5 hours. The time from one rising to the next is just over 11 hours. The motion of Phobos among the stars can be noticed within minutes.
Deimos is one of the smallest moons in the Solar System at 9.3 by 7.6 by 6.8 miles. It is only 12,500 miles above the Martian surface and close enough so it would never be visible above the horizon at latitudes above 82 degrees. Deimos would appear only twice as large in the sky as Venus appears from Earth being around 2 arc minutes across and shining at magnitude –0.1. The phases would be barely visible and seeing any surface detail would be almost impossible. Deimos takes a little over 30 hours to orbit Mars and would remain above the horizon for 60 hours for those observers below Mars’s 82-degree latitudes. Unlike Phobos, Deimos would rise in the east and set in the west. The gravity is so weak on Deimos that anyone could launch themselves into space by running at a speed of 7 miles per hour and then jumping up.
Mars experiences many solar eclipses thanks to having two moons, but they would not be total. Phobos would cross the Sun 1300 times per year taking 19 seconds to cross the disc, while Deimos would show an average of 130 transits each taking 1 minute and 48 seconds. To someone on Mars there would be a quick dimming of sunlight as a huge shadowy curtain swept across the dusty sky, but there would be no total darkness as Phobos eclipsed the Sun. A solar eclipse with Deimos would be similar, but more subtle and graceful. The twin rovers, Spirit and Opportunity, observed and photographed several eclipses of the Sun with both moons in early 2005. The silhouette of Phobos is like a potato against the Sun while Deimos is much smaller and appeared rounder.
Phobos and Deimos are probably captured asteroids that wandered too close to Mars, or resulted from a larger asteroid that fractured in two under the influence of Mars’s gravity. An amusing theory came from the Russian astrophysicist Iosif Shklovsky in 1959 who thought these moons were artificial satellites launched into orbit by an ancient Martian civilization. The rationale for this was that if Phobos was slowly being drawn closer to Mars by atmospheric drag, then the moons had to be hollow. Scientists know that this is inaccurate as it is tidal drag, not atmospheric drag, that will cause the eventual demise of Phobos in about 100 million years. On that fateful day Phobos will plunge into Mars and blast an impact crater over 60 miles across. Deimos is far enough from Mars and small enough that it will never be in danger of impacting Mars.
Several spacecraft have been actively operating at Mars every day since 1997 and several have turned their cameras and instruments towards Phobos and Deimos. The Spirit and Opportunity Rovers have observed the motions of these moons at night besides observing and photographing the solar eclipses. Mars Express and the Mars Reconnaissance Orbiter have returned highly detailed and full color photos of Phobos showing craters and grooves everywhere, including the huge crater Stickney, which is 6 miles in diameter. Unfortunately Deimos is a bit too far away to be imaged up close as the orbiters orbit 100 to 6000 miles from the surface of Mars most of the time and are much closer to Phobos, occasionally coming less than 100 miles from it. Deimos has fewer craters and appears smoother, perhaps covered with pulverized material from all of the impacts. Only so much can be learned from flybys and the best way to study a world is to land on it, even better would be to return samples to Earth for study in the labs. Russia has designed a mission called Phobos-Grunt (Russian for soil, the “G” is pronounced like an “H”). It will launch in November and arrive at Mars during October 2012. It will land on Phobos during February 2013 and collect a small sample, no more than 200 grams of soil, and return it to Earth by August 2014. If successful, this will be a historic accomplishment that will pave the way for other sample return missions from other worlds.
Phobos and Deimos may have only been points of light before the Space Age, but a whole new understanding of them is well under way. Regardless, it does not change the fact that Mars is keeping dangerous company with Phobos as its fate has long been sealed.