The surface of Mars
February night sky
NASA is landing a pair of rovers on the surface of Mars. As I write this article, Spirit is just departing its lander to start its explorations of Gusev crater. Perhaps many of those living in the northeastern United States would like to join it for a balmy escape from their cold weather. On January 14, the high at 1:00 p.m. in Syracuse, New York,. was -4 degrees Fahrenheit. In Mount Washington, New Hampshire, it was -36. According to the rover's infrared instrument on the same day, in the early afternoon at the Gusev Crater, it was +12 degrees Fahrenheit. But don’t plan on moving just yet; even an Eskimo would not enjoy a night on Mars. That same night, after sundown, the temperature plummeted to about -130 degrees Fahrenheit.
With the rovers active on Mars, and President Bush’s announcement that we are looking toward landing a man on Mars, I began wondering just what we would see in the night sky there. From Gusev Crater, we can gaze into the Martian sky and see distant stars and not-so-distant planets. Some are surprisingly familiar, but others are strange to us on Earth.
Mars is remarkably similar to Earth in its rotational rate, which determines the duration of day and night. A day on Mars is just slightly longer than Earth: twenty-four hours, thirty-seven minutes compared to the twenty-four hours we're used to. The stars and constellations would move across the sky at about the same rate. The main difference is the Martian year. Mars is farther from the Sun than Earth and takes longer to make a trip around the Sun. That makes a Martian year about twice as long as an Earth year.
The Martian atmosphere is thin, about 1 percent as dense as Earth's, and the temperatures so frigid that astronomical observing from Mars would be equivalent to observing from space: the stars would not twinkle. Under these conditions, the average human eye would be able see celestial objects down to magnitude 6.6 during a clear night free of low-level clouds or Martian dust storms. (To denote brightness in magnitude, lower numbers are the brightest and negative numbers are the brightest of all.) Magnitude 6.6 is about the same as what is ultimately possible on Earth. But here there is so much light pollution from cities and even rural streets and porches, that few viewers achieve totally dark skies.
If you set up your telescope around sunset on Mars, the sky would gain a pinkish cast, caused mainly by the scattering of reddish dust particles suspended in the lower atmosphere. As the Sun sets and night takes over, the stars begin to make their entrance. Surprisingly, you would notice that the stars create the same familiar patterns we see from Earth. Stars also have the same apparent brightness as they do when viewed from the Earth. For example, Sirius is still the brightest star in the sky.
While stars are in familiar positions, planets are another story. Earth is the most notable difference. In fact, Earth is one of the brightest objects in the Martian night sky, outshining all stars when Earth and Mars are at their closest. The pale blue glow of Earth is surpassed only by Venus. Although further away from Mars than Earth on average, Venus manages to outshine Earth due to its highly reflective atmosphere. Though difficult to discern in the bright glow of Earth, Earth's Moon would also be visible without a telescope.
The most notable difference in the sky would be Mars’s two moons, Phobos and Deimos. The moons are so small that even in large telescopes from Earth they appear as mere points of light. Phobos, the larger of the two, measures fourteen miles across, while Deimos is just eight miles wide. 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 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 per year and conceivably could crash into Mars in forty to fifty million years. Deimos is a bit farther out at a distance of 12,400 miles. As a comparison, Earth's Moon is, on average, 238,900 miles from the planet.
Standing near the equator the two moons move across the sky in quite different ways. Like our own Moon, both Deimos and Phobos move from west to east. Deimos takes thirty hours and eighteen minutes to make one swing around Mars. Since Mars rotates only a bit faster than Deimos revolves around Mars, 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 thirty-three hours to reach that point directly overhead. It would then take another thirty-three hours before we would see it finally set in the west. We would have to wait another sixty-six hours before it again reappears above the eastern horizon.
In contrast, Phobos, takes only seven hours and thirty-nine 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 two hours and forty-eight minutes after Phobos has risen, it is already overhead. And after another two hours and forty-eight minutes it is setting; an astronaut on Mars could witness it rising twice during a single Martian night.
Perhaps, if we on Earth have the will, early in the next century our descendants will be able to see these familiar yet strange sites in person.