Mars has two tiny moons. Phobos, the larger of them, is a spud-shaped object about 14 miles (22 km) across. Deimos (DEE-mohs or DYE-mohs) is a bumpy ball with an average diameter of 8 miles (13 km). Compared to our moon’s 2,159 mile (3,474 km) diameter, these guys are truly small potatoes.
Phobos passing overhead after sunset as seen by NASA’s Curiosity Rover last month. Time-lapse images were taken over 27 minutes.
Both the Curiosity rover and the Spirit and Opportunity rovers have taken time out from staring at the ground to occasionally look up at the moons of Mars at dusk or dawn. This week NASA released a short video of Phobos crossing through the sky after sunset. I like the fresh perspective. Seeing how the Martian sky differs from Earth’s makes that planet feel all the more an alien.
If you could stand next to Curiosity and look up, you’d see that Phobos looks considerably smaller than Earth’s moon – only a third as big. Like our moon, it occasionally crosses in front of the sun, an event known as a transit.
While it’s too small to completely eclipse the sun it makes a striking silhouette in the video.
Deimos looks even smaller both because of its smaller size and it’s more than twice as far from the planet as Phobos. To the naked eye, Deimos would look like a brilliant star to everyone but keen-eyed skywatchers who might glimpse its teensy shape with a little concentration.
Because they resemble asteroids in size and composition, many astronomers think Deimos and Phobos were captured by Mars in the distant past. Nowadays they circle the planet in 30.3 hours (Deimos) and 7.6 hours (Phobos). While they’re every bit a moon like our own familiar orb, their behavior in the sky is something altogether different.
Phobos orbits at an average distance of 5,830 miles from Mars. Its extreme closeness to the planet means it moves rapidly across the sky as seen from the ground. Phobos rises in the west and sets in the east, crossing the sky in just 5 1/2 hours. Wait another 5 1/2 and you can watch it rise again in the west. On a long winter night, an astronaut on Mars would see Phobos rise twice and set once!
Deimos, which orbits further from the planet, rises in the east like a normal moon or star, but because its orbital period is so close to that of Mars (24 hours and 37 minutes) it moves very slowly upward from the horizon and doesn’t set in the west until almost 3 days later. One moon’s on steroids; the other putters about as if it had all the time in the world.
Curiosity also snapped a few photos of sunset on Mars on June 22. Martian sunsets and sunrises aren’t quite the visual feast they are on Earth. There’s so much dust suspended in the planet’s atmosphere, the sky glows a monotone reddish brown with a large pale blue aureole surrounding the sun.
“The blue color comes from the way Mars’ dust scatters light,” says Mark Lemmon, associate professor of atmospheric sciences and a camera operator on the twin rovers Spirit and Opportunity.
“The blue light is scattered less, and so it stays near the sun in the sky, while red and green are all over the sky. On Earth, blue light is scattered all over by gas molecules, but there are not enough of these on Mars, which has less than 1 percent of Earth’s atmosphere, to accomplish this.”