NASA’s Curiosity rover whipped out its eclipse glasses over the last few weeks to capture these amazing images of the moons Phobos and Deimos eclipsing the sun. The rover snapped pictures Phobos, the larger of the two planet’s moons at 16 miles (26 km) across, on March 26. Deimos (10 miles / 16 km) had its day in the sun on March 17.
Phobos doesn’t completely cover the sun, so this would be considered an annular solar eclipse. Annular eclipses on Earth happen when the moon is farthest from the Earth and only covers the center of the sun, not the edges. At maximum eclipse, a brilliant ring (annulus) of sunlight surrounds the blackened moon.
Deimos is so small compared to the disk of the Sun, we call it a transit instead. It reminds me a lot of the dual transits of Venus we saw in 2004 and 2012. Through a solar filter Venus was plainly visible as a perfectly round black dot without a telescope; Deimos would be even easier. If we could watch from Mars it would appear 2½ times as big.
In addition to capturing each moon crossing in front of the sun, one of Curiosity’s Navigation Cameras caught the shadow of Phobos during sunset against the dusty Martian atmosphere. It’s a pretty incredible sequence. As the moon’s shadow passed over the rover during, it momentarily darkened the light. Notice the photo caption (below) reads “as Phobos was rising.” Phobos orbits so close to Mars that it completes an orbit every 7 hours 39 minutes. That’s fast enough to outpace the planet’s rotation, making Phobos rise in the west and set in the east!
Because Phobos orbits close to Mars and almost in line with its equator, annular eclipses of the moon are visible most days of the year from somewhere on the planet. The sun looks about a third smaller from Mars than it does from Earth (21 arcminutes vs. 30 arcminutes wide), because Mars is half again as far from the sun than the Earth.
Besides just being amazing to look at these events help researchers fine-tune the orbits of the moons. Before the Spirit and Opportunity rovers landed in 2004 we only had an approximate idea of . The first time one of the rovers tried to get an image of Deimos in eclipse, the moon was 25 miles (40 km) off from predictions. The orbits of these small objects are always changing because of the pull of Mars, Jupiter and even the slight tug each moon exerts on the other. Every eclipse Curiosity observes adds precision to predicting the next. To date, eight Deimos and 40 Phobos events have been recorded.
For fun I used Stellarium to simulate the March 25th Phobos sunset eclipse. A nearby rise blocked the view for Curiosity, but if you had walked to the top you would have seen something like the image above. Wow …!