How This Little Orbiting Rock Can Tell Us a Whole Lot About Mars

We can't look at directly at solar eclipses without going blind. That's why some nerds (like me) carry eclipse glasses in their cars; eclipse glasses block all the harmful sunlight while allowing you to see the disk of the Sun. Curiosity, NASA's latest nuclear powered Mars rover, is the same way. Looking right at the sun will damage its cameras, so it also keeps eclipse glasses in the car. Specifically, it has a neutral density filter on its Mastcam. Which came in handy a few sols ago when it caught a Phobian eclipse, that is, a partial eclipse of the sun by Mars' moon Phobos.

It wasn't an accidental eclipse sighting. Curiosity didn't exactly look up and say, "Golly! An eclipse! I ought to take pictures to show my science team!" The science team knew the eclipse was coming. They got the filters on the Mastcam and snapped 1,000 shots of the sun, 200 of which show the eclipse. But seeing an eclipse from Mars isn't where the interest in this astronomical phenomenon stops. There's some really cool science to come from the pictures. Scientists will analyze the eclipse pictures to figure out how much Phobos' orbit has changed over time, how much the tiny moon changes Mars' shape, and what the internal composition of planet and moon might be. Now that's a wicked thing to do with pictures of an eclipse.

All the nitty gritty details of how the team will glean so much from 100 pictures of a partial eclipse will come in the final report in a few months, but mission scientists did give a bit of a preview during a press teleconference on Wednesday.

Phobos is, quite literally, an odd ball. Even for a moon it's abnormal — compared to the other moons we know about in the solar system, that is. It's tiny, just 16 miles across at its widest point, but it dwarfs Mars' other moon; Phobos is 7.24 times as massive as Deimos. Phobos orbits Mars at an incredible low altitude, just 5,840 miles about the surface. For some comparison, our Moon orbits an average distance of about 250,000 miles. To maintain an orbit that low, Phobos has to move really fast to counter the pull of Mars' gravity. Phobos makes one full orbit around Mars every seven hours and 39.2 minutes. That's over three times faster than Mars rotates on its own axis. Seen from the surface, the little moon rise in Mars' western sky and is so low that you wouldn't be able to see it from the poles at all. And finally, being so small and irregularly shaped, neither Phobos nor Deimos will ever fully eclipse the sun. Eclipses on Mars are always partial, often labeled as transits rather than eclipses.

But that doesn't make them any less significant.

Dynamicists have the fascinating but daunting job of knowing how and where every body in the solar system is at any given time. They also know more or less when things like eclipses will happen. Knowing where the sun was relative to Mars during the eclipse, studying the images of the Phobian crossing the disk of the sun can tell dynamicists exactly where Phobos was in its orbit at the time. Comparing this to their predictions, they can figure out how much Phobos' orbit has changed since their last measurements.

Just like our Moon is slowly getting further from the Earth, Phobos is getting closer to Mars all the time. Eventually it will get so close that tidal forces from Mars will overcome the gravity that's keeping Phobos together. It will likely break up, giving Mars at least a temporary ring system before the pieces dip into the atmosphere and eventually impact the surface.

Eclipse (or transit) pictures can also tell scientists about the effects Phobos has on Mars. Although we're not yet at the point of drilling into the surface to study Mars' insides directly — that's the goal of the 2016 InSight mission — scientists can measure the transits with enough precision to draw conclusions about the effects of tidal forces. Just like our Moon creates noticeable changes in water tides, it also creates much less noticeable land tides. Phobos does the same on Mars.

It's an astounding wealth of knowledge captured in 200 pictures that, to the untrained eye, don't look like much of anything.