Mars has two moons, Deimos and Phobos. Unlike our Moon, which is pretty big and spherical, the Martian moons are tiny and irregular. Deimos is a little over a dozen kilometers wide, and Phobos is more like 20 km across.
Phobos is in a very low orbit around Mars, just about 6,000 km above the planet’s surface. It orbits so rapidly that it goes around Mars faster than the planet spins, so it rises in the west and sets in the east! That would be weird to see.
Mars Express is a European Space Agency spacecraft that’s in a highly elliptical orbit around Mars, and the dance of orbital mechanics has it passing relatively close to Phobos three times per year. Just a few weeks ago, on 17 November 2019, it passed the wee moon at a distance of about 2,400 km. As it passed, it took a series of images from different angles, which makes for a dramatic animation of the encounter:
Whoa. At the beginning of the video we’re looking down on the huge impact crater Stickney, which is about 9 km in diameter. You can see parallel grooves all across the moon, which have been mystery for decades. A recent bit of research points to Stickney as their source: Rocks and debris ejected from the impact rolled and bounced across the moon’s surface, carving those tracks.
As the animation continues you may notice that Phobos appears to brighten and then darken again. That’s a real effect! When the video starts, the Sun is down and to the left. As Mars Express moved, the angle between the Sun, the spacecraft, and Phobos narrowed until they were all very nearly in a line, and the camera was looking down onto landscape with the Sun directly behind it — Phobos was "full," seen at high noon. But the spacecraft kept moving, and then slides away from the Sun-Phobos line.
That angle from the Sun to Phobos to the spacecraft is called the "phase angle," and it changes the appearance of the illumination of the surface. From a high phase angle shadows are obvious, and they provide contrast so that craters and other features are easy to spot. As the phase angle narrows, as Mars Express moves closer to being between the Sun and Phobos, the shadows disappear. Contrast is lower, and topological features harder to spot. But the surface also looks brighter, because shadows are shorter! There's more illuminated surface visible, so the brightness increases.
On top of that is another effect called heiligenschein (German for “halo”), where dusty surfaces tend to reflect light back in the direction it comes. That means sunlight is preferentially reflected back toward the Sun, so if you’re in the line between the Sun and that surface, it gets even brighter. You may have seen this yourself: When you appear to have a halo around the shadow of your head as you walk on a dusty dirt field, or on grass wet with dew in the morning. This makes Phobos, in this case, appear even brighter. Another name for this is the “opposition effect” or “opposition surge”, because you see the Sun in the opposite direction of the surface.
If this sounds familiar, yes, it’s why the Moon goes through phases (hence the term phase angle!) and appears so much brighter when it’s full.
The images Mars Express took of Phobos at different angles can tell scientists about the texture of the surface, too, as the brightness surge depends on what the surface materials are made of, their grain size, and so on.
That’s important. Even after all these years, we still don’t know how the Martian moons formed. It was thought for a long time they were captured asteroids from the Main Belt. The materials composing the two moons are similar to many asteroids, so that’s a plus in favor of the idea, but it’s extremely difficult for a smallish planet like Mars with a thin atmosphere to capture asteroids that fall in from the Main Belt. Most likely they’d just fly on past.
A newer idea is that they formed from the debris after a big asteroid or comet impact on Mars itself, the force of the oblique collision blasting ejected material literally into space. Recent work indicates that this could result from an impact with an object the size of Vesta one of the largest objects in the Main Belt, at about 500 km across.
In that case the moons would form mostly from material from the planet itself, but heated to a degree that a lot of lighter material (like water) would be lost to space. Simulations indicate it’s possible an impact like this could explain the moons' major features.
The only way to know is to look at the moons more closely to understand them better. Landing on them and looking at samples in situ, or even returning them to Earth, would be fantastic — and Japan is working on just such a mission.
Until then, though, images like those from Mars Express will tell us more about these weird little moons, and hopefully untangle some of their mysteries.