Roaring away on top of a tongue of flame, the Japan Aerospace Exploration Agency’s (JAXA’s) ambitious Martian Moons Exploration (MMX) spacecraft launched on its way to Mars on October 19. Its mission: to study the Martian moons Phobos and Deimos, setting down on the surface of the former (along with a small rover) and even returning a 10-gram sample of Phobian material to Earth for later study.
The two tiny moons—Phobos is a lumpy space potato that is about 26 kilometers in length, and Deimos is an oddly shaped rock that is about 16 km long—were discovered in 1877 but still hold many mysteries tight. Despite close-up images taken of both by various Mars missions over the decades, there is much we still don’t understand about them, even basics such as how they formed. In fact, this is such a big question that answering it is actually MMX’s primary goal.
I’m excited about this mission! MMX will take a year to voyage to the Red Planet and, once there, will slip into an orbit around Mars that will keep it close to Phobos. It will map Phobos’s surface to measure the moon’s shape and scout for potential landing sites (and will also occasionally observe Deimos from a distance). Once engineers back on Earth choose an appropriate spot on Phobos, MMX will deploy a rover to roam the surface, and soon thereafter, the mission’s main spacecraft will touch down. It will collect a small sample of the surface regolith—the powdery material made from rocks pulverized by meteorite impacts and thermal weathering—and eventually blast off into space once again. The plan is to then fly by Deimos for some close-up observations and to subsequently head back to Earth to return the surface sample in 2031.
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The rover’s design is pretty clever. It’s a joint effort by France’s National Center for Space Studies (CNES) and the German Aerospace Center (DLR). Called Idefix (after the dog in the French Asterix comic), it has a mass of about 24 kilograms. It’s not much bigger than a large shoebox, although, once the solar panels extend, it will span just more than a meter across.
The deployment is pretty gutsy—if successful, it will make Phobos the smallest body ever visited by a wheeled rover. It starts with MMX slowing to a stop about 40 to 100 meters above Phobos and then releasing Idefix. From there, the moon’s weak gravity (only about a thousandth as strong as Earth’s) will pull the rover slowly down; the descent could last several minutes even from such a low height. The impact will be gentle, probably less than one meter per second, roughly human walking speed.
After that, the locomotion system is designed to flip the rover upright no matter how it lands, putting the wheels down so it can move. Even that is tricky; Idefix will weigh only about 15 grams, or roughly half an ounce, on Phobos! A house sparrow on Earth weighs twice as much. Idefix is designed to measure the moon’s surface temperature and mineralogy, but it will also snap pictures of its wheel tracks on Phobos to better understand the cohesion strength and size of the regolith grains.
So why is there interest in the Martian moons? Mostly it’s because their origin story is so baffling.
Mars sits on the inner edge of the main asteroid belt, so some scientists have proposed that these wee moons are captured asteroids. One point in this idea’s favor is that the compositions of both moons resemble those of common asteroids.
It’s difficult, however, for a planet to grab a passing rock and keep it; orbital mechanics dictate that an asteroid must shed a lot of velocity in order for Mars to ensnare it. Such an object would have to be on a fantastically lucky trajectory to, say, dip down and drag through the upper Martian atmosphere to slow to just the right speed for capture.
It’s far easier, though, if the incoming asteroid is binary: two rocks in orbit around each other. In that case, one of the pair falls into orbit while the other is ejected from the system and carries away the excess energy. That means there must have been two separate events to snag both moons; their orbits are very different, with Phobos a mere 6,000 km above the Martian surface and Deimos a little more than 20,000 km up, so they were unlikely to have been captured together.
Also, both moons have circular orbits directly over Mars’s equator, whereas captured asteroids would be expected to fall along more elongated trajectories around the planet. Such highly elliptical orbits could have been circularized via atmospheric drag and tidal forces from Mars acting on the moons. But this process takes so long (billions of years) that it’s not clear that enough time would have elapsed for Deimos to have settled onto its current path that way.
Another possibility is that a large asteroid impact on Mars could have thrown material into space that then coalesced as the moons. This mechanism has precedent; our own moon came to be that way when a small planet (about the size of Mars, ironically enough) slammed into Earth not long after the formation of our world,: the impact ejected huge amounts of rock that rapidly condensed to form our faithful companion.
Indeed, there are hints that, in some ways, the Martian moons’ compositions resemble that of the Red Planet. But if they formed from the same event, why are they in such different orbits? Perhaps many such objects were formed from the event, and the current moons are the only survivors. A more recent hypothesis is that one moon formed (either together with Mars or from a later impact) but was itself blown apart by an impact to create the extant pair; this idea can explain the circular equatorial orbits, as well as the current differences in their orbital heights.
Phobos is lower-density than expected for a solid rock, so it’s likely to be at least partially a rubble pile, composed of countless smaller rocks held together by their mutual gravity. Many, if not most, smaller asteroids are rubble piles, and we’re still not completely sure how this happens. It almost certainly results from impacts, with the leading candidate being a smashup energetic enough to shatter the asteroid but not quite sufficient to dissipate it. Afterward, the broken pieces fall back together via gravity. If this is true for Phobos, MMX will tell scientists a lot about how it’s changed over time.
There have been previous attempts to send dedicated spacecraft to Phobos (such as the former Soviet Union’s Phobos 1 and 2 missions and the Russian-Chinese mission Phobos-Grunt), but they all failed in different ways to get there. In fact, many missions to Mars have failed, prompting the dark joke in the space community that a monster on Mars called the Great Galactic Ghoul was eating them. Mind you, Phobos and Deimos are named after the minor gods of fear and terror, both sons of Ares!
I hope MMX has more luck. It’s well past time humanity was successful in sending a dedicated probe to these weird little Martian moons. The images and data will be wonderful, and hopefully many mysteries will be unraveled—as inevitably more will pop up. That’s science, and that’s why we explore. There are always more fun things to discover.