The History and Future of Phobos, Mars' Nearest Moon

Phobos and Mars, artwork

The Martian moon Phobos is one of two small worlds circling the Red Planet. It's often mentioned as a possible target for future astronauts to explore. In cosmic terms, Phobos has a fascinating long-term fate, with clues to its future buried in its enigmatic formation story billions of years ago.

Phobos orbits close to Mars, at a distance of just over 9,000 kilometers (nearly 6,000 miles), and measures 27 by 22 by 18 km (16.7 by 13.6 by 11 miles). The other Martian moon, Deimos, is about half the size of Phobos. Both worlds are irregularly shaped, and their makeup is much more like an asteroid's would be. For that reason, planetary scientists have long thought they could be asteroids that happened to stray too close to Mars in the distant past. They got captured by the Red Planet's gravitational pull and have stayed in orbit ever since. It's also possible that the moons were part of a collision that peppered Mars with craters and an impact basin in the distant past.

Their names, Phobos and Deimos, mean "fear" and "terror" (after two characters in Greek mythology), and both were discovered in 1877 by astronomer Asaph Hall. Those names went along with the idea of Mars being named after the ancient Roman god of war.

Fascinating Clues to a Hectic Past

Phobos is a very interesting case study of a moon. Its rocks are similar what are called "carbonaceous chondrites", a key material in some asteroids. They are essentially carbon-based material along with other types of rocks. It's quite possible that the rocks that form Phobos are also mixed with ice below the surface.

The moment you see a picture of Phobos, you notice that it looks very rugged and battered. It's very heavily cratered, meaning that it's been a target of incoming space debris for its whole life. The largest crater is called Stickney, and it covers about 9 km (nearly 6 miles) of this tiny moon's surface. Whatever hit it nearly broke Phobos apart.

Along with the craters, Phobos has long, narrow grooves and streaks in its landscape. They're not very deep, but some extend nearly the length of this moon. The surface itself is covered with a deep layer of very fine dust, probably created as Phobos gets hit by incoming meteoroids.

What Do the Clues Tell Us?

You can tell from its craters, grooves, and dust pits that Phobos has a tumultuous past. Interestingly, more clues to its early history also exist on Mars itself. As scientists study the Red Planet in detail, they're finding evidence of huge impacts that smacked the planet millions or billions of years ago. There are regions on the planet that have different types of rocks than the "standard" Mars rocks. For example, the North Polar Basin was created by a giant impactor that plowed into the planet 4.3 billion years ago. An asteroid slammed into Mars and that sent huge piles of debris into space. Some of that material became a ring around Mars, some fell back to the surface. The rest probably clumped together to form one or more moons.

It's possible that this event (or one very like it) was the birth of Phobos. Ever since then, this tiny world has whirled around in an orbit that is slowly taking it closer to Mars. At some point, it will stray past what's called the Roche limit. That's the distance (about 2.5 times the radius of Mars) where the tidal forces imposed by Mars's gravity are strong enough to break apart a moon. Once Phobos gets inside that invisible boundary, it will begin a long, slow breakup. That process will take about 70 million years, and create a new ring around the Red Planet.

Future Exploration of Phobos

Phobos has been explored by orbiting spacecraft for many years, including the European Space Agency's Mars Express and Exomars orbiter,  the Indian Space Agency's Mars Orbiter mission, and NASA's Mars Reconnaissance Orbiter and the MAVEN mission (which is studying the Martian atmosphere). Their images and data show great details of the surface, including its mineral makeup. All that data will come in very handy when the first human missions land on this moon to study in greater detail.

Astronauts may land on Phobos within the next two decades, establishing scientific outposts and "caches" of supplies for later missions. Once there, explorers will take soil samples and dig deeper into the surface. This information would help fill out the tale of Phobos's past.

One mission idea on the drawing boards at NASA is a precursor trip to Phobos that would establish a beachhead on this tiny moon before people would proceed on to Mars. It's more likely that people will get to Mars first and then establish an outpost on Phobos for purely scientific reasons. It remains an interesting target for studies that might well fill in some gaps in our knowledge of its formation and of conditions in the very early solar system 4 billion years ago.