Planets

Mars Mission Buzzes Weird Moon Phobos

This eerie portrait of Mars' moon Phobos in ultraviolet light was snapped by the NASA orbiter MAVEN as their orbits crossed paths.

This eerie portrait of Mars' moon Phobos in ultraviolet light was snapped by the NASA orbiter MAVEN as their orbits crossed paths.

Phobos is the nearer and stranger of Mars' two moons - in the 1950s and 1960s, some scientists thought that its unusual orbit, spiraling inward, suggested that it might be a hollow, artificial body. The little moon has long, shallow grooves along its sides, likely caused by the pull of Mars, and it moves about 6.6 feet (2 meters) closer to the Red Planet every hundred years, NASA officials have said.

ANALYSIS: Spacecraft Ready to 'Skim' Mars Moon Phobos

In fact, Phobos is the closest moon to its planet in the solar system, circling Mars at just 3,700 miles (6,000 kilometers) above its surface. It's so close that it orbits within the sphere of Mars' thin atmosphere - the blue pixels that form the new portrait's background are the ultraviolet light scattered from hydrogen gas in Mars' extended upper atmosphere. The orange comes from longer ultraviolet wavelengths of sunlight reflected from the moon's surface.

NASA's MAVEN (short for Mars Atmosphere and Volatile Evolution) mission snapped the shot with its Imaging Ultraviolet Spectrograph in December 2015 as its orbit crossed paths with Phobos', and researchers are hoping MAVEN's measurements will help determine what the moon is made of, where it came from and whether it has organic molecules on its surface. (A previous measurement, by the Mars Express spacecraft, found evidence of such organic molecules.)

NEWS: Mars Express Orbiter Buzzes Martian Moon Phobos

"The images will allow MAVEN scientists to better assess the composition of this enigmatic object, whose origin is unknown," NASA officials said in a statement.

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NASA's Mars Atmosphere and Volatile Evolution (MAVEN) orbiter grabbed this ultraviolet image of Mars' moon Phobos from within 186 miles (300 kilometers). The orange is mid-ultraviolet sunlight reflected off the moon's surface, and the blue is far ultraviolet light from the extended upper atmosphere of Mars.

Mars has two small moons, Phobos and Deimos, that orbit above the Red Planet's surface. Phobos has been in the news lately because the moon is doomed, and scientists have some new findings about its possible fate. What else do we know about Phobos? Read on to find out.

RELATED: Mars' Moon Phobos Is Double-Doomed

Phobos sports an impact crater that is large relative to its size. Called Stickney, it's about 9.5 kilometers (5.9 miles) across and likely came from an impactor that was so big, Phobos almost broke apart. Secondary craters created by the impact mess up a commonly used technique called "crater counting" to estimate how old the surface of a moon or planet is, according to the

European Space Agency

. An area with fewer craters could be more eroded -- and older -- than an area with more craters. But this does not work on Phobos.

So we all know that Phobos is doomed, but precisely what is happening to it is still a subject of debate. These grooves you see in the picture were once believed to be the result of whatever hit Phobos and created Stickney crater. New research, however, suggests these are

instead stress fractures produced by Mars' gravity

. Phobos is supposed to hit Mars in 30 million to 60 million years, but these marks would show the moon will never make it. Instead, it would rip apart and create a small moon around Mars.

ANALYSIS: Mars Will Become a Ringed Planet When Phobos Dies

Phobos is a really dusty place. Observations from Mars Global Surveyor suggest that there is dust about one meter thick on the small moon, suggesting a lot of erosion from impact craters over the years. You can see evidence of some of these craters in the picture above.

NEWS: How did Mars Moon Phobos Become so Groovy?

Phobos orbits quite close (6,000 km or 3,700 miles) to Mars, appearing half as big as Earth's moon in the sky despite its diminutive size. It whips around the planet every 7 hours and 39 minutes, which means that unlike Earth, it rises in the west and sets in the east. The moon is in fact so close to Mars that it isn't visible from all points on the planet; the curvature of Mars gets in the way.

NEWS: Mars Express Orbiter Buzzes Martian Moon Phobos

It's not very clear if Phobos was captured by Mars long ago, although the European Space Agency

points out its low density and "large voids" inside

hint it could have been an asteroid. But other evidence suggests it could have formed at Mars because its orbit is circular, or that (because its composition appears so primitive) perhaps it could have been the leftovers of a past moon or "ejecta" from Mars' surface.

NEWS: Moon Phobos: A Chip Off The Martian Block

Image: Asteroid 951 Gaspra (top) compared with the two moons of Mars: Deimos (lower left) and Phobos (lower right).

To get a body or feature on a body officially named in space, you need to go through the International Astronomical Union. The IAU has naming rules for each of the planets and moons in our solar system, among other locations. On Phobos, this is what it says is needed: "Deceased scientists involved with the discovery, dynamics, or properties of the Martian satellites, and people and places from Jonathan Swift's 'Gulliver's Travels'." The latter sounds a bit strange, but was decided because Swift mentioned two Martian moons in his fictional 1726 book. Phobos wasn't discovered until 1877.

Image: The first edition of "Gulliver's Travels", a 1726 book by Jonathan Swift.

Even though dozens of missions have gone to Mars, nothing has been able to get all that close to Phobos -- at least yet. Not to say people haven't been trying. In 1988, the Soviet Union attempted to send two probes called Phobos 1 and Phobos 2, both of which failed en route. Russia also attempted to send a mission called Phobos-Grunt in 2011, which got stuck in Earth orbit after its launch (pictured here). Luckily for science, several Mars missions have been able to capture imagery of the satellite -- imagery you have seen in this slideshow.