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Sunday 15 May 2016

Mars' teeny, weird, moons, and it's strange eclipses..

A solar eclipse is one of nature's wonders, and they come in dfferent types:
  • Total, where the Moon completely covers the Sun:

  • Annular, where the moon is a bit further away and the Sun peeks out from around the edge:
Courtesy of astrobob
  • Partial, where the Moon only covers part of the Sun:
Courtesy of astrobob

Now, thanks to the Mars rover Curiosity, a fourth kind has been seen: Martian. Mars has two moons - far smaller than our Moon, but also a lot closer to Mars than the Moon is to Earth. So, although Martian eclipses aren't as dramtic as Earth's they do happen - and the Curiosity rover caught one on camera. Take a look:

Above: A solar eclipse on Mars. The moon responsible is the twenty km wide Phobos.

It's one of the first times such a thing has been seen from the surface of another planet. Compare it to a total eclipse on Earth...

Above: A solar clipse seen from Earth. Ours is bigger Mars, neeeer!

...and some of the differences between Earth's Moon and Mars's moons are pretty obvious: Our Moon is round, because it's big enough for gravity to pull it into a ball, whereas Mars's moons are so tiny that their gravity is barely noticeable. Still, for all their tiny size, Phobos and Deimos are pretty interesting little worlds. They present a major mystery: They seem to be incredibly ancient captured asteroids, but they've somehow ended up on nearly perfectly circular equatorial orbits - very unlikey for a random capture.

The innermost Moon, Phobos, is being gradually pulled closer and closer to Mars, and in about fifty million reas Martian gravity will rip the teeny moon apart. That might be a long way off, but the cracks are literally already showing.

Above: Phobos, a teeny world just 20 Km long, with a plethora of odd surface features. Courtesy of NASA/JPL.

Phobos would also be a great way to get samples of the Martian surface back to Earth on the cheap: The little moon gets sprayed with Martian rock every time a major impact hits the red planet. That means there's a lot of Martian rock on it's surface, and it's weak gravity would make a return trip to Earth much easier than the return trip from Mars.

Russia tried to send a probe there a few years back, but it malfunctioned before it could break Earth orbit. Beyond getting a cheap Mars return, Phobos' own geological makeup is a bit of a mystery, and it has some baffling surface features, like the infamous Phobos monolith.

Above: The Phobos monolith - almost certainly just a big rock, and not an alien marker. But I'm only going with 'almost', just in case the first probe to reach it finds the words 'Earthlings suck' engraved on it or something. Courtesy of NASA/JPL.

Deimos, the smaller and further out moon, is much less well explored than Phobos. From the martian surface it's barely bigger than Venus in the sky, so it doesn't eclipse the Sun in any meaningful way... but aving two Moons does meran Mars gets a kind of eclipse we don't get on Earth at all: One Moon eclipsing another, which has also been seen by Mars rovers:

Above: Phobos eclipsing Deimos, as seen by the Curiosity rover. Courtesy of NASA/JPL
We may or may not one day land see humans on Mars, and I know I'll never see these sights in person - but I like knowing that they're out there - even if the only audience they ever get is robots....

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