Moons
Moons

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Moons

3.3 Phobos and Deimos

Small moons have too little gravity to pull themselves into spherical shape. Meet the tiny moons of Mars, moons orbiting asteroids, Saturn’s diverse moons (big and small), and gear up for the first visit to Pluto's moons.

Here you become better acquainted with some of the smaller moons in the Solar System. You begin with the best-known, which are the two moons of Mars: Phobos and Deimos.

Unfortunately Phobos-Grunt failed after launch in November 2011 (shortly after this video was made) when rocket failures left it stranded in low Earth orbit.

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Transcript

NARRATOR
Circling the red planet Mars are two tiny moons, Phobos and Deimos, each less than 15 miles across
EMILY BALDWIN
The really interesting thing about Phobos and Deimos to me, is that I've always thought of them as kind of underdogs of the Solar System in terms of the way all the other moons are. They might not immediately jump out as being places to look for water or life, but they do still have a very interesting story of their own to tell.
ANDREW BALL
They're quite different from our Moon in that they're very much smaller, and are perhaps related to asteroids from the asteroid belt. But their origin isn't completely clear
JOHN MURRAY
I first became interested in Phobos and Deimos really because they were so exciting, and exotic, and so different from what we have everyday here on Earth.
The discovery of the satellites of Mars occurred in 1877, which was a year when Mars was much closer to the Earth than it had been for some time. And an American astronomer Asaph Hall actually used a new telescope in Washington, the 26-inch refractor, and he found not just one satellite, but two.
NARRATOR
The moons Asaph Hall discovered were just tiny pinpoints of light. It was to be another century before we learned anything about them.
ANNOUNCER
Lift off.
LAUNCH TECHNICIAN
Roger. 137, we're on our way.
ANNOUNCER
Roger.
ANDREW BALL
Spacecraft first started to visit Mars in the 1960s, initially by flying by. And then in the 1970s, the first orbiters went into orbit around Mars. In terms of Phobos and Deimos, they're always there for these Mars missions. And so they've often been kind of an add-on rather than the focus of a mission.
JOHN MURRAY
The first thing that was extraordinary about them of course, was that they weren't round. All the other moons that we'd seen are round.
ANDREW BALL
Small bodies don't have enough gravity to pull themselves into a spherical form. So Phobos is indeed like most asteroids in fact - potato-shaped.
JOHN MURRAY
The very first close-up pictures of Phobos taken by the NASA Viking missions in 1976, were really quite spectacular. They showed the surface of Phobos to be covered with these parallel grooves right the way across it. And these were astounding. Nothing like this had ever been seen before. And they've remained really, since that time, one of the wonders of the Solar System.
EMILY BALDWIN
These strange grooves looked like they were coming out of the Stickney crater, which is the largest crater on Phobos. And at first, it seemed likely that perhaps these grooves had something to do with that crater.

[ANNOUNCER SPEAKING RUSSIAN]

NARRATOR
In 1988, the Russians decided to take a closer look with their spacecraft Phobos 1 and Phobos 2.
ANDREW BALL
The highlight of the mission was going to be a low hovering phase over the surface of Phobos, and the deployment of two landers - a stationary lander and a hopping lander.
EMILY BALDWIN
But the Russians really weren't too lucky. And in fact, Phobos 1 failed en route. And Phobos 2 failed shortly before it was going to take all these fantastic close-up images of the surface.
JOHN MURRAY
That was really disappointing because I thought, well, we have a chance this time to photograph all of Phobos, and to photograph it from very close up. Because the ideas I've been thinking of really required better imagery than we got from Viking.
EMILY BALDWIN
It's really with the modern era of spacecraft, with these really high-resolution cameras that we're seeing incredibly beautiful and detailed pictures of these moons. And some of the images of Phobos are truly spectacular. We're seeing the giant Stickney crater close up in incredible detail, to the point that we can even see mini avalanches of material that has slumped down inside the crater walls. And they're just truly spectacular.
Because Phobos is that bit closer to Mars and because these missions are Mars exploration missions, it's just a function of that that Phobos is the one that tends to be photographed in much more detail than Deimos.
JOHN MURRAY
Deimos is the kind of poor relation of Phobos and Deimos, I think. It's much smaller and it doesn't have those spectacular grooves. And the surface is also very much smoother and rather more rounded than Phobos.
EMILY BALDWIN
Until we actually get a bit closer to Deimos, that part of the Phobos and Deimos story is going to have to wait for a later mission.
NARRATOR
Better photography has also led John Murray at the Open University to develop an intriguing theory explaining the mysterious grooves of Phobos.
JOHN MURRAY
They didn't really look like fractures, which was the main idea that people were proposing at the time. So then I started thinking, well, they must be impact craters from somewhere. And then it suddenly struck me - of course, it's very, very close to Mars. There are a lot of big impacts on Mars. A lot of debris is being thrown out all the time.
EMILY BALDWIN
Bearing in mind, Phobos is only 6,000 km away from the surface of Mars. It's a very short distance indeed. So when Phobos is travelling in its orbit around Mars, it just happened to intersect this material.
JOHN MURRAY
Well, here we've got a picture of Phobos, and you can see these grooves running across it. These two, for example, are completely parallel. They look almost as if they could've been ruled with a ruler. They're so straight. Now these, I believe, were formed by impacts of material into Phobos. And so this end of the groove would have been formed first. And as Phobos moves in this direction, so you get this string of impacts occurring on the surface. Rather like a machine gun firing at a moving target.
NARRATOR
Undaunted by their earlier failures, the Russians are now planning to go back to Phobos.
ANDREW BALL
Phobos-Grunt, this new mission from the Russians, is a fantastically ambitious and interesting mission. They're attempting to not just go back to Phobos, but to take a sample from the surface and bring it back to Earth. This will be the first time that a sample has been returned from another body from the Solar System since 1976.
JOHN MURRAY
Phobos-Grunt means Phobos-Ground. And that's because it's a lander mission, and will be sampling part of the ground of Phobos that is the soil of Phobos. So it'll be something like the soil in my garden in terms of grain size, except that it won't have any plants or organisms in it. And that is actually called, not soil, but regolith.
EMILY BALDWIN
The Phobos-Grunt mission is going to scoop up samples of this regolith and bring them back to Earth. So we'll be able to find out just exactly what Phobos is made of, and how old this material is.
ANDREW BALL
Of course, having a sample in the lab is fantastically useful. There's only so much information that you can get from in situ data. And people say that a picture is worth a thousand words. Well, a sample is worth a thousand pictures.
EMILY BALDWIN
Phobos and Deimos are the only two other moons in the inner Solar System - aside from Earth's own Moon, of course. So in that respect, they're the next accessible moons to study. And perhaps understanding where they came from will help us to learn more about how the Solar System itself was put together as a whole.
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