Not very! The dimensions of Phobos are said to be 27 by 22 by 18 kilometres -quite an odd shape for a moon, isn’t it? This is why we simply say that it has an average diameter of around 22 kilometres! Its mass is around 11 quadrillion kilograms, nothing compared to the Earth’s 6 septillion kilograms.
Further, the gravitational pull on Phobos is only about 1/1700th that on the Earth. This means that if you weigh 50 kilograms on the Earth, on Phobos you will hardly weigh 30 grams – the “Earth weight” of six grapes!
With such a low gravity, Phobos does not have an atmosphere either. But why is this Martian moon so light? For one, Phobos has a low density, a little less than twice that of water. (For comparison, the Earth’s density is 5.5 times that of water!)
Back in the 1950s, there were even rumours that Phobos might be hollow within and may be an artificial satellite made up of sheet metal! But this was later disproved.
Today, scientists say that Phobos is not a solid body but one made up of pieces of rock. It is quite porous and may even have ice hidden beneath its surface!
Picture Credit : Google
