• Question: how can planets have an atmosphere

    Asked by Calobster159 to Helen, Hugh, Julian on 20 Nov 2014.
    • Photo: Hugh Osborn

      Hugh Osborn answered on 20 Nov 2014:

      Because gravity. The weight of a planet pulls gas molecules towards the surface, stopping them from wandering off into space. The bigger the planet, the greater the gravity, the bigger the atmosphere.

      Jupiter, more than 300x bigger than Earth, has a huge atmosphere of hydrogen and helium that is effectively trapped their by gravity. Mars, on the other hand, has very little atmosphere because its gravity is so low. Hope that helps!

    • Photo: Julian Onions

      Julian Onions answered on 20 Nov 2014:

      There is even a moon with an atmosphere – Titan! Its a very yellow atmosphere, but thick enough so you could fly unaided in it.
      Titan can keep an atmosphere because it is quite big for a moon, and very cold so the gas is very sluggish.

    • Photo: Helen Johnson

      Helen Johnson answered on 21 Nov 2014:

      Yep, all about the gravity. The outer planets are all made up of mostly Hydrogen and Helium gases, and don’t really have a surface to land on. If you stuck Saturn in a huge pool of water it would actually float!

      Mercury is tiny and doesn’t have any atmosphere at all, and so doesn’t have the same protection as we do from incoming space junk – that’s why it’s all cratered and pockmarked. Venus has an atmosphere made up of mostly carbon dioxide, so it’s incredibly hot because this gas traps the heat in under the clouds – just like the greenhouse effect / global warming on Earth! Venus’ atmosphere is actually a lot heavier than ours – if you could stand on its surface the pressure is almost 100 times higher!!

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