Asked by Lana Freedman
Mercury’s atmosphere is incredibly thin. With a mass of about 5.5 per cent that of the Earth this planet has a far weaker surface gravity and cannot hold on to much atmospheric gas. Couple this with its proximity to the Sun, and a combination of heat and bombardment from the solar wind (a stream of high energy charged particles from the Sun) and you get a planet that has been stripped of the majority of its early atmosphere.
However, since 2008, NASA’s Messenger spacecraft has been studying the thin atmosphere of Mercury showing that nearly half of it is oxygen, with sodium, hydrogen, helium and potassium making up most of the rest of it.
One interesting find is the presence of water vapour in the atmosphere. While much of it may be captured from comets, the rest may originate from deep craters in the polar regions that remain constantly dark and cold, allowing water ice to remain.
Answered by Sophie Allan from the National Space Centre
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