There’s water on Earth, obviously. And large quantities of water ice on Mars and in the outer Solar System. But astronomers have wondered, is there water on Mercury? There obviously aren’t any lakes and oceans of water on Mercury. We would have detected them from Earth, and definitely would have seen them when the various missions to Mercury skimmed past the planet, taking close up photographs. But does Mercury have water in any form?
The conditions on Mercury sure rule it out. As Mercury slowly rotates, the side facing the Sun experiences extremely high temperatures. At noon on the equator, the temperature rises to 700 Kelvin. And then dips down to just 100 Kelvin at night, since there’s no atmosphere to hold in the temperature. Any water on the surface of Mercury would boil away quickly and escape into space because of Mercury’s low gravity, and the constantly blowing solar wind.
But you might be surprised to know that astronomers have discovered water on Mercury. Not liquid water, but deposits of water ice at the planet’s poles. This is because there are craters at the north and south poles of Mercury which are eternally in shadow. The ice in these craters is never warmed by the Sun, and so it always remains frozen at the bottom of these craters.
When NASA’s MESSENGER spacecraft flew past Mercury in 2008, it also discovered the presence of water vapor in the thin atmosphere that surrounds Mercury. This atmosphere, or more precisely “exosphere”, is created when particles from the Sun’s solar wind bombard Mercury and kick up atoms into its atmosphere. It’s possible that the solar wind is kicking up water from the ice deposits at Mercury’s poles, or maybe it’s coming from cometary fragments. Or maybe the solar wind is depositing the oxygen and hydrogen atoms on Mercury’s surface in the first place.
So there is water on Mercury, just not very much.
We have written many stories about Mercury here on Universe Today. Here’s an article about a the discovery that Mercury’s core is liquid. And how Mercury is actually less like the Moon than previously believed.
We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.
De agua sobre el mercurio