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What Gases Make Up Mercury?

Mercury, the closest planet to the sun, is a tiny world hardly bigger than Earth's moon. In fact, Mercury and the moon are similar in that, since they both have little to no atmosphere, they are covered in craters. Despite its proximity to the Sun, since Mercury has no real atmosphere, it doesn't retain heat, so temperatures on the planet's day side and night side range greatly, from 800 degrees Fahrenheit to -290 degrees Fahrenheit, respectively.

Atmosphere
- Instead of an atmosphere like Earth's, due to the intense heat and solar winds from the Sun, Mercury is able to maintain only an exosphere of atoms blown up from the surface of the planet by intense solar winds and meteorite impacts. On Earth, the exosphere is the outermost, lowest density, layer of the atmosphere. On Mercury, the exosphere is so thin that its atoms escape into space, but is constantly reformed by the continual influx of atoms from the surface and from the solar wind itself.
Gases in the Exosphere
- The exosphere is a thin, low-density layer of particles and, as such, contains only the lightest of atoms. Mercury's exosphere contains trace amounts of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor. The pressure of the exosphere is negligible. The hydrogen and helium atoms probably come from the solar wind, while the rest of the trace gases come from the planet's surface.
Atmospheric Tail
- Mercury is much closer to the Sun than any other planet in the solar system. The photon particles that makeup light exert a pressure on surfaces that they make contact with, though that pressure is negligible within normal human experience. However, on Mercury, the pressure from sunlight is much stronger than on Earth. There, the light actually pushes atoms away from the Sun and Mercury's exosphere, creating an atmospheric tail behind Mercury. The tail has been observed to contain sodium, calcium and magnesium gases. The tail extends about 11,000 miles from the planet.
History
- It is believed that in the early solar system, Mercury may have had a much thicker atmosphere, much like Earth's. However, due to its low gravity and proximity to the Sun, it wasn't able to hold onto it. The main culprit being the Sun's solar winds blasting Mercury's atmospheric particles into space. Many astronomers believe that water was brought to Mercury by comets impacting the surface. Some of this water my remain frozen at Mercury's poles, while the rest is present in Mercury's atmosphere in trace amounts.

Astronomy