Mercury’s Sodium Tail (Sun not to scale)
Mercury’s Calcium Exosphere (Sun not to scale)
Mercury has an extremely thin atmosphere, called an exosphere, which is made up of mostly hydrogen, oxygen and exotic gasses like sodium, potassium and calcium. The density at the surface is about 10 quadrillion (1016) times less than the atmospheric density at the surface of the Earth. In fact, Mercury’s atmosphere is so thin that it is a better vacuum than we can commonly produce in a laboratory on Earth! The solar wind pushes the exosphere outward and creates a “tail” extending away from the planet in the direction opposite the Sun. Since atoms in the exosphere heavier than hydrogen and helium predominantly come from the surface of Mercury, observations of the exosphere help us understand more about the composition of the surface and which processes generate and maintain the exosphere. In the left image the green region to the right of the planet (opposite the Sun) represents sodium in Mercury’s tail. At certain times of the year if you could stand on Mercury’s surface, sodium would appear to glow a faint orange in the night sky similar to sodium lamps here on Earth. MESSENGER also observed magnesium in the tail region of Mercury’s exosphere, which was suspected to be present but had never been observed. Other instruments confirmed the presence of magnesium on Mercury’s surface. The image at right shows a model of calcium in Mercury’s exosphere, which shows the source region is on the dawn hemisphere of the planet. The calcium there is so hot that it is most likely created by micrometeroids vaporizing when they impact Mercury’s surface.