There is no evidence of the presence of water on Mercury, either now or in the past. Its outgassing must have been quite complete, with every
molecule of water decomposed by ultraviolet solar radiation and swept away by solar wind. The planet lacks atmosphere and resembles our Moon, both in size and its heavily cratered surface.
Planetary scientists have become increasingly convinced the planet's polar regions are dotted with thick, permanent slabs of water ice. Evidence of this
first arose in 1991, when radar probings of this planet turned up mysterious, highly reflective deposits at both poles. The existence of such reflective poles revealed the possibility that ice could be the
source of the reflections.
The existence of ice on the hottest planet in our Solar System in which day-time temperatures reach 825 K is actually very possible due to Mercury's unique rotational axis. Mercury spins on an
that is almost perpendicular to the plane of its orbit, and therefore the planet does not have true seasons. Because the Sun is kept almost directly over Mercury's equator, the poles are always at the edge of the sunlit side of the planet. Thus, flat areas on the poles receive very little solar energy and long shadows keep polar crater floors in virtual darkness. These shadowed crater floors have maintained temperatures as cold as 60 K (-213 degrees Celsius), and this is well below the 112 K temperatures needed to retain water ice for billions of years.
This is why most recent studies have led scientists to the conclusion that water ice likely exists in the polar craters of Mercury. These radar examinations were done from Earth using the Arecibo telescope, the
Goldstone antenna, and
the Very Large Array (VLA). The Arecibo results show that the radar reflective areas on Mercury's poles are concentrated in crater-sized spots. The location of the largest reflective area on the south pole coincides with the large crater called Chao Meng-Fu. The smaller reflective areas on the south pole are also located near other identified craters. At the north pole, the majority of the area containing bright radar spots was not imaged by the Mariner 10 spacecraft which took many close-up pictures of Mercury as it flew by back in 1974 and 1975. This is due to the fact that the same hemisphere of Mercury was sunlit for the entire voyage (due to Mercury's extensive day length). Furthermore, the Mariner spacecraft ultraviolet spectrometer (UVS) identified traces of hydrogen and oxygen in the Mercurian atmosphere and this could be the result of exhalations from ice existing at its poles.
The most recent radar results of Mercury's poles are indicative of the existence of water
ice, and represent the same radar features displayed by the ice-rich southern polar cap of Mars and the icy Galilean satellites Europa and Io. One scientist revealed the implications of these radar detections: "The measurements require an ice thickness of no less than a couple of meters." These ice deposits could very likely be topped of with thin layers (tens of centimetres) of dust and other loose material that would insulate the ice from slow erosion by interstellar radiation. The existence of such an insulation would be relatively transparent to the radar probing and therefore would not effect the results of such tests.
Scientists are also greatly excited by the possibility of the ice being truly ancient, having accumulated in layers over billions of years. The source of this
polar ice was most likely either meteorite bombardment or planetary outgassing. Past meteorites or outgassing of water from the planet's interior could have carried quantities of water to Mercury's surface. The permanently shadowed crater areas at the poles likely acted as "cold traps" to freeze the water and preserve it so that it is able to exist today.