Ice on Mercury

Scientists in the 1990’s through the use of radar telescopes beaming signals 
at the poles of Mercury have evidence of ice on Mercury. But how could this 
be? Mercury is one of the hottest places in the solar system, excluding the 
sun, with temperatures reaching over 700° K (800° F, 427° C) how could ice 
exist here? It turns out, at the very bottom of some craters at the poles, 
where the sun doesn’t shine, so it may be cold enough for ice to exist.

Studies:

Scientists have found this evidence by beaming radar signals at Mercury using 
the Arecibo radio telescope, the Goldstone antenna and the Very Large Array 
(VLA). The Goldstone and VLA’s were used in one study where the NASA Deep 
Space Network 70-m Goldstone dish antenna transmitted a right circularly 
polarized radar wave at 8.51 GHz and 460kW at Mercury and the National Radio 
Astronomy Observatories 26 VLA antennas received the reflected signals. The 
results showed a high reflectivity (radar-bright) with depolarized signatures
at the north pole. This is indicates ice. The second and third studies were 
done by the Arecibo radio telescope. The Arecibo telescope aimed S-band (2.4 
GHz), 420 kW, circularly polarized coded radar wave at Mercury and also 
received them. The filtered and processed data turned up a radar reflectivity 
map of Mercury with a resolution up to 15 km. They found about 20 anomalously
highly reflective and depolarized circular features at the north and south 
poles of Mercury.

INTERPRETATION OF DATA:

Highly reflective radar spots are thought to be ice because first, they are 
totally different from their surroundings. The silicate rock that makes up 
most of Mercury is not highly reflective. The supposed icy spots also sent 
back radar that was depolarized, which indicates water-ice. The places of 
high radar reflectivity also coincide with the craters. At the south pole 
the largest high reflective radar spots sit in the Chao Meng-Fu crater (see 
picture below) and smaller areas also coincide with some other identified 
craters. However pictures of the north pole have not been taken, probably 
because when the Mariner 10 passed by it couldn’t see the north pole then, 
so the radar spots can’t be matched up to any craters. Those craters at the 
poles could provide permanent shading and cold enough temperatures for ice 
to exist a long time. The radar images indicate that the ice on Mercury is 
not really contaminated. Pure ice reflects radar signals much better. The 
reflectivity of the ice on Mercury was not that high or as high as the ice 
found on other bodies like Europa, Ganymede, and Callisto. This could mean 
that the ice on Mercury is thinly covered with dust or has patches of rock 
between the ice. Scientists are not absolutely sure that the radar reflective 
patches are actually ice, since no one has been there physically to make sure.
The coinciding of shadowed craters and the reflective spots point to the 
presence of ice. Although, the radar reflections could actually be some 
other highly radar reflective material like metal sulphides or other 
metallic condensates or precipitated sodium ions.

HOW'D IT GET THERE?:
      For ice to exist on Mercury it has to be cold at all times. Mercury 
has very little atmosphere so ice on the surface of Mercury is exposed to 
the vacuum of space at all times, and it would naturally sublime into a gas 
and escape into space, unless it were kept very cold. The Mariner 10 is the 
only spacecraft to have visited Mercury and subsequently the only source of 
pictures of the surface. About only half of the planet was imaged so we don’t
know of any other spots that could have deep shadowed craters. Theoretical 
studies using typical crater dimensions have determined (though not for sure)
that temperatures at the poles never rise above 102° K (-171° C) and flat 
surfaces at the poles don’t exceed 167° K (-106° C).

Ice could of ended up on Mercury in a few ways, either a meteorite with ice 
on it crashed into Mercury, or planetary outgassing. Especially far back then, 
meteorites could have carried lots of ice on them to Mercury. Planetary 
outgassing refers to the gases released from the planets interior. Mercury 
outgassing is purely speculative but is a possibility. The water would have 
gotten trapped at the very cold places in the shadowed craters because it 
would of frozen into ice at the surface. Radar images indicate that the ice 
is practically uncontaminated, which also indicates that it was put there 
all at once or in small rapid events by say, a large comet. Other water that 
didn’t outgass at the poles or land there could have randomly wandered its 
way there and been trapped there.