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° Kelvin (800° Fahrenheit, 427° Celsius) 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.

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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.

Picture of the Chao Meng-Fu Crater. The dark areas could have ice on them.

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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 planet's 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.

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