For a long time now scientists have pondered the idea that Mercury, the first planet of out solar system, harbors water ice on its polar regions. This was first strongly indicated by data from Arecibo radio-telescope, when it was noticed that the radar reflections from some places on the pole seemed much brighter than that from the rest of the surface. These brighter reflections suggested that there might be water ice on surface of Mercury, which is, as we thought, not really a planet on which we'd expect to find frozen water. However, due to its extremely small rotational tilt, which is less than 1° (which means that the rotational axis of Mercury is almost perpendicular to the plane of its orbit around the Sun), some craters near the pole, or rather floor of some craters, never gets hit by the light from the Sun. And, true enough, these bright radar reflections coincided with big craters pictured by the Mariner 10 in the 1970s.
|MESSENGER in orbit around Mercury (artist's impression).|
MESSENGER is a robotic spacecraft which arrived at Mercury last year, becoming the first spacecraft ever to enter into the orbit around it. Since then, it had presented us with data such as we'd never see before. And now it seems to have confirmed the 'water ice on poles' hypothesis. Pictures taken by its Dual Imaging System show that the radar-bright patches are indeed within the shadowed regions around both poles.
|Red: permanently shadowed regions; yellow: polar deposits indicated by Earth-based radar|
You'll notice how well yellow points correlate to craters. This really is the best place to expect water ice on Mercury, since, as said before, these are the parts of its surface that never get warm enough for water to evaporate. So there we have a strong indication of water ice on one of the hottest places in our solar system.
Next, using neutron spectroscopy, the average hydrogen concentrations in these radar-bright regions are measured. The data from this experiment shows a hydrogen-rich layer, 0.5 to 20 meters thick, a few dozen centimeters below the surface. The amount of hydrogen in this layer is consistent with the amounts found in almost pure water ice. So this data also strongly corroborates the hypothesis.
One other amazing thing was found, too. There are indications that some regions in the craters are dark, while others are bright. The best explanation for this phenomenon is the presence of organic molecules. Now, don't jump out of your chair just yet - this does not mean there is life on Mercury! It does show, however, that the basic building blocks of life can survive some pretty harsh conditions. This organic matter is carried around our solar system on comets and asteroids, and seems to be quite abundant (it might have seeded the Earth, too, a long time ago). And with one more place with (almost) confirmed water, we can now see that the ingredients for life as we know it are much more common in the vast expanse of our solar system, and, very likely, the entire universe, then we previously thought.
If you want to read more about MESSENGER mission and this most recent discovery, I recommend these two links: