Mercury is the planet closet to the sun, and it is the smallest of the planets. Because of its closeness to the sun, its temperature is over 427 degrees (C), and since it has no atmosphere to trap greenhouse gases, that temperature is due to its closeness to the sun. Because Mercury can't be seen very well with earth-bound telescopes, there are active scientific programs to determine characteristics of the planet. As explained by Wikipedia,
The first of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of the planet’s surface from 1974 to 1975. The second is the MESSENGER spacecraft, which attained orbit around Mercury on March 17, 2011, to map the rest of the planet.
Other science articles follow.
Now researchers from NASA, MIT, the University of California at Los Angeles and elsewhere have discovered evidence that the scorching planet may harbor pockets of water ice, along with organic material, in several permanently shadowed craters near Mercury's north pole.
"MESSENGER's instruments are capturing data that can be obtained only from orbit," says MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. "We have imaged many areas of the surface at unprecedented resolution, we have viewed the polar regions clearly for the first time, we have built up global coverage with our images and other data sets, we are mapping the elemental composition of Mercury's surface, we are conducting a continuous inventory of the planet's neutral and ionized exosphere, and we are sorting out the geometry of Mercury's magnetic field and magnetosphere. And we've only just begun. Mercury has many more surprises in store for us as our mission progresses."
On March 17, the tiny MESSENGER spacecraft completed its primary mission to orbit and observe the planet Mercury for one Earth-year. The bounty of surprises from the mission has completely altered our understanding of the solar system's innermost planet. As reported in one of two papers published recently on Science Express, scientists have found that Mercury's core, already suspected to occupy a greater fraction of the planet's interior than do the cores of Earth, Venus, or Mars, is even larger than anticipated. The companion paper shows that the elevation ranges on Mercury are much smaller than on Mars or the Moon and documents evidence that there have been large-scale changes to Mercury's topography since the earliest phases of the planet's geological history.

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