The planet Mercury will be favorably placed for viewing in the evening sky during February. Despite having been known about since the 14th century BC, Mercury is not all the easy to see. It basically spends its time bobbing back and forth between the evening and morning sky, as it circles the sun slightly more than four times each year. For this month, an observer with a fairly clear view of the western sky could see Mercury from the 2nd to the 23rd, although it will be best placed for general viewing on February 16.
Mercury is a difficult object to study because it spends so much of its time lost in the glare of the sun, as seen from Earth. This means observations using ground-based telescopes frequently have to peer through the murkiest part of Earth’s atmosphere to see the planet. Even so, we learned long ago that Mercury did not have an atmosphere and occasional markings were seen on its surface. Back in 1974 and 1975, the spacecraft Mariner 10 flew by Mercury for the first close-up photos. These revealed a planet whose appearance was not that much different from the Moon.
During 2004, NASA launched the spacecraft MESSENGER to go into orbit around Mercury and study it in detail. MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. The spacecraft took its time traveling to this planet, as it used the inner planets to adjust its course to save on fuel. It flew passed Earth in 2005, Venus in 2006 and 2007, and Mercury in 2008 (twice) and 2009. It finally entered orbit around Mercury on March 18, 2011. The initial mission was to spend a year mapping the surface, studying the magnetic field, and trying to understand the geology of a planet that sits so close to the sun. The spacecraft performed so well, that it was put on an extended mission to continue studying the planet, as well as the upcoming solar maximum.
Among the many discoveries made by MESSENGER is that Mercury, the second hottest planet in the solar system, which reaches temperatures of 800 degrees Fahrenheit, has ice within craters sitting around its north and south poles. How does ice exist on such a hot planet? As noted earlier, Mercury has no atmosphere. Without an atmosphere, there is no way for heat to be carried to the entire surface of the planet. So, although it is 800 degrees F when sunlight strikes its surface, it is -370 degrees F where sunlight does not strike…which includes the bottoms of craters sitting on the north and south poles. The ice likely comes from the impacts of comets.