A Close Look at Mercury

It took six years but on Thursday, March 17, NASA's Messenger spacecraft finally settled into orbit around the small planet Mercury. Now, for the first time, a spacecraft will observe Mercury for at least a year and may answer some questions.

Why, for example, is Mercury so much denser than Earth, Venus and Mars? Scientists know the basic reason: Mercury has less rock and a higher percentage of iron than the other planets, and iron is denser than rock. But how did Mercury end up that way?

Another question is whether there could be ice hiding in craters at Mercury's poles. Radar soundings taken from Earth show that there's something highly reflective inside, and because the crater floors are in perpetual shadow, it's cold enough for ice to exist. The spacecraft's neutron detectors will look for signs of hydrogen — the H in H2O — which would suggest it might be ice.

However, the most mysterious question is why Mercury has a magnetic field. In theory, you need molten iron in the core to generate one, but Mercury is so small, its core should have solidified.

And there's plenty more: Messenger carries no fewer than seven different instruments, including high-resolution cameras that will map just about every inch of the planet in extraordinary detail, picking out objects as small as 18 m across.

But anyone who follows astronomy knows that there's an important rule in space exploration: When you try a new, more powerful instrument on a celestial object you're sure to raise more questions than you answer. That is when the fun really begins.

By Michael Lemonick at www.time.com. Picture by NASA