NASA/Johns Hopkins
The MESSENGER spacecraft transmitted to Earth the first high-resolution
 image of Mercury by a spacecraft in over 30 years. The false-color image
shows details of the surface but it is not exactly duplicating what a person
would see.

By Jesse Emspak | February 2, 2011 5:10 PM EST
Next month, the first space probe in nearly 40 years will approach the planet Mercury, with an array of instruments that could help answer fundamental questions about how planets form.

The mission is called MESSENGER, for Mercury Surface, Space ENvironment, GEochemistry and Ranging. On March 17 it will pull into orbit around mercury, after more than six years of maneuvering between the Earth, Venus and Mercury itself.

Mercury is the closest planet to the sun, completing one of its revolutions in only 88 days. Surface temperatures on during the day top out at 426 degrees Celsius (798 degrees Fahrenheit) - hot enough that lead and zinc would melt like ice on a hot day. Nighttime temperatures plunge to -173 degrees C (-279 F), cold enough to liquefy neon gas. Mercury also spins very slowly, and does so in such a way that a single day on Mercury lasts 176 days - two of the planet's years.
So far the only space probes to get a close look at Mercury are Mariner 10 in the mid-1970's, and MESSENGER itself as it has passed the planet on previous flybys.
MESSENGER will address several questions. One is why the planet is so dense. David Blewett, senior staff scientist at Johns Hopkins Applied Physics Lab, said it's an important question because Mercury would offer clues as to how the other planets formed as well.

Mariner established that the planet has a magnetic field, if much weaker than Earth's. That only opened up more questions, though, because most planetary scientists would say a magnetic field requires a molten or plastic core, along with the planet's own rotation. But there wasn't any evidence that Mercury's core was like that. The surface didn't look like it had been buckled, twisted or modified in the way the Earth's has, via plate tectonics.
Mercury also has a very high iron to rock ratio, Blewett said. That's unusual. "Evidence from the modeling is that all the smaller protoplanets would have had a fair amount of mixing, so people would have expected more normal iron to rock ratio," he said.
Mercury seems to have an iron core that is much bigger relative to the planet than Earth's, some 42 percent of the planet's volume. On Earth that ratio is 17 percent. The planet is only about 40 percent the size of Earth but it is far denser, like a ball of iron surrounded by a thin layer of rock.
There are two major hypotheses. One is that the early sun went through a hotter and more active phase and evaporated the planet's outer layers, leaving what is essentially the core of a world that was once much larger. The second is that the outer layers got blown off in a massive impact, similar to the one scientists theorize created Earth's moon.
Other questions are what the exact structure is of Mercury's core, and a more exact picture of the planet's magnetic field than Mariner was able to see. Those two pieces of information will reveal much about how the planet maintains its magnetic field.
MESSENGER will also get a much better visual picture of Mercury because Mariner was only able to see one side. That's because Mercury spins three times on its axis for every two orbits. Mariner happened to pass by when the same side of the planet was in daylight, so it only got pictures of half of the surface. Since MESSENGER will be orbiting Mercury for a year at least, it will get a much closer view and see the whole planet in much more detail. It also has a spectrometer, which will allow it to see what the surface rocks are made of.
Because Mercury, unlike Earth, is not tilted relative to its orbital plane, there are areas near the poles where the sun would never come up over the horizon. Those areas are a lot colder than the rest of the planet. Most scientists think the mystery material is water. These regions were discovered when ground-based astronomers bounced radio waves off the planet.
MESSENGER has what is called a neutron spectrometer, and it can detect hydrogen near the surface. "If it turns out there are enhanced abundances of hydrogen in the shallow areas, that's a pretty good case for water ice," Blewett said.
MESSENGER is one of the tougher spacecraft NASA has built. It has to handle extreme temperature changes and even has a sunshade to keep it cool. At that distance the sun is many times more intense than it is on Earth. "On the bright side the solar panels don't have to be very large," Blewett said.

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