Mercury

Eleven articles have been published in the most recent issue of the journal Science on the January flyby of the planet Mercury by the MESSENGER probe. Messenger is an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. This picture has shown up in a number of news sources over the last few days, and it is beautiful.

I should be very clear that this is a false-color image. A number of different shots are taken with optical filters that each pass a different band of wavelengths ("colors"), creating a number of different black and white images. I don't know in this case how many bands were used or at what wavelengths, but often the bands are either in ultraviolet or infrared, outside the range we actually perceive with our eyes. The idea of "color" in these ranges isn't meaningful, except you can image them, then convert them to colors we do perceive. You can study these images individually, and you can convert each to a particular color that highlights features of interest and combine all the images into one, as was done above.

Mercury in visible light looks like this:

This last picture looks very similar to our moon. The expectation was that, at least on its surface, Mercury would resemble the moon in its composition.

It doesn't.

The moon is covered with basalt, an iron-rich rock, as are Venus and Mars for the most part. Most of Earth is covered with basalt: the ocean basins are created as intrusions and extrusions of basalt at the mid-ocean ridge system. But Earth is unique in the Solar System in having active plate tectonics. Processes arising out of Earth's tectonism create conditions in which magma (molten rock underground) can segregate into different fractions with differing compositions.

In the first picture, the big light orange splotch (labeled "C") is the Caloris Basin, one of the larger known impact basins in the Solar System- not the biggest but a big one. The colors here indicate a much lower proportion of iron than one would expect if the rock was basalt. The black arrows point to what are thought to be explosive volcanic centers, which are also iron-deficient. The white arrows point to areas of rock that appear to be similar in composition to those of the Caloris Basin. This presents a puzzle: how is Mercury's composition so different than we expected? How does our new knowledge of Mercury's composition inform our ideas of the planet's evolution? Scientists love puzzles like this. I'm planning and gathering information to lay out what we know of the early history of the Solar System, how rocks are formed here on Earth, what we know of the geology of Mars and Venus, and how all that relates to our ideas about Mercury and its history. I just love this stuff, and I'm pretty excited about this new information.

As I mentioned, MESSENGER made its first flyby in January. It will make another in October of this year, a third in September of next year, then settle into an orbit around the planet in March of 2011. So more data will come in over the next few years, with the best in about 3 years. Scientists will be puzzling over the innermost planet for some time to come. In the Meantime, here's the MESSENGER homepage (check out the orbital diagrams here). And here's the most detailed article I've found- I haven't laid my hands on the Science issue yet. The Website does post the introductory essay here, but the articles aren't open to the public. You can, however, read the abstracts; links are posted below the intro essay.