MESSENGER's color filter imaging capability reveals variations
in color on Mercury too subtle for the human eye.
Photo credit: NASA/MESSENGERLike a snow-bird relative vacationing in warmer climate localities and sending back picture postcards of their trip, NASA's MESSENGER spacecraft has made yet another swing past our Solar System's innermost planet, Mercury. But, like the traveler who just can't seem to get enough sightseeing in, this was another whirlwind flyby set to the furious tempo of a camera snapping pics–about 1200 in all…

Did MESSENGER find anything new, since its first flyby back in January? Here are a few highlights:

• Prominent "ejecta" rays streaking out from several large craters–previously revealed only by radar imaging from Earth, now photographed for the first time.

• 30% more of Mercury's largely unexplored surface than had been revealed by the Mariner 10 flybys in the 70's and MESSEGNER's own first flyby last January (spacecraft–namely Mariner 10 and MESSENGER–have now imaged 95% of Mercury's surface).

• "Hyper-color" (my own word) imaging of surface features that reveal variations in color too subtle for the human eye to notice, providing information on soil and rock composition.

I'm a planet junkie–and Mercury has always had a special place in my imagination. One might think of Mercury as the least interesting planet, in our Solar System as well as among dozens of "exoplanet" systems yet discovered. After all, it's a small, dry, cratered, and airless lump of rock and dust, resembling for the most part Earth's Moon. Consider, however, the point of view of someone who's favorite place on Earth is dry, dusty Death Valley, and my enamorment might not come as such a surprise.

In my imagination I see towering cliffs, enormous, deep crevasses, wide, flat dusty plains, bright brights in sunlit patches and dark darks in shadow….

But it's really its differences from Earth that make Mercury such an appealing exotic vision. Being where it is, 36 million miles from the Sun (about a third the Earth-Sun distance), the sunlight striking the Mercurian landscape is six times brighter–imagine that! And not just the visible light spectrum, but all the wavelengths of light the Sun puts out are free to impact Mercury's surface, unimpeded by an atmosphere: infrared, ultraviolet, X-rays, and potent burst of gamma rays rain down intensely on the planet's plains, mountains, and craters.

Speaking of the Sun, its behavior in Mercury's skies is, to say the least, zany. Mercury orbits the Sun in about 88 days (Earth days), but rotates so slowly that a single Mercurian day (the time from one high noon to the next) is about 115 Earth days. Not only does that mean sun-up to sun-down lasts roughly a couple of months, but that Mercury's orbital motion has a greater effect than its rotation on the Sun's apparent motion through its sky. The complicated relationship between Mercury's year and its day also causes the Sun to go "retrograde" at times–that is, periodically halt its progress from one horizon to the other and temporarily go in the opposite direction.

So, our prodigal vacationer MESSENGER has its itinerary straight: a climate with the brightest, warmest sunlight, pristine landscapes, long sunny days, and big skies that perform tricks for its amusement. Now, if only there was a beach…

A limb shot of Mercury's horizon taken by the
MESSENGER spacecraft on January 14, 2008.
Photo Credit "NASA/MESSENGER"

If you can take a name like "Mercury Surface, Space Environment, Geochemistry and Ranging" and craft it into a neat acronym like MESSENGER, then you may have a future working with NASA….

And no, this blog isn't about NASA acronymizations, but rather the heat-resistant robot behind one of them. MESSENGER is the space probe that NASA sent to Mercury to give the Solar System's innermost planet the first up-close look since 1975, when Mariner 10 flew by.

Though MESSENGER's main mission will begin in earnest when it returns to Mercury and finally settles into an orbit around the planet, on March 18th 2011, we were given a tantalizing peak last January 14th when the probe made its initial flyby.

What did this quick, on the fly snapshot tell us that we didn't know before? Well-a lot, considering Mercury has been one of the least understood planets in the Solar System, and was for a long time thought to be similar in character to our own Moon. Mercury is shaping up to be a lot less like Earth's Moon than its gray, cratered, airless appearance would mislead.

One key difference: density-how much material is packed into the planet; or how heavy a standard sized chunk of it would be. Our Moon is a lightweight on this score, with an average density of only 3.4 grams per cubic centimeter, while Mercury weighs in at a hefty 5.427 g/cc-almost as dense as Earth.

Another key difference: magnetic field. Planets like Earth and the Gas Giant worlds (Jupiter et al) generate respectable magnetic force fields, useful for everything from deflecting plasma flowing from the Sun (the "solar wind") to properly directing magnetic compass needles. Venus, Mars, and our Moon do not possess magnetic fields worth mentioning, as it turns out.

Mercury, on the other hand, does. Planetary magnetic fields are believed to be generated by currents in a planet's liquid outer core-like how the electric current in the wire coil of an electromagnet generates a magnetic field. Mercury's magnetic field suggests it still has some activity in its core-molten metals circulating in currents as the core slowly cools off. And speaking of Mercury's core, it appears to comprise 60% of the planet's mass-about twice what is "typical" for Terrestrial (solid) planets.

I've often imagined Mercury to be a cosmic goldmine, with its apparent richness in metals and its density. I wonder if an astronaut could just walk along and pick up chunks of gold from its surface….

Another interesting find by MESSENGER is that some of the flat plains on Mercury may have been formed by volcanoes, long ago. In particular, MESSENGER imaged a number of volcanoes along the edge of the Caloris Basin, a large impact basin-one of the largest in the Solar System, at 1550 kilometers across.

The news coming out of the innermost region of the Solar System makes me giddy. Too bad I have to wait until 2011 for my next look at Mercury. These things take time.