Artist concept of MESSENGER arriving at Mercury.
Credit: NASA

History has been made yet again: NASA's MESSENGER spacecraft is now in orbit around the solar system's innermost planet!

This is exciting to me. When I was a child, Mercury was a contender in my mind for "favorite planet"—right up there with Pluto, being so far away and mysterious, and Uranus, being such a pretty shade of blue. Mind you, that was back before spacecraft had visited any of them….

Being closest to the Sun gave Mercury a claim to fame—just as Venus being the hottest planet or Earth being the home planet or Mars being the red planet or Saturn having rings…okay, they're all special.

At first glance, Mercury presents the appearance of Earth's Moon, but doing just a little math tells you there's more to it. Just divide its mass by its volume and you get 5.43 grams per cubic centimeter—Mercury's average density. That's only a shade less than Earth's average density of 5.51 g/cc, which indicates that Mercury contains more heavy elements than the Moon—elements like iron, for example. (Personally, I suspect there's gold on that thar planet, too….) In fact, after Earth, Mercury is the densest known planet in the solar system. What does it hide under that cratered and radiation-baked surface?

An interesting tidbit about Mercury: because of its high density—because of the amount of material packed within its smallish confines—the surface gravity on Mercury is about the same as on Mars, even though Mars is larger. (Mercury has a diameter of just over 3000 miles, while Mars is about 4200 miles across.) On both worlds, you'd weight about 38% what you weigh on Earth.

And now, Mercury has joined a new club: planets that have been orbited by spacecraft—in this case, NASA's MESSENGER. MESSENGER has been en route to its final orbit around Mercury for over six and a half years. Since its launch on August 3, 2004, MESSENGER has been winding its way through the inner solar system, swinging back past Earth about a year after launch, making a couple of close passes by Venus, then three fly-bys of Mercury itself, before settling into orbit on March 17th.

This long and meandering path was a great fuel saver: by making flybys of planets, MESSENGER's trajectory and speed were altered to make the transition from Earth orbital velocity to a stable orbit around Mercury without burning a lot of fuel to do so. Other spacecraft have used gravitational "slingshot" maneuvers to change direction or gain speed for free, including the Voyagers and New Horizons (on its way to Pluto).

At Mercury, sunlight is up to ten times more intense than at Earth—and since Mercury has no atmosphere to filter out ultraviolet and X-rays, let's just say the sunburn you'd get standing on its surface would be nothing short of lethal. Never mind the lack of air….

To keep cool in the intense solar radiation environment around Mercury, MESSENGER takes advantage of the same physical principle that keeps the dark side of Mercury itself cool (as cold as negative 185 degrees F): insulation. Down on Mercury, the night-time surface is protected from the Sun's intense rays by an entire planet of material. With no atmosphere to trap heat, at night the temperature plummets as heat radiates from rocks and soil directly into space.

MESSENGER possesses a shield: a "sun screen" made of heat-resistant ceramic cloth, situated between the Sun and spacecraft much like someone holding a parasol to stay in the shade.

Poking out from behind its shield, MESSENGER wields two solar panels that provide the spacecraft all of its power—one distinct advantage of the otherwise troublesome intensity of Mercurial sunlight.

Now that MESSENGER is in orbit, what do we hope to discover in the days and months ahead?

Is there ice at the bottom of polar crater floors? How is Mercury's magnetic field generated? Does Mercury have, or did it have, plate tectonics? What's it made of? How does Mercury interact with plasma flowing from the Sun—the solar wind? Are there any strange obelisks on its surface? …to make just a partial list…. Answers to follow (for most of them, anyway).

37.8148 -122.178

MESSENGER's color filter imaging capability reveals variations
in color on Mercury too subtle for the human eye.
Photo credit: NASA/MESSENGER

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…

37.8148 -122.178

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.

37.7631 -122.409