Just two weeks ago, NASA's Messenger spacecraft fired its braking rocket and settled into a polar orbit around the innermost planet — a historic first.
Today key mission personnel provided an update to let everyone know that the spacecraft's instruments are already hard at work scrutinizing the innermost planet as never before.
Eric Finnegan, the team's engineering leader, proudly noted that the craft ended up remarkably close to its desired orbit, a highly elongated loop that brings the spacecraft to a point 129 miles (207 km) from the planet's surface every 12.07 hours. While a close-in circular orbit would have been optimal for science, it would have posed harsh engineering challenges (particularly repeatedly going into and out of the planet's shadow).
OK, OK, the spacecraft is healthy and functioning normally — important for mission success, to be sure. But what does Mercury look like, up close and personal? To the casual eye, it's got craters, craters, and more craters. If you didn't know better, you'd swear the spacecraft was really showing obscure areas of the Moon.
But the two could hardly be more different. Whereas the Moon has just a tiny core, Mercury is a cannonball world. Its iron-dominated core takes up 75% of the planet's radius and nearly half its volume. (In fact, after adjusting for compression effects, Mercury is actually denser than Earth.) Something happened at the dawn of solar-system history that set it distinctly apart from its terrestrial siblings.
As lead scientist Sean Solomon explained today, Messenger (short for Mercury Surface, Space Environment, Geochemistry and Ranging) is just going through the first laps of its planned year-long observation plan. In that time the planet will spin beneath the spacecraft's gaze six times. The entire globe will be imaged at eight different visible and near-infrared wavelengths by wide-field and telephoto cameras. Seven other instrument packages will assay its composition, map all of its highs and lows, measure its magnetic field, and scan the surrounding space for charged particles.
Meanwhile, just by being in orbit, the spacecraft will be letting radio telescopes on Earth know how much its motion is sped up or slowed in response to subtleties in the planet's gravitational attraction — key to understanding how Mercury's interior is put together and, in particular, where all that iron came from.
One of three things happened: (1) somehow Mercury came together with hardly any lower-density silicates, the kind found abundantly in Earth's crust; (2) it endured a period of extreme heating from the young Sun, which caused many elements simply to boil away; or (3) something really big collided with Mercury and stripped away the lion's share of its crust and mantle.
Mariner 10, the only other spacecraft to make the long interplanetary trip inward, swooped past Mercury three times in the mid-1970s. But it could only observe about half the planet in detail (a consequence of Mercury's unique spin-orbit resonance), and it lacked the infrared spectrometer than could have coaxed out Mercury's mineral makeup.
Messenger's spectroscopic observations should settle the argument. If (1) is correct, then the surface will be covered with a mixture of common minerals. By contrast, the evaporation model (2) would have left the planet depleted in "volatile" elements like potassium and sodium — but then why are these atoms oozing into space from its surface? And if the impact hypothesis is correct, then the silicate minerals on the planet's surface should have compositions more mantle-like than metal-poor crustal rocks.