Mercury Gets a Second Look

Thirty-fours years ago, as a cub reporter on the Sky & Telescope staff, I headed to the Jet Propulsion Laboratory in Pasadena, California, on my first real assignment. I have a vivid recollection of looking over 8-by-10 glossies of Mercury, taken weeks before by Mariner 10. Back then, scientists created whole-planet mosaics by carefully cutting up photos and pasting them together.

Mercury coverage by Mariner 10 and Messenger
Click on this map to see a larger view that compares the coverage of Mercury obtained by Mariner 10 in 1974-75 with that from Messenger during its first and second flybys in January and October 2008, respectively.
These days, making beautiful, seamless composite views is a snap. Exhibit A is the set of wonderful views of Mercury unveiled during a NASA press briefing yesterday. These came courtesy of Messenger, short for "Mercury Surface, Space Environment, Geochemistry, and Ranging," which made its second flyby of the innermost planet three weeks ago.

NASA's press release makes the point that much of the territory recorded on October 6th's pass has never been seen at close range. (Due to Mercury's quirky rotation, three full spins every two orbits, Mariner 10 saw only 45% of the surface despite flying past three times in 1974-75.) All told, Messenger has now scrutinized the lion's share its target, and some "global" properties are becoming apparent.

For example, after examining the topographic heights measured along a 1,000-mile (1,600-km) swath of terrain, investigator Maria Zuber (MIT) concludes that the just-seen hemisphere is about 30% smoother than the one seen last January. Planetary scientists have seen these two-faced appearances elsewhere, on the Moon, Mars, and Iapetus, for example. In Mercury's case, the dichotomy might reflect cratering differences (rougher implying older), or it might be a manifestation of goings on in the planet's interior.

Mercury's magnetic field
The magnetic field strengths measured (in nanoteslas) during Messenger’s first two flybys show striking similarity. The magnetopause and bow-shock crossings also occurred where expected.
Mariner 10 discovered that Mercury is magnetized, and Messenger's second flyby showed that the strength of the planet's magnetic field is nearly equal on opposite sides of its globe. As Brian Anderson of JHU's Applied Physics Laboratory explained, this matchup means that the planet's field is strongly dipolar (as is Earth's), very nearly centered on the planet's core, and aligned within 2° of the rotation axis. All this suggests that Mercury generates its field by churning motion within a partially liquid core.

OK, time for a pop quiz: Does Mercury have an atmosphere?

Answer: Yes and no.

There are wisps of vapor around the planet, but technically they're part of an exosphere — atoms that aren't really permanently bound by gravity but rather are streaming out into space. What's got the science team scratching its collective head is that Messenger has detected emissions from emission from sodium, calcium, magnesium, and hydrogen atoms, in amounts that differed between the flybys. These atoms must be coming from somewhere on the surface, but just how, where, and why are TBD.

To the eye, Mercury's surface would appear neutral gray (left). But adding a near-infrared view and enhancing the contrast (right) brings out subtle colors that suggest compositional differences across the surface. Click here for a larger view and here to see a close-up comparison.
Ah, yes, the pictures! To the eye, this world looks remarkably color free. "No matter how we combine the images," notes principal investigator Sean Solomon, "Mercury comes out pretty gray." But on this flyby alone, the spacecraft snapped its cameras' shutters 1,287 times through 11 different filters — plenty of fodder for geologists to start extracting the surface's secrets.

For example, Mark Robinson (University of Arizona) notes that mysteriously dark and ever-so-slightly-blue patches, first seen by Mariner 10, appear here and there all over Mercury's surface. The source of this material seems to be deep seated — it's unearthed by largish impacts — but he points out that it's not around every large crater.

Robinson and the mission's other geochemists will have a much easier time deducing the origin of these mysterious deposits once the spacecraft starts orbiting Mercury in 2011. For now, he can only speculate that the blue-tinged stains might contain the mineral ilmenite (iron-titanium oxide) or perhaps small grains of metallic iron.

A few weeks ago, there was giddy rush of adrenalin as these images came streaming in from the spacecraft. "It's sobering, it's exhilarating, and it's great fun," Solomon admitted, "to see an expanse of planetary surface bigger than South America" for the first time. But now the team has settled down to the task of figuring out what it all means. So consider yesterday's announcements just a status report, with more discoveries to come in the weeks and months ahead.