The Space Age was barely two years old when, in October 1959, the Soviet Union sent Luna 3 to photograph the far side of the Moon. Astronomers were ecstatic once they'd finally glimpsed the never-before-seen half of the lunar landscape. Luna 3's 40 crude images showed that the far side was different than the side facing Earth, with very few dark, basalt-filled maria.
Planetary scientists have been likewise awaiting a chance to glimpse details on the half of Mercury that went unseen by Mariner 10 in the mid-1970s. And, thanks to Messenger's close flyby on January 14th, they've gotten a good start on filling in the blanks. They provided a few peeks at what the craft radioed to Earth during a press conference on January 30th.
Before I forget, let me note that the mission's actual name is Mercury Surface, Space Environment, Geochemistry, and Ranging — that's why everyone just calls it "Messenger."
But what's in a name? The spacecraft and all seven of its instruments really delivered. Before turning the spotlight over to the imaging specialists, principal investigator Sean Solomon noted that the planet's magnetic field — unique among planets in its similarity to Earth's — hasn't changed since Mariner 10's visit. But the sodium and hydrogen atoms found streaming away from Mercury (more so from the north half than from the south) will require more observations to puzzle out.
Messenger's views are gorgeous, though they lack the color and variety seen most everywhere else in the solar system. To appreciate Mercury, ya gotta love craters — to the casual eye, that's all the pictures show. Big ones, little ones, splashy ones, dark ones, and a few double-ringed ones.
The biggest, baddest crater on Mercury — and the one most likely to reveal how this planet came to be — is Caloris basin. Mariner 10 saw only a slice of Caloris, but Messenger has now imaged all of it. At 960 miles (1,550 km) across, it's actually 20% bigger than previously thought.
Lots of curious features lie inside this Alaska-size saucer. One, nicknamed "the spider," is a 25-mile-wide crater with scores of fractures radiating away from it like bicycle spokes. It's the kind of pattern you'd get if something were pushing up on the crust from below.
The press conference was as interesting for what wasn't said as for what was. No one really commented on the surface composition, though I'm sure there's already plenty to say about it. And the team was mum on what they've learned about the planet's interior. (By flying by so closely, the spacecraft was yanked around by the planet's gravity in subtle yet revealing ways.)
Mercury's density is too high — there's too much iron inside it — for the planet simply to have assembled from the mix of compounds present near the Sun when the solar system formed. So planetary scientists think one of three things happened: (1) somehow Mercury came together with a paucity of 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 minerals to simply boil away; or (3) something really big collided with Mercury and stripped away the lion's share of its crust and mantle.
The compositional data returned by Messenger probably offer crucial clues to which of these scenarios is correct. But the Messenger scientists didn't address this subject during the press conference — and they'll have little more to say until mid-March, when they'll present a far more complete analysis at the annual Lunar & Planetary Science Conference in Houston, Texas. I'll be out in the audience, scribbling furiously.