This week marked the first anniversary of an historic event in planetary exploration. One year ago, NASA's Messenger spacecraft fired a braking rocket and slipped into a looping orbit around Mercury.
And what a year it's been! Previous posts here have described the planet's strange surface chemistry (almost no iron, but rich in sulfur), its weird magnetic field, the strange pitting inside many of its craters, and hints about how Mercury might have formed. Clearly, there's been no shortage of things to tell you about!
But, honestly, I've been waiting all year for three results from Messenger that involve nothing you could see by eye — and today I got my wish when the mission's scientists made a series of presentations at a huge meeting of planetary specialists in Houston.
The first involves what might be termed the "getting the lay of the land."
One of the spacecraft's instruments uses a near-infrared laser to zap the surface eight times per second, all told more than 10 million times in the past year, and timing how long it takes these pulses to reflect off the planet. In doing so it's revealed the highs and lows of Mercury's northern half in exquisite detail. (Because of its highly eccentric orbit, the spacecraft swoops down close over the northern hemisphere but remains too far from the southern half for this technique to work.)
This laser altimeter has revealed that the planet has a broad, pronounced depression encircling its north pole. This low-lying plain is covered with thick volcanic layers that spewed from the interior early in the planet's history. In that respect it's not unlike one of the big lunar maria — except that its polar location is strategic. In the current installment of Science Express, the altimeter team (led by Maria Zuber of MIT) suggest that this broad lowland might have been flipped into that polar perch when Mercury's spin axis reoriented early in solar-system history.
Another topographic oddity involves the giant, 1,000-mile-wide Caloris basin, Mercury's largest impact feature. Inexplicably, part of the floor of Caloris stands higher in elevation than its rim. Zuber has a hunch this bulge might be related global stress, caused by contraction of the young planet's crust as the interior cooled — but it's just a hunch. "How indeed does the interior of Caloris end up higher than its rim?" asks geophysicist Sean Solomon, Messenger's principal investigator. "We'll be working on that question for a while."
Number Two on my Messenger wish list is a map of the planet's gravity field, painstakingly assembled by tracking the spacecraft's motion as it loops around every 12 hours. In a companion Science Express article, David E. Smith (NASA-Goddard) and a host of geophysicists report that Mercury's gravity field reveals an interior structure far stranger than expected. For one thing, Mercury's iron core — already known to be enormous — takes up nearly 85% of the planet's diameter. It's got to be at least partially molten, though its center might be solid.
Such an oversize core leaves just the outermost 250 miles (400 km) for Mercury's mantle and crust! Yet, despite their unexpected thinness, the crust and mantle appear to contain nearly half of Mercury's rotational inertia. This would make sense if the solid exterior were rich in dense metals, but it's not. Instead, according to an analysis presented today by Steven A. Hauck II (Case Western Reserve), there must be a dense solid layer — iron sulfide ("fool's gold") seems most likely — sandwiched between the mantle and core.
No other planet, rocky or otherwise, has internal layering as complex as Mercury's. But in one respect the dense extra layer makes sense: Mercury's magnetic field isn't as strong as it should be, given the massiveness of its iron core, and an iron-sulfide shell would tend to weaken the field.
And Number Three? About 20 years ago, astronomers discovered that the poles of Mercury "light up" when illuminated with radar beams from giant ground-based antennas. As implausible as it seems, they concluded that the radar-bright patches might be due to water ice. I am not making this up! These "snowballs in hell" are just barely conceivable because Mercury's axial tilt is very nearly 0°, so the floors of many polar craters remain permanently shadowed from the Sun's searing light.
Messenger's cameras can't see the darkened crater interiors, but the spacecraft carries a neutron spectrometer that can tell if the deposits are water ice. The details are a little thorny, but when cosmic rays strike Mercury's surface, atoms in the rocks emit neutrons — and if a neutron collides with a light atom (like hydrogen) before escaping to space, it will lose energy. If Messenger spots enough of these "slow" neutrons coming from near Mercury's north pole, water ice is the only likely cause.
Images of the polar region show that the radar-bright deposits appear to be a dead match to permanently shadowed crater walls and floors. I've yet to see the laser altimeter's maps of the polar region, which determines the terrain's exact shape and slope, regardless of whether it's in sunlight of shadow. Those results will be more telling.
So is water ice hiding in those same spots? It's too early to know for sure — it takes a long time for the spectrometer to record enough neutrons. But the evidence in hand suggests that the water ice isn't just lying exposed on the surface. That's the same conclusion reached by a team that's modeled north-polar temperatures: it's too warm for water ice right at the surface, but go just a few inches down and it could remain stable for eons.
According to neutron-spectrometer team leader David Lawrence (Applied Physics Laboratory), if water ice is there, then it's either buried under a thin insulating layer or some other hydrogen-bearing compound is mixed in with it. An important clue is that the topmost layer of these putative deposits appears to be very dark, based how well it reflects the altimeter's near-infrared laser light. One crazy-but-plausible possibility is that the water ice is mantled by a thin blanket of hydrocarbon compounds — which are abundant in comets and meteorites, less volatile than water, and often very dark.
The good news is that Messenger's neutron spectrometer — and all of its instruments — will have more time to scrutinize the innermost planet. NASA managers recently gave the mission a one-year extension. One important change will come next month, when the spacecraft fires its engine to make its orbit less loopy. The apoapsis (high point) will drop from 9,500 miles (15,000 km) to about 6,000 miles (10,000 km). This means that images and other measurements of Mercury's southern hemisphere will get even better.
So this time next year we'll be celebrating its second anniversary — and, I can only hope, some equally amazing scientific results.