It isn't just science fiction that has given Mercury short shrift. In the first fifty years of space exploration, the planet got one—and only one—visit by a space probe: Mariner 10, which did three flybys in 1974-75, returning fuzzy images of 45% of the surface.
Part of the problem of visiting Mercury is that getting there is nearly as hard as getting to Jup-iter. Unless you want to carry an inordinate amount of fuel, the best route involves braking flybys of the inner planets. And not just one flyby: a bunch of them. The current Mercury spacecraft, called MESSENGER (for ME rcury S urface, S pace EN vironment, GE ochemistry, and R anging mission)[3], took fifteen Mercury years to achieve its first flyby, and will take nearly six and a half Earth years before it can finally settle into orbit.
Launched on August 3, 2004, it flew back past Earth (once), twice past Venus, and has now passed Mercury three times. Its fourth encounter with Mercury is scheduled for March 18, 2011, at which time it will (hopefully) brake into orbit and begin systematic observations.
Why all that effort for a planet thought to be an overheated duplicate of the Moon?
Well, partly, it's because Mercury has become one of the least explored pieces of real estate in the Solar System. And wherever we've gone before, we've found things we didn't expect. Also, as it turns out, Mercury isn't really all that much like the Moon. Bob Strom, a planetary scientist at the University of Arizona's Lunar and Planetary Laboratory who's been studying it since the Mariner 10 days, lists a half-dozen major differences between Mercury and not just the Moon, but anywhere else we've ever visited:
* It's the densest planet.[4] Since the demotion of Pluto, it's also the smallest.
* Its orbit is odd: inclined 7 degrees from the plane of the ecliptic. That gives it the greatest inclination of any planet, a fact that might or might not mean anything. Its orbit is also the most eccentric.
* It's the closest planet to the Sun.
* It has zero obliquity, or tilt. This means that Mercury does not have seasons as we know them.[5] More importantly, the bottoms of craters at its pole are permanently shadowed. This means they might have water, in the form of ice. In fact, current thinking is that they do have water.
* It has a dipole magnetic field, like the Earth's, albeit only one-sixtieth as strong. That's weak, but no other inner planets have any dipole field at all (although Mars appears to have once had one). What this means is that Mercury has liquid iron in its core.
* And, finally, Mercury has the greatest surface temperature range of any planet, varying from 467 degrees C (872 degrees F, far above the melting point of lead, but well below that of silver or gold) to -183 degrees C (-297 degrees F, colder than Saturn's moon Titan).
Rather than, why go there, the question is nearly reversed. “This is a planet of superlatives,” Strom says.
* * * *
Heart of the Matter
The full story will emerge when MESSENGER is in orbit and begins systematic mapping. But three flybys in 2008 and 2009 have already done much to extend our knowledge.
To begin with, they've confirmed that the magnetic field is the result of an active dipole, rather than a remnant, frozen in the crust. Until the second MESSENGER flyby in 2008, that wasn't a sure thing. Each of the magnetic measurements made by Mariner 10, and the first one by MESSENGER, had by unlucky coincidence been made on the same side of the planet. They were consistent with a dipole, but didn't mandate one. It wasn't until the second flyby that we finally got a look at the opposite side of Mercury's magnetic field and confirmed it really was shaped like Earth's. “It's the field of a dipole that's aligned with the spin axis within two degrees,” Sean Solomon of the Carnegie Institution of Washington said last year at a meeting of the Geological Society of America (GSA).[6] “This is the kind of geometry you get from dynamo models, but not a remnant field. So it's a vote for a dynamo model."
A strong vote, in fact. And as we noted before, it means Mercury has at least a layer of molten iron in its core.
Mercury's core, in fact, is one of its greatest puzzles. To begin with, in a planet that small, it's a surprise that it's still molten. (We'll get back to that later.) But it's also enormous, at least compared to Mercury's size: three-quarters of its radius, or 42% of its volume. By contrast, Earth's core is 16% of its volume. No other planet has a comparably large core.
There are three possible explanations:
1. There might have been more iron in the materials that condensed to form a planet that close to the Sun. Or perhaps the rocky silicate materials that make up our own much-thicker mantle didn't accrete as well in the hot inner-system environment.
2. Maybe the normal mix of materials did accrete, but the intense heat from the Sun drove most of them back into space.
3. Perhaps Mercury was clobbered by a big (read that “near planet-sized") rock that blasted most of the silicates back into space.
Of these, the impact theory is the most interesting. It requires a truly giant impact—enough to blow away about three-quarters of Mercury's original mantle. But there is growing evidence that each of the inner planets was truly clobbered early in its evolution. Mars may have been hit by an asteroid large enough to produce its northern Borealis Basin, 10,000 kilometers in diameter. A Mars-sized object hitting Earth is the leading explanation for the Moon. And a large impact on Venus could explain its unusual retrograde rotation (it rotates backward, compared to most other bodies in the Solar System). If something similar happened on Mercury it would be an indication that such impacts were the norm.
One of the instruments carried on MESSENGER is a spectrometer. One hope is that by revealing the composition of surface rocks it can give clues to the makeup of Mercury's interior, helping scientists determine exactly what happened, so long ago.
* * * *
Cold Ice and Hot Pyroclastics
Also of interest is the question of whether Mercury has any significant quantity of vola-tiles, either on the surface or in its interior.
On the surface, radar shows “dark” polar deposits that look suspiciously like ice. “They are in permanently shadowed craters where the temperature is less than -170 degrees C,” says Strom. “[They are] very, very similar to the icy Galilean satellites."
These deposits cover approximately 30,000 square kilometers. If they're as little as two meters thick, that's sixty cubic kilometers of water. Not enough for an ocean, but plenty for a good-sized lake. “I think it's relatively pure ice,” Strom says.
If so, where did it come from? One prospect is cometary impacts. But comets originate far out in the Solar System, and by the time they hit Mercury they're moving fast. “An impact would not only vaporize the water, it would disassociate it into its component parts,” Strom notes. Rather than depositing in permanent cold traps at the poles, these gases would simply escape back into space.
But comets aren't the only possible source of water. H2O is also locked up in minerals found in rocky asteroids. “I think bound water is more likely,” Strom says.
But there are also signs that Mercury might have lightweight elements in its interior. To begin with, MESSENGER has found traces of sodium, potassium, calcium, and magnesium in space surrounding the planet.[7] “We don't know if they are indigenous to Mercury or were delivered by meteorites or some other exogenous source,” Solomon says. But they definitely exist.
Also, he notes, the only way for the core of Mercury to be molten enough to produce a dipole is if the iron is mixed with a lighter element, like sulfur, which lowers its melting point. Otherwise, as we noted earlier, even that close to the Sun, Mercury would have had its core long since solidify.
Even stronger evidence is coming from photos of volcanic zones that appear to be pyroclastic flows. On Earth, such flows are the deadliest of volcanic eruptions: fast-moving clouds of superheated ash that travel downhill at enormous speeds, searing everything in their paths. On Mercury there's nothing to scorch, but MESSENGER has seen flows whose smooth contours certainly look like pyroclastics
.
What's interesting about this is that pyroclastic flows are like ground-effect vehicles for lava: to occur, they require magma with a lot of dissolved gas. “We're getting an intriguing look that even as close to the Sun as Mercury, there may be some process for delivering and retaining volatiles to the interior,” Solomon says.
* * * *
Lava, Lava Everywhere
Pyroclastic flows aren't the only type of volcanism on Mercury. Like the Moon and Mars (and presumably the early Earth) large portions of its surface appear to be lava.
This might not seem like a particularly important finding: we'd known since Mariner 10 that Mercury has lots of flat plains, both between craters and on the crater floors. They certainly look a lot like lava flows. But appearances can deceive. We got a good lesson about that during the Apollo landings on the Moon, cautions James Head III, a geologist from Brown University, who notes that areas thought to be volcanic plains turned out to be impact breccia (rock melted and reformed by the heat of the impact). Nor did the Mariner 10 photos reveal any signs of volcanic vents or sinuous lava flows—the traditional smoking guns for proving that a plain is comprised of lava rather than something else.
In an effort to resolve this, Head and a group of colleagues even went so far as to attempt to prove it was impossible to create a terrestrial-style planet without volcanism. Unfortunately, he says, “we found that it's actually pretty easy."
MESSENGER, however, has resolved the stalemate. “I'm here to tell you as a volcanologist that we're saved,” Head said at the 2009 GSA meeting. “Mercury has significant evidence for volcanic activity."[8]
The evidence is a bit subtle: nobody suddenly found an Olympus Mons or Mt. Fuji rising thousands of feet above the plains. Instead much of it comes from “embayments"—places where one feature appears to have lapped against the side of an older one.
Many of the newer images, for example, show the rings of small, partially buried craters rising above the otherwise smooth floors of big ones. That tells us a lot about the order in which things happened. First, there was the big crater. Then, subsequent impacts produce the smaller ones. After that, something flowed across its interior, partially burying them.
What this means is that the “something” can't have been formed by the process that created the original crater. It had to have come along afterward. That rules out impact melt and ejected debris. The only option we're left with is lava. We can even estimate its thickness (a couple of kilometers in some places) from the size of the partially buried features.
We see all sorts of features of this type, says Head. One of the most dramatic wasn't discovered until the third flyby (on September 29, 2009), so recently that it didn't even have a name when geologists discussed it at the GSA meeting. Instead, the MESSENGER scientists were informally dubbing it “Twin” because it looked a lot like a previously seen crater called Raditladi.
Both craters are about 260 kilometers in diameter, and both are striking double-ring structures, with central plains, inner rings, outer annulus-shaped plains, and outer rims. What makes “Twin” exciting it that there are far fewer small craters pocking its inner basin than in the annulus. This means the inner basin hasn't been exposed all that long to meteor bombardment. “It's the youngest terrain we've yet seen on Mercury,” says Clark Chapman, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. “It's hard to say, but it could be less than a billion years old."
That's exciting enough, but what's really exciting is the difference in crater count between the inner basin and the annulus. It's substantial, about a factor of three, so it's not just a statistical fluke: the floor of the inner basin and the floor of the annulus were formed at different times . . . very different times.
There's only one thing that could resurface the floor of an impact crater in two stages. “That's just absolute proof of volcanism,” Chapman says.
* * * *
Color Coding
All of these overlapping features—embayments, craters, lava plains, etc.—have another use as well. Once we sort them out, we can determine the order in which they were formed, combining that into a stratigraphic map of the entire planet.
MESSENGER is equipped to look at Mercury in eleven different wavelengths, carefully chosen to reveal key minerals in the rocks. Some of these have already been assembled into false-color maps, readily available online.[9] Although the planet isn't really that colorful (the images are strongly enhanced) they're useful for tracing strata across long distances and guessing at their depths. In some places, for example, the floors of bigger craters are a different color than those of smaller craters, indicating that the smaller impacts weren't powerful enough to punch all the way through the surface rocks. “This gives us evidence for stratigraphy,” says Head.
Maps like this have been used to put together remarkably detailed histories on other worlds, like the Moon. There, for example, the order of the largest craters has been worked out in some detail, telling us much about the process that mysteriously peppered the Moon (and probably all of the inner planets) with asteroids about 3.9 billion years ago, late in the planets’ formation. Recent data, for example, presented at the same GSA meeting by David Kring, a planetary geologist at the Lunar and Planetary Institute in Houston, Texas, suggest that Jupiter must somehow have shifted its orbit, sweeping the Asteroid Belt with gravitational resonances that knocked large numbers of asteroids out of orbit.[10] Who knows what new hints we'll learn about the early days of the Solar System by putting together similar maps of Mercury?
* * * *
Crumpled Crust
Volcanoes and lava flows aren't the only signs of geological activity on Mercury's surface. The planet is also crisscrossed with fault lines and zones where its surface obviously moved.
Some were seen by Mariner 10. The most dramatic are long scarps of the type produced on Earth when crustal blocks are squeezed together hard enough that one segment rides up onto another. Known to geologists as thrust faults, these occur on Earth as a result of the crushing forces of continental drift.
But continental drift requires plate tectonics, and it's not at all clear that Mercury ever had plate tectonics. Thanks to the giant impact (or whatever) that stole most of Mercury's silicates, its remaining mantle is only 600 kilometers thick (compared to 2,900 kilometers for the Earth's), a problem, because plate tectonics are driven by convection currents rising from the deep mantle.
Nevertheless, in a 2008 paper in Nature Geoscience,[11] Scott King, a geophysicist from Virginia Polytechnic Institute, was able to produce such scarps by modeling currents in Mercury's thin mantle. But mantle convection isn't the only thing that might have produced them. One side of Mercury is dominated by the Caloris Basin, a 1,550-kilometer-wide impact crater formed late in the planet's evolution. One theory is that the faults are the result of this impact. Another is that the scarps were formed by tidal forces from the Sun.
One odd thing about the scarps is that there are no similar extensional features. On Earth, thrust faults are balanced by rifts, either in the middle of continents, like Africa's Rift Valley, or on the seafloor. These are places where crustal plates are pulling apart at the same rate they're bashing into each other elsewhere, producing a global balance between extension and compression. But on Mercury, there are few extensional faults except in the bottoms of craters (which we'll talk about in a bit). Whatever formed the scarps therefore involved something compressing the entire planet, like a hand crushing a wadded-up Kleenex. For this reason, the leading theory is that they're the result of shrinkage of the core as it cooled, after the crust had solidified. It doesn't take a lot of shrinkage to do the trick: a kilometer or two reduction in radius would account for everything we see.
* * * *
Custard-Cup Craters
When you look at a global view of Mercury, of course, what first catches your attention isn't the scarps or lava flows. It's the craters. They are why Mercury, on first glance, looks so de
ceptively like the Moon. But only on first glance. As we peer more closely at them, Mercury's craters prove to be just as intriguing as everything else.
Several, for example, show rings of cracks in their inner basins, roughly paralleling the crater walls. They look like a baked custard whose center dropped, producing ring-shaped cracks.
One of the most interesting of these is Rembrandt Basin, a dramatic 715-kilometer crater first seen on MESSENGER's initial flyby. “It's been through a wringer,” says Louise Prockter of the Planetary Exploration Group in Laurel, Maryland. “It's had all sorts of things happen to it."
First of all, its center shows a wheel-and-spoke configuration of ridges and troughs. Further out, another ridge forms a near-circle, 375 kilometers in diameter. Cutting across the top of all of this is one of the many fault scarps mentioned earlier, indicating that whatever caused the planet-wide shrinking occurred well after Rembrandt Basin had been created.[12]
All told, Prockter says, it appears that Rembrandt Basin saw four successive stages of tectonic upheaval. The first two created the wheel-and-spoke features: probably by subsidence in the crater surface after the interior lava flows solidified.[13] Then the entire center of the crater uplifted, creating the circular wrinkle-ridge. Finally, the planet contracted, projecting a thrust fault across the crater as a whole.
Think that sounds complex? It's nothing compared to the Caloris Basin.
Caloris is the largest crater on Mercury: one of the largest in the entire Solar System, in fact. It is the result of an impact so strong that seismic waves ran all the way through the planet, creating a chaotic landscape on the other side, where the terrain is chopped into a checkerboard of hills and faults. “We think this is the result of focused seismic waves,” says Strom.
Analog SFF, October 2010 Page 8
