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The Case of the Lobate Scarps A Noir Look at Mercury's Mysterious Surface Evolution
Mercury's horizon, as seen by the MESSENGER mission.
The name's Basin, Caloris Basin, and I'm a planetary science detective. Perhaps you've heard of me. Of all the planets in all the solar systems in the cosmos, I'm interested in Mercury. It's a classy place, with a great surface to boot.
So, until just a couple of days ago, things weren't going too well for me. I'd been stonewalled with a lack of knowledge about ALL of Mercury's surface. It was tough, and I was down to my last... well, let me tell you the whole story.
It was late on a Friday afternoon in mid-January. Business was slow. It had been for years, ever since the Cassini mission had launched, followed by New Horizons. Everybody's attention was turned toward the outer solar system, or near-Earth asteroids, or dwarf planets beyond Neptune.
And, it seems that ever since I'd cracked the case of the sulfuric plumes in the Venusian atmosphere, inner-solar-system detective work had just dried up. Pancake eruptions on Venus were so last-century. Even Martian dust storms weren't getting as much press as they used to. Oh, sure, the occasional asteroid-impact threat on Earth raised a little stir now and again, but in the main, it seemed like nobody cared about the inner planets any more. A pity.
I mean, there was Mercury, waiting to be explored again. Even though Mariner had given it a quick look back in the 1970s, its glory days weren't over. Not by a long shot! Sure, its surface would be at home on a black-and-white scene from a 1940s detective movie set (without the rain and fog, though). And sure, it's a bear to observe from Earth. But, Mercury's got as many mysteries as those outer planets, and it's a darned sight more rocky!
Still, all the hot researchers and their grants (and grad students) were out there at Saturn, and using Hubble and ground-based telescopes to poke around Pluto and the Kuiper Belt. They were flush with success, invoking cryovolcanism right and left to explain what they were finding! Yet, for my NSF grant money, there was a lot of good science to be done in the inner solar system. So, I resigned myself to having to wait for a while. I knew that soon I'd eventually have my day in (or actually near) the Sun.
Well, there wasn't much work waiting for me that day, so after feeding the boa constrictor that had been with me since my grad school days, I settled in with a six-pack of Red Bull and a stack of back issues of Icarus to wade through. I had just put my feet up on the desk and was reading "Fugitives from the Vesta Family" (Nesvorny, et al., 193, January 2008, p. 85-95) when I heard a knock on the door. It was the sound I'd programmed on my computer to let me know when a potential planetary science discovery was waiting to be investigated.
There it was: a GoogleNews alert about the MESSENGER spacecraft. It seems that it had finally gotten a look at a place that had intrigued me for years, ever since I first observed the planet Mercury at greatest eastern elongation back in grad school. I was so taken with the place that I'd gone on to study what little we know about this planet closest to the Sun.
And that's when I learned about the mysterious lobate scarps. For my money (at the time, low grad-student slave wages), those scarps (cliffs to you regular joes) were evidence. Of what, I wasn't sure. But was going to find out. Once I saw them in old Mariner Mercury images, they haunted me. I had to know more about how they turned Mercury into a wrinkly prune of a planet in the prime of its life. I asked the standard detective questions. How did they form? When? And how many were covered up by impact-event melt rock? These were key questions, not to a crime, but to the mystery of Mercury's surface evolution.
The knocking on the door sounded again, jerking me out of my former-grad-student reverie and back to the present. As the images scrolled up on the monitor, I knew that my waiting had finally ended. The Mercury MESSENGER mission was sending back a flood of images, and not just of regions I'd already studied. These were scenes of the "far side" of Mercury that we never see from Earth because of Mercury's complicated spin-orbit resonance with the Sun. And what scenes they were! I leapt out of the chair, scattering magazines and empty cans across the desk. My time had finally come—and from a planet that had kept my investigative instincts alive for years!
Sure, there were the usual craters (new AND old), but what caught my attention were the scarps. MORE scarps, including one huge wrinkle that appears to be one of the largest ever found on Mercury. In another scene, old lobate scarps seem to be cutting across craters. Classic evidence you'd see at any surface evolution scene. I could hardly contain myself. And it was ONLY the beginning of MESSENGER's data dump.
I quickly set to work making notes, studying each picture for evidence of the story of Mercury that had only begun to be told when I was first in grad school. I remembered those old lectures as if it was yesterday, and yet, even today, parts of the story are still a classic detective tale.
Mercury began like all the rocky planets, hot and molten. While the others basked in relative coolness out away from the Sun, Mercury stayed hot for quite a while. As it cooled, its surface was blasted with impacts, digging out those craters we see all over the place. The craters weren't the mystery though. We know how they happen, and that they continue to happen. No, the case of the lobate scarps were what piqued my attention.
These cliffs are huge and jagged. What could have caused them? For a long time, that was the central mystery. But, eventually, we figured out the answer: Mercury cooled. Then it shrank. The shrinkage compressed and wrinkled the rocky surface. Early Mercury might have been all wrinkles. We'll never know for sure, since those impacts came in and covered up some of the evidence. But, there's enough left to give us a pretty good general picture of Mercurian surface evolution.
Now, I don't pretend to know the whole cooling and bombardment history of Mercury. That's a mystery I'm still working on. And, to be fair, all the MESSENGER scientists are working on it, too. I'll We'll have to examine each picture and each region on Mercury to figure out which came first: the cracks or the craters. It's standard planetary science detective work. But, I'm up to it; as long as MESSENGER sends pictures, I'll be on the case, cracking the case of the mysterious lobate scarps of Mercury. And, now that we have pictures of Mercury's polar region, maybe I'll tackle a new challenge: the mystery of Mercury's purported polar ices.
Is there ice hidden on shadowed crater walls at Mercury's poles? Visit the MESSENGER web site for the latest details and images from the mission.
Mark and I just announced the fulldome incarnation of our long-popular show MarsQuest—something that's been a long time coming. The show itself has had several incarnations, beginning in 1988 when we created a show about Mars called "The Mars Show" and it was basically a slide show with a soundtrack. (Why that title? We could never think of a better one, so it kept that name for quite a while.)
In 1996, we got together with a group of people at the Space Science Institute in Boulder, Colorado, to talk with them about a traveling exhibition they were creating called MarsQuest. They wanted a planetarium show, and by golly, here we were with a planetarium show that we wanted to update. After a few meetings, we had a deal, and the rest, as they say, is history. The MarsQuest exhibition has finished its run around the country and is retired to a museum in Florida. But, MarsQuest the planetarium fulldome show is still very much alive and kicking, bringing info about the Red Planet to all and sundry.
It seems that I write about Mars every few years, and people often ask why. It's simple: I've always been taken with the Red Planet. It all goes back to a game we used to play when I was a kid, about exploring Mars. And that's part of it. As I got older, I read more about the planet, especially when in 1976 we actually landed a spacecraft there.
So, it was only natural that I'd eventually end up writing a documentary script about it. And revisiting it as more spacecraft send back more images and data about this planet. Not only did the game from my childhood spurred MarsQuest, and a scene in SkyQuest, a show we did for the National Air and Space Museum's planetarium. So the game I played keeps coming back in one form or another.
And it continues. As more Mars images and data come in, I continue to work on other Mars-related presentations. For me, this dry and dusty desert planet is also one of the most tantalizing places in the solar system, and if I were of the right generation, a place I could have once considered exploring in first person.
While checking my daily science sources, I ran across this interactive tour of Titan at the Cassini web site. It lets you peer beneath the heavy clouds that hide this world from our view.
Titan is the largest moon orbiting Saturn and is a fascinating blend of organic materials in its atmosphere and on its surface. The Cassini mission to Saturn will pass by this fascinating place 45 times during its extended exploration. What planetary scientists are finding here may well rewrite the books on many aspects of solar system science.
Titan as shown in composite imagery from two Cassini flybys in 2006. Courtesy NASA and the Cassini Mission.
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