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.