Category Archives: Mercury

Mercury, MESSENGER, planetary history, planetary science

It’s Not Dead, says Jim

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Mercury Continues to Surprise People

I’ve been reading up on the Mercury MESSENGER mission lately. Its findings are fascinating because they seem to refute the old “Mercury’s a dead planet” meme that was popular in planetary science circles for a while. MESSENGER’s measurements of Mercury’s magnetic field, for example, show that there is a dipole field (meaning it has north and south poles), and that it’s still being generated by a dynamo deep inside the planet.

In addition, images of the planet show that it was quite the poppin’ place back in the early days of the solar system. Its geologic history is much more complex than anybody thought, and it includes episodes of volcanic eruptions, particularly around the huge Caloris Basin impact crater site. Here’s what planetary scientist Jim Head had to say about Mercury’s turbulent past:

“By combining Mariner 10 [which first imaged and studied Mercury] and MESSENGER data, the science team was able to reconstruct a comprehensive geologic history of the entire basin interior,” explained James Head of Brown University, the lead author of one of several reports that were published earlier this summer in the journal Science. “The Caloris basin was formed from an impact by an asteroid or comet during the heavy bombardment period in the first billion years of Solar System history. As with the lunar maria, a period of volcanic activity produced lava flows that filled the basin interior. This volcanism produced the comparatively light, red material of the interior plains intermingled with impact crater deposits. Subsidence caused the surface of the Caloris floor to shorten, producing what we call wrinkle-ridges. The large troughs, or graben, then formed as a result of later uplift, and more recent impacts yielded newer craters.”

Mapping a Volcano

What I personally find fascinating are the volcanic vents that MESSENGER has imaged. This figure shows a mosaic of images taken of the largest volcano yet found on Mercury. The sketch map below identifies the major features in the image. The “irregularly-shaped depressions” probably correspond to volcanic vents. The “margin of the dome-like feature” shows the outer limits of lava flows from the vents. Those flows probably covered up the underlying surface of “hummocky plains” that existed earlier. The unlabeled double line outlines bright material associated with the volcano. That material could be pyroclastic deposits ejected during volcanic eruptions at the vents.

The “highly-embayed impact crater” seems to have had lava flow up to its rim; a more distant impact crater is “relatively fresh” and unchanged by any lava. (“Relatively fresh” means that it hasn’t been cratered over, and is younger than the surrounding terrain.) The volcano is located just inside the rim of the Caloris impact basin, labeled as “Caloris basin rim units” on this map.

This map (Credit: Figure 1 from Head et al., Science, 321, 69-72, 2008) and many others are what planetary scientists are using to understand the processes that have shaped Mercury since its formation.

Mercury, MESSENGER, solar system exploration

The Case of the Lobate Scarps

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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. Continue reading The Case of the Lobate Scarps →