As I write this, the Rosetta spacecraft has completed its close flyby of (2867) Steins, a rare E-type asteroid. First images and data presented from the flyby, plus scientific commentary starting at 11:55 CEST on Saturday, September 6. (For those of us in the U.S., that’s 5:55 a.m. EDT, 02:55 PDT.) If you’re up that early, check out the press conference, which will be streamed live. This should be interesting, and I can’t wait to post the real images here!
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.”
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.