The Messenger probe's second flyby of Mercury earlier this month has provided the first global view of the pockmarked planet, hinting at a complicated mineral structure beneath the surface and more glimpses of the planet's volcanic history.
Messenger flew just 200 kilometres above the planet's surface on 6 October to image 30% of the planet that had never been photographed by spacecraft. The manoeuvre was the second of three flybys designed to place the probe into orbit around Mercury in 2011.
The pass revealed more patches of an unknown dark, bluish material that had previously been seen on the planet's surface.
Initially researchers weren't sure how widespread the material was, Mark Robinson of Arizona State University in Tempe, said in a teleconference with reporters on Wednesday. "Now we see [the dark patches] are spread all around the planet, and they appear to be excavated from depth."
No layer cake
The material is darker than the rest of the surface, and seems to be kicked up from below ground by impacts. But not all craters of similar depth show the material, suggesting the dark deposits are not evenly distributed beneath the planet's surface. This hints that Mercury does not have an "even, layer-cake geology," Robinson says.
The composition of the dark material is unknown, but one idea is that it contains opaque minerals like the iron-titanium mineral ilmenite, which is fairly common on the Moon. The dark stuff might also be silicate rock that contains iron atoms arranged in a particular fashion. Researchers hope to learn more after the probe enters orbit around the Sun-baked planet.
The second flyby also revealed some new surface features that hint at the planet's complex history.
The probe's laser range-finder measured less variation in the height of the planet's surface than it did in the first flyby in January, which imaged a different part of the planet. That suggests the newly imaged western half of the planet's equator is some 30% smoother than the other side, says Maria Zuber of MIT.
The probe's altimeter also found a 600-metre-high "wrinkle ridge" in the planet's crust. The feature is twice as high as similar features on Mars, Zuber told reporters, and could provide an additional clue to find out how much the planet shrunk as it cooled.
The new images also revealed more evidence of volcanic activity, which was hinted at by Mariner 10, the only other mission to the planet.
Craters of the same size are supposed to have roughly the same depth. But Messenger found a crater that was four times shallower than a similarly wide neighbour.
That suggests the shallow crater had been filled with an estimated 15,000 square kilometres of lava. "That's an awful lot of volcanic material in one place for such a little planet," Zuber said.
The best information on the planet is expected to come after 18 March 2011, when Messenger enters orbit around Mercury, where it will operate for at least a year.
To slow itself down enough to enter into orbit, Messenger will fly past Mercury once more on 29 September 2009.
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