Along Came a Spider

As anyone who’s recently cleaned their attic can tell you, unexpectedly finding a large spider sitting in a dark, hidden part of your home can elicit excitement, consternation, and sometimes a family squabble.

Apparently it’s no different if you are a planetary scientist, even when the home in question is the solar system and the “spider” is a mysterious formation sitting in a crater on Mercury.

In January the MESSENGER spacecraft beamed back images of a side of Mercury no one on Earth had seen before.

The suite of new data from the probe’s first flyby of the innermost planet revealed lots of volcanism, asteroid impacts, and an odd feature the team dubbed the spider—a network of more than a hundred raised, narrow troughs radiating outward from a central structure.


—courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The whole formation sits in the middle of the Caloris Basin, a massive 3.8-billion-year-old impact crater that had not been seen in its entirety before the flyby.

Today Sean Solomon, principle investigator for the MESSENGER mission, presented at the 3rd European Planetary Science Congress his theory that the spider is the product of a meteorite impact.

His team’s computer models are based on the idea that the floor of the basin was pre-stressed by years of crustal deformation. Hurtling a virtual meteorite into the crater created a pattern not unlike what MESSENGER saw.

“… The formation of the crater relieved the stress build-up and weakened the central area, allowing the troughs to spread out like cracks in a windscreen,” Solomon said in a press release.

Working hypotheses about the nature of the universe have certainly been based on less solid evidence (yes, dark energy, I’m looking at you). But even on the MESSENGER team, not everyone is convinced an impact is to blame.

Co-investigator Jim Head is more in favor of a volcanic origin for the cosmic arachnid. Instead of stress from above, stress from below in the form of upwelling magma might have created a pocket of hot rock and a radial web of dykes, he suggests.

“Given the amount of volcanic activity we’re discovering in that area, I wouldn’t want to rule out a volcanic cause just yet,” Head cautioned in the release.

Luckily for this family feud, MESSENGER still has two more Mercury flybys—mark your calendars, the next one’s on October 6—before it settles into orbit around the planet in March 2011. Then the real fun can begin.

Human Journey