Mercury is a passingly strange place. It's hot, and dense, and covered with craters. One of the most intriguing features on the diminutive planet is a vast impact basin, called Caloris Basin. This circular monstrosity is nearly 1600 kilometers across (960 miles), which is pretty frakkin' impressive given that Mercury is just under 5000 km (3000 miles) in diameter. Whatever hit Mercury was big, and hit hard.
Caloris has been known for decades, but recent MESSENGER images have revealed new features inside it. The weirdest of them all is a series of radial cracks, dubbed The Spider (the real name is Pantheon Fossae), which appear to be centered on a small impact crater called Apollodorus, which is about 40 km (25 miles) across. The cracks radiate away for hundreds of kilometers. They appear to be associated with the crater -- the fact that it sits right at the center is awfully suspicious -- but no one was sure why. Most models proposed uplift in that region (from pressure buildup underneath, or possibly from the crust of the planet shrinking as it cooled, putting pressure on the region). But nothing really explained the formation well, and moreover, it's unique: no other feature like this is seen on Mercury or its nearest analog, the Moon.
However, new results just released at the European Planetary Science Congress 2008 by MESSENGER Principal Investigator Sean Solomon may finally explain the weird structure. His team made a three-dimensional model of the region around Apollodorus. They started with the same idea as earlier -- pressure from below building up stress in the crust -- but then modeled what would happen if an asteroid smacked into the stressed region. What they found was that the impact relieved the stress catastrophically, and the crust fractured. The end result would be a series of radial cracks hundreds of kilometers long, centered on the impact.
There is some argument over the idea, though. Another proposal is that magma upwelling could explain the fossae, with the cracks being dikes.
But I'm not so sure. Remember, the Spider is unique, which indicates an unusual event to create it. A large impact in a region already under stress is pretty unusual, and it does seem to be the parsimonious explanation.
Either way, I'm delighted! We have bizarre things still left to explore out there, and Mercury seems to have its share of them. MESSENGER will swing by the planet again on October 6 -- in just two weeks! -- and we'll get even more data. A year later it'll make its third pass, and finally settle into orbit around Mercury in March 2011. And then we'll get all the pictures our hearts desire.
By the way, if you want to know more about Mercury and MESSENGER, I made a video about it that is up on YouTube.
I'm really excited about this next pass in October. What new things will we see?