Bad Astronomy

Mercury hides a monster impact

How the heck do you hide a terrifyingly huge impact basin easily big enough to stretch from San Francisco to Los Angeles?

Easy. Put it on Mercury.

This new MESSENGER image of Mercury was taken when the spacecraft swept past the tiny planet back in October 2008. The big circular feature is the Rembrandt impact basin, which is a ginormous 715 kilometers (440 miles) across. Incredibly, this feature has never been seen before on Mercury! That goes to show you how hard it’s been to study the planet in the past. From Earth, Mercury never gets very high above the horizon so it’s always a fuzzy blob, and previous spacecraft encounters only imaged parts of the planet.

Head-on view of Rembrandt
Rembrandt seen head-on. Colorado
would comfortably fit inside it.
Plus, sometimes it’s hard to see something that’s really big. I remember standing on the lip of a volcano with a caldera 5 km across, and it was so huge it was hard to tell it was a volcano! In the case of the Rembrandt basin, its location on Mercury, its size, and its somewhat subtle rim made it difficult to see in the past. MESSENGER’s phenomenal cameras, however, were able to catch it in a mosaic of several images.

It looks like a relatively young feature given how well-preserved it looks. Yet it still has plenty of craters in it, indicating it’s been around a while at least. Note the inner circle, too: that’s a common feature in huge impact basins like this, and the reasons the multiple rings form is still being debated.

Rembrandt is cut by a 1000-km-long scarp on the left – the longest yet seen on Mercury – and also has a series of ridges and troughs in the floor unlike anything seen on Mercury or any other planet. Those are somehow causes by crustal deformation, but the specific mechanism isn’t understood yet.

Imagine! Here is an entire planet, one of the closer ones in the solar system, and yet it still holds many mysteries… including some so big they are almost literally right in our face, just waiting to be unveiled and understood. What will MESSENGER see in September, on its third pass of the planet? What will it see once it settles into orbit in March 2011?