When someone says “comet,” what’s the first image that comes to mind? A glowing orb streaking across the sky with a long tail, right? So where is the tail on Comet 67P/Churyumov-Gerasimenko – aka Comet 67P – and what are those odd little water jets we’re seeing in the latest pictures being sent back by the Rosetta spacecraft?
Let’s start with the lack of a tail. The vapor trail we’re accustomed to seeing being comets is caused by ice melting on the comet’s surface as it approaches the heat of the sun. The vapor is not just water steam but also ammonia, ice crystals and debris lifted by the rising gases. Comet 67P is not yet close enough to the sun for vaporization to occur in any great quantity, but it’s close enough that NASA moved quickly on picking a landing spot for the Philae lander. This will lessen any damage that might be caused by the vapor and allow for a pinpoint landing since the tail can alter the flight patch of a comet. Getting there early will put Philae in a prime spot for observing the creation of the comet’s tail. Comet 67P’s lack of a tail is also due to its age. It has already shed most of its surface ice and is now covered primarily with dust.
So what are those little spouts seen on the latest photos? It appears that Comet 67P has some subsurface ice which, as it melts and vaporizes, is being pushed out under high pressure through cracks and fissures in its crust. That could account for the small geyser-like sprays and the odd directions they’re spurting in.
That’s the theory. Once Philae lands on November 11th, it will drill into the surface, collect samples and analyze them. Together with measurements of surface and subsurface temperatures and radar images of the interior structure, the mystery of the jet plumes may be solved.
If Comet 67P wants us to solve it …