One of the big differences between Hubble’s first and latest image of Comet ISON is its jet – or lack thereof. In Hubble’s April image, computer modeling showed a jet streaming away from the comet’s nucleus. In the October image, that jet seems to have vanished.
Where did it go? Well, first let’s talk about why it was there to begin with.
The surface of a comet’s nucleus isn’t smooth. It’s porous and uneven. Because of this structure, the surface area doesn’t warm up equally, and the comet’s ice vaporizes unevenly. The Sun heats the frozen carbon dioxide beneath the surface of the nucleus, changing it directly into a gas, a process known as sublimation. The heated gas wants to expand and escape, and it picks a weak spot in the surface of the comet to break forth.
Gas and dust bursts forth like a geyser – in fact, even the dynamics are somewhat similar: An area below the surface warms up and punches through the surface with explosive force due to its gas content. In comets, the emerging gas carries a stream of dust along with it, instead of the water you would see with a geyser.