These three pairs of images from NASA's EPOXI mission demonstrate that a dust jet and gaseous carbon dioxide are being released from comet Hartley 2 at the same time, and from the same location on the comet. The observations suggest that carbon dioxide is driving the jet and taking tiny grains with it as it spews out of the nucleus of the comet. This is the first time this type of jet has been observed.
The top row consists of three images showing carbon dioxide gas being released by the comet at different points in time, from when the comet was at its minimum brightness to its maximum brightness. The bottom row of images shows dust coming from a jet on the comet at the same three points in time. The observations demonstrate that the gas and the jet are coming from the same location on the comet at the same time. This, in turn, suggests that the carbon dioxide is driving the jet.
The presence of this jet tells the scientists that the comet is made of chunks rich in solid carbon dioxide, sort of like chocolate chip chunks in frozen cookie dough. What's more, this variability in the comet's composition implies that the ingredients for both comets and planets must have been mixed up early on in the formation of our solar system. Without this mixing, comets would have more homogenous composition -- in simple terms, this would be having comets made of just "dough," and comets made of just "chocolate chunks."
The top-row images show data taken by the spacecraft's infrared spectrometer, a part of the High-Resolution Instrument. The bottom row images were taken in visible light by the spacecraft's Medium-Resolution Instrument.