Combining ground- and space-based observations of Comet Hartley-2 reveal that although the overall composition of the comet remains consistent, its core is made up from a patchwork of at least two different types of ices.

“We haven’t seen a comet like this before,” says Michael Mumma of NASA’s Goddard Space Flight Center, reporting the results in the Astrophysical Journal Letters. “Hartley-2 could be the first of a new breed. Something subtle is happening. We’re not sure what that is.”

Last November the EPOXI spacecraft flew by the comet, finding jets of carbon dioxide and water ice spewing from the surface, activated when the sunlight warmed the comet turning frozen carbon dioxide ice into gas that then escapes through open holes in the surface.

Extensive ground-based observations were also conducted before, during and after the flyby. Using Hawaii and Chile-based telescopes, an international team of astronomers studied the components of the gaseous, dusty coma of Hartley-2 at near infrared wavelengths in order to make inferences about the composition of the core, based on the amount of each type of molecule present. Since earlier ground-based observations had revealed the comet to be rotating quickly – once every 18 hours – the goal was to learn how the production levels of those molecules changed during that timeframe.

The astronomers found that the amount of water changed dramatically night by night and even within a single night, in some cases doubling, and, more surprisingly, they saw that as the amount of water varied, so did the other gases.

“The fact that the gases all vary together is somewhat puzzling, because EPOXI found a large variation in the release of carbon dioxide relative to water,” says the head of the EPOXI science team, Michael A’Hearn, of the University of Maryland. The close-up observations of Hartley-2's jets suggested that chunks of water ice are glued together inside the comet by frozen carbon dioxide – dry ice – the latter evaporating before the water ice. “The carbon dioxide gas drags with it chunks of ice, which later evaporate to provide much of the water vapour in the coma. In other comets that have been visited, most of the water appears to be converted into gas below or at the surface, we have not seen icy grains, or at least, very few, being dragged into the coma.”

EPOXI also saw that different amounts of carbon dioxide and water are released from different sections of the comet, again suggesting that the composition of the core changes from one region to another. Meanwhile, Mumma's ground-based team found even more evidence for the core's varying composition. Looking at different gases and the directions they travelled after release, they saw that water and methanol are found together, implying that they come from the same chunks of ice. “So, we have water ice with methanol in it, and we have carbon dioxide ice. Both are in the comet’s core,” Mumma says. “We may also have a third type of ice, made from ethane.” The ethane, Mumma reports, was released strongly in one direction. “This is actually rather profound. It suggests that some molecules, such as methanol, may be mixed with water, while others, such as ethane, are not. This isn’t the way we’ve thought of comets, before now.”

Meanwhile, amateurs from around the globe have worked alongside EPOXI scientists to contribute to a research paper published in the Astrophysical Journal, which highlights their global campaign of monitoring, data reduction and light curve analysis on the comet. Astronomy Now's Equipment Consultant Nick Howes, who is a contributor to the paper with his own observations and light curve/rotation calculations using the two-metre Faulkes North Telescope in Hawaii comments: "To work with professional astronomers and NASA on such a project was a real pleasure, and whilst the author list is vast, it genuinely credits so many of the people involved in the daily monitoring and data reduction work on the comet. All of the contributors, who worked closely with Dr Karen Meech in Hawaii consider it a great honour to be a part of this project. The support we got from NASA/JPL and in particular the University of Maryland with this project was immense, and it again shows what amateur astronomers can do when they put their minds to it."

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