Data collected by Mumma’s team and detailed images of the comet taken by NASA’s EPOXI mission reveal that the Hartley-2 core is not uniform. According to Mumma: “We have evidence of two different kinds of ice in the core, possibly three, but we can also see that the comet’s overall composition is very consistent. So, something subtle is happening. We’re not sure what that is.”
Mumma and his team observed Hartley-2 several times during the summer, fall and winter of 2010, both before and after the EPOXI mission’s Deep Impact spacecraft had its November rendezvous with the comet. Using telescopes perched high in the mountains of Hawaii and Chile, Mumma’s team studied the comet’s coma—the aura of gas, dust and ice particles that surround the core.
The gases and rocky particles that make up the coma are the clues that astronomers use to deduce what the core is made of, and thus its origin. To see which types of molecules are there, researchers check for signatures in the near-infrared region of light, (from around 2.9 to 3.8 micrometers).
The release of the molecules depends a great deal on exposure to the sun. The researchers knew that in 2009 ground-based observers had detected telltale signs that the core was rotating quickly. So the team was interested in what would happen to the production levels of these molecules as the comet rotated every 18 hours, giving each of its faces a turn to bathe in sunlight. Turns out, they saw something that nobody has seen before.
The team discovered something very strange: “The amount of water changed dramatically night by night and even within a single night—in some cases, doubling in that time,” says Mumma. But, in truth, Hartley-2 isn’t the only comet to get caught being fickle.
What surprised the researchers was this: as the amount of water went up, so did the amounts of the other gases. And as the amount of water went down, the others did, too. “This is the first time anyone has seen an entire suite of these gases change in the same way at the same time,” says Mumma.
This result is important for astronomers, he notes, because they often study the gases in a comet’s coma one at a time. “But this suggests that if you look at one gas on one night and another the next night, the production rates might change quite a bit. The findings could be different than if you measured the two gases together,” he says. “And in the worst case, you could get the wrong idea about the composition of the comet.”
Beyond that, Mumma says, “this tells us that the overall composition of the gas in the coma did not change.” Taken by itself, this might seem to imply that the core of the comet is uniform. But when the findings of the EPOXI science team are considered, the picture gets more complicated.
More research needs to be done, and whether all comets behave like Hartley-2 isn’t known, Mumma adds. “But now that we know what this one does, we have a baseline to compare other comets against.”