[meteorite-list] At the Heart of Hartley-2, a New Breed of Comet?

[Ron Baalke]

http://www.nasa.gov/mission_pages/epoxi/hartley-2.html

At the Heart of Hartley-2, a New Breed of Comet?
Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Md.
May 17, 2011
 
At the heart of every comet lies a remnant of the dawn of the solar system. 
Or is that remnants? Astronomers don't know, but the answer would give them
a clearer picture of exactly how comets were born eons ago at the birth
of the Solar System. Did thin tendrils of dust and ice get drawn slowly
inward and pack themselves into a single, uniform mass? Or did a
hodge-podge of mini-comets come together to form the core for a comet of
substance?

For Hartley-2, the answer so far is neither. "We haven't seen a comet
like this before," says Michael Mumma of NASA's Goddard Space Flight
Center in Greenbelt, Md. "Hartley-2 could be the first of a new breed."

Both data collected by Mumma's team and detailed images of the comet
taken by NASA's EPOXI mission reveal that the comet's core is not
uniform. "We have evidence of two different kinds of ice in the core,
possibly three," says Mumma. "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."

The researchers observed Hartley-2 six 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 findings of Mumma and his colleagues at Catholic
University of America in Washington, D.C., the University of Missouri in
St. Louis, the University of Hawaii in Honolulu, the California
Institute of Technology in Pasadena, the Max Planck Institute for Solar
System Research in Katlenburg-Lindau, Germany, and Rowan University in
Glassboro, N.J., are being reported in a special issue of Astrophysical
Journal Letters on May 16, 2011

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
telltale signatures in the near-infrared region of light, at wavelengths
from 2.9 to 3.8 micrometers. In this way, it's also possible to tell how
plentiful each type of molecule is.

Ices in Hartley-2 are mostly made of water, along with traces of many
other types of molecules, the team learned. This is in addition to the
plentiful carbon dioxide detected in the comet in 1997 by the European
Space Agency’s Infrared Space Observatory. Mumma and colleagues paid
close attention to the levels of water and seven other molecules that
evaporate easily. The molecules remain frozen either on or below the
core's surface until the warming rays of the sun vaporize them; then,
they are swept into the coma.

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.

First of all, they saw the comet's wild side. "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.

"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. "At this point the interpretation is pretty
speculative."

EPOXI's Deep Impact spacecraft had a rendezvous with the comet in
November 2010. The rich images taken then of the comet's surface
revealed small, volcano-like "jets" spewing out carbon dioxide gas and
water ice at one end. The jets activate when sunlight warms that end of
the comet, turning the frozen carbon dioxide (aka dry ice) below the
surface into gas that escapes through open holes.

The researchers think that chunks of water ice are glued together in the
comet's core by the frozen carbon dioxide, which evaporates before the
water ice. "The carbon dioxide gas drags with it chunks of ice, which
later evaporate to provide much of the water vapor in the coma," A'Hearn
explains.

Researchers had never seen this before. "In other comets that have been
visited, most of the water appears to be converted into gas below or at
the surface," says A'Hearn. "We have not seen icy grains, or at least,
very few, being dragged into the coma."

But the whole core is not made the same way. EPOXI revealed that the
carbon dioxide jets are not found at the large end of the comet, and in
the middle region, water vapor is released without any carbon dioxide.
"So clearly, when we look at the comet up close, the composition of the
core changes from one region to another," Mumma says.

Mumma's team found more evidence that Hartley-2's core is not uniform.
They did so by looking carefully at four types of gas to see in which
directions their molecules traveled after release. They saw that water
and another gas, methanol, came off the comet in all directions.
"Because they are found together, we infer that they come from the same
chunks of ice," he explains.

"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."

That possibility is based on the fact that ethane, unlike water and
methanol, was released strongly in one direction. "This is actually
rather profound," says Mumma. "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."

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."