Writing this week in Science, an international team with lead author Samuel Gulkis of the Jet Propulsion Laboratory, Pasadena, describes the first millimeter-wave measurements made from June through September 2014, of the comet鈥檚 nucleus, subsurface temperature and of water vapor and other molecules in the coma of gas and dust that is beginning to form around the comet as it approaches the Sun.
The researchers used NASA鈥檚 Microwave Instrument on the Rosetta Orbiter (MIRO), launched as a part of the European Space Agency鈥檚 Rosetta Mission, to study heat transport in the comet鈥檚 nucleus along with outgassing and development of the coma as interrelated processes. Data from MIRO鈥檚 millimeter-wave radio receivers help to map temperature and spectral information during the day, night and across seasons as the comet approached the sun.
Schloerb, a planetary scientist who brings special expertise in radio astronomy and remote sensing to the team, says such measurements and monitoring of the interplay between gas emissions and surface temperature over time by MIRO provides key information that allows scientists to better understand the evolution of the gas and dust that comprise the comet鈥檚 coma.
Observations of the comet at millimeter wavelengths allow astronomers to measure the temperature beneath the surface of the comet鈥檚 nucleus and properties of the gases in the coma as well.
鈥淎n important contribution is that we鈥檙e sensing not only the subsurface, we鈥檙e also looking at the important properties of gases inside the coma. Our instrument puts the two together, providing an integrated look at the important physical processes that make the comet work,鈥� the UMass Amherst astronomer says.
Schloerb adds, 鈥淭hat鈥檚 valuable because we can measure temperature within the surface and then see the gases that are produced directly as the nucleus heats up. The surface and subsurface temperatures vary quite a lot as the comet rotates and changes its orientation towards the sun during its orbit. It will be fun to relate this driving force behind the gaseous emissions and then see how the molecules produced behave in response as the comet moves toward the sun.鈥�
鈥淚n our initial results, not surprisingly, we found important day-night and seasonal variations in the temperature of the nucleus. It is northern summer now, so the northern latitudes are quite warm, while some southern latitudes are un-illuminated at this time and very cold. We鈥檙e the only instrument that can make measurements of this un-illuminated part of the comet, so our measurements are completely unique in that area.鈥�
Schloerb says the team鈥檚 observations from comet 67P show a comet spectrum was not consistent with water vapor coming out the same in all directions. Instead, water vapor flowed out mainly from the illuminated side.
鈥淩ight away, by looking at the shape of the spectrum, we knew the emissions were very asymmetrical, with most gas being produced on the dayside where the comet was being heated up. This is not a new result, we鈥檝e known this in a general way for 30 years from ground based observations, but it was sure interesting to see it up close where we can look at the detailed behavior.鈥�
Another of the important properties of gases inside the coma, besides composition, is their speed as they escape, Schloerb says. MIRO has the ability to measure the outflow speed of the gasses, which is fundamental to the interpretation of observations of the coma and a key parameter needed to check theoretical models.
The UMass Amherst astronomer, who has studied comets for decades and led the worldwide radio observations of Halley鈥檚 comet as a part of the International Halley Watch in the 1980s, says, 鈥淭his has been a pretty neat experience. I鈥檝e never been on a spacecraft project before, but as a planetary scientist I always wanted to do this. After all those years of sitting and looking through a telescope, to see the spectra come in and make the first maps ever of the comet nucleus is really satisfying.鈥�
A component of one of the receivers was built at UMass Amherst by Neal Erickson, a world expert on millimeter and submillimeter receivers.
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