Researchers using a new radio-telescope array high in Chile's Atacama Desert have made the most detailed maps yet of simple organic molecules in the halos of two comets.
The 3D maps not only reveal the molecules present, but where in the halo, or coma, they appear. Their distribution offers clues about their origins – either coming from the nucleus as it heats and the ices it harbors morph straight from ice to gas to form the coma and tail, or produced through chemical reactions in the coma itself.
Such comet chemistry is of keen interest for researchers trying to uncover the conditions in the early solar system some 4.5 billion years ago, as the planets formed. Comets, especially those coming from a distant, extended reservoir of objects known as the Oort Cloud, are thought to harbor evidence of these early conditions in the dust, rock, and ice that make up their cores.
Moreover, comets are thought to have delivered water as well as molecular building blocks for compounds important for the emergence of life on Earth.
The observations from the Atacama Large Millimeter/Submillimeter Array (ALMA) presented fresh insights on two levels, say the researchers involved in the study.
"We wanted to find out what we could about comets," says Martin Cordiner, a postdoctoral research associate at NASA's Goddard Space Flight Center in Greenbelt, Md., and the lead author of a formal report on the results set to appear in the Sept. 1 issue of The Astrophysical Journal Letters. "But we also wanted to show that we could detect comets at all."
ALMA is designed to use 66 mobile dish-shaped antennas in a coordinated fashion to provide a level of detail comparable to a singe dish as wide as 10 miles across. Yet comets move through Earth's neighborhood very quickly. The observatory needed to show, among other things, that multiple antennas could track a comet while keeping the object rock steady and dead center in their collective field of view.
The initial test object was comet C/2012 F6 (Lemmon), which the team observed in June 2013 with 25 dishes active. The results were sufficiently encouraging that the team next took aim at comet C/2012 S1 (ISON), which they observed with 28 dishes in mid-November 2013.
Both comets were making their closest approach to the sun. Comet ISON, the brighter of the two, was making its first and only pass as a sun-grazer, breaking up hours before its closest approach Nov. 28. Both comets came from the Oort Cloud.
The international team conducting the observations focused on three well-known cometary molecules: hydrogen cyanide, hydrogen iso-cyanide, and formaldehyde. Of the three, hydrogen iso-cyanide and formaldehyde are ubiquitous in the dense molecular clouds that give rise to stars and the disk of dust and gas that forms around them – disks that under favorable conditions form planets.
These molecules had been seen in comets before, but the researchers note that previous observations were unable to determine where in the comet the molecules formed. Knowledge of the reactions that formed them was even more hazy.
The formaldehyde in comet Lemmon was clumpy and too far away from the nucleus to have come from there given the comet's distance from the sun, says Stefanie Milam, an astrochemist at Goddard and another member of the research team. Even on ISON, where the formaldehyde was concentrated near the nucleus, the distribution was sufficiently uneven and still extended farther from the nucleus than it should have if the nucleus was its source.
The made-in-coma label on formaldehyde on comet Lemmon confirmed work Dr. Milam had performed on data taken from Hale-Bopp, which also showed clumpiness at a distance too far away from the nucleus to come from there. But the earlier results were somewhat controversial because they came from one object via hardware that was state of the art at the time, but lacked ability to see the coma with ALMA's sensitivity and level of detail.
When the ALMA data came in, she and Anthony Remijan, with the National Radio Astronomy Observatory in Charlottesville, Va., a member of the research team who worked with Milam on Hale-Bopp, "were completely gratified" that ALMA was revealing the same features on another comet.
The team also noticed that to varying degrees on both comets, hydrogen iso-cyanide also appeared in blobs. For both molecules, the aligned blobs suggested that jets on the nucleus were erupting, sending gases into the coma to be broken down, perhaps by temperature or sunlight, into formaldehyde or hydrogen iso-cyanide.
The nature of these precursors is unclear, although more-complex molecules bound up in dust that the comet also ejects could be a source, the researchers suggest.
Some of the best tests will come when the European Space Agency's Rosetta spacecraft, which arrived at comet 67P/Churyumov-Gerasimenko Aug. 6, deposits its lander on the comet's surface.
Back on Earth, however, ALMA appears to be exceeding the expectations of astronomers interested in comets and cosmochemistry. It's sensitivity, ability to pick out fine details, and the ability to use spectra to measure the velocities of ejected molecules at very fine scales bodes well for the future of comet research, team members say.
These features, and additional antennas still to come, will allow ALMA to gather previously inaccessible information on the processes affecting a broad range of comet types at a wide range of distances.
"With ALMA, you don't need a comet of the century anymore to test your theories," Dr. Remijan says.