First Rosetta Science Results Are Surprising

The first suite of science papers from the Rosetta mission has been published, giving new insights about comets.

Comet 67P/Churyumov–Gerasimenko has become the most-studied comet in history. Earlier comet probes paved the way, as the Introductory Article on Science Magazine recounts:

Such space-based investigations of comets began in the 1980s with a flotilla of spacecraft: the European Space Agency’s (ESA’s) first deep space mission, Giotto, which pursued comet 1P/Halley; Deep Space 1 at 19P/Borrelly; Stardust at 81P/Wild 2; Deep Impact and Stardust NeXT at 9P/Tempel; and EPOXI at 103P/Hartley 2.

Now that Rosetta has been in orbit about “Comet 67P” for months, and has landed the probe Philae on its surface, what has been discovered? There’s far too much information in the collection of papers to summarize, but we can search on the word “surprise” for indications of the most dramatic new findings.

• Eric Hand writes in Science Magazine this headline: “Comet close-up reveals a world of surprises.” Among them is “a surprising diversity of features on the 4-kilometer-long duck-shaped comet.” Certain pits on the surface “contained some surprises. In one, OSIRIS revealed a delta of material that had flowed out, like ooze from a baking-soda-and-vinegar volcano. Thomas says the flow is a sign that pockets of pressure can build up so high in the icy interior that fluid mixes of material occasionally erupt.” This finding is bound to generate discussion: “In the walls of other pits, OSIRIS has spotted what could be features dating back to the comet’s formation: what the team calls ‘goosebumps’ or ‘dinosaur eggs,’ nodules about 3 meters across that could represent the fundamental chunks of material that coalesced into 67P.” In short, “A boring lump of ice and dust it’s not.“
• Science Daily quotes researcher Hans Nilsson: “We are also surprised how much structure we see in our data — the comet atmosphere appears to be very unevenly distributed around the nucleus.”
• The paper on subsurface properties says, “To our surprise, the highest water column densities are often observed above shadowed neck regions where the nucleus thermal continuum—and, consequently, the surface and subsurface temperatures—is low….” Some of the pits have “remarkably flat floors.”
• Southwest Research Institute (SWRI) headlines its press release, “Rosetta data reveals more surprises about comet 67P.” Notable among them is this quote: “It was certainly a surprise when we saw time variations from 200 km away. More surprising was that the composition of the coma was also varying by very large amounts. We’re taught that comets are made mostly of water ice. For this comet, the coma sometimes contains much more carbon dioxide than water vapor.“
• PhysOrg quotes a member of the dust measurement team: “Because comets have very little gravity, dust and gas flow freely into space. But we were surprised to find a cloud of particles orbiting the comet that are large and heavy enough to defy the sun’s radiation pressure.“

The authors of the papers tend to be a little more reserved in their emotions when writing formal papers, but from the quotes in the popular media, they were excited and surprised by a number of things. One is the dumbbell-shaped structure of the nucleus. They’re not sure if it eroded down that way, or consists of two “cometesimals” that somehow got glued together. This comet challenges the old “dirty snowball” model taught for decades. The surface has deep pits and steep cliffs, and some flow structures, as described above. Nature says “the body is relatively fluffy and porous — with a density of around half that of water,” making it the density of meringue or cotton, quite an upset for what was thought to be mostly a water-ice snowball.  In fact, it has “an abundance of opaque, organic compounds, but very little water ice.”

Reporters tend to experience visions of hydrobioscopy when they hear the word “organic,” but organic just means molecules with carbon, which can be very simple and have nothing to do with life.  Some exotic organics are present in small amounts, but the “organic” compounds mostly refer to carbon monoxide (CO) and carbon dioxide (CO₂) ices blanketing the surface. One of the papers mentions that particles detected near the orbiter, far from the comet, consist “mostly of water but also of organic material, fragments of hydrazine, and vacuum grease (fluorine).” When Eric Hand says that the organics “could include carboxylic acids—precursors to amino acids,” he is not saying that either has been detected. Some reporters, however, ran with it as evidence that comets either might have life or might have brought life’s building blocks to earth. Actually, the unexpectedly high deuterium-to-hydrogen ratio (D/H), three times higher than ocean water, rules out this kind of comet as a bearer of water to earth.

It’s not clear how typical Comet 67P is. The primary paper on organic molecule detection says “67P represents a different species in the cometary zoo.” As a Jupiter-Family Comet (JFC), it may have been captured from the Kuiper Belt.

These are some of the initial discoveries. More are sure to follow as the comet heats up and emits more gas and dust as it approaches perihelion in August. There is some hope that increasing sunlight might wake up the Philae lander at long last, providing more “ground truth” to complement the orbital data.

This is the first of three historic “space firsts” we are fortunate to watch in 2014-2015. The second will be the orbit of asteroid Ceres by the Dawn Spacecraft in March; already,  images are getting better and better as it approaches. The third will be the long-awaited flyby of Pluto by New Horizons in June (see BBC News review). Pluto was still a bona fide “planet” at launch, but should be no less thrilling to see as a “dwarf planet” with four moons, including the large moon Charon.

As with all planetary encounters, expect surprises! These are like the grand voyages of discovery, when sailing ships in the Victorian Age brought back exotic tales from distant lands. They are the memories you can tell your grandchildren about. These events have much more staying power than football scores. Who remembers much about sports in 1969, when Apollo astronauts landed on the moon? Pay close attention to these groundbreaking discoveries, so that you can make your stories come alive.