Rosetta has finally ended its dance around Comet 67P. After more than ten years spent on its mission, it took its last picture about 20 meters from the surface of 67P and then parked itself on the “head” of the rubber-duck-shaped comet.
The actual impact was more of a bump, bump, settle: It probably happened at 2 mph, which is about walking pace. The spacecraft shouldn’t have been destroyed in the impact; instead, ESA mission scientists set a software trigger that put it in indefinite “airplane mode” immediately upon physical contact with the comet. This way Rosetta’s radio emissions wouldn’t impede communication with other spacecraft.
The science results Rosetta and Philae have returned represent a wealth of data that can confirm or dispel our ideas about the formation of our planet, and even the origins of life in our solar system. One of the key findings from Rosetta is that the water ice found on 67P is a different “flavor” than water on earth: it has a different ratio of deuterium to hydrogen (D/H). Like the dozen or so comets we’ve sampled, 67P has more deuterium in its water than does Earth. In fact, only one comet even comes close to Earth’s D/H ratio: 103P, also called Hartley 2.
Meteorites that originally came from asteroids in the Asteroid Belt, though — those do match the composition of Earth’s water. Even if each asteroid has much less water than an iceball comet, a hail of many asteroids could still have deposited enough water to explain the volume we have on Earth.
67P is a wanderer: Thanks to gravitational interactions, 67P has changed position from beyond between Mars and Jupiter to between Earth and Mars. The comet itself is more like a snowball than an iceball. Rosetta’s CONSERT instrument measured the density of 67P and found that it’s “fluffy” — suggesting that it’s made of “cometesimals” that fell together at speeds even slower than the stately pace at which Rosetta settled onto the comet’s surface, almost like drifting snow. This would mean that the comet’s insides are a lot like a snowbank in late winter: loose bits of ice and rock partially settled and partially melted together. It periodically sheds hunks of ice a meter on a side, which is bigger than we thought the pieces of comet in its tail would be.
That was far from the only reason we sent a probe to 67P, though. Rosetta also went to see what chemicals there were on the surface and in the comet’s fuzzy atmosphere. It found organic compounds, phosphorus (an important ingredient in cell walls and DNA), and the amino acid glycine. Rosetta’s instruments directly detected the glycine in the comet’s halo, letting us rule out the possibility of earthly contamination.
For the curious, there is no D-glycine nor L-glycine; because its substituent group is just an extra hydrogen atom on that central carbon atom, there’s no difference in chirality and therefore only one stereoisomer. Terrestrial life produces mostly L-amino acids and D-sugars, but there’s no energetic reason for life to have “chosen” one chirality over the other. There’s nothing that demands L-amino acids be more common in the universe. Finding R-amino acids off-world would support the idea that meteorites rich in amino acids and their precursors provided a nutritious primordial soup from which the first enzymes could have developed.
“There is still a lot of uncertainty regarding the chemistry on early Earth and there is of course a huge evolutionary gap to fill between the delivery of these ingredients via cometary impacts and life taking hold,” said mission scientist Hervé Cottin. “But the important point is that comets have not really changed in 4.5 billion years: they grant us direct access to some of the ingredients that likely ended up in the prebiotic soup that eventually resulted in the origin of life on Earth.”
While Rosetta’s operations are done, the science part of the mission is just getting revved up. Between some 80,000 images and the huge body of readings and data the spacecraft beamed back, scientists will be busy for a while.