Emily Lakdawalla • Sep 29, 2016
Rosetta spacecraft may be dying, but Rosetta science will go on
The Rosetta mission will end tomorrow when the spacecraft impacts the comet. ESA took advantage of the presence of hundreds of members of the media to put on a showcase of Rosetta science. If there’s one thing I learned today from all the science presentations, it’s this: Rosetta data will be informing scientific work for decades to come. My apologies for the lack of images in this post, but it’s all I could do to throw this together late in the evening after the talk sessions, before the dramatic events of tomorrow. Here, have a random portrait of this strange and now strangely familiar comet.
There were nine science talks today, and unlike most such events it wasn’t each instrument’s principal investigator touting the achievements of their own instrument. Instead a variety of science team members, several of whom I hadn’t seen before, gave talks on scientific themes, using data from multiple instruments to show how Rosetta has advanced work in that area. It was a refreshingly different way to run a mission science overview. Here are lightly edited notes from the presentations.
Mohamed El-Maarry began with a talk on the “magical” landscapes of the comet. It has a rich textural diversity. Broadly speaking, most areas can be separated into one of two types of terrain. One is rough materials that appear like rocks and are pervasively fractured. The other is a smooth terrain, material that is coated by dust. The dusty regions appear smooth and featureless from higher orbits, but when one such place (Agilkia) was seen up close by Philae, you can see it’s covered with grains of different sizes. This dusty material, El-Maarry said, is interpreted to be stuff that was ejected during cometary activity.
Before the comet reached perihelion, most of the activity was from the sunlit northern side of the comet neck, and models show that stuff ejected from there at lower than escape velocity will collect on the comet’s north poles, where the smooth regions are seen. After perihelion, activity shifted into the south – but models show that stuff ejected from the south still settles preferentially in the north. Indeed, Rosetta sees a north-south dichotomy on the comet, explained by the fact that the south experiences short, very intense summers around perihelion, and therefore erodes at a faster rate than the north.
Valerie Ciarletti talked about the interior structure of the comet, mostly as revealed by CONSERT but also using some other Philae data sets. She was happy to report that two quite distinct data sets (mass and volume from gravity and imaging, and also from electrical properties with CONSERT) both produced similar estimates of the comet’s internal porosity, of more than 70%. Thus the comet is made of ice and dust but mostly vacuum! But when Philae measured the porosity of the material at the surface at the two sites where it landed (using Philae permittivity data and also thermal inertia data), it found a much lower porosity of around 50%. So Ciarletti proposed that the interior of the comet is mostly vacuum, but there’s a harder, denser rind. Imaging has shown the comet to appear to be built of many meter-scale round bodies. These are, frustratingly, just below the size of internal structure that CONSERT could detect – if the comet were made of many bodies of 5 or 10 meters in size with denser rinds and airier interiors, CONSERT would have seen that, and it did not. The comet appears homogeneous to CONSERT. Therefore any internal structure must be finer-scale than that. She said in her talk that the recent discovery of Philae’s exact location and orientation “provided a missing piece of our ground truth” and will provide inputs into ongoing data interpretation.
Thurid Mannel gave a cool presentation on the structure of the comet’s dust particles as seen by three different instruments: GIADA, COSIMA, and MIDAS. She said that each is sensitive to particles in a different size range: GIADA can count the number, speed, and mass of particles of millimeter sizes; COSIMA collects and images particles at scales of 10 to a few hundred microns; and MIDAS measures the shapes of the very smallest, micron-sized particles. All found that the dust is extremely fragile; it fragments upon very slow impacts into showers of smaller particles. The particles have fractal structure across 5 orders of magnitude – they are agglomerations of agglomerations of agglomerations of agglomerations of particles. As a result, many of the largest particles (the ones detected by GIADA) have a density less than that of air! Among the very smallest ones, those detected by MIDAS, some are “fluffy” like this, while some are compact. The particles have a variety of compositions, including high-molecular-weight hydrocarbons, primitive silicates, and iron sulfide.
Jean-Baptiste Vincent gave a presentation on the comet’s activity. He said that dust and gas streams that Rosetta’s OSIRIS camera has seen on the comet can be traced back to specific topographic features: cavities, cliffs, and pitted terrains on smooth plains. Although the jets are very dynamic features, most of them repeat precisely from one comet day to the next. By combining VIRTIS and OSIRIS images, they have documented a water cycle on the comet, seeing ice sublimating during the comet’s day from the Hapi region, and then seeing it redeposited on the surface as frost during the night. They have also documented cycles of other volatile species like carbon dioxide; the abundance of these species that Rosetta observed on the surface of the comet changed as a function of season and orbital position. One of the big surprises, he said, was the way erosion was taking place on the comet. Before Rosetta, they imagined the comet shrinking radially throughout its perihelion passage. But the erosion on the comet was actually taking place laterally, not vertically; the fastest-eroding surfaces were the steep slopes of cliffs. “Activity and topography are interdependent on all scales.” He also showed that Rosetta saw the comet spin up during its perihelion passage, with its day length getting shorter by 21 minutes, from a period of 12 hours 24 minutes at the time of Rosetta’s arrival, to its period now of 12 hours 3 minutes. Curiously, the comet spun up by 21 minutes during its pervious perihelion passage, too.
André Bieler, a member of the ROSINA ion and neutral mass spectrometer team, talked about how much the composition of the gases ejected by the comet’s activity varied over time. “Comets don’t care for taxonomy. What you find depends on where you look and when you look at them. Snapshots are not representative measurements.” He dropped the surprising fact that the team has analyzed only a tiny fraction of their compositional data (about 5%), and even that has required a million hours of computer time.
There was a talk about plasma and magnetic fields by Charlotte Goetz…but unfortunately here I had a jet-lag-induced brain lapse, and I realized at the end of her talk that I had spaced out and not written down any notes. My apologies to plasma fans!
I refocused for Cecelia Tubiana’s talk, on ground-based observations of comet Churyumov-Gerasimenko. She made the case for why ground-based observations are still important when you have a flagship mission in orbit at the comet for more than 2 years. The comet’s coma and tail extend far beyond Rosetta’s orbit, so provides large-scale context to the Rosetta results; and the ground-based observations also help connect Rosetta’s studies of 67P to ground-based studies of other comets. She said that a total of 1300 hours of ground-based telescope time has been dedicated to the Earth observation campaign, a huge data set. One interesting fact from her presentation is that the long-term observations show that the source regions for the comet’s jets are the same now as they were in the past.
Kathrin Altwegg gave a lively presentation on the “zoo” of chemical species detected in the comet. She said the team was surprised by the variety of heavy compounds they found, all kinds of hydrocarbons of higher masses. Rosetta has detected dozens of compounds in 67P that have never been detected on comets before. Some are long carbon chains, polycyclic aromatic hydrocarbons, and macromolecules – “we have a lot of carbon in this comet.” Many of these compounds are prebiotically important. Only one amino acid has been found in 67P, glycine, which she said makes sense because it’s the only one you can form without liquid water. “Imagine if you throw 67P into the ocean, and it starts to melt,” she said. Many of these compounds react with water, and in water they could also react with each other, and maybe that’s how life started on Earth. The newest addition to the zoo, she said, was noble gases, notably argon and xenon. She said that the xenon in the comet might solve a puzzle about why xenon isotopes in Earth’s mantle don’t match those in the atmosphere, suggesting that maybe comets delivered Earth’s atmospheric xenon. And she hinted at more exotic “animals” to come in the zoo.
Finally, Björn Davidsson gave a fascinating talk summarizing the current state of understanding of the origin of the solar system, and what Rosetta has added to that understanding – my fingers were really flying for this talk, and I want to do the subject better justice by devoting more time to it in a later post. But if you can’t wait for that, a Twitter commenter pointed out that you can read Davidsson’s summary of his work at his own blog here.
It was an excellent scientific session, and has really served to ameliorate any sadness I have felt over the impending demise of Rosetta. The spacecraft will die tomorrow, but the data that it has returned will provide valuable science for decades to come. During the active mission, scientists have mostly only had time to reduce and examine data from one or maybe a couple of closely associated instruments. Over time, as the teams get a handle on their own instruments’ data, they’ll have more time to compare and incorporate and expand upon it by bringing in more and more data sets, each one multiplying the value of the whole with its contribution. So much science yet to do!
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