Tag: Comet 67P/Churyumov-Gerasimenko

Data from Rosetta has found that Comet 67P/C-G has changed color and brightness since the spacecraft’s arrival.

Even when Rosetta first rendezvoused with the comet far from the Sun, ices hidden below the surface were being gently warmed, sublimating into gas, and escaping, lifting some of the surface dust away and contributing to the comet’s coma and tail. VIRTIS shows that as the ‘old’ dust layers were slowly ejected, fresher material was gradually exposed. This new surface was both more reflective, making the comet brighter, and richer in ice, resulting in bluer measurements.

On average, the comet’s brightness changed by about 34%. In the Imhotep region, it increased from 6.4% to 9.7% over the three months of observations

The changes are similar to those found on the Moon when the surface there is disturbed. The old surface is very dark. When hit by a meteorite or scrapped by an astronaut’s foot, it brightens. In the case of Comet 67P, the underlying ice pushes outward when heated, the dust is removed, and the surface gets brighter.

Cool image time! As part of its research plan, Rosetta has been moved outward from Comet 67P/C-G for the next few weeks in order to better study its coma and tail. In this new position, engineers were able to maneuver the spacecraft so that it was flying about 600 miles farther from the Sun and could look back and see the Sun being eclipsed by the comet.

Thanks to the combination of a long, four-second exposure, no attenuation filter and a low-gain setting on the analogue signal processor of NAVCAM (a setting that is used to image bright targets), the image reveals the bright environment of the comet, displaying beautiful outflows of activity streaming away from the nucleus in various directions. It is interesting to note hints of the shadow cast by the nucleus on the coma below it, as well as a number of background stars sprinkled across the image.

In the next week the spacecraft will move back in close to the comet.

Cool image time! As Rosetta completes a several weeks of in-close observations it took the above oblique view (cropped and reduced to show here) of Comet 67P/C-G. The image was taken from about 7.5 miles with a resolution of about 3 feet per pixel. It shows the Imhotep flat area with the large 80-foot-high Cheops boulder in the center. It is worthwhile to compare this image with one taken in January.. Though the angle is far different, you can recognize the same areas in Imhotep where some of the surface has apparently evaporated away.

What I especially like about today’s image is that it really gives one a feel for what it would be like to stand on the surface here. The light gravity allows some strange rock configurations, such as that giant weird balancing outcrop on the horizon. If you were standing in Imhotep that outcrop would hang above you threateningly.

The spacecraft is now moving away from the comet for the next few weeks

Following the brief encounter at these close distances, Rosetta is now heading out on an anti-sunward excursion to around 1000 km to investigate the comet’s wider coma, tail and plasma environment. Today, 24 March, Rosetta is already over 200 km away from the comet. The current plan is for Rosetta to make a 30 km zero phase flyby around 9 April, before entering back into closer bound orbits by 21 April.

Cool image time! As Comet 67P/C-G moves away from the sun and cools down, the Rosetta science team has been able to move the spacecraft back in close to the comet. The image on the right was taken on March 5 from only 12.6 miles above the comet’s surface, and has a resolution of 14 inches per pixel.

I have brightened the image and cropped it to show it here. At this scale, if they managed to photograph the location where Philae sits we would see it with no problem at all. As it is, the detail is remarkable. For example, look at the slope below the cliff in the lower right. You can see what look like a very faint series of terraces, suggesting the existence of onion-like layers below the surface.

Go to the link. There is a second high resolution image there that is as amazing.

Scientists using Rosetta have finally detected the expected bubble or region surrounding Comet 67P/C-G where there is no magnetic field and the Sun’s solar wind does not enter.

The bubble is caused by the material being ejected from the comet. Scientists had detected the same thing around Halley’s Comet back in 1986, but it turns out the bubble around Comet 67P/C-G is larger than expected based on those previous measurements, and also fluctuates in size more than predicted.

The Rosetta science team has determined that Comet 67P/C-G has no voids or large caverns in its interior, and that its low density is because its dust and water ice have mixed to produce a “fluffy” density.

In a new study, published in this week’s issue of the journal Nature, a team led by Martin Pätzold, from Rheinische Institut für Umweltforschung an der Universität zu Köln, Germany, have shown that Comet 67P/Churyumov-Gerasimenko is also a low-density object, but they have also been able to rule out a cavernous interior. This result is consistent with earlier results from Rosetta’s CONSERT radar experiment showing that the double-lobed comet’s ‘head’ is fairly homogenous on spatial scales of a few tens of metres.

The most reasonable explanation then is that the comet’s porosity must be an intrinsic property of dust particles mixed with the ice that make up the interior. In fact, earlier spacecraft measurements had shown that comet dust is typically not a compacted solid, but rather a ‘fluffy’ aggregate, giving the dust particles high porosity and low density, and Rosetta’s COSIMA and GIADA instruments have shown that the same kinds of dust grains are also found at 67P/Churyumov-Gerasimenko.

A new paper based on accumulated data from Rosetta has given scientists a better understanding of the behavior of water ice on Comet 67P/C-G, including the process by which it escapes and is also covered by dust on the surface.

Although water vapour is the main gas seen flowing from comet 67P/Churyumov–Gerasimenko, the great majority of ice is believed to come from under the comet’s crust, and very few examples of exposed water ice have been found on the surface. However, a detailed analysis by Rosetta’s VIRTIS infrared instrument reveals the composition of the comet’s topmost layer: it is primarily coated in a dark, dry and organic-rich material but with a small amount of water ice mixed in.

In the latest study, which focuses on scans between September and November 2014, the team confirms that two areas several tens of metres across in the Imhotep region that appear as bright patches in visible light, do indeed include a significant amount of water ice. The ice is associated with cliff walls and debris falls, and was at an average temperature of about –120ºC at the time.

Note that many media sources today are falsely reporting the “discovery” of water by Rosetta on the comet. This is ridiculous, as water has been detected there for years. To suggest that “discovery” indicates a remarkable level of stupidity and ignorance by these news organizations about science. Either they think their readers are dumb, or they themselves don’t know anything.

Unfortunately, I worry that the answer is both.

The Rosetta science team has released a new more complete 3D shape model of Comet 67P/C-G.

They have also released data that will allow someone to print a 3D model of the comet.

The uncertainty of science: Unexpectedly scientists using Rosetta data have discovered oxygen in the coma of Comet 67P/C-G.

It was not immediately clear where the oxygen came from. The team discovered that water and oxygen were often found together — an indication that similar processes released both molecules. But Bieler and his colleagues ruled out many scenarios in which oxygen arises as a by-product when energetic particles such as photons and electrons split apart water. Instead, the researchers argue that the oxygen is a remnant from when 67P formed billions of years ago, a process that may have trapped the gas in small grains of ice and rock that coalesced to create the comet’s solid core.

But many models of the early Solar System rule this out because most oxygen tends to pair off with hydrogen. Given this affinity, it is tricky to adjust models of the early Solar System to allow for the survival of gaseous O2, says Mike A’Hearn, an astronomer at the University of Maryland in College Park and a co-investigator on Alice. But he adds that it may be possible with the right chemical abundances and temperature conditions.

Newly released Rosetta data has shown, for at least one area on the surface of Comet 67P/C-G, the process by which the surface ice is replaced by water ice from below as the comet rotates and sunlight causes the surface ice to evaporate away.

The data suggest that water ice on and a few centimetres below the surface ‘sublimates’ when illuminated by sunlight, turning it into gas that then flows away from the comet. Then, as the comet rotates and the same region falls into darkness, the surface rapidly cools again. However, the underlying layers remain warm owing to the sunlight they received in the previous hours, and, as a result, subsurface water ice keeps sublimating and finding its way to the surface through the comet’s porous interior.

But as soon as this ‘underground’ water vapour reaches the cold surface, it freezes again, blanketing that patch of comet surface with a thin layer of fresh ice. Eventually, as the Sun rises again over this part of the surface on the next comet day, the molecules in the newly formed ice layer are the first to sublimate and flow away from the comet, restarting the cycle.

They discovered this process when they noticed surface ice evaporating in this region during the comet’s 6-hour day and then getting resurfaced with ice during the comet’s 6-hour night.

Meanwhile, Rosetta is about to move as much as 1500 kilometers away from the comet for several weeks so that its scientists can study its coma more broadly.

Cool image time! On August 22, just days after its closest approach to the sun, Rosetta caught the outburst, image above, from the larger lobe on Comet 67P/C-G.

The image scale is 28.6 m/pixel and the image measures 29.3 km across. Although the activity is extraordinarily bright even in the original (below), the image above has been lightly enhanced to give a better view of the outline of the nucleus in the lower part of the image, as well as to show the full extent of the activity.

The most interesting images, I think, will actually come later, when the activity dies down and they can bring Rosetta in closer again. We will then be able to compare the nucleus both before and after this outburst, getting a sense of how the comet changes with each close pass to the sun.

Cool image time! As Comet 67P/C-G approaches perihelion, Rosetta is detecting and imaging more and more activity coming from the nucleus, including a power outburst lasting less than a half hour.

In the approach to perihelion over the past few weeks, Rosetta has been witnessing growing activity from Comet 67P/Churyumov–Gerasimenko, with one dramatic outburst event proving so powerful that it even pushed away the incoming solar wind.

The comet reaches perihelion on Thursday, the moment in its 6.5-year orbit when it is closest to the Sun. In recent months, the increasing solar energy has been warming the comet’s frozen ices, turning them to gas, which pours out into space, dragging dust along with it.

The three pictures above were taken 18 minutes apart. The first shows nothing, and in the last the jet has almost completed dissipated. In the middle image, however, the jet is well defined, and data from the spacecraft indicated that it was so strong that it “had pushed away the solar wind magnetic field from around the nucleus.”

Accumulating data from Rosetta is now giving scientists an excellent picture of how this comet interacts with the solar wind as it moves in towards its closest approach to the Sun.

They have seen that the number of water ions – molecules of water that have been stripped of one electron – accelerated away from the comet increased hugely as 67P/C-G moved between 3.6AU (about 538 million km) and 2.0AU (about 300 million km) from the Sun. Although the day-to-day acceleration is highly variable, the average 24-hour rate has increased by a factor of 10,000 during the study, which covered the period August 2014 to March 2015.

The water ions themselves originate in the coma, the atmosphere of the comet. They are placed there originally by heat from the Sun liberating the molecules from the surface ice. Once in gaseous form, the collision of extreme ultraviolet light displaces electrons from the molecules, turning them into ions. Colliding particles from the solar wind can do this as well. Once stripped of some of their electrons, the water ions can then be accelerated by the electrical properties of the solar wind.

Not all of the ions are accelerated outwards, some will happen to strike the comet’s surface. Solar wind particles will also find their way through the coma to hit home. When this happens, they cause a process called sputtering, in which they displace atoms from material on the surface – these are then ‘liberated’ into space.

There’s more at the link, including animations and simulations.