Tag: comets

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.