ESA's space probe Rosetta has reached the destination of its more than ten year long journey through space. At 11:30 AM (Central European Summer Time) ESA's control station in Darmstadt (Germany) picked up the long awaited signal: Rosetta has arrived at the comet 67P/Churyumov-Gerasimenko.
The most recent images taken by the onboard scientific imaging system OSIRIS on August 3rd reveal a world of bizarre beauty – with steep slopes, prominent pits, and wide planes.
“We have been approaching 67P for such a long time, it is almost surreal to now actually be there“, says OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany. “Today, we are opening a new chapter of the Rosetta mission. And already we know that it will revolutionize cometary science“, he adds.
With a spatial resolution of 5.5 meters per pixel, the most recent images that were obtained from a distance of 285 kilometers above 67P's surface surpass all pictures earlier space missions had taken of cometary surfaces. „It's incredible how full of variation this surface is“, Sierks says. „We have never seen anything like this before in such great detail.”
Apart from the strong variations in albedo found on the surface, especially the many sharp-edged structures are tantalizing. The images show steep precipices and eye-catching protruding structures. In addition, the surface exhibits areas that seem almost smooth. „It's too early to tell, whether these planes are really smooth“, says Sierks. „The current images only show that the surface structures there are smaller than our current resolution.”
The scientists assume that the comet's activity formed these bizarre landscapes. When on their orbit through the solar system comets approach the Sun, volatile substances escape from their surface tearing jets of dust with them. If these particles are too heavy or too slow to leave the body's gravitational field, they sink back to the surface where they are locally re-deposited. Comet 67P is believed to have passed through the inner solar system only a few times before. The next months will show, whether the structures now becoming visible were truly formed during these advances.
In the next months, Rosetta will come closer than 10 kilometers to the comet’s surface. One of the main goals will be to search for an appropriate landing site for the Philae lander. Philae is scheduled to touch down on the surface in the autumn of this year. Even afterwards, Rosetta will accompany „her“ comet until the end of 2015. „We will have the unique opportunity to witness, how the comet's activity forms and changes its surface“, says Sierks.
The reflected light from all dust particles seen in this column worked together to create a signal. Now, as the resolution of OSIRIS images increases, a much smaller region − and thus far less dust particles − contributes to one pixel.
Nevertheless, new images dating from July 25th clearly reveal an extended coma shrouding 67P’s nucleus. “Our coma images cover an area of 300 by 300 square kilometers”, says Luisa Lara from the Instituto de Astrofísica de Andalucía. However, most likely these images show only the inner part of the coma, where particle densities are highest. Scientist expect 67P’s full coma to actually reach much farther.
Meanwhile, new images of the comet’s nucleus confirm the collar-like appearance of the neck region which presents itself brighter than most parts of the comet’s body and head. The reason for this feature is still subject to discussion. Possible explanations range from differences in material or grain size to topological effects.
“This is unlike any other comet we have ever seen before”, says OSIRIS project manager Carsten Güttler from the MPS. “The images faintly remind me of a rubber ducky with a body and a head”, he adds with a laugh. How 67P received this duck-like shape is still unclear. In the next months, the scientists hope to determine more of the comet’s physical and mineralogical properties. These could help decide, whether the comet’s body and head were formerly two individual bodies.
In order to get an idea of what seems to be a very unique body, the researcher interpolate the observed image data to create a smoother shape. “There is, of course, still uncertainty in these processed, filtered images and the surface will not be as smooth as it now appears”, Güttler points out.
The images remind the scientists of comet 103P/Hartley. This body was visited in a flyby by NASA’s EPOXI mission in 2010. While Hartley’s ends show a rather rough surface, its middle is much smoother. Scientists believe this waist to be a gravitational low: since it contains the body’s centre of mass, emitted material that cannot leave the comet’s gravitational field is most likely to be re-deposited there.
Whether this also holds true for 67P’s neck region is still unclear. The Osiris team hopes to receive spectra data of the camera system within the next few weeks. With the help of several filters, Osiris can select several wavelength regions from the reflected light allowing for an identification of the characteristic fingerprints of certain materials and compositional features.
At the same time, the team is currently modelling the comet’s three-dimensional shape from the camera data. Such a model can help to get a better impression of the body’s shape.
The two-part nucleus is not the first notable observation of the comet. On June 6, Microwave Instrument for Rosetta Orbiter, MIRO, had identified water steam released into space by the comet. At the time, Churyumov-Gerasimenko was still at a distance of 583 million kilometres from the sun. Since then, water vapour has been found every time MIRO has been pointed towards the comet. "We always knew we would see water vapour outgassing from the comet, but we were surprised at how early we detected it,” says Sam Gulkis, the instrument’s principal investigator at NASA’s Jet Propulsion Laboratory in Pasadena, California, USA, and head of the MIRO Group.
In addition to carbon monoxide, methanol and ammonia, water is one of the most important volatile components of a comet. As the comet moves closer towards the sun, these gases evaporate from its surface and feed the so-called "coma", a shell of gas and entrained dust. MIRO, the microwave instrument of the Rosetta orbiter is able to identify these gases and determine their production rates. The instrument for analysis of the microwave radiation emitted from the gas molecules. Water and other substances imprint.
"The signals left by the water molecules in our data can be particularly well detected," says Paul Hartogh from the Max Planck Institute for Solar System Research, under whose leadership the developed chirp transform spectrometer of MIRO was developed and built. "The instrument is especially sensitive to it." The scientists expect that 67P/Churyumov-Gerasimenko will also outgas other gases which sublime as water at lower temperatures. From the current distance between spacecraft and comet, however, these cannot be detected.
The detection of water vapour from such a great distance is, in Hartogh's words, impressive evidence of the performance of MIRO. The researcher finds a good comparison: It's like discovering, from Earth, the evaporation of a cup of hot tea on the moon.
Dr. Birgit Krummheuer | Max-Planck-Institute
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