26 January 2015
ESA’s Rosetta mission is providing unique insight into the life cycle of a comet’s dusty surface, watching 67P/Churyumov–Gerasimenko as it sheds the dusty coat it has accumulated over the past four years.
The COmetary Secondary Ion Mass Analyser, or COSIMA, is one of Rosetta’s three dust analysis experiments. It started collecting, imaging and measuring the composition of dust particles shortly after the spacecraft arrived at the comet in August 2014.
Results from the first analysis of its data are reported today in the journal Nature. The study covers August to October, when the comet moved along its orbit between about 535 million kilometres to 450 million kilometres from the Sun. Rosetta spent the most of this time orbiting the comet at distances of 30 km or less.
The scientists looked at the way that many large dust grains broke apart when they were collected on the instrument’s target plate, typically at low speeds of 1–10 m/s. The grains, which were originally at least 0.05 mm across, fragmented or shattered upon collection.
The fact that they broke apart so easily means that the individual parts were not well bound together. Moreover, if they had contained ice, they would not have shattered. Instead, the icy component would have evaporated off the grain shortly after touching the collecting plate, leaving voids in what remained.
By comparison, if a pure water-ice grain had struck the detector, then only a dark patch would have been seen. ...
Last week a suite of eight peer-reviewed papers were published in Science magazine, detailing the first results of the Rosetta mission. The papers are open-access, so you can go read them all if you want. And don't miss the supplemental materials! The publication of these papers means that the OSIRIS camera team has finally released a large quantity of closeup images of comet Churyumov-Gerasimenko, taken in August and September of last year. Since OSIRIS images and derived data products are such a rare treat I figured I'd post them all here and discuss them based on the two papers and also my notes from the Rosetta presentations at the American Geophysical Union (AGU) meeting, which I never wrote up because I lacked the images to illustrate them! ...
Read more and see the images at the blog - I'll just link to this wonderful image. MMc
ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA Comet landscape from 8 kilometers (OSIRIS)
In October 2014, Rosetta ventured to within 8 kilometers of the surface of comet Churyumov-Gerasimenko -- lower than the cruising altitude of a passenger plane -- to capture this photo of smooth plains and steep cliffs with its OSIRIS science camera.
These are the two most popular questions currently being asked of the mission – especially on our social media channels – and ones that we will try to answer in this post, including inputs from the OSIRIS team, and from the Lander Control Centre at the German Aerospace Center (DLR). ...
ESA’s Rosetta probe is preparing to make a close encounter with its comet on 14 February, passing just 6 km from the surface.
Click to play embedded YouTube video.
Yesterday was Rosetta’s last day at 26 km from Comet 67P/Churyumov–Gerasimenko, marking the end of the current orbiting period and the start of a new phase for the rest of this year.
Today, Rosetta is moving into a new path ahead of a very close encounter next week. First, it will move out to a distance of roughly 140 km from the comet by 7 February, before swooping in for the close encounter at 12:41 GMT (13:41 CET) on 14 February. The closest pass occurs over the comet’s larger lobe, above the Imhotep region.
“The upcoming close flyby will allow unique scientific observations, providing us with high-resolution measurements of the surface over a range of wavelengths and giving us the opportunity to sample – taste or sniff – the very innermost parts of the comet’s atmosphere,” says Matt Taylor, ESA’s Rosetta project scientist.
... It was always planned to change from ‘bound orbits’ to flyby trajectories at this point in the mission, based on predictions of increasing cometary activity. The range of flyby distances also balances the various needs of Rosetta’s 11 instruments in order to optimise the mission’s scientific return.
During some of the close flybys, Rosetta will encounter the comet almost in step with the rotation, allowing the instruments to monitor a single point on the surface as it passes by.
Meanwhile, the more distant flybys will provide the broader context of a wide-angle view of the nucleus and its growing coma.
... In the month before perihelion [ on 13th August 2015], as activity is reaching a peak, the team are planning to study one of the comet’s jets in greater detail than ever.
“We hope to target one of these regions for a fly-through, to really get a taste of the outflow of the comet,” adds Matt.
After perihelion and once the comet’s activity begins to subside, the mission team will determine if and when to return to a bound orbit around the comet, and how long Rosetta might be able to operate beyond the end of 2015.
M
Little White Blobs
Posted: Fri Feb 06, 2015 5:50 pm
by MargaritaMc
There was a discussion on the Apod thread on Feb 3rd relating to the size of "grains" emitted by and/or orbiting comet 67P/C-G. The image then was from OSIRIS, taken on 22 November 2014
This NAVCAM image, from today's Rosetta blog post, was taken on 31 January 2015 and is quite similar. There are more little white-ish dots surrounding the comet and it's possible to see that some of them are in between the comet and Rosetta, and so are not stars.
At that time Rosetta was about 28km from the comet. This is what the blog says:
This four-image mosaic comprises images taken from a distance of 28.0 km from the centre of Comet 67P/Churyumov-Gerasimenko on 31 January. The image resolution is 2.4 m/pixel and the individual 1024 x 1024 frames measure 2.4 km across. The mosaic has been slightly cropped, and it measures 4.6 x 4.3 km.
Four image mosaic comprising images taken on 31 January 2015 by Rosetta's Navigation Camera (NAVCAM). Rotation and translation of the comet during the imaging sequence make it difficult to create an accurate mosaic, and there may be some spurious spatial and intensity features as a result of the mosaic-making process, so always refer to the individual frames before performing any detailed comparison or drawing conclusions about any strange structures or low intensity extended emission. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
...The large number of small white blobs and streaks in the image are likely specks of dust or other small objects in the vicinity of the comet.
The four-image montage and the individual 1024 x 1024 frames are provided below:
I wonder how big those blobs are? Really near Rosetta, thus tiny? Or nearer to the comet and thus not so tiny? Given that the image resolution is given as 2.4 metres per pixel. Any thoughts???
I'll link this post to the Apod discussion.
M
PS. For those who haven't read it, this ESA Rosetta blog post of 22 January mentions "grains" up to two metres in size in the bound orbit cloud (which begins about 150 kilometres from the comet, if my memory serves me correctly) and says
When the comet starts to eject bigger clumps (metre-sized clumps at next perihelion, as already observed in the bound cloud as being remnants from the last perihelion)...
67P/C-G is still six months from perihelion, but if has upturned expectations of its behaviour before.
Re: Little White Blobs
Posted: Fri Feb 06, 2015 6:49 pm
by Chris Peterson
MargaritaMc wrote:I wonder how big those blobs are? Really near Rosetta, thus tiny? Or nearer to the comet and thus not so tiny? Given that the image resolution is given as 2.4 metres per pixel. Any thoughts???
I'm skeptical that we're seeing much more here than stars. However, assuming that there are also particles, we can't infer anything about their size from the pixel scale. Everything will be much smaller than a pixel. So size estimates need to be made based primarily on intensity, which is still problematic because in most cases the distance wouldn't be known.
The population of bound particles were determined from images that showed their motion, allowing their orbital parameters to be calculated.
Re: Little White Blobs
Posted: Sat Feb 07, 2015 4:49 am
by alter-ego
Chris Peterson wrote:
MargaritaMc wrote:I wonder how big those blobs are? Really near Rosetta, thus tiny? Or nearer to the comet and thus not so tiny? Given that the image resolution is given as 2.4 metres per pixel. Any thoughts???
I'm skeptical that we're seeing much more here than stars. However, assuming that there are also particles, we can't infer anything about their size from the pixel scale. Everything will be much smaller than a pixel. So size estimates need to be made based primarily on intensity, which is still problematic because in most cases the distance wouldn't be known.
The population of bound particles were determined from images that showed their motion, allowing their orbital parameters to be calculated.
As I posted in the APOD, there are stars. The numbers and positions of the ones I identified are too many and too exact to be random particles or artifacts. However, it seemed that there were way too many other specs to be ccd artifacts, either intrinsic or environment induced. It took a lot of time to extract the stars from all the specs. I can easily believe there are particles big/bright enough to be captured in the image. I didn't think about that earlier. The fact that comet bits are a plausible contributor makes sense. I'm just spec-ulating of course.
Re: ESA: Rosetta: 100 days to wake-up
Posted: Sat Feb 07, 2015 5:28 am
by geckzilla
The only reason I am not more skeptical about seeing the presence of larger particles floating around 67P is that a while back I was looking through archival images of 103P and was surprised to see many dozens of particles which were more than the fine mist or dust I expected. Here's a video that shows it pretty well:
Click to play embedded YouTube video.
I'd be surprised if Rosetta wasn't recording similar, even better sequences of images as it orbits.
Re: ESA: Rosetta: 100 days to wake-up
Posted: Sat Feb 07, 2015 1:18 pm
by MargaritaMc
Many thanks. There's discussion about this here at unmannedspaceflight.com
Later edit: this 2010 press release from NASA has info about the Hartley 2 ejecta that geck mentioned
I've found this public lecture, delivered in November 2013, to be very useful in giving me background information about comets and the missions that have explored them. It has some very interesting images, and the lecturer, the Gresham Professor in Astronomy, Dr Carolin Crawford, is an excellent speaker.
I'm posting the link here in case it is of interest to other readers of this thread. Rosetta is mentioned at the very end as the next step in the exploration.
One of the images that fascinated me was of Comet 73P/Schwassmann–Wachmann, which is in the process of disintegrating. Here is one of the images that is on Wikipedia.
The southern side of Rosetta’s comet will change dramatically in the next months. Under the influence of the Sun it may lose a surface layer of several meters.
February 09, 2015
The northern and southern hemisphere of Rosetta’s comet 67P/Churyumov-Gerasimenko experience sun-driven erosion to a very different extent. This is the result of a recent analysis performed by Rosetta’s OSIRIS team. Based on data acquired by the scientific imaging system OSIRIS, the scientists used a thermal model to estimate how much material both hemispheres lose during one orbit as ice sublimates from the comet’s surface carrying grains of dust with it. While the southern hemisphere may be widely reshaped shedding a layer of several meters, the northern hemisphere will be much less affected. Since Rosetta’s arrival, the comet’s southern hemisphere has been facing away from the Sun. Starting in May it will be illuminated again. The scientists expect to see dramatic changes then.
...With an angle of 52 degrees, this inclination is much larger in the case of comet 67P/Churyumov-Gerasimenko than in the case of Earth. Together with its complex shape and strongly elliptical orbit this leads to a very uneven distribution of summer and winter months among both hemispheres. While the comet’s northern hemisphere experiences a long summer lasting for 5.6 years when the comet is far away from the Sun, the southern hemisphere has a short, but intense hot season of about ten months.
...“Assuming that four times more dust is emitted than ice, our model leads to very different scenarios for the northern and southern hemisphere”, says OSIRIS scientist Stefano Mottola from the Institute for Planetary Research of the German Aerospace Center (DLR). “While during its short but intense summer the southern hemisphere may lose a surface layer measuring up to 20 meters in thickness, this value should be much smaller for the northern hemisphere. According to our estimations, only very few prominent peaks and cliffs may erode by more than ten meters over the course of one orbit.”
The scientists therefore expect the southern side to change dramatically as 67P approaches perihelion in August. “Quite possibly, after perihelion 67P will no longer be the comet we have grown used to in the past months”, says Sierks. “Witnessing these changes from up close will be an unsurpassed adventure”, he adds.
The neck area between the comet’s two lobes is particularly weakly insolated. At the same time, it has displayed the strongest and earliest dust activity in the past months. The scientists therefore believe that possibly this region has a different composition than the rest of the comet. ...
OSIRIS images taken during Rosetta’s close flyby on 14 February show 67P’s surface in remarkable detail – and the shadow of the spacecraft encircled in a wreath of light.
Several days after Rosetta’s close flyby of comet 67P/Churyumov-Gerasimenko on 14 February 2015, images taken on this day by OSIRIS, the scientific imaging system on board, have now been downlinked to Earth. With a resolution of 11 centimeters per pixel, these data from OSIRIS’ Narrow Angle Camera reveal highly detailed structures on the comet’s surface. Since at closest approach Sun, spacecraft and comet were almost perfectly aligned, few shadows are visible in the images. With one exception: as a side-effect of this exceptional observational geometry Rosetta’s shadow on the surface can be seen surrounded by a bright halo-like region.
The image released today shows an area near the edge of the comet’s belly at the boundary of the Imhotep region covering 228 meters x 228 meters on the comet’s surface. A mesh of steep slopes separates smooth looking terrain from a more craggy area. The image was taken from a distance of six kilometers from the comet’s surface thus making structures with a pixel scale of only 11 centimeters visible. In resolution it is only surpassed by images taken by Philae’s camera ROLIS on 12 November, 2015 during descent. ...
Re: MPS: OSIRIS catches glimpse of Rosetta’s shadow
The Hapi region on the neck of Rosetta’s comet 67P/Churyumov-Gerasimenko reflects red light less effectively than most other regions on the comet. It thus appears slightly blueish. The Hapi region is located between the comet’s two lobes and has in the past months proven to be particularly active and the source of spectacular jets of dust and gas. Scientists from the OSIRIS team are using images obtained with the color filters of Rosetta’s scientific imaging system to study the reflectivity properties of 67P’s surface. Their analyses confirm that the Hapi region is unique. Its bluish coloring might point to the presence of frozen water mixed intimately with the dust at the surface.
When seen with the human eye, comet 67P/Churyumov-Gerasimenko is grey – all over. With its color filters Rosetta’s scientific imaging system OSIRIS, however, can discern tiny differences in reflectivity. To this effect, scientists from the OSIRIS team image the same region on the comet’s surface using different color filters. If, for example, the region appears especially bright in one of these images, it reflects light of this wavelength especially well.
“Even though the color variations on 67P’s surface are minute, they can give us important clues”, says OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany. In a recent analysis performed by the OSIRIS team, the Hapi region clearly stands out from the rest of the comet: while most parts of 67P display a slightly reddish reflectivity spectrum as is common for cometary nuclei and other primitive bodies, the reflection of red light is somewhat lower in this region.
“We know that the reflectivity properties are closely correlated to the surface morphology”, says OSIRIS scientist Sonia Fornasier from the Paris Observatory. Where the smooth surface of the Hapi region gives way to the more rugged terrain of the surrounding areas, the reflectivity, too, changes. The scientists believe that Hapi’s special reflectivity properties hint at a higher abundance of frozen water at or near the surface. Earlier comet missions had observed similar behavior on comets 103P/Hartley 2 and 9P/Temple 1 and associated the bluish spectrum to the presence of frozen water. While OSIRIS can only study a limited number of spectral bands, Rosetta is equipped with other instruments such as VIRTIS than can unambiguously identify the spectral signature of water molecules in infrared reflection. “We are excited to see, whether our suspicion will be confirmed”, says Sierks. ...
Measurements made by Rosetta and Philae during the probe's multiple landings on Comet 67P/Churyumov-Gerasimenko show that the comet's nucleus is not magnetised.
Studying the properties of a comet can provide clues to the role that magnetic fields played in the formation of Solar System bodies almost 4.6 billion years ago. The infant Solar System was once nothing more than a swirling disc of gas and dust but, within a few million years, the Sun burst into life in the centre of this turbulent disc, with the leftover material going into forming the asteroids, comets, moons and planets.
The dust contained an appreciable fraction of iron, some of it in the form of magnetite. Indeed, millimetre-sized grains of magnetic materials have been found in meteorites, indicating their presence in the early Solar System.
This leads scientists to believe that magnetic fields threading through the proto-planetary disc could have played an important role in moving material around as it started to clump together to form larger bodies.
But it remains unclear as to how crucial magnetic fields were later on in this accretion process, as the building blocks grew to centimetres, metres and then tens of metres across, before gravity started to dominate when they grew to hundreds of metres and kilometres in scale. ...
The nonmagnetic nucleus of comet 67P/Churyumov-Gerasimenko - Hans-Ulrich Auster et al
[img3="Spacecraft: ESA/ATG medialab; comet, left: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; comet, top right: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0; data: Feldman et al"]http://www.esa.int/var/esa/storage/imag ... mage_2.jpg[/img3][hr][/hr]
Rosetta’s continued close study of Comet 67P/Churyumov–Gerasimenko has revealed an unexpected process at work, causing the rapid breakup of water and carbon dioxide molecules spewing from the comet’s surface. ...
One instrument, the Alice spectrograph provided by NASA, has been examining the chemical composition of the comet’s atmosphere, or coma, at far-ultraviolet wavelengths.
At these wavelengths, Alice allows scientists to detect some of the most abundant elements in the Universe such as hydrogen, oxygen, carbon and nitrogen. The spectrograph splits the comet’s light into its various colours – its spectrum – from which scientists can identify the chemical composition of the coma gases. ...
For this study, the team focused on the nature of ‘plumes’ of water and carbon dioxide gas erupting from the comet’s surface, triggered by the warmth of the Sun. To do so, they looked at the emission from hydrogen and oxygen atoms resulting from broken water molecules, and similarly carbon atoms from carbon dioxide molecules, close to the comet nucleus.
They discovered that the molecules seem to be broken up in a two-step process.
First, an ultraviolet photon from the Sun hits a water molecule in the comet’s coma and ionises it, knocking out an energetic electron. This electron then hits another water molecule in the coma, breaking it apart into two hydrogen atoms and one oxygen, and energising them in the process. These atoms then emit ultraviolet light that is detected at characteristic wavelengths by Alice.
Similarly, it is the impact of an electron with a carbon dioxide molecule that results in its break-up into atoms and the observed carbon emissions. ...
Even after nightfall Rosetta’s comet 67P/Churyumov-Gerasimenko remains active and emits dust jets into space.
[img3="This image of Rosetta’s comet taken on 25 April, 2015 from a distance of approximately 93 kilometers shows clearly distinguishable jets of dust after nightfall. Credit: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA"]http://www.mps.mpg.de/3998373/zoom.jpg[/img3][hr][/hr]
When night falls on Rosetta’s comet 67P/Churyumov-Gerasimenko, the bizarrely shaped body remains active. This can be seen in new images of the Ma’at region located on the comet’s “head” captured by OSIRIS, the scientific imaging system on board the Rosetta spacecraft. They were taken approximately half an hour after the Sun had set over the region and show clearly distinguishable jets of dust escaping into space. Researchers from the OSIRIS team believe that the increasing heating-up of the comet is responsible for the newly observed phenomenon.
"Only recently have we begun to observe dust jets persisting even after sunset”, says OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany. In the past months, the comet’s activity originated from illuminated areas on the day side. As soon as the Sun set, these jets subsided and did not re-awake until after the next sunrise. An exception poses an image from 12 March, 2015 showing the onset of a dust jet on the brink of dawn.
According to OSIRIS scientists, the jets now occurring even after sunset are another sign of the comet’s increasing activity. “Currently, 67P is rapidly approaching perihelion in mid-August”, says Sierks. At the time the image was taken, comet and Sun were only 270 million kilometers apart. “The solar irradiation is getting more and more intense, the illuminated surface warmer and warmer”, Sierks adds. ...
Cavities measuring up to a few hundred meters in diameter can be found under the surface of Rosetta’s comet 67P/Churyumov-Gerasimenko. They can be instable and collapse in a kind of sinkhole process. This is the result of a new study led by researchers from the Max Planck Institute for Solar System Research (MPS) in Germany, which analyses images of the comet’s surface. The images show peculiar, pit-like recesses that are unlike ordinary craters and that emit dust and gas into space. In their study the researchers argue that these pits arise when cavities beneath the surface cave in. The results will be published this Thursday in the journal Nature.
Researchers under the lead of Jean-Baptiste Vincent from the MPS have studied 18 peculiar pit-like depressions all occuring in the northern hemisphere of Rosetta’s comet 67P/Churyumov-Gerasimenko. The scientists analysed images of the comet obtained by OSIRIS, the scientific imaging system on board ESA’s Rosetta spacecraft, in the period from July to December 2014. The pits' diameters vary between ten and a few hundreds of meters. They exhibit nearly vertical sidewalls and are exceptionally deep with the largest ones extending up to two hundred meters into the comet’s interior. The walls of these depressions are characterized by layers and terraces, their bottoms are mostly flat. ...
[c] Outburst in Action - Credit: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA[/c][hr][/hr]
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 period around perihelion is scientifically very important, as the intensity of the sunlight increases and parts of the comet previously cast in years of darkness are flooded with sunlight.
Although the comet’s general activity is expected to peak in the weeks following perihelion, much as the hottest days of summer usually come after the longest days, sudden and unpredictable outbursts can occur at any time – as already seen earlier in the mission.
On 29 July, Rosetta observed the most dramatic outburst yet, registered by several of its instruments from their vantage point 186 km from the comet. They imaged the outburst erupting from the nucleus, witnessed a change in the structure and composition of the gaseous coma environment surrounding Rosetta, and detected increased levels of dust impacts.
Perhaps most surprisingly, Rosetta found that the outburst had pushed away the solar wind magnetic field from around the nucleus. ...
Re: ESA: Rosetta: Comet 67P
Posted: Tue Aug 11, 2015 7:43 pm
by BMAONE23
The central and right image have almost enough difference to create a Cross Eyed 3D image
ESA's Rosetta spacecraft has provided evidence for a daily water-ice cycle on and near the surface of comets.
[img3="The water-ice cycle of Rosetta's comet. Credit: Data: ESA/Rosetta/VIRTIS/INAF-IAPS/OBS DE PARIS-LESIA/DLR; M.C. De Sanctis et al. (2015); Comet: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0"]http://sci.esa.int/science-e-media/img/ ... _1280w.jpg[/img3][hr][/hr]
Comets are celestial bodies comprising a mixture of dust and ices, which they periodically shed as they swing towards their closest point to the Sun along their highly eccentric orbits.
As sunlight heats the frozen nucleus of a comet, the ice in it – mainly water but also other 'volatiles' such as carbon monoxide and carbon dioxide – turns directly into a gas.
This gas flows away from the comet, carrying dust particles along. Together, gas and dust build up the bright halo and tails that are characteristic of comets.
Rosetta arrived at Comet 67P/Churyumov–Gerasimenko in August 2014 and has been studying it up close for over a year. On 13 August 2015, the comet reached the closest point to the Sun along its 6.5-year orbit, and is now moving back towards the outer Solar System.
A key feature that Rosetta's scientists are investigating is the way in which activity on the comet and the associated outgassing are driven, by monitoring the increasing activity on and around the comet since Rosetta's arrival. ...
The diurnal cycle of water ice on comet 67P/Churyumov–Gerasimenko - M. C. De Sanctis et al
The noble gas argon has been detected in the coma of Comet 67P/Churyumov-Gerasimenko for the first time, thanks to the ROSINA mass spectrometer on-board Rosetta. Its detection is helping scientists to understand the processes at work during the comet’s formation, and adds to the debate about the role of comets in delivering various ‘ingredients’ to Earth.
[c]Four image NAVCAM montage of Comet 67P/C-G comprising images taken on
20 Oct 2014, during the timeframe of the ROSINA measurements reported
today. The images were taken about 7.4 km from the comet surface. (Credits: ESA/Rosetta/NAVCAM)[/c][hr][/hr]
The new results are reported in Science Advances today and describe data collected on 19, 20, 22, and 23 October 2014, when the comet was around 465 million km (3.1 AU) from the Sun, and Rosetta was in a 10 km orbit around the comet.
During the time spent close to the comet, the ROSINA instrument was able to take an inventory of the key constituents of the comet’s coma, with many ingredients already reported. Determining the chemical make-up of comets is a necessary step to understanding their role in bringing water and other ingredients to the inner planets during the Solar System’s early history.
The so-called noble gases (helium, neon, argon, krypton, xenon, and radon) rarely react chemically with other elements to form molecules, mostly remaining in a stable atomic state, representative of the environment around a young star in which planets, comets, and asteroids are born.
In addition, their abundance and isotopic compositions can be compared to the values known for Earth and Mars, and for the solar wind and meteorites, for example. The relative abundance of noble gases in the atmospheres of terrestrial planets is largely controlled by the early evolution of the planets, including outgassing via geological processes, atmospheric loss, and/or delivery by asteroid or cometary bombardment. Thus the study of noble gases in comets can also provide information on these processes.
However, noble gases are very easily lost from comets through sublimation, and so this first detection of argon at Comet 67P/C-G is a key discovery. Not only that, but it is also an important step in determining if comets of this type played any significant role in the noble gas inventory of the terrestrial planets. ...
How Rosetta's Comet Got Its Shape ESA | Rosetta | 2015 Sep 28
Two comets collided at low speed in the early Solar System to give rise to the distinctive 'rubber duck' shape of Comet 67P/Churyumov-Gerasimenko, say Rosetta scientists.
The origin of the comet's double-lobed form has been a key question since Rosetta first revealed its surprising shape in July 2014.
Two leading ideas emerged: did two comets merge or did localised erosion of a single object form the 'neck'?
Now, scientists have an unambiguous answer to the conundrum. By using high-resolution images taken between 6 August 2014 and 17 March 2015 to study the layers of material seen all over the nucleus, they have shown that the shape arose from a low-speed collision between two fully fledged, separately formed comets.
"It is clear from the images that both lobes have an outer envelope of material organised in distinct layers, and we think these extend for several hundred metres below the surface," says Matteo Massironi, lead author from the University of Padova, Italy, and an associate scientist of the OSIRIS team.
"You can imagine the layering a bit like an onion, except in this case we are considering two separate onions of differing size that have grown independently before fusing together." ...
The Two Independent and Primitive Envelopes of the Bilobate Nucleus of Comet 67P/C-G - Matteo Massironi et al
Rosetta's First Peek at the Comet's Dark Side NASA | JPL-Caltech | ESA | Rosetta | 2015 Oct 01
[img3="Image of the southern polar regions of Comet 67P/C-G taken with Rosetta's OSIRIS imaging system on 29 Sep 2014, when they were still experiencing southern winter. Credits: ESA/Rosetta/MPS for OSIRIS Team
MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA"]http://www.jpl.nasa.gov/spaceimages/ima ... 969_ip.jpg[/img3][hr][/hr]
Since its arrival at comet 67P/Churyumov-Gerasimenko, the European Space Agency's Rosetta spacecraft has been surveying the surface and the environment of this curiously shaped body. But for a long time, a portion of the nucleus -- the dark, cold regions around the comet's south pole -- remained inaccessible to almost all instruments on the spacecraft.
Due to a combination of its double-lobed shape and the inclination of its rotation axis, Rosetta's comet has a very peculiar seasonal pattern over its 6.5-year-long orbit. Seasons are distributed very unevenly between the two hemispheres. Each hemisphere comprise parts of both comet lobes and the "neck."
For most of the comet's orbit, the northern hemisphere experiences a very long summer, lasting over 5.5 years, while the southern hemisphere undergoes a long, dark and cold winter. However, a few months before the comet reaches perihelion -- the closest point to the sun along its orbit -- the situation changes, and the southern hemisphere transitions to a brief and very hot summer.
When Rosetta arrived at 67P/C-G in August 2014, the comet was still experiencing its long summer in the northern hemisphere, and regions on the southern hemisphere received very little sunlight. Moreover, a large part of this hemisphere, close to the comet's south pole, was in polar night and had been in total darkness for almost five years.
With no direct illumination from the sun, these regions could not be imaged with Rosetta's OSIRIS (the Optical, Spectroscopic, and Infrared Remote Imaging System) science camera, or its Visible, InfraRed and Thermal Imaging Spectrometer (VIRTIS). For the first several months after Rosetta's arrival at the comet, only one instrument on the spacecraft could observe and characterize the cold southern pole of 67P/C-G: the Microwave Instrument for Rosetta Orbiter (MIRO). ...
Dark side of 67P/Churyumov-Gerasimenko in Aug-Oct 2014. MIRO/Rosetta
continuum observations of polar night in the southern regions - M. Choukroun et al
