In many ways, stars are the engines of creation. Their energy drives a whole host of processes necessary for life. Scientists thought that stellar radiation is needed to create compounds like the amino acid glycine, one of the building blocks of life.
But a new study has found that glycine detected in comets formed in deep interstellar space when there was no stellar energy.
In August 2014, the ESA’s Rosetta spacecraft arrived at its destination, Comet 67P/Churyumov-Gerasimenko, after a 10 year journey. Rosetta carried a small companion, the Philae Lander. On November 12th, Philae was sent to the surface of Comet 67P. Unfortunately, things didn’t go exactly as planned, and the lander’s mission lasted only 63 hours.
During that time, it gathered what data it could. But mission scientists weren’t certain of its precise location, meaning its data was difficult to interpret accurately. Only when scientists knew precisely where Philae was located on the comet, could they make best use of all of its data.
It seems that comet 67P/Churyumov–Gerasimenko is not the stoic, unchanging Solar System traveller that it might seem to be. Scientists working through the vast warehouse of images from the Rosetta spacecraft have discovered there’s lots going on on 67P. Among the activity are collapsing cliffs and bouncing boulders.
The European Space Agency’s (ESA) Rosetta mission spent two years at the comet 67P/Churyumov-Gerasimenko. At the end of September 2016, its mission was ended when the spacecraft was sent on a collision course into the comet. During its time at comet 67P, it captured a vast amount of images.
The ESA made all those images freely available at their Rosetta website, and now an astro-photographer working with those images has found something interesting: a chunk of ice travelling through space with 67P.
Ever since we’ve been able to get closer looks at comets in our Solar System, we’ve noticed something a little puzzling. Rather than being round, they’re mostly elongated or multi-lobed. This is certainly true of Comet 67P/Churyumov-Gerasimenko (67P or Chury for short.) A new paper from an international team coordinated by Patrick Michel at France’s CNRS explains how they form this way.
The European Space Agency (ESA) spacecraft Rosetta visited 67P in 2014, end even placed its lander Philae on the surface. Rosetta spent 17 months orbiting 67P, and at its closest approach, Rosetta was only 10 km (6 mi) from 67P’s surface. Rosetta’s mission ended with its guided impact into 67P’s surface in September, 2016, but the attempt to understand the comet and its brethren didn’t end then.
Though Rosetta’s pictures of 67P are the most detailed comet pictures we have, other spacecraft have visited other comets. And most of those other comets appear elongated or multi-lobed, too. Scientists explain these shapes with a “comet merger theory.” Two comets collide, creating the multi-lobed appearance of comets like 67P. But there’s been a problem with that theory.
In order for comets to merge and come out looking the way they do, they would have to merge very slowly, or else they would explode. They would also have to be very low-density, and be very rich in volatile elements. The “comet merger theory” also says that these types of gentle mergers between comets would have to have happened billions of years ago, in the early days of the Solar System.
The problem with this theory is, how could bodies like 67P have survived for so long? 67P is fragile, and subjected to repeated collisions in its part of the Solar System. How could it have retained its volatiles?
In the new paper, the research team ran a simulation that answers these questions.
The simulation showed that when two comets meet in a destructive collision, only a small portion of their material is pulverized and reduced to dust. On the sides of the comets opposite from the impact point, materials rich in volatiles withstand the collision. They’re still ejected into space, but their relative speed is low enough for them to join together in accretion. This process forms many smaller bodies, which keep clumping up until they form just one, larger body.
The most surprising part of this simulation is that this entire process may only take a few days, or even a few hours. The whole process explains how comets like 67P can keep their low density, and their abundant volatiles. And why they appear multi-lobed.
The simulation also answered another question: how can comets like 67P survive for so long?
The team behind the simulation thinks that the process can take place at speeds of 1 km/second. These speeds are typical in the Kuiper Belt, which is the disc of comets where 67P has its origins. In this belt, collisions between comets are a regular occurrence, which means that 67P didn’t have to form in the early days of the Solar System as previously thought. It could have formed at any time.
The team’s work also explains the surface appearance of 67P and other comets. They often have holes and stratified layers, and these features could have formed during re-accretion, or sometime after its formation.
One final point from the study concerns the composition of comets. One reason they’re a focus of such intense interest is their age. Scientists have always thought of them as ancient objects, and that studying them would allow us to look back into the primordial Solar System.
Though 67P—and other comets—may have formed much more recently than we used to believe, this process shows that there is no significant amount of heating or compaction during the collision. As a result, their original composition from the the early days of the Solar System is retained intact. No matter when 67P formed, it’s still a messenger from the formative days.
ESA scientists have found one additional image from the Rosetta spacecraft hiding in the telemetry. This new image was found in the last bits of data sent by Rosetta immediately before it shut down on the surface of Comet 67P/Churyumov–Gerasimenko last year.
Planetary astronomer Andy Rivkin noted on Twitter that for size context, he estimates the block just right of center looks to be about the size of a hat. That’s a fun comparison to have (not to mention thinking about hats on Comet 67P!)
The picture has a scale of 2 mm/pixel and measures about 1 m across. It’s a really ‘close’ close-up of Comet 67P.
“The last complete image transmitted from Rosetta was the final one that we saw arriving back on Earth in one piece moments before the touchdown at Sais,” said Holger Sierks, principal investigator for the OSIRIS camera at the Max Planck Institute for Solar System Research in Göttingen, Germany. “Later, we found a few telemetry packets on our server and thought, wow, that could be another image.”
The team explains that the image data were put into telemetry ‘packets’ aboard Rosetta before they were transmitted to Earth, and the final images were split into six packets. However, for the very last image, the transmission was interrupted after only three full packets. The incomplete data was not recognized as an image by the automatic processing software, but later, the engineers in Göttingen could make sense of these data fragments to reconstruct the image.
You’ll notice it is rather blurry. The OSIRIS camera team says this image only has about 53% of the full data and “therefore represents an image with an effective compression ratio of 1:38 compared to the anticipated compression ratio of 1:20, meaning some of the finer detail was lost.”
That is, it gets a lot blurrier as you zoom in compared with a full-quality image. They compared it to compressing an image to send via email, versus an uncompressed version that you would print out and hang on your wall.
Rosetta’s final resting spot is in a region of active pits in the Ma’at region on the two-lobed, duck-shaped comet.
Launched in 2004, Rosetta traveled nearly 8 billion kilometers and its journey included three Earth flybys and one at Mars, and two asteroid encounters. It arrived at the comet in August 2014 after being in hibernation for 31 months.
After becoming the first spacecraft to orbit a comet, it deployed the Philae lander in November 2014. Philae sent back data for a few days before succumbing to a power loss after it unfortunately landed in a crevice and its solar panels couldn’t receive sunlight.
But Rosetta showed us unprecedented views of Comet 67P and monitored the comet’s evolution as it made its closest approach and then moved away from the Sun. However, Rosetta and the comet moved too far away from the Sun for the spacecraft to receive enough power to continue operations, so the mission plan was to set the spacecraft down on the comet’s surface.
And scientists have continued to sift through the data, and this new image was found. Who knows what else they’ll find, hiding the data?
The Rosetta spacecraft learned a great deal during the two years that it spent monitoring Comet 67P/Churyumov-Gerasimenko – from August 6th, 2014 to September 30th, 2016. As the first spacecraft to orbit the nucleus of a comet, Rosetta was the first space probe to directly image the surface of a comet, and observed some fascinating things in the process.
For instance, the probe was able to document some remarkable changes that took place during the mission with its OSIRIS camera. According to a study published today (March. 21st) in Science, these included growing fractures, collapsing cliffs, rolling boulders and moving material on the comet’s surface that buried some features and exhumed others.
These changes were noticed by comparing images from before and after the comet reached perihelion on August 13th, 2015 – the closets point in its orbit around the Sun. Like all comets, it is during this point in 67P/Churyumov-Gerasimenko’s orbit that the surface experiences its highest levels of activity, since perihelion results in greater levels of surface heating, as well as increased tidal stresses.
Basically, as comets gets closer to the Sun, they experience a combination of in-situ weathering and erosion, sublimation of water-ice, and mechanical stresses arising from an increased spin rate. These processes can be either unique and transient, or they can place over longer periods of time.
“Monitoring the comet continuously as it traversed the inner Solar System gave us an unprecedented insight not only into how comets change when they travel close to the Sun, but also how fast these changes take place.”
For instance, in-situ weathering occurs all over the comet and is the result of heating and cooling cycles that happen on both a daily and a seasonal basis. In the case of 67P/Churyumov-Gerasimenko’s (6.44 Earth years), temperatures range from 180 K (-93 °C; -135 °F) to 230 K (-43 °C; -45 °F) during the course of its orbit. When the comet’s volatile ices warm, they cause consolidated material to weaken, which can cause fragmentation.
Combined with the heating of subsurface ices – which leads to outgassing – this process can result in the sudden collapse of cliff walls. As other photographic evidence that was recently released by the Rosetta science team can attest, this sort of process appears to have taken place in several locations across the comet’s surface.
Similarly, comets experience increased stress because their spin rates speed up as they gets closer to the Sun. This is believed to be what caused the 500 meter-long (1640 ft) fracture that has been observed in the Anuket region. Originally discovered in August of 2014, this fracture appeared to have grown by 30 meters (~100 ft) when it was observed again in December of 2014.
This same process is believed to be responsible for a new fracture that was identified from OSIRIS images taken in June 2016. This 150-300 meter-long (492 – 984 ft) fracture appears to have formed parallel to the original. In addition, photographs taken in February of 2015 and June of 2016 (shown above) revealed how a 4 meter-wide (13 ft) boulder that was sitting close to the fractures appeared to have moved by about 15 meters (49 ft).
Whether or not the two phenomena are related is unclear. But it is clear that something very similar appears to have taken place in the Khonsu region. In this section of the comet (which corresponds to one of its larger lobes), images taken between May of 2015 and June 2016 (shown below) revealed how a much larger boulder appeared to have moved even farther between the two time periods.
This boulder – which measures some 30 meters (98 ft) across and weighs an estimated 12,800 metric tonnes (~14,100 US tons) – moved a distance of about 140 meters (~460 ft). In this case, outgassing during perihelion is believed to be the culprit. On the one hand, it could have caused the surface material to erode beneath it (thus causing it to roll downslope) or by forcibly pushing it.
For some time, it has been known that comets undergo changes during the course of their orbits. Thanks to the Rosetta mission, scientists have been able to see these processes in action for the first time. Much like all space probes, vital information continues to be discovered long after the Rosetta mission officially came to an end. Who knows what else the probe managed to witness during its historic mission, and which we will be privy to?
Rosetta’s Comet hails from a cold, dark place. Using statistical analysis and scientific computing, astronomers at Western University in Canada have charted a path that most likely pinpoints comet 67P/Churyumov-Gerasimenko’s long-ago home in the far reaches of the Kuiper Belt, a vast region beyond Neptune home to icy asteroids and comets.
According to the new research, Rosetta’s Comet is relative newcomer to the inner parts of our Solar System, having only arrived about 10,000 years ago. Prior to that, it spent the last 4.5 billion years in cold storage in a rough-and-tumble region of the Kuiper Belt called the scattered disk.
In the Solar System’s youth, asteroids that strayed too close to Neptune were scattered by the encounter into the wild blue yonder of the disk. Their orbits still bear the scars of those long-ago encounters: they’re often highly-elongated (shaped like a cigar) and tilted willy-nilly from the ecliptic plane up to 40°. Because their orbits can take them hundreds of Earth-Sun distances into the deeps of space, scattered disk objects are among the coldest places in the Solar System with surface temperatures around 50° above absolute zero. Ices that glommed together to form 67P at its birth are little changed today. Primordial stuff.
Watch how Rosetta’s Comet’s orbit has evolved since the comet’s formation
There are two basic comet groups. Most comets reside in the cavernous Oort Cloud, a roughly spherical-shaped region of space between 10,000 and 100,000 AU (astronomical unit = one Earth-Sun distance) from the Sun. The other major group, the Jupiter-family comets, owes its allegiance to the powerful gravity of the giant planet Jupiter. These comets race around the Sun with periods of less than 20 years. It’s thought they originate from collisions betwixt rocky-icy asteroids in the Kuiper Belt.
Fragments flung from the collisions are perturbed by Neptune into long, cigar-shaped orbits that bring them near Jupiter, which ropes them like calves with its insatiable gravity and re-settles them into short-period orbits.
Mattia Galiazzo and solar system expert Paul Wiegert, both at Western University, showed that in transit, Rosetta’s Comet likely spent millions of years in the scattered disk at about twice the distance of Neptune. The fact that it’s now a Jupiter family comet hints of a possible long-ago collision followed by gravitational interactions with Neptune and Jupiter before finally becoming an inner Solar System homebody going around the Sun every 6.45 years.
By such long paths do we arrive at our present circumstances.
This week’s special guest is Dr. Voula Saridakis, a professor at Lake Forest College in Illinois specializing in the history of science and astronomy, who runsthe History of Astronomy on Twitter at @histastro
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The Rosetta team has released the final batch of images taken by the NAVCAM during the last month of its two years of investigations at Comet 67P/Churyumov-Gerasimenko. It’s a big batch and they are absolutely stunning, but its sad to know they are the last NAVCAM images. The image set covers the period from September 2-30, 2016 when the spacecraft was on elliptical orbits that sometimes brought it to within 2 km of the comet’s surface, so you’ll see a wide variety of imagery with a variety of geology and lighting conditions.
Take a look below:
While these are the final NAVCAM images, there may be more images coming from the OSIRIS camera. Also, many other instruments will be releasing data, as they were active as long as possible before impact. Many of the science instruments were expected to return their last data from between 20 meters to 5 meters above the surface.
ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) collected data on the density of gas around the comet and its composition while GIADA (Grain Impact Analyser and Dust Accumulator) measured the dust density.
RPC’s (Rosetta Plasma Consortium) instrument suite provided a look at interaction between the solar wind and the surface of the comet. Alice, an Ultraviolet Imaging Spectrometer similar to the one on New Horizons, took high resolution ultraviolet spectra of the surface. RSI (Radio Science Investigation) got the most accurate measurements of the gravity field during descent.
And here’s one of the last five images from Rosetta’s NAVCAM as it descended to its controlled impact on September 30 onto Comet 67P, taking incredible, close-up images during descent, this one just 18.1 km up. It shows the “drippy icing” landscape on this portion of the comet: