#Bizwhiznetwork.com Innovation ΛＩ |Technology News
A study published this week in the journal Science summarizes the types of surface changes — including the growing fractures, collapsing cliffs, rolling boulders and moving material — observed during Rosetta’s two years at 67P/Churyumov-Gerasimenko.
Showcase of the different types of changes identified in high-resolution images of 67P/Churyumov–Gerasimenko during more than two years of monitoring by ESA’s Rosetta spacecraft. The approximate locations of each feature are marked on the central context images. Dates of when the ‘before’ and ‘after’ images were taken are also indicated. Note that the orientation and resolution between image pairs may vary, therefore in each image set arrows point to the location of the changes, for guidance. Image credit: ESA / Rosetta / NAVCAM / MPS for OSIRIS Team / MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA / CC BY-SA 3.0 IGO.
Most comets orbit the Sun in highly elliptical orbits that cause them to spend most of their time in the extremely cold outer Solar System.
When a comet approaches the inner Solar System, the Sun begins to warm the ice on and near the comet’s surface. When the ice warms enough it can rapidly sublimate. This process can occur with variable degrees of intensity and time-scales and cause the surface to change rapidly.
ESA’s Rosetta spacecraft spent two years (from August 2014 to September 2016) orbiting 67P/Churyumov-Gerasimenko, most of it at distances that allowed surface characterization and monitoring at submeter scales.
“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,” said lead author Dr. Mohamed Ramy El-Maarry, from the University of Bern in Switzerland and the University of Colorado, Boulder.
The changes, which were either unique transient phenomena or taking place over longer periods, are linked to different geological processes: in situ weathering and erosion, sublimation of water-ice, and mechanical stresses arising from the comet’s spin.
In situ weathering occurs all over the comet, where consolidated materials are weakened — such as by heating and cooling cycles on daily or seasonal timescales — causing their fragmentation.
Combined with heating of subsurface ices that lead to outflows of gas, this can ultimately result in the sudden collapse of cliff walls, the evidence of which is apparent in several locations on the comet.
A completely different process is thought to be responsible for the 1,640-foot- (500 m) long fracture spotted in August 2014 that runs through the comet’s neck in the Anuket region, and which was found to have extended by about 98 feet (30 m) by December 2014. This is linked to the comet’s increasing spin rate in the lead up to perihelion.
Furthermore, in images taken in June 2016, a new 492-984-foot- (150-300 m) long fracture was identified parallel to the original fracture.
Close to the fractures, a 13-foot- (4 m) wide boulder moved by about 49 feet (15 m), as determined by comparing images taken in March 2015 and June 2016. It is not clear if the fracture extension and movement of the boulder are related to each other or caused by different processes.
A substantially larger boulder, some 98 feet wide and weighing 12,800 tons, was found to have moved an impressive 460 feet (140 m) in the Khonsu region, on the larger of the two comet lobes.
It is thought that the boulder moved during the perihelion period, as several outburst events were detected close to its original position. The movement could have been triggered in one of two ways: either the material on which it was sitting eroded away, allowing it to roll downslope, or a forceful outburst could have directly lifted it to the new location.
Erosion caused by the sublimation of material, and deposition of dust falling from outbursts, are also thought to be responsible for sculpting the landscape in different ways, either uncovering previously hidden surfaces or depositing material elsewhere.
For example, scarps in several smooth plains have been observed to retreat by tens of feet and at a rate of up to a few feet per day around perihelion.
“Scarp retreats were observed before on Comet Tempel 1, inferred by comparing images taken during flybys of the comet by NASA’s Deep Impact in 2005, and Stardust-NExT in 2011. What we were able to do with Rosetta was to monitor similar changes continuously, and at a higher resolution,” Dr. El-Maarry said.
“Our observations additionally tell us that scarp retreat seems to be a common process on comets, specifically in smooth-looking deposits.”
Furthermore, in the smooth plains of Imhotep, previously hidden circular features, along with small boulders, have been exposed by the removal of material.
In one location, a depth of about 10 feet (3 m) had been removed, most likely through the sublimation of underlying ices.
Changes were also noted in the comet’s smooth neck region near the distinctive ripples that were likened to Earth’s sand dunes when they were first identified.
Close monitoring of the ripple formations showed this location to also display expanding circular features in the soft material that reached diameters of 328 feet (100 m) in less than three months. They subsequently faded away to give rise to new sets of ripples.
Dr. El-Maarry and co-authors speculate that the repeated development of these unique features at the same spot must be linked to the curved structure of the neck region directing the flow of sublimating gas in a particular way.
Another type of change is the development of honeycomb-like features noticed in the dusty terrains of the Ma’at region on the comet’s small lobe in the northern hemisphere, marked by an increase in surface roughness in the six months leading up to perihelion.
Similar to other seasonal changes, these features faded substantially after perihelion, presumably as a result of resurfacing by the deposition of new particles ejected from the southern hemisphere during this active period.
The authors also note that although many small-scale localized changes have occurred, there were no major shape-changing events that significantly altered the comet’s overall appearance. Ground-based observations over the last few decades suggest similar levels of activity during each perihelion, so they think that the major landforms seen during Rosetta’s mission were sculpted during a different orbital configuration.
“One possibility could be that earlier perihelion passages were much more active, perhaps when the comet had a larger inventory of more volatile materials in the past,” Dr. El-Maarry said.
_____
M. Ramy El-Maarry et al. Surface changes on comet 67P/Churyumov-Gerasimenko suggest a more active past. Science, published online March 21, 2017; doi: 10.1126/science.aak9384
This article is based on text provided by the European Space Agency.
