ESA's Rosetta spacecraft has observed a dramatic increase in activity from the comet 67P/Churyumov-Gerasimenko, seemingly brought on by a landslide. The new data will help scientists to further characterize the activity that continues to shape 67P to this day.
Whilst the Rosetta spacecraft and her Philae payload have done an admirable job in probing the secrets of the comet 67P since their arrival in August 2014, our understanding as to the nature of these wandering oddities is yet relatively juvenile.
One of the key issues faced by scientists hoping to observe and learn from comet activity is the unpredictable nature of 67P. We are aware that, as the traveller makes its closest approach to the Sun, known as perihelion, heat from our star causes a massive increase in outgassing activity. This knowledge allowed mission controllers to send Rosetta into a more distant orbit in the months before and after perihelion in order to protect the probe.
However, perihelion is now long gone, and 67P is racing back into the cold depths of our solar system. Now, for Rosetta to catch sight of one of the comet's unpredictable and dramatic bursts of activity, luck is the key factor. If the probe were to capture such an event, it would have to occur with Rosetta in the right place, at the right time, with her instruments actively observing the comet.
At around 9:40 GMT on the 19th of February, Rosetta got lucky. 67P let forth a deluge of activity from the Atum region, located on the comet's large lobe, and nine of the probe's instruments were in a position to document the entire event. In the two hours following the initial detection, the probe recorded elements of comet activity sometimes reaching a factor of 100 times the intensity prior to the outburst.
During the event, Rosetta was orbiting at a distance of 35 km (21.7 miles) from the comet, and was able to harvest the most comprehensive data set detailing the characteristics of an outburst from 67P to date. These measurements were taken by a wide range of instruments, including the probe's gas and plasma analysers, the OSIRIS wide-angle camera, and the GIADA dust impact instrument.
At the height of the activity, GIADA was struck by 200 particles, where normally the instrument would collect 3 – 10 over the course of a full Earth day. The probe also recorded a 30 ºC increase in the temperature of the surrounding gas, alongside a brightening of the coma.
Diagram of the Rosetta orbiter. Instruments in bold represent the tools used to characterize the outburst of activity
As would be expected following such a period of activity, Rosetta recorded a significant increase in the quantity of gas and plasma present in the comet's coma following the event. The sheer amount of material escaping the surface of 67P had the effect of scattering the light from distant stellar bodies, which interfered with the probe's star trackers.
Star trackers are navigational instruments mounted at a 90-degree angle on the side of the spacecraft. The interference experienced by the nav tools helped Rosetta scientists to create a 3D model of the outburst, and to track its evolution.
An analysis of the data, paired with the fact that the dust cone was very broad in nature, suggests that the unexpected activity resulted from a landslide on the surface of 67P rather than a release of fresh material from deep within the comet.
The Rosetta team believe that the surface shift, which took place on a steep hill, was created by thermal pressures brought about as the Atum region transitioned into a period of sunlight. The resultant landslide uncovered shallow deposits of water ice, which, in the direct glare of the Sun's light, were immediately transformed into gas. Dust was then carried along with the escaping gas, and subsequently recorded and captured by Rosetta's instruments.
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