Rosetta images show changes on Comet 67P as it approached the Sun
Two separate studies of images captured by the European Space Agency’s (ESA) Rosetta probe highlight increasing activity on the surface of Comet 67P/Churyumov–Gerasimenko as it approached perihelion, the point in its orbit closest to the Sun.
Rosetta orbited the comet for two years between August 2014 and September 2016, conducting the first close-up study of a comet as it approached the Sun and subsequently receded from it.
A study published in the March 21 issue of the journal Science confirms activity such as collapsing cliffs, rolling boulders, and growing fractures as Comet 67P headed toward the Sun. As a result of this activity, the comet’s surface regularly changed, with some features being buried and others uncovered.
“As comets approach the Sun, they go into overdrive and exhibit spectacular changes on their surface. This is something we were not able to really appreciate before the Rosetta mission, which gave us the chance to look at a comet in ultra-high resolution for more than two years,” said study leader Ramy El-Maarry of the University of Colorado at Boulder, who is also a member of the U.S. Rosetta science team.
Surface changes on comets are triggered when their ice begins to warm on approach to the inner Solar System. At that point, the ice can quickly sublimate, transforming directly from solid to gaseous.
That sublimation can cause extreme surface changes such as deep cracks and movement of large rocks. Cameras on board Rosetta observed a major crack in the neck of Comet 67P and the movement of truck-sized boulders over distances the length of one-and-a-half football fields, El-Maarry noted.
Large boulder movement
One particularly massive rock 100 feet (30 meters) wide and weighing about 282 million pounds (130 million kilograms) was transported approximately 150 yards (140 meters) from its initial position on the comet’s nucleus, the movement most likely triggered by increasing outbursts on the comet’s surface.
In addition to sublimation, weathering, erosion, and stresses from increased spin can cause both short-term and long-term surface changes.
Regular heating and cooling on either a daily or seasonal basis weaken surface materials, accelerating a comet’s fragmentation.
The warmer temperatures that comets encounter on approach to perihelion speed up the comets’ rotation rates, which can also cause surface fractures.
Fracture on Comet 67P’s neck
A 1,600-foot (500-meter) long fracture observed on Comet 67P’s neck in its Anuket region in August 2014 widened by about 100 feet (30 meters) in just four months.
By June 2016, 10 months after perihelion, another parallel fracture between 500 and 1,000 feet (150 and 300 meters) was observed.
The researchers attribute both fractures to the comet’s increased spin on approach to perihelion rather than to sublimation and weathering.
“The large crack was in the ‘neck’ of the comet – a small central part that connects the two lobes,” El-Maarry said. “This helps us better understand the conditions of the early Solar System, and possibly even how life started.”
Scientists are uncertain as to whether the comet began its life as a single object or was formed by the merger of two separate ones.
Comet cliff collapse
A second study published March 21 in the journal Nature Astronomy confirms a link between the collapse of a prominent cliff on Comet 67P’s surface and an outburst of gas and dust from the comet’s nucleus.
Brief, frequent outbursts were observed during the two years Rosetta orbited the comet. These appear to be linked to collapses of eroded areas on its surface in response to sudden heating and exposure of volatile materials.
As buried ices increasingly sublimated into gases, surface dust was released into space, producing the outbursts.
One example of this is a fracture on a cliff edge known as Aswan on the comet’s larger lobe. Following an outburst Rosetta observed on July 10, 2015, this region was found to have a new, sharp edge and numerous boulders at the cliff’s foot.
“The last time we saw the fracture intact was on 4 July, and in the absence of any other outburst events recorded in the following ten-day period, this is the most compelling evidence that we have that the observed outburst was directly linked to the collapse of the cliff,” said study leader Maurizio Pajola of the NASA Ames Research Center.
The exposed cliff face was six times brighter than the rest of the comet’s nucleus following the outburst but faded by half just five-and-a-half months later, indicating most of the water ice had sublimated into gas.
Over a year later, in August 2016, the cliff face was back to its normal brightness with the exception of one bright block.
Because comets contain primitive material dating back to the formation of the Solar System, close-up observations of them help scientists piece together the dynamics of the early Solar System as well as determine whether they played a role in the start of life on Earth by delivering water and organic molecules to the planet.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.