Rosetta makes startling ice discovery on comet


67P/Churyumov–Gerasimenko's close encounter with the sun is giving investigators much to study. Richard A Lovett reports.


Comet 67P/Churyumov–Gerasimenko continues to intrigue and surprise researchers.
Maurizio de Angelis/Science Photo Library

Scientists studying images returned from the European Space Agency's Rosetta mission to comet 67P/Churyumov–Gerasimenko have found many intriguing changes in its surface during its close passage to the sun, including a giant avalanche that offered a surprising glimpse into the comet’s icy interior.

Mapping such changes was one of the primary goals of the Rosetta mission, says Mohamed Ramy El-Maarry, a planetary scientist at the University of Colorado’s Laboratory for Atmospheric and Space Physics in Boulder, Colorado.

It was made possible, he adds, because the Rosetta spacecraft was able to travel with the comet for two years between 2014 and 2016, monitoring changes as it moved from 539 million kilometres out from the sun to a closest approach of 185 million kilometres – about 23% farther out than the Earth – and then back again.

Many changes were observed, El-Maarry and colleagues reported in a paper published this week in Science and at the 2017 Lunar and Planetary Sciences Conference (LPSC) in The Woodlands, Texas. Among these changes were steep slopes that retreated as fast as 5.4 metres per day, a boulder the size of an office building that was dislodged and rolled 140 metres downhill, and dune-like ripples created by wind from erupting gases blowing across the comet’s surface at speeds up to 500 metres per second.

The scientists also spotted a large crack in the “neck” of the duck-shaped comet. When initially observed, this crack (which may eventually spit the comet in two) was about 500 metres long. By the time the comet had completed its dive into the inner solar system, it had lengthened by another 30.

Most of these changes, the scientists believe, are due to solar heating that raised 67P/Churyumov–Gerasimenko’s daytime side above freezing, causing dust and vapor to jet out into space and into its tail. But the growth in the neck fracture probably came from a change in the comet’s spin, El-Maarry says. That occurred, he adds, because much of the vapor and dust lost by the comet came from its higher elevations, causing its rate of spin to speed up. It wasn’t a huge change, but it was measurable, shortening the comet’s 12.4-hour “day” by about seven minutes.

“In essence it’s what a ballerina would do when she puts her hands together so she will spin faster,” he says. “When it spins up, it generates more stress in the neck region.”

But the most dramatic find may have been a giant landslide that occurred simultaneously with a big outburst of the dust. The landslide did more than blast dust into the comet’s tail.

High-resolution images revealed that when the slab of material, 12 metres thick, 70 metres wide and 65 high, peeled off the top of a 134-metre cliff and fell onto the talus slope below, it exposed a patch of underlying material that was extremely bright. It was so bright, says Maurizio Pajola, a planetary scientist at NASA Ames Research Center, Moffett Field, California, that the glare from it saturated Rosetta’s cameras, meaning it was reflecting at least 40 percent of the sunlight that fell on it.

The only thing that could possibly be that bright, Pajola reported in another presentation at the LPSC and in a paper in this week’s issue of Nature Astronomy is water ice. And to be that bright, the ice has to be nearly pure.

It’s a remarkable finding, says Humberto Campins, a planetary scientist at the University of Central Florida, Orlando, because scientists hadn’t expected to find high-purity ice on the comet. Campins, who was not part of either study team, adds that the presumption was that the comet’s ice would be so thoroughly mixed with dust that none of it could possibly appear that bright.

The question now becomes how it could have such pure ice. Perhaps it’s primordial, Campins suggests. Or perhaps it’s the result of internal processes that somehow cause clean ice to condense into underground cavities or other areas, one of which was exposed by the landslide.

“I don’t know,” he says, “but it opens up some interesting questions.”

All told, he notes, the nature and diversity of the changes seen by Rosetta are revealing a lot about how comets evolve as they make their brief passages into the inner solar system. They may also provide important clues to their interior structures and to how comets form.

“I don’t know exactly what these things are telling us,” he says, “but it is clear that they are revealing a very complex morphology.”

Contrib ricklovett.jpg?ixlib=rails 2.1
Richard A. Lovett is a Portland, Oregon-based science writer and science fiction author. He is a frequent contributor to COSMOS.
  1. http://10.1126/science.aak9384 (2017).
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