Rosetta data sheds light on comet dust jets

In unprecedented observation, five science instruments studied a single dust jet for several hours.
By Laurel Kornfeld | Oct 31, 2017
In a stroke of good luck, five science instruments on board the European Space Agency's (ESA) Rosetta spacecraft, which orbited Comet 67P/Churyumov-Gerasimenko from 2014-2016, successfully observed the surface location on the comet from which a dust jet erupted.

The multiple observations, which shed new light on the process by which comets emit dust into space, were unusual because dust jets are impossible to predict, and when they do occur, can usually be observed with just one instrument.

On July 3, 2016, Rosetta's path took it directly through a cloud of dust the comet was emitting into space just as the Sun rose over its Imhotep region, warming up the surface and triggering the dust jet.

At that same time, the probe's OSIRIS scientific camera system directly observed the surface region of the comet from which the dust was emitted.

Over the next several hours, an unprecedented five instruments on Rosetta studied multiple aspects of the dust jet.

The Grain Impact Analyzer and Dust Accumulator (GIADA) and the Cometary Secondary Ion Mass Analyzer (COSIMA) actually captured dust particles from the jet, enabling them to analyze the particles' speeds, sizes, and densities.

For COSIMA, this was the first opportunity to study particles released from a single dust jet as opposed to those from multiple sources collected over several weeks.

COSIMA found the particles to be more fragile than typical dust particles released by comets.

"They must either be very fast or relatively loosely built," said COSIMA Principal Investigator Martin Hilchenbach of the Max Planck Institute for Solar System Research (MPS) in Germany.

Alice, Rosetta's spectrograph, recorded the brightness increase caused by the dust jet and identified tiny ice particles within the dust cloud.

One of Alice's star sensors, which help the orbiter navigate its position in space, measured increased radiation levels from the comet's coma over several hours.

High-resolution images of the surface location from which the jet originated allowed scientists to pinpoint a ten-meter-wide circular region containing frozen water as the jet's source.

The most significant discovery of these observations is that dust jets such as the one emitted that day are not produced solely by the sublimation of frozen water as previously thought.

Because the jet contained far more dust than expected, scientists now believe it and jets like it are driven by another, energetic process just beneath the comet's surface.

That process could be a release of compressed gas as the surface is warmed by the Sun, or a release of energy as subsurface ice transforms from an amorphous state to a crystallized one, also triggered by warmth from sunlight.

Findings of the study have been published in the journal Monthly Notices of the Royal Astronomical Society.

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