Rosetta scientists unveil the source of ice and dust jets on comet 67P

September 25, 2015 by Monica Grady, The Conversation
Rosetta scientists unveil the source of ice and dust jets on comet 67P
Comet 67P and its mysterious jets. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA, CC BY-SA

After a decade-long journey through space, the Rosetta spacecraft has spent the past year less than 100km from the nucleus of comet 67P Churyumov-Gerasimenko, capturing some stunningly detailed images. But despite this wealth of visual evidence for researchers there is a lot we still don't know about the comet – including why it is covered in organic material rather than just ice and what causes its powerful jets of dust and ice.

One of the big surprises of the Rosetta mission has been discovering just how dark 67P is: completely unlike an "dirty snowball", which was how astronomer Fred Whipple described comets in the 1950s. Although images from missions to other comets have shown surfaces that are more likely to be mixtures of ice and rock, findings from an instrument called the Visible and Infrared Thermal Imaging Spectrometer onboard Rosetta have shown that 67P is rocky and almost completely covered in a layer of organic compounds – which was not expected. So where is all the ice?

Now researchers using the same instrument have come up with an answer. They discovered a day-night cycle of ice sublimation (where solid turns to a gas without first turning to liquid) and re-condensation linked to the amount of sunlight. In short, when parts of the are in shadow, it is cold enough for ice to form; when the sun shines on the surface, the ice disappears.

Yes, it might sound a bit obvious, but to dismiss this as "not exactly rocket science" is woefully unjust. The effects are not only on the surface, but extend to depths of several centimetres, leading to production of a layer where water could be active.

The information has even been used to uncover the mechanism by which the jets which shoot up from 67P's surface form – one of the major goals of Rosetta's mission. When the comet is lit up by the Sun, the boost in surface temperature triggers release of gas as the water ice sublimes. The study also helps explain how the comet's surface has eroded: regions which are more deeply in shadow will experience a more reduced cycle of sublimation and condensation than regions where more sunlight can fall, leading to different patterns of rock fracturing and dust removal.

Rock as far as the eye can see. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA, CC BY-SA

It is a beautiful example of how one set of results can help interpret a whole set of apparently unrelated phenomena. The idea of temperature cycles could be taken further to look at transport of molecules in the sub-surface layers and to investigate whether there is sufficient energy below the surface to allow chemical reactions to occur.

Probing the solar wind

Having found out how the jets of dust and ice form, Rosetta is now taking a three week break away from the nucleus, to travel to a more exotic location closer to the Sun. However, Rosetta will not be on holiday – far from it. Although the spacecraft will be 1500km away from the nucleus, it will still be observing the comet very closely. The excursion takes Rosetta to the bow of the comet, the region where the comet meets the .

Imagine that you are walking head-on into a fierce wind. As the wind hits you, it hurts! This is what is happening to 67P. The solar wind is a fast-moving stream of charged particles (mainly protons and electrons); 67P is a solid object moving against the flow. When the two meet, it is 67P which comes off the worst. The dust and gas around the nucleus get swept away, to form the tails of the comet.

The reason why Rosetta is taking the trip is because the bow shock of a comet has never been observed in detail before. Other cometary missions have flown past or through the shock region, but have never lingered. Studying the interface between the solar wind and the plasma region around another body is important because it will help us understand the physics of the interactions that are taking place between the particles. This is necessary because we have our own bow shock region where the Earth meets the solar wind.

Currents in the solar wind, which are modified at the , lead to changes in space weather (which affect satellite communications). While it might seem a bit of a stretch to suggest that measurements taken at a comet 270m kilometres (168m miles) away will help your mobile phone signal, it is by making such measurements that we will eventually be able to predict and ameliorate the effects of the solar wind at the Earth. Electronic devices work using information brought to them as electromagnetic radiation, but charged particles from solar flares can interfere with such devices, producing random signals.

So Rosetta's excursion to the Sun will help keep us all in touch. Now, where's Philae's phone number..?

Explore further: Comet skirts past Sun with Rosetta in tow (Update)

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plasmasrevenge
1.3 / 5 (3) Sep 25, 2015
No mention here of the fact that the observed rate of erosion from 67p appears to be around an order of magnitude faster than expected.

I think nobody had any doubt that the dirty snowball model would be adjusted to reflect observations, but the rapid rate of erosion of what appears to be rocky material over the past couple of months seems in line with suggestions of electric discharge machining as well. That "ice" they claim to be seeing could just as easily be OH.

The act of contrasting two competing worldviews in light of observations is the recipe for teaching critical thinking in the sciences. It says a lot about academia when theorists appear so allergic to that process. The public would be wise to simply pay attention to both worldviews.
r_archer
not rated yet Sep 26, 2015
I believe that comets are the seeds of life, creating the complex organic molecules that are precursors to life. Life has been found in very surprising locations on earth. It will be found in other locations in our solar system and is ubiquitous in space.
Few species are intelligent enough to get off their planet and establish a space-faring civilization.
We managed to get some tourists into low earth orbit.
If our global civilization has an insatiable appetite for resources. To grow, it must grow beyond the earth.
r_archer
not rated yet Sep 26, 2015
Comets are valuable resources, giant chunks of ice, gas, and organic material.
It would be easy to divert them with a rocket engine. The comets themselves are the reaction mass.
Place them in lunar orbit and drop a cable for a cheap and easy lunar elevator as well as all the water you would ever need.
We need to build a self-sufficient industrial base on the moon before our next great recession crashes our global-economy.
jonesdave
5 / 5 (1) Sep 26, 2015
No mention here of the fact that the observed rate of erosion from 67p appears to be around an order of magnitude faster than expected.

I think nobody had any doubt that the dirty snowball model would be adjusted to reflect observations, but the rapid rate of erosion of what appears to be rocky material over the past couple of months seems in line with suggestions of electric discharge machining as well. That "ice" they claim to be seeing could just as easily be OH.

The act of contrasting two competing worldviews in light of observations is the recipe for teaching critical thinking in the sciences. It says a lot about academia when theorists appear so allergic to that process. The public would be wise to simply pay attention to both worldviews.


How about you actually do some critical thinking? OH cannot be mistaken for H20! What they saw was neutral H20. It was detected as far back as 1986 at Halley. It has a completely different signature to OH!

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