Saturday, November 20, 2010

Comet Hartley Snowstorm

Two items here and a few photos of the Hartley comet.  We have learned a lot and I am sure that actual sampling is now needed to finish the job.  This is a fascinating look-see.

It appears that this object formed as a gas ball with heavier molecules forming a crust that creates the asteroid shaped object.  Observe the rounded surface.  We do not know the elemental content of the crust itself.  It could even be elemental carbon as I have conjectured.

However carbon dioxide out gassing from the structure and carrying frozen water vapor appears plausible and is certainly a constituent.

The interesting question is what may be driving the out gassing phenomena. It is assumed that this is a warming event and perhaps it is.  I also have proposed that it is possibly an electrical event caused by the charge build up produced as the comet approaches the sun.

My reason for that is that the temperature thresholds are significant barriers and right now these objects look like great insulators. A charge buildup would affect the whole structure and mobilize all free molecules not tied down.

Of course, no one is effectively looking for elemental carbon and with all that CO2, would be unlikely to discern it at all.

Surprise at a Comet Visited by NASA: A Snowstorm

Part of Hartley 2’s nucleus, with the sun illuminating it from the right and a distinct cloud of individual particles.

Published: November 18, 2010

A peanut-shaped comet was spewing hundreds of tons of fluffy ice chunks every second as a NASA spacecraft swung by it two weeks ago.

 “To me, this whole thing looks like a snow globe that you’ve simply just shaken,” Peter H. Schultz, a Brown University professor working on the mission, said Thursday during a news conference.

The Deep Impact spacecraft passed within 435 miles of Comet Hartley 2 two weeks ago, producing a series of photographs that showed bright jets coming off a rough surface.

What fascinated the mission scientists most was that the chunks of water ice in the jets were not being lifted off the surface by the force of water vapor heated by the sun, but rather by jets of carbon dioxide. This was the first time that such carbon dioxide jets had been observed at a comet.

Frozen carbon dioxide — dry ice — turns to carbon dioxide gas at a temperature of about minus 100 degrees Fahrenheit. Water ice stays frozen until it reaches much higher temperatures. Thus, it appears that the carbon dioxide gas on the comet is blowing apart the still-frozen water ice, along with dust particles, and sending it into space.

The scientists who analyzed the photos of Hartley 2 said that the frozen carbon dioxide within the comet must date from the beginnings of the solar system, because once it turns to gas, it disperses into space. “If it’s there, it’s primordial,” Lori M. Feaga, another member of the science team, said in an interview.

While the bright specks seen in the images ranged in size from golf balls to perhaps basketballs, the spacecraft did not suffer any damage as it flew through the storm at a speed of 27,000 miles an hour.

From the absorption of light by the specks, the scientists deduced that the chunks were not solid ice.

“We’re not seeing hail-size softballs or even ice cubes,” said Jessica M. Sunshine, a deputy principal investigator on the mission. “What we’re seeing are fluffy aggregates of very small pieces of ice. And so, they’re akin more to maybe a dandelion puff that is very empty air that can be easily broken apart.”

The carbon dioxide jets are coming off the two lobe-shaped ends of the comet, which is just three-quarters of a mile long. The spacecraft also found large amounts of water vapor — and not carbon dioxide — emanating from the central narrow waist region between the two lobes. This may mean that, for reasons not yet deciphered, the middle part of the comet does not contain much carbon dioxide, and thus the water ice in it can warm into vapor.

“We wouldn’t expect this at all,” Dr. Sunshine said. “This comet is doing two things at once.”

The Hartley 2 findings differ significantly from what the Deep Impact spacecraft found when it visited Comet Tempel 1 five years ago. At Tempel 1, Deep Impact found water vapor emissions similar to those coming from the middle of Hartley 2, but no carbon dioxide jets and no visible chunks of water ice.

After the Tempel 1 visit, NASA decided to reuse the spacecraft, which still had ample maneuvering fuel left, to head to a second comet. The mission was renamed Epoxi, an amalgamation of two acronyms: Epoch, or Extrasolar Planet Observation and Characterization, which has been using one of the spacecraft’s cameras to look at stars known to have planets, and Dixi, or Deep Impact Extended Investigation, for the second comet fly-by.

“It has emphasized how different comets are from one another and how understanding them is a much more complex problem than the rather simplistic approach I like to normally take,” said Michael F. A’Hearn, Epoxi’s principal investigator.

This article has been revised to reflect the following correction:

Correction: November 18, 2010

A previous version of this article omitted the word "minus" in describing the temperature at which dry ice turns to carbon dioxide.

Comet Snowstorm Engulfs Hartley 2

Nov. 18, 2010:  NASA has just issued a travel advisory for spacecraft: Watch out for Comet Hartley 2, it is experiencing a significant winter snowstorm.

Deep Impact photographed the unexpected tempest when it flew past the comet's nucleus on Nov. 4th at a distance of only 700 km (435 miles). At first, researchers only noticed the comet's hyperactive jets. The icy nucleus is studded with them, flamboyantly spewing carbon dioxide from dozens of sites. A closer look revealed an even greater marvel, however. The space around the comet's core is glistening with chunks of ice and snow, some of them possibly as large as a basketball.

This contrast-enhanced image obtained during Deep Impact's Nov. 4th flyby of Comet Hartley 2 reveals a cloud of icy particles surrounding the comet's active nucleus. [larger image]

"We've never seen anything like this before," says University of Maryland professor Mike A'Hearn, principal investigator of Deep Impact's EPOXI mission. "It really took us by surprise."

Before the flyby of Hartley 2, international spacecraft visited four other comet cores—Halley, Borrelly, Wild 2, and Tempel 1. None was surrounded by "comet snow." Tempel 1 is particularly telling because Deep Impact itself performed the flyby. The very same high resolution, high dynamic range cameras that recorded snow-chunks swirling around Hartley 2 did not detect anything similar around Tempel 1.

"This is a genuinely new phenomenon," says science team member Jessica Sunshine of the University of Maryland. "Comet Hartley 2 is not like the other comets we've visited."

The 'snowstorm' occupies a roughly-spherical volume centered on Hartley 2's spinning nucleus. The dumbbell-shaped nucleus, measuring only 2 km from end to end, is tiny compared to the surrounding swarm. "The ice cloud is a few tens of kilometers wide--and possibly much larger than that," says A'Hearn. "We still don't know for sure how big it is."

Data collected by Deep Impact's onboard infrared spectrometer show without a doubt that the particles are made of frozen H2O, i.e., ice. Chunks consist of micron-sized ice grains loosely stuck together in clumps a few centimeters to a few tens of centimeters wide.

This plot compares the infrared spectra of particles surrounding Comet Hartley 2 (black crosses) to spectra of pure water ice grains in the laboratory (purple lines). Micron-sized grains provide the best match. What it means: Hartley 2's snowballs are made of small bits of H20.

"If you held one in your hand you could easily crush it," says Sunshine. "These comet snowballs are very fragile, similar in density and fluffiness to high-mountain snow on Earth."

Even a fluffy snowball can cause problems, however, if it hits you at 12 km/s (27,000 mph). That's how fast the Deep Impact probe was screaming past the comet’s nucleus. An impact with one of Hartley 2's icy chunks could have damaged the spacecraft and sent it tumbling, unable to point antennas toward Earth to transmit data or ask for help. Mission controllers might never have known what went wrong.

"Fortunately, we were out of harm's way," notes A'Hearn. "The snow cloud does not appear to extend out to our encounter distance of 700 km. Sunlight sublimates the icy chunks before they can get that far away from the nucleus."

The source of the comet-snow may be the very same garish jets that first caught everyone's eye.

The process begins with dry ice in the comet's crust. Dry ice is solid CO2, one of Hartley 2's more abundant substances. When heat from the sun reaches a pocket of dry ice—poof!—it instantly transforms from solid to vapor, forming a jet wherever local topography happens to collimate the outrushing gas. Apparently, these CO2 jets are carrying chunks of snowy water ice along for the ride.

An artist's concept of Comet Hartley 2 shows how CO2 jets drag water ice out of nucleus, producing a 'comet snowstorm.' [larger image]

Because the snow is driven by jets, "it's snowing up, not down," notes science team member Peter Schultz of Brown University.

Ironically, flying by Hartley 2 might be more dangerous than actually landing on it. The icy chunks are moving away from the comet’s surface at only a few m/s (5 to 10 mph). A probe that matched velocity with the comet's nucleus  in preparation for landing wouldn't find the drifting snowballs very dangerous at all--but a high-speed flyby is another matter. This is something planners of future missions to active comets like Hartley 2 will surely take into account.

Comet snowstorms could be just the first of many discoveries to come. A’Hearn and Sunshine say the research team is only beginning to analyze gigabytes of data beamed back from the encounter, and new results could be only weeks or months away.
Stay tuned for updates from Comet Hartley 2.

Author: Dr. Tony Phillips | Credit: Science@NASA

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