Puzzling Comet Composition Solved?

How Are Comets Formed?

For years comets have mystified scientists with their compositions that appear to have formed in both warm and cold environments, rather than in one location of a uniform temperature. But new research shows that the reason some comets feature patches of differing surface composition is not because they are made from material that formed in different parts of the Solar System, but because some parts of their surface absorb heat at varying rates. This leads to localized heat sinks and cold traps, according to a new model constructed by David Jewitt and Aurelie Guilbert-Lepoutre from the University of California, Los Angeles (UCLA). Their model shows that the chemical composition of a comet can evolve in the ten million year period during which a comet is classed as a Centaur, migrating from the Kuiper Belt to the inner Solar System.

“The Centaurs are objects which have escaped from the Kuiper belt and are drifting amongst the giant planets,” says Jewitt. “Their lifetimes in these orbits are limited to about 10 million years because they are gravitationally perturbed by the planets to other orbits. At least half are ejected from the Solar System to the interstellar medium. Some are kicked inside the orbit of Jupiter, where the ice begins to sublimate and we call them comets.”

The key is variances in the surface – thermal conductivity, reflectivity (albedo), obliquity (tilt) and even topography such as craters or hilly terrain. This leads to the creation of ‘thermal shadows’.

“Just as it is cooler in the shadow of a building than standing in the full Sun, the region beneath a bright spot or a boulder on the surface of a comet will remain cooler than the surroundings,” says Jewitt. The higher the albedo, the more sunlight is reflected away, keeping that particular patch of the comet 20 to 30 degrees Celsius cooler than its surroundings. The thermal shadows can be maintained “We have calculated the way the cool spot extends down into the interior of the comet, and examined how deep and how long-lived this cool shadow region can be for objects moving on a variety of different orbits.”

Being colder, the thermal shadows attract volatile materials such as water-ice and carbon dioxide from elsewhere on the comet, enhancing the composition there. Consequently the composition of the comet becomes strongly non-uniform, as does the activity on the comet, manifest in jets of the kind seen, for example, by the Deep impact spacecraft on the Comet Hartley 2 in November 2010.

The paper can be found on the astro-ph archive and can be read here.

Fizzy Comet Hartley 2 is Throwing Snowballs

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As Jessica Sunshine said, Comet Hartley 2 might be the smallest of the five comets that our spacecraft have visited, but no doubt it is the most interesting, and for its size, the most active. Sunshine is the EPOXI mission deputy principal investigator, and she and her team have had the chance to analyze images from the Nov. 4 flyby of the comet. Closeup views yielded some big surprises: Hartley 2 is throwing snowballs.

“When we first saw all the specks surrounding the nucleus, our mouths dropped,” said Pete Schultz, EPOXI mission co-investigator at Brown University. “Stereo images reveal there are snowballs in front and behind the nucleus, making it look like a scene in one of those crystal snow globes.”

Estimates of the size of the largest particles ranges from a golf ball to a basketball.

Another surprise, which was noted almost immediately from the flyby images, were that the very active jets on the comet were powered by carbon dioxide. “This is the first time we’ve ever seen individual chunks of ice in the cloud around a comet or jets definitively powered by carbon dioxide gas,” said Michael A’Hearn, principal investigator for the spacecraft. “We looked for, but didn’t see, such ice particles around comet Tempel 1,” the comet that the Deep Impact spacecraft flew by in 2005.

Here are highlights from the press conference last week, along with some of the fantastic imagery of Comet Hartley 2.

Hartley 2 CO2 jet up close. Credit: NASA/JPL-Caltech/UMD/Brown
Comet Hartley 2 can be seen in glorious detail in this image from NASA's EPOXI mission. It was taken as the spacecraft flew by around 6:59 a.m. PDT (9:59 a.m. EDT), from a distance of about 700 kilometers (435 miles). The comet's nucleus, or main body, is approximately 2 kilometers (1.2 miles) long and .4 kilometers (.25 miles) at the 'neck' or most narrow portion. Jets can be seen streaming out of the nucleus. Image credit: NASA/JPL-Caltech/UMD
This image from the High-Resolution Instrument on NASA's EPOXI mission spacecraft shows part of the nucleus of comet Hartley 2. The sun is illuminating the nucleus from the right. A distinct cloud of individual particles is visible. This image was obtained on Nov. 4, 2010, the day the EPOXI mission spacecraft made its closest approach to the comet. Image Credit: NASA/JPL-Caltech/UMD
Infrared scans of comet Hartley 2 by NASA's EPOXI mission spacecraft show carbon dioxide, dust, and ice being distributed in a similar way and emanating from apparently the same locations on the nucleus. Water vapor, however, has a different distribution implying a different source region and process. Image Credit: NASA/JPL-Caltech/UMD
This zoomed-in image from the High-Resolution Instrument on NASA's EPOXI mission spacecraft shows the particles swirling in a 'snow storm' around the nucleus of comet Hartley 2. Scientists estimate the size of the largest particles ranges from a golf ball to a basketball. They have determined these are icy particles rather than dust. The particles are believed to be very porous and fluffy. Image Credit: NASA/JPL-Caltech/UMD
The motion of some icy particles in the cloud around Hartley 2, as seen by NASA's EPOXI mission spacecraft. A star moving through the background is marked with red and moves in a particular direction and with a particular speed, while the icy particles move in random directions. The icy particles are marked in green, blue and light blue. Image Credit: NASA/JPL-Caltech/UMD/Brown
This image shows the nuclei of comets Tempel 1 and Hartley 2, as imaged by NASA's Deep Impact spacecraft, which continued as an extended mission known as EPOXI. Tempel 1 is five times larger than Hartley 2. Visible jets are easily seen in images of Hartley 2, but required extensive processing to be seen in images of Tempel 1. Tempel 1 is 7.6 kilometers (4.7 miles) in the longest dimension. Hartley 2 is 2.2 km (1.4 miles) long. The Tempel 1 image was built up from more than 25 images captured by the impactor targeting sensor on July 4, 2005. The Hartley 2 image was obtained by the Medium- Resolution Imager on Nov. 4, 2010.

Hartley 2 in Motion: Stunning Morph Animation of Flyby Images

The folks from UnmannedSpaceflight.com have done it again. Daniel Machácek created this wonderful animation from just the five initial images of Hartley 2 that were released by the Deep Impact team immediately following its flyby on November 4, 2010, using Sqirlz Morph software. Time in the animation is five times faster than the actual speed of the flyby. Hartley 2 really does look like a flying bowling pin, except this one is 2km (1.25 miles) long and about .2 km in diameter. Thanks to Daniel for sharing his animation.

First Close Images of Hartley 2: It’s a Peanut with Jets

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NASA’s Deep Impact spacecraft came within 700 kilometers (435 miles) of Comet Hartley 2 at 10:01 a.m. EDT (1401 GMT) today, imaging with several cameras. Here are the first pictures released of the closest approach.

The scientific team watched along with viewers online and on NASA TV as the images were returned to Earth, about an hour after the spacecraft made its closest approach. First impressions? It is a peanut with jets.

“This is a type of moment that scientists live for,” said JPL’s Don Yeomans, “to get new results in such a dramatic fashion. The images are clear, taken as spacecraft was approaching, then as it swung past and moved away.”

The Sun is off to right, and visible is the icy surface of the comet throwing dust and gas towards the Sun.


Another view of Comet Hartley 2 during EXPOXI close approach. Credit: NASA

More images will be coming down from the spacecraft and Yeomans said the scientists will be examining Hartley 2, looking for the origination spots of the jets. “Are the jets coming from the surface, or is it coming from well beneath where heat of Sun reaches into the comet? We’ll be looking for how many jets, or if possibly the whole comet outgassing. There is a single obvious jet coming off towards the Sun, but also you can see one at the 7 o’clock position, which was evident in previous images, too.”

Image of Hartley 2 as the EXPOXI spacecraft moved away from comet. Credit:NASA

The spacecraft uses several high-resolution instruments, and one camera can image the entire comet with a resolution of about seven meters (about 23 feet) per pixel. The spacecraft also acquired 199 medium-resolution images.

From previous images taken by EPOXI from a distance and radar images taken from the ground, scientists knew Hartley 2 was a bi-lobate comet, which means peanut- or pickle-shaped. But they didn’t know if it was a solid surface or a contact binary, where two smaller cometesimals were stuck together.

But, Yeomans said, these images show the comet is of a solid, one-piece construction.

EPOXI Principal Investigator Mike A’Hearn agreed. “Every time we go to a comet they are full of big surprises,” he said. “The comets we’ve seen up close all seem to work the same way, but they look very different so there must be some fundamental differences in the ways they work. It could be they came from different parts of the early solar system or that they evolved very differently. Finding out how the solar system formed is really what we want out of this.”

The discoverer of Hartley 2, Malcolm Hartley, was on hand at JPL for the closest approach. He found the comet 26 years ago as a smudge on photographic plates taken at the Siding Spring Observatory in Australia. “I was doing quality control of photographic plates and I noticed faint object with a telltale glow like a comet,” said Hartley, who still works at the same observatory. “It has been very interesting to be here, and it has been interesting for the science team and quite a challenge for the engineers. There’s going to be enough data downloaded to keep researchers busy for several years.”

See the EPOXI website for more images, and more will be coming down from the spacecraft over the next few days.

To see a “quick and dirty” animation of the flyby images, see this link provided by Doug Ellision of Unmanned Spaceflight.com (and JPL).

Hartley 2 Spawns Meteor Shower

Universe Image Gallery

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The comet of the year for 2010 seems to be Hartley 2. Although this comet is receding from Earth now (its closest approach was in the latter half of October) and growing fainter, it seems to have left us with one last hurrah: The spawning a brief meteor shower.


Although other comets, such as 2009 R1 (McNaught) and 2P/Encke have passed earlier this year, none has presented an especially tempting target for amateur astronomers (both McNaught and Encke were too close to the Sun during perihelion to be easily observed). Additionally, Hartley is the target of a flyby of the Deep Impact probe bringing it further attention.

Meanwhile, observationally, the comet has been somewhat difficult to observe. I went out on October 17th to hunt for it with a 4″ telescope, but despite my best efforts, couldn’t find it. Although the comet was predicted to reach 5th magnitude, the growing nucleus has apparently become so diffuse, reaching over 1° in the sky, that it’s hard to spot. Undeterred, I attempted again this past weekend with my 8″ SCT. Again, my attempts were frustrated. Even a 15 second exposure with my camera barely brought out more than a smudge.

Yet that night we observed several bright meteors radiating from near Cassiopeia which is where Hartley had been a few weeks prior. We checked to ensure there weren’t any other annual meteor showers from that region. Sure enough, there weren’t, and we wondered if there might be a connection between Hartley’s passing and the meteors we witnessed.

Sure enough, just such a shower was a predicted possibility. Whether or not the shower would occur would depend on just how much dust Hartley had given off in the past and how diffuse the cloud had grown (on this pass and others) since its closest approach to Earth was still 12 million km. Although the meteors my friends and I witnessed were notable (around 2nd to 3rd magnitude) they came from the wrong direction. Meteors spawning from Hartley should have a radiant in Cygnus, the swan. But while ours may not have caught these “Hartley-ids”, others have been witnessing a far grander show in the past few nights that seem to come from the right direction.

In Seascape California, Helga Cabral caught a bright fireball. “I saw a bright white ball and tail, arcing towards the ocean. It was quite beautiful and it looked like it was headed out to sea and so picture perfect it could have been a movie!” A similar fireball was reported the same night near Boston, Massachusetts by Teresa Witham. The predicted peak of this shower occurs tonight so if you have a chance and clear skies, go out and look. As with most showers, there may be some stragglers just before and after so you may be able to catch some for the next few nights if conditions tonight aren’t favorable.

Meteors from Hartley 2 will have a relatively low velocity upon entering our atmosphere since the comet is traveling roughly in the same direction. As such, the expected velocity as it hits our planet is a mere 7 miles a second. The result of this is that they will likely travel slowly across the sky, taking perhaps as much as a few seconds. In contrast, the Leonid showers coming later this month have a relative velocity of 45 miles per second, which causes the meteors to streak across the entire sky in less than a second. The lower velocity for the Hartley-ids will also mean they won’t undergo as much frictional heating and will likely glow fainter shades of reds and yellows.

Watch Live Coverage of EPOXI’s Hartley 2 Encounter on Nov. 4

Comet 103P/Hartley 2 Animation, created by images taken by Patrick Wiggins, NASA/JPL Solar System Ambassador to Utah. Used by permission.

Watch live coverage of the EPOXI mission’s close flyby of Comet Hartley 2. Live coverage begins on November 4, 2010 at 9:30 a.m. EDT (6:30 a.m. PDT) from mission control at the Jet Propulsion Laboratory. You can watch NASA TV’s Media Channel online at this link, (and make sure you click on the “Media Channel” tab on the right side of the “tv” screen). You can also watch on JPL’s UStream channel online. Coverage includes closest approach, an educational segment, and the return of close-approach images. Emily Lakdawalla of the Planetary Society Blog has posted a very detailed timeline of the encounter.
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