Scientists at the University of Arizona have found convincing evidence for the presence of liquid water in a comet, shattering the current paradigm that comets never get warm enough to melt the ice that makes up the bulk of their material.
"Current thinking suggests that it is impossible to form liquid water inside of a comet," said Dante Lauretta, an associate professor of cosmochemistry and planet formation at the UA's Lunar and Planetary Laboratory. Lauretta is the principal investigator of the UA team involved in analysis of samples returned by NASA's Stardust mission.
"In our samples, we found minerals that formed in the presence of liquid water," Berger said. "At some point in its history, the comet must have harbored pockets of water."
Comets are frequently called dirty snowballs because they consist of mostly water ice, peppered with rocky debris and frozen gases. Unlike asteroids, extraterrestrial chunks made up of rock and minerals, comets sport a tail –- jets of gas and vapor that the high-energy particle stream coming from the sun flushes out of their frozen bodies.
"When the ice melted on Wild-2, the resulting warm water dissolved minerals that were present at the time and precipitated the iron and copper sulfide minerals we observed in our study," Lauretta said. "The sulfide minerals formed between 50 and 200 degrees Celsius (122 and 392 degrees Fahrenheit), much warmer than the sub-zero temperatures predicted for the interior of a comet."
Discovered in 1978 by Swiss astronomer Paul Wild, Wild-2 (pronounced "Vilt") had traveled the outer reaches of the solar system for most of its 4.5 billion year history, until a close encounter with Jupiter's field of gravity sent the 3.4 mile-wide comet onto a new, highly elliptical orbit bringing it closer to the sun and the inner planets.
Scientists believe that like many other comets, Wild-2 originated in the Kuiper belt, a region extending from beyond Neptune's orbit into deep space, containing icy debris left over from the formation of the solar system. Wild-2 is thought to have spent most of its time in the Kuiper belt, transiting on unstable orbits within the planetary system before Jupiter's gravity hurled it into the neighborhood of the sun.
The discovery of the low-temperature sulfide minerals is important for our understanding of how comets formed –- which in turn tells us about the origin of the solar system.
In addition to providing evidence of liquid water, the discovered ingredients put an upper limit to the temperatures Wild-2 encountered during its origin and history.
"The mineral we found –- cubanite –- is very rare in sample collections from space," Berger said. "It comes in two forms; the one we found only exists below 210 degrees Celsius (99 degrees Fahrenheit). This is exciting because it tells us those grains have not seen temperatures higher than that. "
Cubanite is a copper iron sulfide, which is also found in ore deposits on Earth exposed to heated groundwater and in a particular type of meteorite.
"Wherever the cubanite formed, it stayed cool," she added. "If this mineral formed on the comet, it has implications for heat sources on comets in general."
According to Berger, two ways to generate heat sources on comets are minor collisions with other objects and radioactive decay of elements present in the comet's mixture.
Heat generated at the site of minor impacts might generate pockets of water in which the sulfides could form very quickly, within about a year (as opposed to millions of years). This could happen at any point in the comet's history. Radioactive decay on the other hand, would point to a very early formation of the minerals since the radioactive nuclides would decay over time and cause the heat source to flicker out.
The presence of the cubanite and the other sulfide minerals helps scientists better understand cometary heat sources. The interior of the comet must have been warm enough to melt ice yet cool enough –- below 210 degrees Celsius –- to form cubanite.
"Such detailed thermal constraints will allow for detailed analysis of the role temperature played during the history of comet Wild 2," Lauretta said.
Each sample Berger's team analyzed consisted of a microscopic speck of cometary dust about the size of a bacterial cell. The group then studied the chemical composition by electron microscopy and X-ray analysis, during which the chemical elements revealed their presence by giving off a characteristic beam. Turning the sample in different orientations gave the scientists clues about its crystallographic structure.
According to Lauretta, the findings show that comets experienced processes such as heating and chemical reactions in liquid water that changed the minerals they inherited from the time when the solar system was still a protoplanetary disk, a swirling mix of hot gases and dust, before it cooled down enough for planets to form.
The results demonstrate the increasingly apparent connections between comets and asteroids.
"What we found makes us look at comets in a different way," Lauretta said. "We think they should be viewed as individual entities with their own unique geologic history."
"This study shows the high science value of sample return missions," Lauretta said. "These grains would never have been detected by remote sensing or by flying a spacecraft past the comet to make observations without collecting a sample."
On 2 January 2004, NASA's Stardust spacecraft successfully survived flying through the coma (dust and gas cloud) surrounding comet 81P/Wild 2, captured thousands of fresh cometary dust particles released from the surface just hours before, and is now on its way home for Earth return set for January 2006.
During the flyby, the highest resolution images ever taken of a comet's nucleus were obtained and have been the subject of intense study since the flyby. A short exposure image showing tremendous surface detail was overlain on a long exposure image taken just 10 seconds later showing jets.
"This spectacular composite image [below] shows a surface feature unlike any other planetary surface see to date in our solar system", says Prof Donald Brownlee, the Stardust Principal Investigator from the University of Washington. "Other than our sun, this is currently the most active planetary surface in our solar system, jetting dust and gas streams into space and leaving a trail millions of km long."
"The overall shape of the nucleus resembles a thick hamburger patty with a few bites taken out", says Thomas Duxbury, the Stardust Project Manager from JPL. "The surface has significant relief on top of this overall shape that reflects billions of years of resurfacing from crater impacts and out-gassing".
The Daily Galaxy via University of Arizona and stardust.jpl.nasa.gov