EMBARGOED FOR Comets used to be cherished as unvarnished messengers from the earliest days of planet formation, primordial objects frozen in the outskirts of the solar system and virtually unchanged since birth.
That view has been changing for three decades and is totally sullied by a new report.
Comets suffer heating, sandblasting, radiation damage and other tainting effects during their often shattered lives. In fact, many comets that grace our night skies are nothing like the utterly primitive time travelers we've been led to believe.
Passing stars, interstellar dust and even distant stellar explosions can alter comets at their surfaces and even more than 65 feet (20 meters) into their insides, according to an overview to be published in the Aug. 7 issue of the journal Nature. Crashes between comets mean many of them are young siblings with new faces.
"A variety of subtle but important evolutionary mechanisms operate on comets during their long storage, so they can no longer be regarded as wholly pristine," writes Alan Stern of the Southwest Research Institute in Boulder, Colorado.
Subtle clues to all this cometary change began emerging in the 1970s, and the new view has vaulted to prominence in recent years, as more comet data has been collected, Stern says in his summary.
Faulty cryogenics
Comets used to be thought of as solid objects. Most are now believed to be loosely bound rubble piles of rock, dirt and ice from water and other elements, Stern explains. The frozen concoctions typically range in diameter from about a half-mile to 9 miles (1-15 kilometers).
Comets essentially originate in two places: the faraway
Kuiper Belt, a fairly flat plane of planetary leftovers beyond Neptune; or the more distant and diffuse Oort Cloud, a huge spherical region that extends about a fifth of the way to the next known star and envelops the entire solar system.Now and then a comet from either reservoir is gravitationally redirected toward the inner solar system. Conventional theory used to state that on a first pass near the Sun, a comet would pack nothing but the original raw materials of planet formation -- the chemicals that swirled around the newborn Sun 4.5 billion years ago -- all neatly bound in some original packaging.
As a comet nears the Sun, extreme erosion is well documented. Volatile surface material is boiled away -- sublimated, in scientific terms -- by increasingly intense solar radiation. This creates the
familiar fuzzy glow of a comet head and, sometimes, a tail. Some comets are so fragile they break apart under all this pressure. Researchers are only now recognizing that similar though less intense processes are at work throughout a comet's life, Stern suggests.
As theory once had it, a comet spends most of its existence preserved in cryogenic storage, much like a turkey sandwich in your freezer stays pristine compared to one rolling around in the back of the SUV.
But deep space poses a host of hazards that don't affect your freezer.
Passing stars and wild wrecks
The Oort cloud's immensity means passing stars "regularly penetrate it" on cosmic time scales, Stern says, resulting in elevated temperatures that could strip some the most volatile cometary gases, such as neon and oxygen. Comets in the Kuiper Belt are less affected by this heating.
Much more distant exploding stars, called supernova, could heat the Oort Cloud from afar, too, Stern calculates. As many as 30 explosions close enough to make an impact could have occurred in the past 4 billion years.
Collisions between comets play an important role. The Oort cloud is sparsely populated. Comets are typically up to 310 million miles (500 million kilometers) apart. But theory holds that the Oort cloud comets were formed closer in and booted to their new orbits as the giant outer planets formed between 4.5 billion and 4 billion years ago.
"Although collisions in the Oort cloud itself are rare and apparently of little effect, recent work indicates that collisions probably greatly affected the surfaces and interiors of many of these bodies during their ejection from the planetary region to the Oort cloud," Stern says.
Collisions in the Kuiper Belt would be far more common, he says, and almost all of the smaller Kuiper Belt comets are likely chips off old, larger blocks.
Old stuff, new look
"Whereas Oort cloud comets are still expected to be (damaged) relics of the formation era, most Kuiper Belt comets must be young," Stern says. The smaller comets are, in effect, new creations -- siblings carved from older parents. "Many of the short-period comets we see aren't even ancient," as Stern puts it.
High-speed
interstellar dust grains erode cometary surfaces, too, like a house sandblasted by a slow but persevering contractor. Over the eons, somewhere between 1 and 65 feet (1-20 meters) of a typical Oort Cloud comet's surface would be flung into space, one tiny impact at a time. Kuiper Belt comets, typically having younger surfaces because of the higher collision rate, would suffer less from this effect.
Finally, even the low doses of solar radiation that reach the Kuiper Belt and Oort Cloud provide a constant source of change, as individual photons persistently erode surface chemicals.
Stern says this new way of thinking about comets should help researchers better understand the surface observations they collect -- comets from the Kuiper Belt ought to be different from Oort Cloud comets. He also suggests that the search for information about the early solar system requires scratching below the surface and probing deeply into the interiors of comets, the closest thing we have to primordial messengers.
Comet Photo Gallery 
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