‘Snowy Dirtball’ Is Melting Old Theories
Rather than a dirty snowball darting across the solar system, the comet targeted by a NASA probe July 4 turns out to be a “snowy dirtball” instead, made mostly of dust, according to the first detailed results from the Deep Impact comet-hunting mission.
Mixed in the dust is a soup of potentially life-spawning organic compounds all wrapped within a ball of space fluff with less consistency than a snowbank, scientists reported Tuesday.
The outer surface “is unbelievably fragile,” said Michael A’Hearn, an astronomy professor at the University of Maryland. “The comet is mostly empty, mostly porous.”
Until Deep Impact, scientists had ideas but no detailed knowledge of the inside of a comet, one of the most familiar but least understood celestial objects. The probe that smashed into comet Tempel 1 set off an explosion of ice and dust seen by 80 observatories around the world.
The data from the $333-million mission, published in this week’s issue of the journal Science, indicate that the probe penetrated into the surface of the comet 30 feet or more and blasted out a crater the length of a football field.
The blast caused the comet to release an estimated 11 million pounds of water, along with so much dust that Earth-based telescopes have struggled to pinpoint the exact impact site.
One of the more interesting findings was the large number of organic molecules detected in the material blasted off the surface of the comet, including methyl cyanide, hydrogen cyanide, ammonia and acetylene. Scientists had remotely detected these compounds in other comets. But the large amounts of some of them, particularly methyl cyanide, was a surprise, scientists said.
The findings offer support for the theory that comets seeded the barren infant planet of Earth with the chemical precursors of life.
“I would argue this makes it more likely,” A’Hearn said in a teleconference with reporters.
It is not known how typical Tempel 1 is among the family of comets in the solar system, but in its outward appearance, it is not vastly different from others, scientists said.
An infrared spectrometer also helped scientists chart the surface temperature of a comet for the first time. They discovered that the nucleus of the comet was porous, with as much as 80% consisting of empty space, like a sponge. This allows the surface to heat up and cool down almost instantly in response to sunlight. Which in turn means that heat is not easily transferred to the interior, and thus material deep inside the comet is preserved from the earliest days of the solar system, 4.6 billion years ago.
Deep Impact traveled 280 million miles over a six-month mission before shooting its 820-pound impactor into the comet, orbiting near Mars, in a maneuver some scientists compared to hitting a bullet with a bullet. The resulting collision increased the light of the comet about fivefold.
The data from the impact showed that there was more dust than ice in a comet. The readings couldn’t pinpoint the exact ratio, but scientists said dust could make up from 50% to 90% of the comet.
One of the greatest mysteries about comets was whether they contained rocky cores. Some space scientists predicted that Tempel 1 would have one, but A’Hearn said that that theory might be wrong. Based on what he’s seen from Deep Impact, “I’m not convinced there is a solid layer under there.”
The probe didn’t penetrate farther into the interior because it hit at an oblique angle, instead of straight on, A’Hearn said.
The fluffiness of the comet may signal trouble for a scheduled mission to a comet in 2014 by the European Space Agency. The mission aims to land a probe on a comet’s surface, instead of crashing into it.
“You can’t really land on the surface,” said Horst Keller of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany. “You would sink in.”
