This composite image, taken by the impactor targeting sensor on Deep Impact, shows two areas (arrowed) where the surface is smooth instead of spotted with depressions (Image: Science)Millions of kilograms of fine dust particles and water and a "surprisingly high" amount of organic molecules sprayed into space when NASA crashed its Deep Impact spacecraft into Comet 9P/Tempel 1 on 4 July 2005, reveal a trio of new studies.
The observations bolster theories that comets may have seeded Earth with the raw materials for life and suggest they may be sponge-like Ė rather than hardened Ė at their cores.
On 4 July, about 80 telescopes on Earth and in space trained their sights on Comet Tempel 1 when a 370-kilogram copper impactor was sent hurtling into its path. Just after the smash, a bright vapour plume spewed from the surface at about 5 kilometres per second, followed quickly by a stream of particles that spread into a cone.
The cone appeared to remain attached to the comet's surface for about 22 hours before separating into a detached arc. Researchers used this gravitational attraction to estimate the mass and density of the comet's main body, or nucleus. They found that the 72 trillion kilogram-nucleus was extremely porous, with as much as 80% of its volume taken up by empty space.
"That tells me there is no solid layer all the way down to the centre," says Mike A'Hearn, the mission's principal investigator at the University of Maryland in College Park, US. He says he had expected that the ice might become denser towards the core of the nucleus, but that instead "probably all the way in, ice is all in the form of tiny grains".
A touch crumbly
"Itís like a sponge, with a lot of cavities," agrees Horst Uwe Keller, an astronomer at the Max-Planck Institute for Solar System Research in Germany. He observed the event with Europe's Rosetta spacecraft and says the discovery confirms previous observations suggesting other comets are also porous. "When you touch it, it just crumbles under your hands."
Observers estimate the impact released about 5 million kilograms of water from beneath the comet's surface and between two and five times as much dust. There was so much dust, in fact, that mission members have not been able to see the impact crater with the high-resolution camera on the mission's flyby spacecraft, about 500 km away.
To add to the problem, that camera was malfunctioning but now image-processing techniques may have revealed a glimpse of the crater and team members may release the image later on Wednesday.
The team estimates the impact blasted away a crater about 100 metres wide and up to 30 m deep. Crucially, organic molecules were among the material ejected. Neither the full range of molecules nor their abundances have been determined yet, but researchers say they have found a surprisingly high amount of methyl cyanide, a molecule seen in large quantities in another comet.
This supports theories that comets may have brought water and the building blocks of life to Earth, and the team hopes to eventually "identify all the species comets brought in abundance to early Earth", says A'Hearn.
The observations have also apparently ruled out another theory Ė that impacts with other objects may be responsible for the occasional stream of gas and dust seen coming off of comets. Although Tempel 1's surface is pockmarked with craters ranging from 40 m to 400 m across, astronomers watching the comet both before and after the impact noticed that it released the streams relatively often in spurts of activity apparently triggered by sunlight.
"I donít think the hypothesis that outbursts are caused by impacts is really valid," says A'Hearn. "Probably comets undergo outbursts like this very frequently and the fact that everyone was looking intensively [at this comet] for an extended period allowed us to see phenomena that are probably common and werenít seen before."[/QUOTE]