The high-speed collision of Comet Tempel 1 with NASA鈥檚 Deep Impact probe on 4 July 2005 released an additional 250,000 tonnes of water over a period of 12 days, according to researchers working with data from the Swift satellite.
But that amount of water is 10 to 20 times greater than that reported by other ground- and space-based observatories. The discrepancy underscores the difficulties scientists face in painting a coherent picture of the Deep Impact collision and its aftermath 鈥� a one-off event observed by dozens of observatories.
鈥淭his outburst is clearly very different from what was monitored from the basic impact聟 Probably the whole impact shook the comet up and it shed water not just from the impact point, but over its whole surface,鈥� Dick Willingale, Swift team member and senior lecturer in physics at the University of Leicester, UK, told New 杏吧原创.
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The international team directed Swift 鈥� normally a gamma-ray burst-seeking craft 鈥� to observe Tempel 1 before, during, and after the Deep Impact smash. Since molecules derived from water glow in X-rays when they interact with energetic particles in the solar wind, researchers can determine how much material a comet loses per second using X-ray telescopes.
Most observatories that watched the Deep Impact collision have reported a brightening plume of gas and dust that reached its brightest point within a day of the impact and decreased to pre-impact levels within about five.
鈥淲hat we saw with Swift was that the X-rays actually peaked about five days after impact,鈥� says Willingale. According to his team, Tempel 1鈥檚 water production rate increased to about 2.5 times the comet鈥檚 average over a 12-day period.
Differing interpretation
But other Deep Impact observers disagree with the Swift team鈥檚 interpretation of their data. Carey Lisse, who led the Deep Impact observations using NASA鈥檚 Spitzer Space Telescope and Chandra X-ray Observatory says that, while the Swift data agree very well with his own group鈥檚, 鈥渙ur interpretations differ quite substantially鈥�.
Lisse told New 杏吧原创: 鈥淲e can show, using measurements of the comet 鈥� from other observatories as well as Swift and Chandra 鈥� that the outburst [from four to six days after the impact] was due to a dramatic upswing in the solar wind hitting the comet, and not due to an increased amount of material coming off the comet.鈥�
Since both the rate of water production and the number of solar wind particles striking the comet鈥檚 environment affect the X-ray signal, it is important to take both factors into account when studying a comet鈥檚 environment. Both groups have taken data from one of NASA鈥檚 solar wind-monitoring spacecraft 鈥� the Advanced Composition Explorer (ACE) 鈥� into account, but in different ways.
Lisse says the X-ray burst is almost all solar-wind driven while Willingale鈥檚 team says it was largely due to an increase in the comet鈥檚 water production rate.
Hard science
Deep Impact鈥檚 principal investigator Mike A鈥橦earn at the University of Maryland, US, says: 鈥淲hat this illustrates is that doing science is hard.鈥� And he emphasises the positive side of such varying interpretations. 鈥淭he fact that we have many separate measurements says that we can catch discrepancies that you wouldn鈥檛 catch in another case,鈥� he says.
Researchers would like to determine the relative abundances of cometary ingredients 鈥� such as water, carbon dioxide and silicates 鈥� within the material kicked up by the event to compare the collision products with those a comet naturally spews without an impact.
They hope to separate the changes that occur at a comet鈥檚 surface through repeated passes by the Sun from what is going on beneath the surface. The goal is to understand comet composition and processes well enough to determine what is deep within a comet鈥檚 nucleus, by observing its surface activity and composition from a distance.
Willingale presented the team鈥檚 results, which have been submitted to the Astrophysical Journal, at the UK鈥檚 National Astronomy Meeting in Leicester on Monday.
