ON 4 JULY, the world鈥檚 TV screens were filled with high-fiving NASA astronomers celebrating the Deep Impact mission鈥檚 direct hit on comet Tempel 1. It was an extraordinary achievement, and fully merited the celebrations. A few weeks later, though, when the cameras had gone, the astronomers were left scratching their heads in confusion.
The Deep Impact team had hoped that, when the impactor spacecraft hit Tempel 1, it would kick up a relatively small cloud of dust, expose an area of pristine icy material underneath, and instigate some spectacular jet activity. This is exactly what didn鈥檛 happen. The dust cloud was more than 10 times bigger than expected, and the effect on Tempel 1鈥檚 activity was almost nil.
We have now had four close encounters with comets, and every one of them has thrown astronomers onto their back foot. This week, at the American Astronomical Society鈥檚 Division for Planetary Sciences meeting in Cambridge, UK, the Deep Impact team will report that comets are defying all attempts to understand them. 鈥淲e really need to think differently,鈥� says Peter Schultz of Brown University in Providence, Rhode Island, a member of the Deep Impact team. 鈥淭hey are like no other bodies in the solar system.鈥�
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Comets have a special place in the hearts of astronomers. These balls of ice, rock and dust originated in the frozen wastes of the outer solar system, but were nudged by the gravitational fields of the giant planets 鈥� and even passing stars 鈥� into the inner solar system. Comets are thought to be related to the icy building blocks that formed the giant planets Jupiter, Saturn, Uranus and Neptune. Many of the moons of these worlds, not to mention the planet Pluto itself, can be thought of as super-sized comets because they, too, are composed mainly of ice and rock.
But unlike planets, comets are far from stable. Each time one passes close to the sun, the heat makes material such as water and carbon dioxide evaporate away into space, creating a tail of dust and gas that stretches behind it for millions of kilometres. Their surfaces also display intermittent 鈥渁ctivity鈥�, shooting out jets of dust and gases. 鈥淭he best way to think of them is that they are in a constant state of disintegration,鈥� says Schultz.
But the details of that disintegration are proving ever more perplexing. Prior to the European Space Agency鈥檚 Giotto mission to study Halley鈥檚 comet in 1985, for example, astronomers believed that as sunlight fell onto a comet, its spin would mean that the heat evaporates a more or less even layer, revealing more icy material beneath. Giotto showed that this idea was hopelessly simplistic. 鈥淎s soon as we saw the nucleus it was clear that activity was confined to individual jets and not coming from the whole surface,鈥� says Giotto project scientist Gerhard Schwehm of the European Space Agency. In fact, only 15 per cent of Halley鈥檚 total surface area was expelling material at the time of the fly-by. The observation has shown astronomers that they are in the dark about even the basics. 鈥淲e still do not know what drives comet activity,鈥� says Schwehm.
Donald Brownlee of the University of Washington in Seattle goes further. 鈥淚t鈥檚 a mystery to me how comets work at all,鈥� he says. Brownlee has good reason to make this claim. He is the principal investigator on NASA鈥檚 Stardust mission, which flew past comet Wild 2 on 2 January 2004. The fly-by images showed 20 active jets spread across the comet鈥檚 sunlit side. So far, so good. Then they saw something that added a new twist to the mystery. Two of the jets were on the night side of the comet.
Astronomers had expected that the jets would simply turn off when the comet turned them away from the warming rays of the sun. For Brownlee it seems to be pointing to an inescapable conclusion. 鈥淚 think that some process is allowing heat to get down below the surface of a comet and drive the activity from the inside out,鈥� he says.
The clue might be in the dark surface layers of the comets. Though it is hardly what you would expect of icy bodies, the exteriors of both Halley and Wild 2 are as black as coal, and these dark layers absorb heat. At the time of the Stardust encounter, when the comet was almost twice as far away from the sun as the Earth, the surface of Wild 2 was a comfortable 18 掳C. Its interior would have been much colder, well below 0 掳C in fact, so heat would naturally flow inwards. That鈥檚 as far as the explanation goes at present. 鈥淚 have no idea about the details of the process,鈥� Brownlee admits.
Enter Deep Impact. The NASA scientists hoped their impactor would not only eject material for them to analyse but also kick-start a new area of research by exposing an area of pristine, icy material inside the comet. And maybe that would provide a few clues to what drives comet activity. Unfortunately, things didn鈥檛 quite go according to plan. The Deep Impact team thought their 370-kilogram impactor would liberate about a month鈥檚 worth of dust, based on normal emission rates, but it now seems more likely that a whole year鈥檚 worth escaped the comet. 鈥淚f I had to choose just one surprising result from this encounter, it would be the amount of material thrown up,鈥� says Schultz.
Deep mystery
The ease with which the dust lifted into space suggests that the comet has a remarkably fragile surface, says Michael A鈥橦earn of the University of Maryland at College Park, Deep Impact鈥檚 principal investigator. 鈥淭he surface material can have no more strength than lightly packed snow, otherwise we would not have seen that amount of dust.鈥�
And there was another surprise in store for the team. As the impactor hurtled towards Tempel 1鈥檚 nucleus at over 10 kilometres per second, it returned pictures of two craters, each a kilometre across. Though they seem to be ubiquitous on every other solid surface in the solar system, craters have never before been seen on a comet. When Giotto flew by Halley鈥檚 comet in 1986 and returned the first ever pictures of a comet鈥檚 icy nucleus, no craters were revealed. Twenty-five years later, NASA鈥檚 Deep Space One flew past comet Borrelly and revealed another surface devoid of craters. Wild 2 did have large numbers of circular depressions on its surface, but their unusual shape suggested to astronomers that these were not created in collisions. 鈥淲e had given up the hope of seeing craters on comets,鈥� says A鈥橦earn.
So where did the holes in Tempel 1 come from? Well, as with Wild 2, they might not be impact craters at all. The depressions have flat floors and their walls appear like giant staircases, and this suggests that they were caused by an explosion within the comet, rather than a hit from outside, according to Laurence Soderblom of the US Geological Survey in Flagstaff, Arizona.
Brownlee believes the porous structure of the comet might allow light to penetrate beneath the surface and heat the interior. The dark layers stop heat escaping, and pressure builds up, eventually resulting in an explosion 鈥� and an unusually shaped crater. It鈥檚 a pretty vague explanation, but the Deep Impact astronomers are looking for some evidence to back it up. A鈥橦earn reckons the numerous jets that they saw as the spacecraft approached Tempel 1 might hold some clues, though it is proving difficult to trace them because no one knows what the features that release jets look like, or how big they are. They could be nothing more than fissures, too small to be picked out by Deep Impact鈥檚 cameras.
So, for the moment, the team is short on clues as to what makes a comet tick. Their detective work has been made even more difficult by the fact that Tempel 1 seems unperturbed by the impact. A week of follow-up observations using the European Southern Observatory鈥檚 Very Large Telescope in Chile revealed that after the initial outburst the comet鈥檚 activity levels remained very much as they were before the encounter. The new jet they had hoped to trigger simply did not materialise. A鈥橦earn believes the amount of dust ejected and the lack of follow-on activity indicate the crater might be wide but not deep, and that the impact merely blasted off the desiccated surface layers without making any serious impression on the icy material buried beneath.
鈥淲ith so much left unknown about the nature of comets, that nine-year wait for Rosetta is going to feel like an eternity鈥�
Unfortunately, the amount of dust released, combined with a focusing fault on Deep Impact鈥檚 high-resolution camera means that the images the team hoped to take of the newly formed crater may now elude them. There may be no way to confirm what happened.
If this is the case, the team will fail in the first two of its stated mission objectives: to observe how the crater forms, and to measure its depth and diameter. They partially succeeded in the third, which is to analyse the composition of the interior of the crater and its ejecta: they鈥檝e analysed the ejecta, but can鈥檛 `the crater. Objective four, which is to determine the changes in the quantity of material ejected by the impact, has been met, even if the answer seems to be a big fat zero.
It鈥檚 disappointing, but it鈥檚 not all bad news. The big cloud the impact kicked up promises a potential science first: a hint of the comet鈥檚 internal structure. 鈥淏y watching the movement of the ejecta cloud with the fly-by spacecraft, we think we can determine the distribution of mass inside the comet,鈥� says A鈥橦earn. Such information will show whether it is a solid object or a conglomeration of pieces, and reveal whether the rock and ice are uniformly mixed throughout the comet, or separated into distinct regions. The Deep Impact researchers are continuing to sift through the images and spectroscopic data transmitted from the spacecraft to piece together all the information they can about Tempel 1.
We may learn a little more about comets next January, when the Stardust mission brings dust from Wild 2 to Earth, but many astronomers are now pinning their hopes on the European Space Agency鈥檚 Rosetta mission to comet Churyumov-Gerasimenko. 鈥淩osetta will be the key to understanding comet activity because it will not be just another snapshot of a comet, it will watch it continuously,鈥� says Brownlee. Upon arrival in 2014, Rosetta will enter orbit around the 2-kilometre-wide nucleus and monitor the comet for two years, during which time it will make its closest approach to the sun and begin to head back out again. Once Rosetta has mapped the comet, a small lander called Philae will descend to the surface. Equipped with harpoons to anchor itself to the comet鈥檚 surface, Philae will examine the composition and structure of the surface in fine detail.
With so much left unknown about the nature of comets, that nine-year wait for Rosetta is going to feel like an eternity to the astronomers meeting in Cambridge this week. And it鈥檚 possible, of course, that Churyumov-Gerasimenko will throw up another set of surprises. When it comes to comets, there鈥檚 only one clear message: expect the unexpected.
Comet conundrums
1: Why do they disintegrate?
IF heat from the sun can become trapped inside a comet, driving later activity, it may also explain one of the most puzzling cometary observations: why some of them simply fall to pieces when they are nowhere near the sun.
About 50 comets are known to have split up in this way. The latest was comet 2005k2 LINEAR, which split into two in June over 100 million kilometres from the sun. Others have broken up much further away. Astronomers think that trapped heat melts the comet from the inside out, increasing the pressure under the frozen surface until finally the comet explodes.
Tempel 1 could be next, if one tentative observation is confirmed. 鈥淲e see a feature running across the nucleus that almost looks like a fault line. But how can that exist? Perhaps Tempel 1 was almost shattered sometime in its past and large blocks are just resting together,鈥� says Michael A鈥橦earn of the University of Maryland at College Park. 鈥淭hat鈥檚 off the top of my head speculation,鈥� he adds.
If Tempel 1 is really a jumble of blocks of ice and rock resting lightly on top of one another, it would not take much to force them apart. But the major puzzle is still how heat can be channelled and trapped inside a comet in the first place.
2: What are they made of?
IF our understanding of asteroids is anything to go by, the solid material in comets could be carbonaceous, silicaceous or metallic. But, as yet, we simply don鈥檛 know enough about comets to generalise about what they are made of. And that鈥檚 a shame because it might tell us more about their history. Donald Brownlee of the University of Washington in Seattle imagines a scenario in which large objects, perhaps as big or bigger than Pluto, formed deep in the outer solar system during the general planet-forming process. Such bodies would generate enough internal heat, by natural radioactivity, for the denser material to sink to the centre, leaving the lighter material to rise to the top. Collisions between these objects could shatter them, creating a shower of comets, all with different compositions depending on where they originated.
If Brownlee is correct it means that astronomers might need to rethink their ideas about comets. Instead of thinking of them as the raw material for new planets, perhaps comets are better described as the debris from failed ones.
3: Where are the impact craters?
SEISMIC tremors caused by small impacts could disturb the surface material on a comet enough to 鈥渇luff it up鈥�, burying or even destroying any craters or other features and creating the smooth plains, suggests Laurence Soderblom of the US Geological Survey, who was a member of the Deep Space One team. 鈥淭he gravity is so low on a comet that it wouldn鈥檛 take much to move the surface material around,鈥� he says.
But if that鈥檚 the case, why do two craters survive on Tempel 1? 鈥淭hat鈥檚 part of the mystery that we have to solve. Perhaps they are not old but young craters,鈥� says Soderblom.
