Comet construction
There is more to a comet than just its spectacular tail
Comets are thought to be among the oldest objects that exist in our Solar System. Many spend the vast majority of their lives slowly drifting through the darkness towards the edges of our planetary neighbourhood. While
▲ An image of Comet ATLAS taken in March 2020 before it broke up. Scientists studied the light from the comet both before and after it fragmented
fragmentation of the comet, which will in turn tell us how the nucleus was held together.”
Not only can studying a comet’s disintegration provide specific clues to its construction, it also has the potential to reveal details about processes that were going on in the Solar System long ago. “One of the most interesting questions to ask is if the fragments have compositional differences,” says Laura Woodney. This can give insights into the distribution of different materials within the nucleus – something that can illuminate the broader formation history of comets and icy planetesimals.
Clues about our Solar System
For Dr Ekaterina Chornaya, and her colleagues at Russia’s Far Eastern Federal University and elsewhere, the demise of ATLAS provided a glimpse at the interior constituents of a comet’s nucleus – one that would also shed light on its origins. “Comets or, speaking more precisely, their nuclei are widely thought to be fossils from the time of our Solar System’s formation,” says Chornaya. “They are remnants of planetesimals which were thrown out of the inner part of the Solar System during or shortly after its formation.”
Some comets might have been flung into the frozen realms at the Solar System’s edge early on in that period, before the Sun even switched on, says Chornaya. There they would have been kept in a largely pristine state. Others may have lingered for a while in our star’s warmth, prior to being ejected outwards.
“In the latter case, [a comet’s] surface may experience a significant weathering due to frequent heating and micro-meteoroid bombardments,” she explains.
Piecing together the details of such ancient history from their apparent composition is one of the challenges planetary scientists face with comets. “When observing a comet upon its return to the inner Solar System, in general, we see not only its pristine materials, but also significantly altered materials,” says Chornaya. “The point is that we do not know the fractions of each type of materials in a given comet.”
As ATLAS broke apart it opened up an extraordinary window onto the pristine contents of its heart, however. Utilising a half-metre aperture telescope sited at the Ussuriysk Astrophysical Observatory, Chornaya and her fellow researchers
studied the light from ATLAS both before and after it fragmented. “We were lucky to catch the comet prior to its disintegration,” says Chornaya. “This makes it possible to draw a whole picture and it is a quite unique chance.”
Ancient treasures
The team’s observations showed the relative amounts of certain substances changing as the comet crumbled – something that could hint at what the make-up of unaltered cometary material is. The researchers also detected a conspicuous rise in the amount of carbonaceous dust particles swirling around the disintegrating comet’s coma; these microscopic particles are relics from the birth of the Solar System and a sign of the truly ancient nature of the comet’s interior. That’s because, today, those tiny flecks would be blown far from our planetary neighbourhood by the light – technically the ‘radiation pressure’ – from the Sun. The fact the signs of them appeared in the light from ATLAS indicates they were incorporated within the comet’s nucleus long, long ago – prior to the point when the Sun began shining as it does now.
At the time of writing, no one knows what’s become of the debris of Comet ATLAS; its orbit has temporarily taken it behind the blazing light of the Sun where it can’t be observed. “If there are any larger bits that survive perihelion, they will continue their journey into deep space and return in another 6,000 years,” says Ye Quanzhi.
Yet even if nothing remains, ATLAS has told us so much already. Perhaps the next time a comet fizzles before our eyes, those initial pangs of disappointment may turn to excitement at what we might learn from the disintegration of these amazing objects.