The death of comet Ison

“We all hoped to make new discoveries thanks to new telescopes in space and on Earth”, explains the astronomer from Liege.  “Every year, we detect some thirty comets originating from the Oort cloud.  But giant comets are very rare, they only show up about once every 10 years at best. In a ten-year period, instruments are improved a lot which usually leads to important progresses”.

One of the most important objectives is to determine the chemical composition of these bodies. “At ULg, we study comets because they are like fossils of the solar system”, he continues. Millions of comets were formed in the region between Jupiter and Saturn 4.6 billion years ago, before being ejected to the outskirts of the solar system. Today, these comets form the “Oort cloud”, a gigantic reservoir of comet nuclei situated at the boundary of interstellar space. Some comets leave their distant orbit when disturbed by the gravitational instability caused by the passage of a nearby star, and plunged towards the Sun. Frozen since billions of years, these comet nuclei enclose dust, ice and molecules dating from the origin of the solar system.

While this ice is made up of 80% water, it also contains a multitude of much rarer elements which make up about 1%. While this may be infinitesimal in terms of quantity it is very important in the eyes of the researchers. “There are rare isotopes, for example, complex organic molecules which we cannot find anywhere, because the Earth and the other planets have undergone huge transformations for 4.6 billion years! Just like paleontologists, we are using comets as fossils to learn more about the first phases of the evolution of the solar system”.

This is why so much hope was placed in Ison. If it had indeed been one of the “great” comets, it would have outgassed in sufficient quantity. And even the trace elements would have been present in large enough quantities to be detected by telescopes and their instruments that are capable of isolating the signature of each molecule.  

A great comet after all ?

This now infamous comet is not without interest, however. “We will be able to explore new leads thanks to the enormous quantity of data gathered by numerous observatories on Earth and in space”, exclaims Emmanuël Jehin enthusiastically. “With the data gathered from TRAPPIST, we will be able to see what kind of matter is ejected during the outburst, see how that varies over the course of time and how the chemical composition evolves... In short, thanks to that, we will better understand comets in general. This is why I think Ison could maybe one day be considered as a great comet after all”.
    
Thanks to the images gathered by TRAPPIST during Ison’s orbit, the team of astronomers from ULg has precious data at their disposal, particularly with regard to the analysis of the small molecules present in the atmosphere of the comet such as the radicals OH, CN, C₂, NH or even C₃, known as “daughter molecules” themselves coming from “mother” molecules (these are bigger and are ejected directly from the nucleus) which break up under the action of the ultraviolet rays of the sun. This makes it possible to better understand the processes that occur in the coma of comets.