New models describe the behavior of the cometary dust trails and their shape
When the comet 17P/Holmes orbiting the Sun between Mars and Jupiter burst out, it spread a vast number of particles on its trail. Academic project leader Maria Gritsevich is involved in multinational research that found out the comet´s trail resembles a gigantic hourglass.

On May, researchers from an international research group from Finland, Canada, and Russia, publicized findings about the dust trail of the comet 17P/Holmes, which orbits the Sun between Mars and Jupiter. The group was led by Maria Gritsevich, adjunct professor at the University of Helsinki and senior researcher at the Finnish Geospatial Research Institute.

– The comet burst out in October 2007, which was the largest known outburst by a comet thus far. A vast number of particles was ejected from the comet which we have been mapping out with the models, says Gritsevich.

– The event was unusual, as the comet became the largest object in the solar system for a while.

The comet orbits the sun in about 7 years. Every year when Earth is crossing the cometary plane is when the geometry of observing the particles released in the phenomenon is most favorable. For the research, astronomers observed the comet using telescopes in Australia, Finland, and the USA in two periods, from 2013 to 2015, and from 2020 to 2021. They have now followed up with the additional observations, confirming the predictions, in 2022. The newest findings were recently presented at the EPSC.

– The models counted that the optimal time for observing this phenomenon on Earth was in the end of February and in the late summer of 2022, says Gritsevich.

Trail of the dust left by the comet’s eruption allows researchers to study the composition of the comet

Thanks to the research, the behavior of comet 17P/Holmes' dust trail and its movements are now predictable. Outbursts on the comet, however, are a different story.

– Other comets in similar orbits around the Sun do not produce this kind of large periodic outbursts, so the 17P/Holmes itself is probably special, says Gritsevich.

– We don´t yet know what actually triggers the outbursts inside the comet. It´s a complicated phenomenon which can´t be accurately anticipated, says Markku Nissinen, member of the Finnish Fireball Network at the Ursa Astronomical Association.

– It´s unusual also the comet´s core has not been destroyed by the outburst.

The outburst leaves a trail of particles behind for hundreds of years.

– The trail is not dense enough to be followed without predicting its position using the modelling, says Gritsevich.

Comet´s outburst left behind a hourglass shape

The research models revealed that the shape of the dust trail is formed by dust particles of various sizes. Due to gravity and solar wind, particles following the comet also have different sized elliptical orbits around the Sun.

The smallest particles have the largest orbits, so they arrive at the nodes last, while bigger particles arrive at the nodes earlier. Two nodes of orbits of particles convergence in the middle, which makes the comet´s trail hourglass-shaped, even when viewed from any angle.

– I found this is a fresh aspect, since I know of no other papers before that brought up this peculiarity of shape of dust trails, says Gritsevich.

The same research models that map out the dust trail can be used to calculate other exciting phenomenon, such as meteor showers.

– I have been modeling well-known meteor streams, such as Leonids and Ursids, which gave the groundwork for the models developed in this study, says Nissinen.

– One thing that this teaches us is that things that seem random in the universe, do have an order behind them – which science can predict, says Gritsevich.

The research was publicized in the astrophysics journal Monthly Notices of the Royal Astronomical Society, and was supported by the Academy of Finland. The models and predictions have been published as open access papers.

Info on Mercury upcoming

Another part of Gritsevich´s Academy project includes studies on the surface of the planet Mercury.

– There are a lot less big rock boulders on Mercury, comparing it to the similarly sized Moon, which is an interesting feature we discovered, says Gritsevich.

The presence of boulders and their geological associations provide information about surface properties and modification processes affecting it. The rarity of boulders on Mercury is due to disproportionally higher micrometeorite flux and a thicker regolith. More intensive thermal cracking and thermal fatigue may also play some role providing shorter boulders’ lifetime.

A follow up study on the Mercury surface properties is about to be released later this autumn, but the major news on the observations of the surface of Mercury will be made public once the BepiColombo shuttle will reach it’s destination after seven years travelling, in 2025.

– Together with Dr. Mikhail Kreslavsky, a collaborator in the project from the University of California, Santa Cruz, we have examined all available high-resolution images of Mercury so it´s nice to be able to obtain more of them, says Gritsevich.




Comet tails

A comet flying on its trajectory is constantly changing, meaning that the material it consists of is slowly disintegrating and forms a tail behind it. The tail consists of stone dust and gases.

- Most of the gas is steam from the ice on the comet. The ice turns into steam when the comet flies close to the sun, and the sun's radiation further breaks down the molecules of water into hydrogen and oxygen, says Antti Penttilä, Post-Doctoral Researcher in space science.

The gravitation of the sun affects comets in other ways, as well, such as causing disturbances in their trajectories, and possibly landslides on the surface of the comet. However, the comet 17P/Holmes studied by Gritsevich is on a trajectory far from the sun, so melting caused by radiation is not likely to be the main cause for disintegration.