Academy Professor Karri Muinonen investigates planetary system with help of scattered light

The way in which sunlight scatters from asteroids, comets, moons or planets tells a story of the composition, structure and topography of each individual body. Key to this is ever-present cosmic dust, tells recently appointed Academy Professor Karri Muinonen.

The Academy of Finland has appointed Professor of Astronomy Karri Muinonen from the University of Helsinki as an Academy Professor. Muinonen also works as a research professor at the National Land Survey of Finland. The new Academy Professors begin their five-year term in September 2021.

Funding provided by the Academy frees up time for research, which in Muinonen’s case is focused on the composition of bodies in the solar system. His research is founded on the interaction between all types of electromagnetic radiation – including radio waves, visible light and X-rays – and matter: the scattering, emission and absorption of radiation. An example of scattering is light reflecting off snow.

Messengers from the early days of, and outside, the solar system

Dark asteroids and comet cores rich in carbon and ice are remnants of the early days of our solar system. Weather phenomena, the climate and life in general are shaping Earth, but the small bodies of the solar system remain largely in their original condition. They constitute a treasure trove for those who wish to know what the system looked like billions of years ago.

To be able to ward off asteroids, it is good to know what they are made of. Asteroid observations, such as their spectrum of light, tell us about the minerals contained in them. To avoid collisions, the orbit and composition of the body in question must be known.

Muinonen describes with enthusiasm the asteroid ʻOumuamua and the comet Borisov, the first small bodies travelling through our solar system that have been identified. Their matter may be originally from another planetary system, which makes it particularly interesting.

Images illustrate their subjects

Currently available telescopes provide an enormous amount of observational data from the sky. The atmosphere blocks X-rays in particular – for the benefit of living organisms – which makes it necessary to send X-ray cameras into space.

The Gaia space observatory of the European Space Agency (ESA) is currently gathering accurate data on, among other things, asteroids in the ultraviolet, visible and near infrared spectra. BepiColombo, a probe that utilises X-ray and hyperspectral cameras launched collaboratively by ESA and the Japan Space Exploration Agency (JAXA), is on its way to Mercury. Once it settles into orbit around the planet in 2026, one of the key questions it will try to answer is the location of iron in the planet. No iron has been identified on the surface of the extremely dense body.

How can so many properties of an object be inferred on the basis of images? This requires complex techniques aimed at solving inverse problems, which utilise the best scattering, absorption and emission models available.

Further advances in modelling will enable an even more accurate overall picture of the birth, present status and future of our solar system.

Astronomical chess problems

Muinonen, who will soon turn 60, was already fascinated by academic research as a young man. In 1990 he defended his doctoral thesis in theoretical physics, and was already at that time investigating the scattering of light at the University of Helsinki observatory. Today, Muinonen heads a group focused on our planetary system at the Department of Physics, University of Helsinki.

He was interested in the unsolved scattering problems associated with bodies in the planetary system, and they were chosen as a topic of research purely out of academic curiosity and personal ambition, not so much in search of any benefit or employment.

Muinonen hopes that young people today will also have the chance to throw themselves into long-term research. What was compelling for him was the consideration of unanswered questions related to astronomical observations, and his career path as an astronomer has wound through several international posts.

In his free time, Muinonen likes to exercise both for fun and for his own benefit. Right now, there’s plenty of snow to shovel around his house. When swimming and badminton had to be put on hold due to the coronavirus pandemic, Muinonen got into Nordic walking.

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