The interstellar asteroid ‘Oumuamua has a surface layer that could hide ice

The interstellar asteroid 'Oumuamua seen by the William Herschel Telescope at La Palma (Image courtesy A. Fitzsimmons, QUB/Isaac Newton Group, La Palma)
The interstellar asteroid ‘Oumuamua seen by the William Herschel Telescope at La Palma (Image courtesy A. Fitzsimmons, QUB/Isaac Newton Group, La Palma)

An article published in the journal “Nature Astronomy” describes the results of a spectroscopic analysis and thermal modeling of the interstellar asteroid 1I/2017 U1 ‘Oumuamua. A team of astronomers led by Professor Alan Fitzsimmons and Dr. Michele Bannister from the School of Mathematics and Physics at Queen’s University Belfast concluded that, due to its exposure to cosmic rays, a superficial layering of organic and insulating materials occurred on the asteroid and it might contain ice.

The discovery of the asteroid subsequently formally classified as 1I/2017 U1 and called ‘Oumuamua attracted considerable interest among astronomers because it seems to come from another solar system. This sparked the public’s imagination, particularly after the announcement by the Breakthrough Listen initiative of the attempt to verify if it’s a spaceship. So far no radio signal from ‘Oumuamua has been detected and everything suggests that it’s a natural object with a strange shape.

Many astronomers tried to examine ‘Oumuamua with the instruments available to them to try to steal some secret from an object that may have formed several light years away. The various observations showed no cometary activity, with emissions of gas originating from sublimated ice while passing in the inner solar system but is that enough to identify ‘Oumuamua as an asteroid composed of rock only?

This research took advantage of the data collected in the weeks after the discovery of ‘Oumuamua performing detections with various telescopes. Spectrographic analysis carried out with instruments like X-shooter, installed on ESO’s Very Large Telescope (VLT), allowed to try to understand the composition of the asteroid to understand if it contains ice and obtain clues on its composition in general.

The result is that ‘Oumuamua reflects sunlight in a way similar to icy objects covered by a dry crust, which in this asteroid’s case has a thickness estimated in about half a meter (about 19″). The existence of a surface of that type is typical of objects exposed to the action of cosmic rays for very long times and this asteroid probably traveled in interstellar space for a few billion years.

Energetic cosmic rays can cause chemical reactions on the surface of an object of this kind, with the consequent creation of a layer rich in organic and insulating materials. Various objects that formed in the outer solar system also show characteristics of that kind. Such an object, even if it was rich in ice beneath the surface, couldn’t behave like a comet when it approaches the Sun because the insulating layer would prevent the ice from being exposed and sublimate.

Dr. Michele Bannister explained that according to her and her colleagues ‘Oumuamua is a planetesimal, meaning an object of the type that under the right conditions can grow over time, with a greyish/red surface and very elongated. The similarities with certain asteroids that formed in the solar system suggests that the formation mechanisms of asteroids and perhaps even planets are similar in different solar systems.

The interstellar asteroid ‘Oumuamua is very interesting but continues to move away from the Earth in its course that will take it out of the solar system. For this reason, studying it is becoming increasingly difficult and researchers must hurry to examin it with as many instruments as possible to try and discover something new about it.

Artist's impression of interstellar asteroid 'Oumuamua (Image ESO/M. Kornmesser)
Artist’s impression of interstellar asteroid ‘Oumuamua (Image ESO/M. Kornmesser)

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