Some clues to Ryugu’s odd colours

When Japan’s Hayabusa2 probe landed on Ryugu in February last year, it provided the opportunity not only to collect a few rocks from the asteroid but also get a close-up view of its surface. 

Credit: JAXA/U. Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Inst. Tech./Meiji U./U. Aizu/AIST

And in doing so, it may have shed light on one of its curiosities – why it appears as different colours in different regions.

A carbonaceous asteroid that orbits mainly between Earth and Mars, Ryugu intrigues researchers, and Hayabusa2 was launched in 2014 with an ambitious plan to bring back some samples.

For around 18 months it studied the asteroid from a distance, capturing stunning images which highlighted one mysterious quirk – Ryugu is blue in the equatorial and polar regions, and red in between. 

Now, high-resolution images and video taken during the probe’s first touchdown (there has since been another) suggest that these variations are likely due to rapid solar heating during a previous orbital excursion near the Sun.

The results are published in the journal Science.

Studying the images and analysing their spectral properties, Tomokatsu Morota from the University of Tokyo and international collaborators observed that the spacecraft’s thrusters disturbed a thin layer of dark grains of material on the surface of the asteroid that corresponded with the red material. 

Interestingly, the chosen landing site initially appeared bluer, and nearby boulders were mostly blue. After the settling of the dust cloud, the area surrounding the lander became redder in appearance.

When the team examined images from when the sampler itself fired into the surface of the asteroid, they also noticed it produced a cloud of dark, fine grain particles.

“These observations suggest that dark, fine grains were originally present on the surfaces (or inside the pores) of darker and redder boulders,” they write. 

During initial surveys and its descent, Hayabusa2 also captured images of the wider asteroid surface, which revealed that recently formed craters were spectrally bluer than their surrounding areas. 

Together the authors suggest these findings are compatible with a theory that the darker red material is not compositionally different to the brighter blue material of Ryugu. Instead, Ryugu’s boulders and surface were originally bluer in colour, but were darkened by solar heating.

Furthermore, they write that the surface reddening has not been occurring throughout Ryugu’s life. Older craters whose spectral characteristics matched the surrounding area and newer craters which were brighter than the surrounding area allowed them to create a timeline of Ryugu’s reddening.

Based on the relative size and distribution of craters compared to the expected frequency of the asteroid being struck by objects, the researchers estimate that the reddening event must have occurred around 8.5 to nine million years after Ryugu formed. 

With there being only 3% as many blue craters compared to red, they estimate that it’s been another 300,000 years to eight million years since that reddening event occurred.

“We suggest that a surface reddening event within a short period of time could be explained if Ryugu underwent a temporary orbital excursion near the Sun, causing higher surface heating,” they write. 

At some point, after its orbit shifted from the main asteroid belt to a near-Earth orbit, Ryugu made a close orbit of the Sun during which the intense heat discoloured and weathered its surface layers, turning it red.

Over time, some of this surface regolith layer moved away from the poles and equatorial region through mass wasting, impacts and thermal fatigue, creating the colour pattern seen today. The red regolith also mixed with the blue, unaltered, layers underneath, creating a mixed layer metres thick. During this time, impacts also blasted away some of the surface layer, leaving young bright craters. 

Hayabusa is currently travelling back to Earth with its rocky cargo and will touch down in Australia in December this year. 

Animation created from ONC-W1 at the bottom and ONC-W2 on the side of the spacecraft. Credit: JAXA/U. Tokyo/Kochi U./Rikkyo U./Nagoya U./Chiba Inst. Tech./Meiji U./U. Aizu/AIST

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