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'Sunburn' discovered on asteroid Ryugu hints at past orbit close to sun

This diagram provided by a research team led by University of Tokyo associate professor Tomokatsu Morota shows the spectrum of colors on the surface of the asteroid Ryugu, which can be observed due to differences in the way sunlight reflects off each part. The reddish areas are believed to have been exposed to extreme solar heat. "TD" marks the area where space probe Hayabusa2 made its first landing. The color of the surface cannot be distinguished and appears all dark to the human eye.

TOKYO -- Researchers in Japan have discovered dark red "sunburned" areas on the surface of the asteroid Ryugu, suggesting it previously orbited much closer to the sun.

The discovery, made by a team of researchers from the Japan Aerospace Exploration Agency (JAXA) and the University of Tokyo, was published May 8 in the U.S. journal Science.

JAXA's space probe Hayabusa2, which is believed to have successfully collected material samples from Ryugu, observed sunlight reflecting off the surface of the asteroid from above, and found areas that had turned a blackish red color, widely dispersed across the surface. The surface of craters that were relatively new, however, did not show such coloring. This and other factors led the team to conclude it is highly likely the asteroid was exposed to extreme solar heat for a short period in the past.

Tomokatsu Morota, an associate professor at the University of Tokyo and a member of the research team, said the traces of charring on the surface of Ryugu suggest that the asteroid was exposed to a high temperature of around 600 to 800 degrees Celsius. Although the path of Ryugu's orbit currently lies between Earth and Mars, the team estimates that the asteroid orbited between the sun and Mercury at a certain point between 300,000 and 8 million years ago. The trajectory of an asteroid's orbit can be changed due to gravitational forces from massive planets like Jupiter.

When Hayabusa2 collected rock samples from the surface of Ryugu in February 2019, it landed in a part that contained both the material altered by the sun and the original substance, and it is likely that the probe was able to collect both. Researchers look forward to uncovering the effects of solar heat on the asteroid by closely examining the samples when they are brought back to Earth.

Seiji Sugita, a planetary science professor at the University of Tokyo who is also a member of the team, commented, "Although it has been known that the orbits of asteroids can largely change, this is the first time that material evidence of this has been obtained, and we are astonished."

(Japanese original by Tomohiro Ikeda, Science & Environment News Department)

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