Some asteroids are more ‘colorful’ than others. Here’s why.

Asteroids may be wondrous windows into the history of the cosmos, but the unfortunate reality is that they are also pretty drab. These rocky amalgamations of space debris generally look pretty much what you probably expect–craggy, ashen gray, and dusty. But they don’t always appear that way when viewed through a telescope from millions of miles away.

Take the asteroids Itokawa and Ryugu. Japan’s space agency (JAXA) confirmed that both are carbon-filled, rubble piles after receiving samples during the historic Hayabusa and Hayabusa2 missions. The same is true for Bennu, a similar asteroid sampled by NASA’s OSIRIS-REx in 2020. In theory, they should reflect light similarly, but they don’t. Ryugu looks somewhat red, while Bennu is slightly blue.

“The question has been why. Why are their spectra different if they have the same kind of minerals?” Purdue University asteroid researcher Michelle Thompson asked.

Thompson is one of the few people in the world tasked with studying the Bennu samples. Prior to getting their hands on the asteroid souvenir, Thompson and her team theorized one explanation may be that the two objects were experiencing space weathering in different ways. 

“Maybe we see different characteristics in one sample compared to the other because of this surface exposure, but what we’re actually seeing is that’s not the case. They are very, very similar in terms of the way that they experience space weathering,” she said.

Thompson and associates recently published three separate papers on Bennu based on years of research and analysis. Among their many findings is some clarity on the colorful mystery, that likely has a lot to do with age.

Rubble pile asteroids like Ryugu and Bennu have their own physiological cycles that slowly shift their surface debris. As it stands, Ryugu’s surface grains have endured space’s harsh conditions for only a few thousand years. However, Bennu,has a surface that’s experienced the same cosmic environment for tens of thousands of years.

“Instead of looking at two different trajectories for how this process is operating on these bodies, instead we’re seeing two different points in one cycle,” explained Thompson. “Their ‘colors’ are changing, meaning their spectral properties are changing relative to their surface exposure age.”

Beyond the color spectrum, the remnants retrieved from Bennu are also allowing Thomson and others to examine the organic molecule contents of an asteroid. Earlier this year, researchers announced their discovery of phosphates in the Bennu sample. These specific types of salts are central components to a living organism’s metabolism and DNA. Although still unconfirmed, many experts believe life began on Earth thanks to the arrival of an asteroid containing these and other evolutionary building blocks. In this sense, Thompson likens Bennu and other space rocks to interstellar time capsules.

“Asteroids are relics of the early solar system,” she said. “Looking at the organic molecules from Bennu, we are getting an understanding of what kinds of molecules could have seeded life on early Earth.”

 

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