Planetary Scientists Find Unexpected Mineral In 496-Million-Ton Asteroid — And It Defies Ryugu's Origin Story

The latest findings from asteroid Ryugu have shocked researchers and forced them to rethink their previous assertions. The findings came from the mineral samples supplied by Japan’s Hayabusa 2 spacecraft in 2020, according to Popular Science. Researchers detected a mineral called djerfisherite from one of the samples, which shouldn't have existed in this asteroid, per their calculations, and dated back to around 1.8–2.9 million years after the birth of our solar system. Masaaki Miyahara, a science and engineering associate professor at Hiroshima University, claims that the discovery challenges the notion that Ryuku is compositionally uniform and brings forth speculation about the kind of interactions that occurred in the early days of the solar system. The examination and its results have been published in the journal Meteoritics & Planetary Science. 

What is Ryugu?

Scientists estimate asteroid Ryugu to be half a mile wide and 496 million tons. Its parent body was a fairly early member of the solar system, created just one to three million years after the formation of the planetary system. Hence, the asteroid is a window through which scientists can glance into the early days of the solar system. Moreover, Ryuku belongs to either the Eulalia or Polana asteroid family, created by the combination of carbon dioxide and water in the outer edges of the solar system. Experts speculate that the decay of radioactive elements inside the asteroid's parent body reached temperatures of 122 degrees Fahrenheit (50 degrees Celsius). In this state, a supposed collision with another asteroid possibly led to the creation of carbon-rich Ryuku.

Surprising discovery in the samples

The speculative formation of Ryugu led experts to believe that the asteroid was composed of rocks that are similar to CI chondrite meteorites, according to Science Daily. These meteorites contain a high level of carbon and have undergone aqueous alterations in the past. In the Ryugu's grain examined in this study, researchers surprisingly detected an element typically found in enstatite chondrites, another kind of meteorite. 

Researchers conducted field-emission transmission electron microscopy (FE-TEM) on several mineral samples from Ryugu to understand the effect of terrestrial weathering on them. They discovered the unexpected mineral in the number 15 grain of the sample plate C0105-042. The mineral in question is djerfisherite, essentially a potassium-containing iron-nickel sulfide. It is the first time the mineral has been found in any Ryugu grain. The discovery of this mineral was surprising, as djerfisherite cannot form under the supposed circumstances to which Ryuku is believed to have been exposed throughout its existence. Experts thought that Ryuku was formed on the outer edges of the solar system. However, as per past examinations, enstatite chondrites that contain djerfisherite are created in the inner region of the solar system. 

"The discovery of djerfisherite in a Ryugu grain suggests that materials with very different formation histories may have mixed early in the solar system's evolution, or that Ryugu experienced localized, chemically heterogeneous conditions not previously recognized," Miyahara claimed. Miyahara claims that these minerals formed in "reduced" environments, which was not the case for Ryugu, as per previous assertions. It implies that the asteroid either faced a local environment that experts are still not privy to or there was some long-distance voyage that took place in the early days of the solar system. "Its occurrence is like finding a tropical seed in Arctic ice," Miyahara added. 

Possibilities of this discovery

Thermodynamic calculations suggested to researchers that djerfisherite in enstatite chondrites could have been created out of high-temperature gas. Also, they considered outcomes from hydrothermal synthesis experiments, which showcased that these djerfisherites could also be formed due to reactions between Fe-Ni sulfides and potassium-bearing fluids in temperatures above 662°F. It led researchers to two theories: either the specific grain arrived at Ryugu's parent body from another source in a different environment, or it formed inside the parent body itself when its temperature rose to 662°F. At present, the evidence collected by experts from the grain indicates that it was formed inside the parent body. Researchers are now looking forward to conducting isotopic studies of this and other grains to learn more about Ryugu's origins. 

For more information, you can read the study, "Djerfisherite in a Ryugu grain: A clue to localized heterogeneous conditions or material mixing in the early solar system," penned by Masaaki Miyahara, Takaaki Noguchi, Akira Yamaguchi, Toru Nakahashi, Yuto Takaki, Toru Matsumoto, Naotaka Tomioka, Akira Miyake, Yohei Igami, and Yusuke Seto in Wiley.