Asteroids are remnants from the early solar system, serving as snapshots of its formation and evolution. While many reside in the asteroid belt between Mars and Jupiter, some venture closer to Earth. One such asteroid, (162173) Ryugu, is a near-Earth object approximately 1 kilometer wide. Originally thought to have formed in the asteroid belt, Ryugu now crosses Earth’s orbit, situated about 300 million kilometers away. Its journey and current position make it an ideal target for studying the processes that shape and alter asteroids over time.
Asteroids, like Ryugu, are subjected to constant bombardment by particles in space, ranging from macroscopic meteoroids to microscopic dust. Recent research, published in The Astrophysical Journal, highlights how even nanometer-sized particles can have significant effects on asteroid surfaces. These findings were made possible through Japan’s Aerospace Exploration Agency (JAXA) and its Hayabusa2 spacecraft, which conducted extensive studies of Ryugu in 2018 and 2019. The mission collected samples from the asteroid, allowing scientists to examine its composition and the processes that have altered it.
One key observation from laboratory analysis of Ryugu’s samples was a pattern of mineral dehydration. Phyllosilicates, such as magnesium-rich serpentine and saponite, showed signs of broken oxygen-hydrogen bonds. These bonds are essential components of the minerals and their hydration states. The study, led by Dr. Daigo Shoji of JAXA, found that even micrometeoroids as small as 2 nanometers can induce this damage.
The mechanism behind this phenomenon lies in the extreme velocities of these tiny particles. Accelerated by the magnetic fields of solar wind plasma, primarily composed of protons, these micrometeoroids can travel at speeds of approximately 400 kilometers per second. Upon impact, the immense kinetic energy of these particles causes molecular bonds to break in a fraction of a second, initiating chemical reactions that alter the asteroid’s surface.
To better understand these processes, researchers used computational molecular dynamics simulations. These simulations examined interactions between silica, magnesium, oxygen, and hydrogen atoms in minerals like serpentine. The reactions occurred on subnanosecond timescales—far too fast to observe directly. The simulations revealed that impacts at velocities of around 20 kilometers per second resulted in approximately 200 broken oxygen-hydrogen bonds. However, at higher velocities, near 300 kilometers per second, this number increased tenfold to about 2,000 bonds. Even at the microscopic scale, the impact craters formed by such high-speed collisions were significant, with diameters reaching 4.4 nanometers. For perspective, a human hair averages 90,000 nanometers in diameter.
Temperature variations on Ryugu’s surface were another factor explored in the study. The asteroid’s surface experiences a wide range of temperatures, from approximately 310 to 340 Kelvin (37–67 °C) during the day to as low as 200 Kelvin (-73 °C) when shielded from sunlight. Despite this, the study found that temperature fluctuations had minimal effect on mineral dehydration. Instead, the kinetic energy from high-velocity impacts was identified as the primary driver of chemical reactions. During these impacts, localized temperatures soared to over 1,000 Kelvin (~727 °C), well above the 600 °C threshold where serpentine becomes unstable, allowing bonds to break more readily.
Interestingly, while these impacts cause dehydration and atom ejection, there may be a compensatory process at play. The dissociated atoms can recombine to form water and silanol functional groups, consisting of silica, oxygen, and hydrogen. This recombination may partially offset the dehydration effects, hinting at a dynamic balance on asteroid surfaces subjected to microbombardment.
This research sheds light on the ongoing processes that shape and weather asteroids like Ryugu. The insights gained not only enhance our understanding of asteroid evolution but also have implications for planetary science and the study of early solar system materials. The findings underline the complexity of these seemingly inert objects, revealing a dynamic interplay of chemical and physical forces at nanoscopic scales that contribute to the broader story of our solar system’s history.