In the realm of planetary science, understanding the formation and evolution of icy planetesimals is crucial, as these ancient space bodies are believed to have played a significant role in delivering volatiles to terrestrial planets and forming icy bodies in the outer Solar System. Researchers Jun Kimura, Ryusei Satoh, Kentaro Terada, and Sho Sasaki from the Institute of Space and Astronautical Science at the Japan Aerospace Exploration Agency (JAXA) have delved into this topic, publishing their findings in the journal Icarus.
The team focused on the thermal evolution of icy planetesimals, which has been a subject of debate due to the diverse data collected from various sources. Samples from the C-type asteroid Ryugu, brought back by the Hayabusa-2 spacecraft, suggest a low-temperature history with aqueous alteration and organic materials. In contrast, iron meteorites with similar isotopic ratios to carbonaceous chondrites indicate exposure to higher temperatures. To reconcile these differences, the researchers developed a comprehensive model that incorporates key evolutionary processes, such as radial growth, impact heating, water phase changes, aqueous alteration, and structural differentiation.
The model considers various factors, including the final radius of the planetesimals (ranging from 10 to 1000 km), the onset of growth (either 1.0 or 2.0 million years after the formation of calcium-aluminum-rich inclusions, or CAI), the duration of growth (0.4 or 4.0 million years), and the mode of growth (linear or runaway). The results of the study reveal that larger planetesimals generally reach higher temperatures, but the timing and mode of growth significantly influence their thermal evolution. Early accretion leads to higher temperatures, with some bodies reaching the Fe-FeS eutectic temperature of 1250 K. In contrast, delayed or prolonged growth reduces heating.
The researchers found that the materials constituting Ryugu, which remained below 40 degrees Celsius, likely formed near the surface of a hydrated mineral layer. This scenario is plausible even within planetesimals several hundred kilometers in size, due to efficient heat transport via convection. If accretion begins 2.0 million years after CAI and completes in 0.4 million years, a wide region within such a body could yield materials similar to those found in Ryugu.
The practical applications of this research for the energy sector are not immediately apparent, as the study primarily focuses on planetary science and the early solar system. However, understanding the thermal evolution of icy planetesimals can provide insights into the distribution and behavior of volatiles, such as water and organic materials, in the solar system. This knowledge could be relevant for future space exploration and resource utilization, including the potential extraction of water and other resources from asteroids or other small bodies. As the energy industry increasingly looks to space for resources and opportunities, research like this can help inform and guide those efforts.
Source: Kimura, J., Satoh, R., Terada, K., & Sasaki, S. (2021). Growth and thermal evolution of icy planetesimals. Icarus, 365, 114529.
This article is based on research available at arXiv.