In 1972, Apollo 17 astronauts collected the rock sample troctolite 76535 from the Moon’s surface. The sample, since then, remains one of the most crucial samples of the Moon due to its pristine nature.
The moon rock that is believed to be originated early in the Moon’s history likely contains essential clues to understanding lunar formation.
Recently, scientists performed sophisticated analysis of this Apollo sample. They used a specialized electron microprobe to perform a high-resolution analysis of troctolite 76535. The study was performed by the University of Hawai’i (UH) at Manoa.
William Nelson, lead author of the study and Earth Sciences, a graduate student in the UH Manoa School of Ocean and Earth Science and Technology (SOEST), said, “Previous reports suggest the minerals in the Apollo sample were chemically homogeneous. Surprisingly, we found chemical variations within crystals of olivine and plagioclase. These heterogeneities allow us to constrain the earliest, high-temperature cooling histories of these minerals using numerical models.”
Using the UH High-Performance Computing facilities, Mana, the team considered the effects of a variety of computer-simulated cooling paths- well over 5 million chemical diffusion models.
Nelson said, “The simulations revealed that these heterogeneities could only survive a relatively short period of time at high temperature.”
The diffusion patterns preserved in the mineral grains and observed with the microprobe were consistent with a rapid cooling history of no more than 20-million-years at high temperatures. The finding challenges previous estimates of a 100-million-year cooling duration and supports the initial rapid cooling of magmas within the lunar crust.
Nelson said, “This is changing our outlook on how an important suite of lunar rocks formed.”
To accommodate high-temperature cooling rates with the generally acknowledged view on how these rocks formed, scientists proposed that perhaps this rock type is formed by a process called reactive infiltration wherein a melt interacts with rock- changing its chemical and physical makeup.
The study also demonstrates the value of re-examining previously analyzed samples using modern techniques and how quickly new data can reshape our understanding of planetary evolution.
- William S. Nelson, Julia E. Hammer, Thomas Shea, Eric Hellebrand, G. Jeffrey Taylor. Chemical heterogeneities reveal early rapid cooling of Apollo Troctolite 76535. Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-26841-4