Cold to Hot

Ethan Wong

July 19, 2024

Since 2009, NASA’s Lunar Reconnaissance Orbiter (LRO) has identified over 200 pits on the surface of the Moon. Utilizing its radar data from 2010, Leonardo Carrer, Riccardo Pozzobon, Francesco Sauro, Davide Castelletti, Gerald Wesley Patterson, and Lorenzo Bruzzone published “Radar evidence of an accessible cave conduit on the Moon below the Mare Tranquillitatis Pit” this past Monday, 2024. Their paper confirmed the discovery of a cave on the moon from the Mare Tranquillitatis Pit, located only 250 miles from the Apollo 11 landing site. While caves had been speculated to exist on the Moon for years, it has now finally been confirmed as true. The data suggests a pit entrance spanning around 100 meters wide with a depth over the length of a football field. Compared to the temperatures of the Moon, ranging 127 to -173 degrees Celsius, the cave has been discussed as a potential base for astronauts on the Moon, as it would shelter them from radiation and keep the temperature around 17 degrees Celsius. 

Scientists believe that the formation of these caves was caused by lava tubes, in which underground tunnels were formed when molten lava flowed underneath the lunar surface billions of years ago (don’t worry, the lava is gone now). If the top surface of the lava tube ends up collapsing, it would create a pit, just like at Mare Tranquillitatis. The idea that the Moon underwent volcanic processes including lava beneath its surface is now common knowledge, yet it wasn’t always supported or popular. That is, until the Apollo 11 moon landing which took place 55 years from tomorrow. 

Before 1969, there were frequent debates surrounding the origin and developing history of the Moon’s formation. One example is Harold Urey’ proposal that the Moon had undergone a cold formation, and that the moon was likely made from primordial material (existing since the beginning of the Solar System). He believed that the Moon had formed similar to the planets through an accretion from a dust cloud at cold temperatures. His theory was popular at the time, yet there were many critics as well. Urey also believed that water had existed on the Moon in the past, an idea which faced backlash from other scientists. In the end, Apollo 11’s rock samples failed to show signs of hydration by water, disproving this proposal. But what about Urey’s cold-Moon theory?

The discoveries following Apollo 11 emphasized a lunar formation through heat with the presence of a “magma ocean” covering the lunar surface. This idea was originally created from the research of John A. Wood and his team after the moon landing. Together, they studied various regolith samples, and highlighted that some light-colored rocks contained plagioclase feldspar (often abundant in anorthosite rocks). Wood then made a series of proposals in which the few rock samples gathered containing signs of anorthosite rocks had been from an impact in the lunar highlands, and were not native to the landing site of Apollo 11. In a collaborative 1970 paper by John A. Wood, John S. Dickey Jr, Ursula B. Marvin, and Benjamin N. Powell, titled “Lunar anorthosites and a geophysical model of the moon,” Wood and his team highlighted the similar chemical composition of the highlands, such as the Tycho crater, with the light-colored rocks found by the astronauts. Wood then proposed that all the Moon’s highlands had been formed of less-dense anorthosite material. 

In the paper, they also proposed that “they are presumably the products of subsequent remelting and magmatic differentiation within the lunar crust” (15-16). In short, they hypothesized that the Moon once had a magma ocean during its formation, allowing the low-density igneous anorthosite rocks to float to the surface (essentially stating that the Moon had melted, rather than cooled, completely destroying Urey’s concept). Another contributing ingredient to the Moon’s surface is KREEP (K is for potassium, REE is for “Rare Earth Elements” and P is for phosphorus), which combined together with radioactive elements, such as uranium, and rose through the magma ocean. While settled underneath, its lava is responsible for the “dark gray spots” on the lunar surface, as impact holes from outside collisions were filled with the lava and cooled to a dark gray color.  

Going back to Wood’s proposals, additional research and missions have been able to greatly support his hot-Moon formation. For instance, Apollo 11 moon samples of basaltic rocks matched textures of igneous rocks, suggesting a geological formation involving…lava. Additionally, their crystallization patterns (arrangement of crystals formed as lava cools) indicated exposure to…lava. But that wasn’t all. During Apollo 16, NASA sent its astronauts to collect samples from the Descartes highlands, where they found more anorthosite rocks, further supporting that the existence of volcanic lava in the past was true. All of this heavily opposed the cold-Moon theory and supported Wood’s claims. 

In 1994, the Clementine mission helped confirm Wood’s idea that all of the Moon’s highlands had anorthosite rocks. Launched in January that year, NASA surveyed the lunar surface and studied the concentration of iron (II) oxide, FeO, in the highlands. Anorthosite rocks have a low FeO concentration, and the Clementine spacecraft successfully found a low FeO concentration in the lunar highlands as well, averaging around 3-6%. 

Currently, NASA aims to send humans back to the Moon and attempt to keep them there through the Artemis program. Their attempts at creating a base (maybe inside a cave) will allow more experimentation and research to be done on the Moon, hopefully uncovering the many mysteries still waiting to be discovered.