The next small steps on the Moon will include a more precise view of the lunar crust
Researchers at Purdue are one giant leap closer to mapping the porosity structure of the Moon by using GRAIL data
With all the talk of possibly building stations on the moon for human activity, it becomes increasingly important to research every aspect of our Moon. But how can scientists have an accurate account of the Moon’s crustal structure and past volcanic activity from so far away here on Earth? In a recent Geophysical Research Letters publication, “Lunar Megaregolith Structure Revealed by GRAIL Gravity Data,” researchers at Purdue University used data from the NASA GRAIL Mission to characterize the porosity structure of the upper lunar crust, known as the megaregolith.
According to the lead author of this publication, Kris Izquierdo, this type of research is an important step for continued lunar research. “Knowing the porosity structure and amount of volcanic material in the lunar crust is essential for constraining the thermal and magmatic history of the Moon,” she says. “Current and future exploration of the Moon, like the one being done by the Chang’E missions, will provide other pieces of information to this puzzle, like the amount of water in the lunar mantle which affects melting.”
In order to get a more complete picture of the history and evolution of our Moon, scientists use the properties of lunar rocks. Though they can make precise measurements from lunar samples, the samples obtained are from only a few of the Moon’s locations.
“For this project, I used gravity data to indirectly constrain the shallow porosity structure of the Moon globally,” Izquierdo says. “A global one-dimensional porosity profile is important because the porosity of the rocks constrains how heat travels and how much volcanic material could reach the crust of the Moon. We found that there is a porosity boundary at a depth between 3–5 km, with a layer of very porous material above this boundary that acts as an insulating blanket. This layer is thicker than what was proposed before, adding more insulation and increasing estimates of heat flux and magmatism.”
Izquierdo is a postdoctoral researcher in the Purdue University Department of Earth, Atmospheric, and Planetary Sciences (EAPS) who is advised by Professor Michael Sori. Both Izquierdo and Sori are authors on this publication in addition to Professor Brandon C. Johnson, Sean E. Wiggins (EAPS graduate student) and Dr. Jason M. Soderblom (research scientist from MIT).
This research used data collected from the NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission. This mission used two twin spacecraft that followed each other in orbiting the Moon for a year. By tracking the location of the spacecraft and the changes in distance between them, it was possible to detect very small changes of acceleration due to the mass of the Moon. When the spacecrafts passed by places with a higher concentration of mass, the acceleration felt by them was higher and their orbit was modified more than when they travelled by a place with less mass. By orbiting the whole Moon, including the near and far side, and flying at a low altitude, GRAIL was able to provide the highest resolution gravity map available of the Moon. This map is also the highest resolution gravity map of all bodies in the solar system, including Earth.
According to Izquierdo, there are still many questions to answer regarding the structure of the lunar crust. It is important to learn more about volcanism and the porosity and composition of the rocky Moon surface.
“Gravity is sensitive to mass,” she says, “which is a function of the porosity and composition of rocks. We explored how a one-dimensional porosity structure fits the global gravity data of the Moon on this paper. Next, we are going to investigate the amount of volcanic material that is present in the crust. Some of this volcanic material is visible as dark regions in the near side of the Moon. We will quantify what is the total volume of volcanic material, visible and not visible, that fits the gravity data. The total amount of volcanic material would constrain the heat flow and timing of the volcanism on the Moon. We will then have a better understanding of both porosity and composition of rocks in the crust.”
Writer: Cheryl Pierce