Lunar Mysteries That Science Still Needs to Solve

The last three Apollo missions all took samples from three major impact craters—Imbrium, Serenitatis, and Nectaris. New evidence suggests that the samples used to date the age of each of these craters, which is crucial to determining whether a period of heavy bombardment occurred, may actually just be debris from the impact that formed the largest crater—Imbrium—about 3.9 billion years ago.

“We’re pretty confident that when Imbrium formed, it spattered the nearside collection areas with its ejecta,” says Nicolle Zellner, a planetary scientist at Albion College. “So when the Apollo astronauts landed in these regions and collected samples, they were very likely to collect samples of Imbrium.”

Zellner says the best way to settle the lunar cataclysm debate will be to visit craters where samples aren’t likely to have been contaminated by the Imbrium impact, such as the south pole or the far side of the moon. If most of those new samples are older than 3.9 billion years, it will cast the theory of the lunar cataclysm in serious doubt and also help scientists better understand conditions in the early solar system.

What Creates the Lunar Ionosphere?

High up at the outer reaches of Earth’s atmosphere is a region of electrically charged particles called the ionosphere. It’s created when the solar wind strips electrons from atmospheric gasses, turning them into ions. In the 1970s two Soviet lunar orbiters discovered that ions also existed in the moon’s ultra-thin exosphere, and scientists have been trying to explain this observation ever since.

The fact that the moon has an ionosphere is not particularly surprising, says Jasper Halekas, an associate professor of physics and astronomy at the University of Iowa. Any planet that has an atmosphere, even one as diffuse as the moon’s, will produce ions when gasses interact with the solar wind. What is surprising, however, is the discrepancies in measurements of how dense the lunar ionosphere is. The figures range from about 1,000 ionized particles per cubic centimeter to about a tenth of a particle per cubic centimeter. As Halekas notes, “Four orders of magnitude is a pretty wide range of discrepancy for measurement, even when it comes to astronomy.”

Better measurements will help scientists understand how the lunar ionosphere is produced. Only a decade ago, some scientists believed that the lunar ionosphere might be created by ionized dust in the atmosphere, which would make the moon’s ionosphere much different from Earth’s. Yet in 2013, when the Lunar Atmospheric Dust and Environment Explorer failed to detect an appreciable amount of dust in the upper lunar atmosphere, this theory was cast into serious doubt. The problem is that if there really are 1,000 ions per cubic centimeter, the ionization of gas in the lunar exosphere can’t account for such a high concentration—there just isn’t enough gas.

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Halekas is the co-investigator on the Lunar Surface Electromagnetics Experiment, which was recently selected by NASA to be one of 12 experiments that will hitch a ride to the lunar surface on a commercial lander. The experiment will measure oscillations in different types of electromagnetic fields, which can be used to determine the density of the ionosphere with unprecedented accuracy. Halekas predicts that the experiment will find low enough concentrations of ions to match the amount of gas present, which would put an end to the debate. But if the experiment detects high concentrations, Halekas says it will be necessary to “go back to the drawing board” to explain how these ions were produced in such large quantities.

Where Did Lunar Water Come From?

Last year, NASA scientists used data from India’s Chandrayaan-1 spacecraft to definitively prove that water ice is present at the lunar poles. Most of this ice exists in permanently shadowed craters at the south pole, where temperatures never rise above -250 degrees Fahrenheit. This is good news for future expeditions to the moon, which plan to use this water ice for everything from life support to rocket fuel. Although it’s unclear what form the water ice is in—big blocks or crystals mixed with lunar regolith—for many scientists the big question is how it got there in the first place.

According to Paul Hayne, a planetary scientist at the University of Colorado, Boulder, there are three main theories for how water originated on the moon. The most “obvious” theory, Hayne says, suggests that the water ice was deposited by asteroid and comet impacts, where it vaporized and eventually made its way to the poles. It’s also possible that ionized hydrogen from solar winds binds with oxygen trapped in regolith and is eventually released as vaporized water due to temperature fluctuations on the surface. Finally, there’s a possibility that water was present in the material that originally formed the moon and was forced to the surface by volcanic eruptions. It could be that all three processes were at work, which makes it a question of how much water each mechanism contributed.

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