The Moon May Have Been Habitable Billions of Years Ago—What Changed?

The Moon today is barren, inhospitable and lifeless. However, there may have been two periods billions of years ago when conditions on the lunar surface were sufficient to support simple organisms, according to a study published in the journal Astrobiology which reviewed the findings of recent space missions and research conducted over the past few years.

In the paper, researchers from Washington State University (WSU) and the University of London suggest these two periods may have occurred around 4 billion years ago, soon after the Moon's formation, and then again around 3.5 billion years ago, when the Earth's only natural satellite was at the peak of its volcanic activity.

During both of these habitable windows, the researchers suggest that the Moon was spewing out large quantities of superheated volatile gases, such as water vapor, from its interior. This process could have resulted in the formation of pools of liquid water on the surface and an atmosphere dense enough to remain for several million years.

"Recent results show that lunar rocks and magma contained more gases and water than previously thought, and some of the rocks are also showing signs of hydrothermal alteration including the presence of clay minerals, suggesting some prior exposure to water," Dirk Schulze-Makuch, an astrobiologist from WSU and author of the study, told Newsweek.

One paper, for example, identified large quantities of water ice on the surface, while other studies have hinted that huge amounts of H2O lie hidden in the Moon's mantle.

Other recent insights—including the discovery of remnant magnetization in lunar rocks—suggest that the young Moon was once protected by a magnetic field which shielded it from potentially deadly particles emitted by the Sun.

Taken together, this body of research indicates that the requirements for life to exist may have been present during the two windows, according to the scientists.

"Life would need an atmosphere to allow water in liquid form to exist on the lunar surface plus a magnetic field to protect the atmosphere from quickly eroding," Schulze-Makuch said. "In addition, life needs some elements such as carbon, nitrogen, phosphorus, and sulfur, which should have been available as well—in the form of gases or as minerals in rocks."

Any life that did exist may have evolved as it did on Earth, although this is unlikely given that the habitability windows may have lasted just a few million years. Another possibility, the researchers speculate, is that a meteorite brought life to the Moon. Life on Earth is thought to have originated around 3.8-3.5 billion years ago at a time when giant meteorite impacts were common throughout the Solar System.

"The meteorite rams into the Earth and catapults some fragments from Earth into space," Schulze-Makuch said. "Some will fall back to Earth, but some will land on the Moon."

If such a meteorite had struck Earth when there was life on the surface, any fragments would likely have been covered in microbes, which could have been transported to the Moon—especially if these microbes were located within the rock or soil and, therefore, not exposed to high pressures and temperatures during an impact.

"Simulations and experiments show that a significant fraction of microbes would survive such a transfer between Earth and Mars, and it should be easier between Earth and the Moon", he said.

An image of the Moon taken by the Galileo spacecraft on December 7, 1992, on its way to explore the Jupiter system. NASA/JPL/USGS

Nevertheless, the researchers stress that there is currently no concrete evidence this happened, or that life evolved any other way on the Moon.

To truly understand whether life once emerged on the Moon, or was transported there, will require extensive lunar exploration programs, according to the scientists. In the meantime, experiments could be conducted in simulated lunar environments to see if microorganisms can survive in the conditions predicted to have existed during the habitable windows.

But if life did once exist on the Moon, why is it not habitable today? Firstly, it has no atmosphere, no surface water, a very weak magnetic field incapable of blocking harmful space particles, none of the complex chemistry necessary for life to emerge, and it experiences wild variations in temperature.

The Moon was probably too small to keep its volcanic activity going, so once its thin atmosphere was eroded, there was not enough gas being released from the interior to replenish it, Schulze-Makuch said.

"Further, at the time of its origin, Earth was very close to the Moon and would have sucked most gases and water into its atmosphere due to its much larger exerted gravitational force," he said. "So, the Moon probably did not have that much water to start with."