NASA recently announced the Artemis lunar exploration program, consolidating its plans plans to land humans on the moon by 2024 and establish a sustainable base there by 2028. This ambitious initiative revives an old question: Will the unique qualities of the lunar surface enable new frontiers in astronomy?
A few decades ago, astronomers had already begun to contemplate different ways their observations could benefit from the absence of an atmosphere on the moon. First, energetic particles such as gamma rays, x-rays, ultraviolet photons or cosmic rays would not be blocked by an atmospheric blanket as they are on earth, and hence they would reach telescopes with large collecting areas mounted to the lunar surface. Second, observatories sensitive to optical, infrared, millimeter or radio waves could reach their diffraction limit without the blurring or absorption associated with passage through turbulent air. Arrays of detectors could therefore constitute giant interferometers with unprecedented angular resolution.
Third, the lack of an ionosphere would allow radio observatories to receive signals at very low frequencies, below the terrestrial cutoff of 10 kilohertz. This would open a new spectral window into the universe, allowing to map the three-dimensional distribution of hydrogen atoms from their first appearance 0.4 million year after the big bang and through the cosmic dawn, using the highly redshifted 21-centimeter line. Although exciting and path breaking in their own right, these visions were all formulated well before the emergence of the frontier of astrobiology associated with the search for extraterrestrial life.
Can the moon provide clues for extraterrestrial life? A new paper I wrote with Manasvi Lingam answers this question in the affirmative. The idea is to consider the moon's surface as a fishing net for interstellar objects collected over time and potentially deliver building blocks of life from the habitable environments around other stars.
The lack of a lunar atmosphere guarantees that these messengers would reach the lunar surface without burning up. In addition, the geological inactivity of the moon implies that the record deposited on its surface will be preserved and not mixed with the deep lunar interior. Serving as a natural mailbox, the lunar surface collected all impacting objects during the past few billions of years. Most of this "mail" comes from within the solar system.
But the solar system also intercepts objects from interstellar space, ranging from dust particles to free-floating planets and stars. A detection of the first interstellar object, 'Oumuamua, with a size on the order of 100 meters was reported in 2017. This year, 'Oumuamua's cousin was tentatively discovered in the form of a meter-size meteor from outside the solar system that burned up in Earth's atmosphere in 2014. And most recently, yet another interstellar visitor may have been identified.
Given the search volume and duration of the surveys that made these detections, it is now possible, for the first time, to calibrate the flux of interstellar objects (assuming they enter the solar system on random trajectories). With this calibration at hand, one can calculate the amount of interstellar material that has collected on the moon's surface over its history. The buildup of interstellar matter can also be observed in real time; another new paper with my undergraduate student, Amir Siraj, showed that a two-meter telescope on a satellite in orbit around the moon can observe interstellar impactors as they crash.
In case some interstellar impactors carry the building blocks of extraterrestrial life, one could extract these biomarkers by analyzing lunar surface samples. Moon rocks delivered to Earth by the Apollo mission were likely contaminated by terrestrial life and are not a viable alternative to a dedicated experimental base on the moon.
Identifying biomarkers from debris of material that originated in the habitable zone around other stars would inform us about the nature of extraterrestrial life. The fundamental question is whether distant life resembles the biochemical structures we find on Earth. Similarities might imply that there exists a unique chemical path for life everywhere or that life was transferred between systems. Either way, a lunar study shortcuts the need to send spacecraft on extremely long missions to visit other star systems.
Getting similar information from a trip to the nearest star system—Alpha Centauri A, B or C—would take nearly nine years round-trip, even if the spacecraft were to travel at the maximum speed allowed in nature, the speed of light; the first half of this period is required for reaching the target and the second half for the information to get back to us. With chemical rockets, this journey would take about 100,000 years, on the order of the time that elapsed since the first modern humans began migrating out of Africa. Excavating the lunar surface for physical evidence of extraterrestrial life is dramatically faster.
Based on the newly calibrated flux of interstellar objects, their debris should constitute up to 30 parts per million of lunar surface material. Extrasolar organics might amount to a fraction of an order of a= few parts per 10 million. Amino acids, which serve as the building blocks of "life as we know it," could amount to a few parts per hundred billion. Standard spectroscopic techniques can be employed to examine individual grains within the lunar regolith and search for signatures that would flag them as extrasolar before unraveling the building blocks of extraterrestrial life within them.
How can extrasolar origin be identified? The simplest flag would be a deviation from the unique solar ratio for isotopes of oxygen, carbon or nitrogen. Laboratories have already demonstrated the feasibility of this method at the required sensitivity levels.
But there is also the exciting opportunity for detecting biosignatures of extinct extraterrestrial life. On Earth, the oldest microfossils, with unambiguous evidence for cells that lived about 3.4 billion years ago, were discovered in the Strelley Pool Formation in Western Australia. It would be tantalizing to find microfossils of extraterrestrial forms of life on the moon. Even more exciting would be to find traces of technological equipment that crashed on the lunar surface a billion years ago, amounting to a letter from an alien civilization saying, "We exist." Without checking our mailbox, we would never know that such a message arrived.
The opportunity to discover signs of extraterrestrial life provides a new scientific incentive for a sustainable base on the lunar surface. The moon is well known for its romantic appeal, but astrobiology offers a twist on this notion. Here's hoping that the moon will inform our civilization that we are not alone and that someone else is waiting for us out there.
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