Footprints on the Moon

Earth and Moon
Distance view of road travelled. Image of the Earth and Moon taken by Galileo spacecraft.
Credit: NASA

The Moon preserves unique information about changes in the habitability of the Earth-Moon system. This record has been obscured on the Earth by billions of years of rain, wind, erosion, volcanic eruptions, mountain building, and plate tectonics. In contrast, much (most?) of the lunar surface still contains information that reflects events at the time of life’s origin and subsequent evolution on Earth. Therefore, lunar research can address critical astrobiology science questions.

In particular, the lunar record allows us to focus on two specific issues in the early solar system–the history of impacts and the history of exposure to radiation. The Moon, as Earth’s closest neighbor, is probably the only body in the solar system where we can address these issues quantitatively. Impacts probably played an important role in the earliest history of life on Earth. Large impacts would have temporarily altered the environment and creating hostile conditions in which life could not survive. Later impacts probably shaped life’s evolution by forcing successive mass extinctions of large numbers of species. The terrestrial impact history is better recorded on the Moon than on the Earth.

Central science goals are to determine the impact rate onto the Moon (and, by extension, the Earth) during the period when life was originating early in solarsystem history, as well as in geologically recent times.

Time lapse exposure of space station silhouette against the lunar backdrop, as seen from Earth at Tracy, CA on November 8, 2003
Credit: Ed Morana

We can use the beautifully preserved record on the Moon to help us to understand the habitability of the Earth at the time of life’s origin and earliest evolution and determine the frequency of impact-driven mass extinctions and the subsequent course of evolution. During Earth’s earliest history, its surface also was bombarded by high-energy particles associated with solar activity (from a solar wind that was enhanced during early history and from solar flares) and galactic cosmic rays, and possibly from nearby supernovae and events associated with gamma-ray bursts. This bombardment must have had deleterious effects on life at the Earth’s surface, and may have severely affected the formation and earliest evolution of life.

The Moon’s surface provides the best and most accessible record of the bombardment history of the Earth and the inner solar system, including changes through time in the mass flux and in the size distribution of impacting objects. The existing data for radiometric ages of returned lunar rocks and for crater densities on the lunar surface are the primary basis for our present understanding of the early bombardment history of the inner solar system and the early Earth (>3.5 Ga). These data constitute one of the most profound scientific legacies of the Apollo program.

These ancient events are recorded in the lunar regolith, formed throughout lunar history by the impact of micrometeorites and which were buried and preserved by subsequent lava flows. Regolith forms continually at the lunar surface as a result of the bombardment by micrometeorites, which break up surface rocks. At present formation rates, a layer up to 2 m thick can be created in a billion years that consists of fine, powdered rock.

Sampling the effects of this radiation within these fossil regoliths, then, provides a window into the energetic-particle environment at the time that the regolith was buried, and sampling many different locations can provide detailed information over time. "Fossil" regoliths formed when such material was buried by lava flows or impact ejecta, thus protecting the previously exposed regolith from subsequent alteration.

Lunar Clementine mission shows the South Pole of the Moon. The permanently shadowed region center showed earlier evidence of meteor cratering and ice never exposed to direct sunlight, but Arecibo radar reveals dust.
Credit: NASA/DOD Clementine

Many of these flows presumably buried ancient regolith, and some specific examples are known, such as Hadley Rille at the Apollo 15 landing site. These can be accessed by trenching, by drilling, in the walls of rilles, or at sites where impacts have done the excavation for us.

Robotic sample return missions would return collected samples to the Earth, where high-precision geochronology and trace-element analyses could be carried out. Human exploration missions would permit detailed documentation of collected samples, field study of their context, and traverse geophysics that can allow us to better understand the character of the craters or basins.

The highest-priority matters would include the following:

Potential for finding ancient Earth rocks on the Moon. The importance of finding ancient Earth materials on the Moon was mentioned previously. If such materials could be identified and were not severely altered by the ejection and impact processes, they would provide a unique and truly exciting window into the early Earth. Similarly, there exists the potential for finding ancient samples from Mars or even Venus, as well as unweathered carbonaceous chondrites.

Processes related to the origin of the Moon. We can use the impact rate onto the early Moon to understand stochastic collisional processes that occurred within the inner solar system and that were related to lunar formation. In addition, samples from additional locations beyond those already obtained during the Apollo era can help us to understand the bulk chemical composition of the Moon for comparison with Earth and other terrestrial planets and the chronology of events at the time of lunar origin.

Moon occulting Venus, the morning star, taken by the lunar probe, Clementine.
Credit: NASA/DOD/

Characteristics, formation, and evolution of primordial crust. The lunar highlands represent the first crust formed after global melting on the Moon, and other terrestrial planets presumably would have had a similar early crust. If we are to understand the evolution of planetary surfaces, we must understand these initial crusts as well. The primordial lunar crust is the best-preserved, and possibly the only, example we have. Evolution of an end-member planetary object. A key issue in astrobiology is to understand the processes responsible for the geological and geophysical evolution of terrestrial planets. In our solar system, the Moon is the smallest of these objects, and determining how interior processes and their coupling to the surface geology occurred over time should constrain how similar processes might play out (or have played out) elsewhere.

Organic chemistry recorded in the polar regions. Studies of the organic chemistry of ices and soils from the lunar polar regions may serve as an accessible analog of radiation-driven processing that occurs on interplanetary dust grains. As the organic molecules resulting from such processes may have played a significant role in the origin of life on Earth, it is important to understand the processes in more detail.

Evaluation of how water and other volatiles were added to the Earth. By examining the volatile content of meteoritic contaminants in the lunar regolith and of volatiles cold-trapped at the lunar poles, we can determine their chemical and isotopic composition and possible source regions. This will help us to determine how volatiles might have been added to the early Earth, and to understand why the Earth has the volatile inventory that it does. This study is complicated, however, by the possibility that polar volatiles and meteoritic debris are relatively recent arrivals on the Moon and, therefore, do not reflect conditions while the Earth and Moon were forming.

Alan Shepard’s contraband six-iron and the only golf shot on the moon. Before climbing back into the lunar module, Shepard took out of his suit pocket "a little white pellet that’s familiar to millions of Americans" – a golf ball – and dropped it on the surface. Then, using the handle for the contingency sample return container, to which was attached "a genuine six-iron," he took a couple of one-handed swings. He missed with the first, but connected with the second. The ball, he reported, sailed for "miles and miles."
Credit: NASA

Recent Lunar Timelines

– Japanese Hiten , Lunar Flyby and Orbiter
– Michael Rampino and Richard Strothers propose Earth could be periodically struck by comets dislodged from orbits when the solar system passes through galactic plane
– US Dept. Defense/NASA Clementine mission, Lunar Orbiter/Attempted Asteroid Flyby
– First commercial lunar mission, AsiaSat 3/HGS-1 , Lunar Flyby
Lunar Prospector launches and enters lunar orbit
Lunar Prospector tries to detect water on the Moon (polar impact)
– Lunar soil samples and computer models by Robin Canup and Erik Asphaug support impact origin of moon
SMART 1 , to launch lunar orbiter and test solar-powered ion drive for deep space missions
– Japanese Lunar-A , Lunar Mapping Orbiter and Penetrator, to fire two bullets 3 meters into the lunar soil near Apollo 12 and 14 sites
– Japanese SELENE Lunar Orbiter and Lander, to probe the origin and evolution of the moon

Related Web Pages

Long, Strange Trips
Astrobiology Roadmap
Moon Meteor Truly Extraterrestrial
Moon Written in Stone
SMART-1: Chips Off the Terrestrial Block
Treasures from the Lunar Attic
Lunar Scarface
End of an Era, Dawn of Another?
Making the Moon