Treasures from the Lunar Attic
Because our moon is lifeless, it is one of the most appealing places to look for the preserved records of life elsewhere. At least according to recent estimates for the amount of ejected rocks that might survive there, the Moon may hold clues from the early history of Mars, Venus and Earth.
|The painting titled "K/T Hit" by artist Donald E. Davis. This impact occured 65 million years ago, ending the reign of the dinosaurs.
Image Credit: Don Davis
Prior to the work of John Armstrong and colleagues from the University of Washington and Iowa State, there were no published "estimates for the abundance of Terran, Martian, and Venusian meteorites on the Moon." To fill this gap, the team undertook computer simulations based on the impact events from 3.9 billion years ago during what is called the Late Heavy Bombardment--the last time that the inner solar system was pelted with asteroid debris. The simulations must take account of the gravity and escape velocities for each inner planet, the orbital paths of debris trails, and finally the Moon's ability to capture strategic samples.
One of the reasons such meteorites might be so valuable is "to substantiate or extend a contested fossil record that begins 3.5 billion years (3.5 Ga) ago", writes Armstrong, thus filling in what early Earth life might have offered. Even shorter spans are available for Venus, where its surface records were catastrophically erased 700 million years ago.
The Washington study indicates that if such meteorites reached Earth- and could be recovered even from the ice plains of Antarctica- a lunar sample would still be preserved as the best recorded history lesson. Whether from wind (atmosphere), water, or fire (volcanism), the Moon's lack of erosion might provide a unique collection compared to anywhere else in our solar system. The authors note: "Most significantly, the Moon lacks the water capable of carrying contaminants into the interior of rocks through cracks."
Boomerang from the Past
Tracking the debris spray from a heavy bombardment proved challenging. Three cases were considered: 1) "direct transfer", where the ejected rocks liftoff the Earth, Mars or Venus with medium velocity, but not too high that the Moon could not have captured them; 2) "orbital transfer", where the meteorite debris leaves at high speed, but comes back later to land on the lunar surface; and finally, 3) "lucky strikes", where the rocks cross paths with the Moon directly.
|Lunar Clementine mission shows the South Pole of the Moon. The permanently shadowed region center shows evidence of meteor cratering and ice never exposed to direct sunlight.
Credit: NASA/DOD Clementine
In the Earth's case, at least, the incoming asteroids or fragments during Late Heavy Bombardment average a whopping 14 kilometers per second (or around 31,000 miles per hour [MPH]). To escape the Earth's gravity (or reach escape velocity), the outgoing rocks also must have a relatively high speed, around 11.5 km/s (37,000 MPH). To complete the lunar capture, a final high-speed event must include an impact on the Moon, or a shock that would complete the sample's journey after a relatively hard-landing at around 2-5 km/s (~10,000 MPH).
One discovery from computer simulations was that the second method of capturing rocks on the moon--orbital transfer-- is dominant. Most (58% by mass) of the terrestrial samples (called Terran) that would be preserved today on the Moon, would have left Earth in all directions, but then later have come back to visit on a centuries-old, boomerang pattern that depends on its orbit and lunar crossing points.
One startling feature of the Moon's pockmarked surface is the cumulative destruction that asteroid and meteor impacts have had already. For instance on the Moon's South Pole (called the Aitken Basin), an impactor weighing 10 quadrillion (1015) tons (1019 kg) left a crater nearly 2200 km (1320 miles), which is at least 100 times the thickness of the Earth's atmosphere. So whatever the source of the lunar South Pole impact was, it had little chance of coming from anything terrestrial, at least not without leaving a similar gash in the Earth's crust.
The Washington study sums up their findings: "The amount of Terran material on the surface of the Moon will depend largely on the age of the surface that is searched. Assuming the regolith (soil) is well mixed, we estimate the total surface abundance of Terran to lunar material to be 7 (parts per million) ppm. This corresponds to ~ 20,000 kg of Terran material over a 10 x 10 square km area."
For the other inner planets, Venusian chunks would be from 1000 to 30,000 times less likely on the Moon, but "an area of 10 x 10 square km should still yield almost 1 kg of Venusian material, if it can be identified as such," as a lower bound, and as high as 30 kg.
Finally, for Mars, approximately fifteen (100-gram) Mars rocks today reach and impact the Earth each year. So if identifiable on the Moon, this translates "to about 180 kg in the same 10 x 10 square km area."
Rummaging for Life in the Lunar Attic
But what kind of evidence would prove a rock's origins? To unravel such a history for a three and half billion year old rock, the researchers considered a cadre of tools to look for what would be evidence of the rock's origins. To estimate the possibilities, these would be "isotopes, significant volatile inventories, organic carbon, and molecular fossils (biomarkers)", according to their study. Could the evidence of another planet survive the high pressures and temperatures of impact and capture?
|Close-up of a Mars meteorite, showing what some argue appears to be fossilized evidence of ancient microbial life.
Image Credit: NASA
The chances of tracing back such a complex history have improved dramatically in recent years, and spawned new investigations within the larger meteorite and astrobiology community. The authors note that estimated survival likelihood has risen dramatically: "Until recently, the prospect that material could escape a planet via a natural process was considered extremely unlikely, much less that the material could do so without being heavily shocked. Experimental and observational evidence has forced a revision of this opinion...In fact, (the Allen Hills, Martian meteorite) ALH84001 apparently traveled from the surface of Mars to Earth without ever exceeding 40 C"--or a mild 104 F.
|Earth in moon perspective
Image Credit: NASA/GSFC Simulation
"Terran materials are abundant and near the surface," they conclude, "with a significant fraction retaining their geochemical and biological signatures for detailed analysis. In addition, since the majority of Terran samples date from the end of the Late Heavy Bombardment, the samples in the lunar 'attic' are a unique probe of the early conditions on Earth, and potentially contain clues to the earliest forms of life."
What the lunar attic might hold of the Earth's past is not entirely a theoretical argument, given that nearly a half-ton of the Moon was brought back during the Apollo missions. As first steps, Armstrong and his coauthors propose looking at what scientists already have vaulted. "Before any such [lunar] mission is attempted, the current stock of lunar material (approximately 400 kg worth) should be searched for Terran material. Given a concentration of 7 ppm, there should be roughly 3 grams of Earth material in the current lunar samples."
A tell-tale sign of what might have originated terrestrially would be what is known as 'hydrated silicates': a remnant of the Earth's watery composition compared to the dry moon. As the authors write: "While this is not likely to yield much in the way of information about the early Earth, it would act as a proof of concept and a baseline for future missions."
Collaborators include Llyd E. Wells (U. Wash.) and Guillermo Gonzalez (Iowa State). This research was supported by the National Science Foundation (NSF-IGERT) training-ship in Astrobiology, an NDSEG fellowship, and the NASA Astrobiology Institute.
Related Web Pages
Great Impact: Part I
Great Impact: Part II
Great Impact: Part III
Great Impact: Part IV
Impact Hazards Website
NASA/JPL Near Earth Object Program
Making the Moon
Review of Theories of Moon-Forming Impact (Planetary Science Institute)
Big Bang, New Moon (SwRI)
Planetary climatic and dynamic factors that convert solid, airless bodies into worlds suitable for life (Dr. Darren Williams - Penn State)