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The Webfooted Astronomer - June 2001

 

Minutes: Thar's Fossils in Them Thar Rilles, Part One

By Greg Donohue

WE frequently store old things in attics. And attics are often spooky places, too. Some even think theirs may be inhabited by poltergeist. Rational folk like you and me scoff at such silly superstition. But according to Dr. Guillermo Gonzales, Earth's attic —the Moon- may really harbor ghosts of things long dead: signs of early Earth Life!

Dr. Gonzales' address to the SAS, "Mining the Earth's Attic for Signs of Early Earth Life," was based on a paper he and two graduate students from the University of Washington Astrobiology Department submitted to the new journal "Astrobiology". The two post-docs are Llyd Wells and John Armstrong (the speaker at this year's SAS banquet).

According to the thesis of the paper, lack of erosion (no atmosphere and no water) allows the Moon to preserve "old things" on its' surface, including fragments from the terrestrial planets. The Earth, being closest to the Moon, should be the best represented.

Meteorites from the Moon, and about a dozen from Mars, have been recovered on the Earth. So there must be some way to blast objects from terrestrial planets and moons without completely melting them. Conversely, one might expect that bits of the Earth must have been blasted away too, to impact mostly on the Moon (and to a much lesser extent on the other planets). And any early Earth life in this material may have left behind fossilized signs of itself. If we could recover and examine some of these Terran meteorite fragments on the Moon, we might well learn a great deal about early life on Earth —signs which, on Earth, have been obliterated over the countless millennia.

At first glance, it might not seem that the Moon should harbor many fragments of Terran material. The Moon currently orbits at a fair distance (about 60 Earth-radii), and is receding at a rate of 3.82cm/year. And recent large asteroid impacts on Earth, such as the one 34 million years ago that carved out the Chesapeake Bay, are rare. So based on this data alone, one would expect to find very few Terran meteorites on the Moon. But in the distance past, the situation was very different.

Lunar samples returned by the Apollo missions allow us to determine the absolute ages of some features on the Moon. There are more than 30 maria scattered over the Moon, dating back some 3.8 to 3.9 billion years ago. During this time —known alternatively as the late heavy bombardment or the lunar cataclysm —the impact rate was much higher.

And the Moon was much closer then. The non-linear nature of the tidal forces involved only allows us to extrapolate the Earth's rotation rate and the Moon's orbital distance back about 2.5 billion years. But we can extend that by employing a neat little trick. Heating due to impacts during the late heavy bombardment left the Moon in basically a plastic state. At the end of the late heavy bombardment, the Moon's current ellipsoidal shape "froze out" as it cooled. Calculations show that the "plastic" Moon must have been about 21 Earth-radii away to achieve its current deformation.

To get an estimate of just how much Terran meteorite material we might expect to find on the Moon, we need to know the cratering rate on Earth during the period of the lunar cataclysm. From that cratering rate, we can calculate the amount of material ejected by the impacts, and then determine how much of it would have ended up on the Moon. But the Earth's cratering rate cannot be directly determined, since craters on the Earth are quickly obliterated by erosion.

Lunar craters, on the other hand, are well preserved. Using the Moon's cratering rate during the late heavy bombardment, we can calculate the cratering rate on Earth during the same period. Owing to so-called focusing factors, namely the Earth's larger surface area and stronger gravitational pull, we find the Earth was hit by 24 times as much asteroidal material as the Moon.

Cratering rates for micrometeorites must be measured directly. These tiny objects are important, because they cause mixing of the lunar surface material. NASA's Long Duration Exposure Facility (LDEF) orbited the Earth for nearly 6 years. Examination of its surface gives us our best current estimate of the micrometeorite flux rate for Earth. Not knowing how this rate might vary with time, Dr. Gonzales' group assumed it to be constant for their calculations.

During the late heavy bombardment, impacts with 100-kilometer size objects were common. Such huge impactors briefly leave a hole in the atmosphere as they plunge through it. The spalls (rock chips and fragments) that are ejected first suffer the least amount of shock, and travel out into space through the atmospheric hole. While an ablation crust will form around this early ejecta, the inside material stays cold during ejection (and during re-entry, if subsequently pulled back down by the Earth's gravitational field).

These Terran fragments can reach the Moon either through direct transfer or orbital transfer. In the direct transfer method, the particles travel out from the Earth in a cone, and the Moon sweeps through the material as it orbits the Earth. This method is very effective for sweeping up the fragments, and was some 7 times more effective when the Moon was only 21 Earth-radii away. The near side of the Moon receives more impacts, and higher velocity impacts, than does the far side.

The Moon's older maria have the highest rate of surface mixing from micrometeorite impacts. The dust in these maria should be about 8 meters deep, and have a ratio of Earth meteorite material of about one part per thousand by volume. RADAR bouncing off the Moon confirms the 8-meter depth calculated by Dr. Gonzales' group.

The second installment of this article, in next month's newsletter, will cover strategies for mining the Moon to find some of these Terran meteorites and to see if they do indeed harbor signs of early Earth life. We'll also examine whether or not life in some of this material could have survived long enough to be capable of re-seeding life on the Earth after impacts with asteroids large enough to sterilize the entire planet.

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