The time immediately after earth’s formation is known as the Hadean Eon. It was a time when earth suffered a heavy bombardment from space. Rocks this age on earth are extremely rare, mostly they have been destroyed by later events – eroded, dissolved, melted or smashed. Scientists searching for rocks formed during these hellish times are starting to turn their eyes away from earth, up to the heavens.
At a recent Lunar Science Event, joint between the Geological Society of London and the Royal Astronomical Society I had a marvellous time. Here’s some of what I learnt.
One of my favourite talks was given by Jay Melosh (Purdue University) who talked about how pieces of planets are thrown off into space and land on other planets. The most unequivocal line of evidence that this happens is the Martian Meteorites – 40 pieces of meteorite that came from Mars but ended up on earth . Dating of these samples suggests 6 ejection episodes, all within the last 20 million years. Magnetic studies suggest that at least some of these samples never reached temperatures greater than 40°C
Apart from Apollo astronauts, the only mechanism that send rock samples through space are large impacts. These are associated with large pressures and temperatures, so how do we explain these samples that were not strongly heated? Numerical modelling, if its very fine-grained, shows that rocks on or near the surface close to the impact are thrown off extremely fast, without being heated. As the shock wave radiates out from the impact site it interacts with the free surface in a process called spallation.
Studies of Martian craters show large sprays of material, up to 1500km from the crater. This material, thrown from the impact can be large enough to form their own craters as they hit the ground again, so called ‘crater secondaries’. It’s a relatively small step to move from throwing material this far to putting it beyond escape velocity and into space. The Zunil crater on Mars is a good example of this and may be a source of earth’s Martian meteorites. The same phenomena is seen on earth – fragments of limestone originally from Germany, near the Ries crater, are found near St Gallen in Switzerland where they were thrown by the impact.
When a probe was due to land on Mars’ moon Phobos , (it never made it), Jay Melosh was asked to model how much Martian material would be on it. They wanted to know if they ought to quarantine Phobos samples in case they contained Martian bugs. The answer was – there would be not a lot, but some.
The clear picture of all this evidence is that impacts on Earth, particularly the early earth when impacts were common, would leave fragments on the moon. Euan Nisbet (Royal Holloway, University of London) gave a whistle-stop tour of the early earth. If pieces of this sit on the moon, we would expect to see zircons (very durable), komatiite lava (olivines with distinctive compositions) and maybe sediments. All very distinctive from lunar rocks.
Dave Waltham (Royal Holloway) talked us through how the earth-moon distance has increased over time. Using an equation created by Charles Darwin’s son and armed with only 3 data points (distance now, zero at moon formation and data from some awesome 620Ma tidal sediments) he constrained distance over time. The moon was very close very early on, but it reached half of the current distance with 10 million years. So the moon was a closer target for bits of the early earth, but not by as much as we’d thought.
One of the joys of the conference was the variety of scientists on display. This ranged from slick suit-wearing committee-men to wild-eyed, wild-haired ‘crazy scientist’ types. There were geochemists who talked about the difficulty of getting good data points, geologists who showed pictures of rocks “because they are pretty” and astronomers who, while restrained, where keen to link events on earth to astronomical causes.
We also had an experimentalist, Mark Burchell (University of Kent). It is a truth universally acknowledged that a man in possession of a good gas gun must be in want of interesting things to shoot out of it. Things such as lumps of shale, yeast-infused samples and seeds are interesting – firing samples at up to 5 kilometers a second from a ‘two-stage light gas gun’ into sand is a pretty good approximation of a piece of earth landing on the moon. Very interestingly, samples of shale retain their biomarkers, chemicals indicative of life. Yeast and bacteria also survive the impact, making ideas of simple life moving between Mars and earth plausible. Plant seeds wouldn’t survive the journey though, so we don’t need to worry about ‘Day of the Triffids’ style alien-plant invasions. Part of me wonders if Mark Burchell wasn’t a little disappointed that the seeds didn’t survive, removing the need to move onto higher forms of life. He strikes me as a man who’d relish the challenge of firing a flea at cosmic velocities.
Finding the evidence
The phrase “the relatively accessible surface of the moon” is definitely something only a scientist would say. For a Professor of Planetary Science such as Ian Crawford (Birkbeck, University of London) the moon is our backyard. He spoke passionately about the scientific benefits of a human presence on the moon.
The samples returned by man to the earth (mostly in Apollo missions) are all from low latitudes on the near side. A return mission to the moon should focus on sampling high latitudes and widening our coverage of lunar material.
Ian Crawford has given much thought to suitable areas to explore lunar geology. He has identified the potential of layers between lava flows as repositories of ancient materials. Consider two overlapping lunar lava flows. Chances are there is a layer of material in between the two that landed on the surface during the gap between eruptions. This material could include earth meteorites, ‘normal’ meteorites, samples of the solar wind and more exotic material from the wider galaxy. Preserved by the upper lava flow, the age of the material can be constrained by dating the surrounding lava flows.
Slightly more prosaically, dating of events on all terrestrial bodies apart from the earth is based on cratering rates derived from the moon. The more craters on a surface on Mars, the older it is. Estimates of its absolute age are derived from studies of lunar cratering. Data of more lava flow surfaces on the moon would help refine our understanding of cratering rate over time and so improve dating of events across the solar system.
Katherine Joy (University of Manchester) spends her time studying rock samples from the moon. Some of these are Apollo samples, some are meteorites found on the earth. She’s already found some amazing things such as a meteorite in a meteorite. A tiny piece of rock floating around the solar system landed on the moon and got incorporated into the surface layers (the regolith). Another large chunk hit the moon and fractured the regolith sending pieces hurtling into space. One of these lunar fragments, containing the older meteorite within it, fell to the earth as a meteorite.
A portion of her research is identifying these fossil meteorites in samples of regolith. She has enough data to identify a suite of 3.9 Ga meteorites that are noticeably different from modern-day meteorites found on the earth. This work involves painstaking analysis to identify the individual fragments and analyse their chemistry. She is able to distinguish between fragments of the moon and pieces of meteorite – these techniques would allow her to recognise a piece of rock from the ancient earth. So one day, maybe right now, a scientist in Manchester will jump up in excitement, eager to share the amazing fact that they have before them a sample from earth’s early history, a little piece of evidence that’s survived four billion years and two journeys through space to bear witness to an otherwise lost time.Image of Moon from ‘via moi’ on Flickr.