The Moon’s Bombardment and Volcanism Combo
The Earth’s surface is subject to great change over geological time periods, due to the movement of tectonic plates, and volcanic activity, as well as long-term weathering and erosion. As a result, craters caused by meteorite, asteroid and comet bombardment long ago is gradually eradicated. We therefore look to the Moon’s cratered surface to provide clearer evidence of the bombardment history of the Earth/Moon binary. That cratering history is then compared to other planets and objects in the inner solar system, allowing astronomers to discern patterns in cratering over long time periods. One of the most significant events is the late, heavy bombardment. Following a period of relative quiet after the formation of the planets, this mass bombardment was thought to have occurred about 3.9 billion years ago:
“Competing models of meteorite-impact rate for the first 2 billion years (Ga) of Earth and Moon history. Note that Earth is believed to have formed about 4.55 Ga before present. Two hypotheses are shown: exponential decay of impact rate (dashes); and cool early Earth–late heavy bombardment (solid curve).” [see right-hand graph] (1).
More recently however, there has been a gradual realisation that this was not a sudden, dramatic event, but rather a sustained period of impacts by what were some colossal bodies:
“Recent high-resolution orbital data and images, more refined techniques for studying small lunar, terrestrial, and other impact samples and a better understanding of their ages, and improved dynamical models based on orbital and sample data have caused a paradigm shift in how we think about the lunar impact rate … The long-held idea of a “lunar cataclysm” at ~3.9 Ga is being replaced by the idea of an extended lunar bombardment from ~4.2 Ga to 3.5 Ga.” (3)
In the same way that Mars appears to have had both its magnetic field and, subsequently, its atmosphere stripped away, there is also a strong possibility that the Moon looked very different during its early period than it does now. Our close cratered neighbour may once have had an atmosphere (4), although not a particularly pleasant one. Between 4 and 3 billion years ago, the Moon was spewing out a variety of gases from the volcanic activity that filled the lunar ‘seas’, and these accumulated across the surface to provide a weak atmosphere:
“The atmosphere was thickest during the peak in volcanic activity about 3.5 billion years ago and, when created, would have persisted for about 70 million years before being lost to space. The two largest pulses of gases were produced when lava seas filled the Serenitatis and Imbrium basins about 3.8 and 3.5 billion years ago, respectively.” (5)
It is interesting to note that this period of lunar volcanism occurs around the same period as what is now considered to be an extended period of bombardment. Volcanism begins shortly after the start of this prolonged period of dramatic cataclysmic activity. The peak atmospheric pressure on the Moon is reached towards the end of the late, heavy bombardment, around 3.5 billion years ago. Following the return to ‘normal’ conditions within the solar system, the lunar volcanism tails off, and the atmosphere is gradually lost. It seems that the lunar volcanic activity, lava flow across the lunar surface, and release of gases into the Moon’s thin atmosphere, is a reactive response from within the Moon itself to collisional bombardment across the surface. This makes some sense, as many of these strikes were truly awesome in scale, melting significant chunks of real estate. Below the lunar surface, another layer of deeper, subsurface craters is now recognised, indicating that our battered neighbour has a truly fractured and pulverised crust (6).
There are many possible causes for a short period of bombardment: comet showers, the influx of interstellar materials as the Sun passes through a nebula, a stellar flyby. Explaining a prolonged period of bombardment, which arises from nothing and lasts ~700 million years is more problematic. It speaks to a significantly different environment within the inner solar system itself, where the Earth/Moon binary encounters objects at a frequency that is orders of magnitude above normal. This is one of the reasons why various models of planetary migration have been suggested, mostly involving significant shifts in position by the solar system’s big planetary players, like Jupiter, and Neptune. However, I am interested in a very different possibility: Was this prolonged catastrophism, and subsequent period of lunar volcanic activity, instead due to the inward migration of the Moon/Earth system itself?
I suggest that an encounter with another rogue planet jolted our world onto a new, inner track, and spilt a huge amount to materials across the inner solar system. The perilous journey undertaken by the inner planets through this massive debris field created the late, heavy bombardment. Eventually, the planets cleared their domains of much of this asteroidal detritus, and settled down into the orbits we see today. The Moon provides a snapshot of this cataclysmic history.
The approximate content of Earth’s early atmosphere has been determined by studying zircon crystals (7). These are igneous minerals which predate Earth rocks. Some are almost as old as the Earth itself. Much can be inferred from their composition about the prevalent conditions during their formation (8). The early Earth’s atmosphere was generated by outgassing from lava melts from surface volcanic activity. By the time of the late, heavy bombardment, the Earth’s atmosphere was not dissimilar to how it is now (7). It contained oxygen-rich compounds like carbon dioxide and sulphur dioxide, as well as water.
The composition of the Earth and the Moon are very similar – the Moon likely having formed early on as a result of an planetary impact between Earth and a Mars-sized body. It should come as no shock that the Moon also has ancient zircon crystals:
“Lunar zircons are thought to form in the lunar magma ocean’s potassium, rare-Earth elements and phosphorus (KREEP) reservoir. And because this reservoir formed towards the end of the magma ocean’s hardening, they provide handy objects to determine when solidification took place. Using a technique called isotope dilution thermal ionisation mass spectrometry – an incredibly sensitive technique that can accurately measure minute amounts of elements such as lead and uranium – [Melanie] Barboni [from the University of California, Los Angeles] and her crew found the zircon fragments crystallised between 50 and 60 million years after the birth of the solar system.” (9)
In the same way that zircon crystals can help to infer the atmospheric conditions prevalent on the early Earth, lunar zircon crystals should be able to spill the beans on the early lunar atmosphere which, quite possibly, dated back almost 4.5 billion years. In other words, early outgassing may have occurred on the Moon as well as the Earth, providing a binary planetary system sharing many key characteristics, including an early, oxygenated atmosphere. Some of the lunar rocks contain zircon crystals, making such research feasible right here on Earth. Similar analyses have been carried out with zircon crystals found in a meteorite which originated from Mars, known as NWA 7533. The zircon grains in question are 4.43 billion years old (10).
My bet is that the Moon had an atmosphere which pre-dated the late, heavy bombardment, and may have been similar to the Earth’s at that time. If the Earth/Moon binary was located further away from the Sun during that early period, perhaps at about 3 astronomical units, then that lunar atmosphere might have been viable at the time, becoming catastrophically depleted only when relocated closer to the Sun.
Written by Andy Lloyd
6-10th August 2018
1) National Research Council “The Scientific Context for Exploration of the Moon”, The National Academies Press, 2007, https://www.nap.edu/read/11954/chapter/4
2) J.W. Valley et al. “A cool early Earth” Geology, 2002, 30: pp351-354
3) N. Zellner “Cataclysm no more: New views on the timing and delivery of lunar impactors” 2017 https://arxiv.org/ftp/arxiv/papers/1704/1704.06694.pdf
4) D. Needham & D. Kring “Lunar volcanism produced a transient atmosphere around the ancient Moon” Earth and Planetary Science Letters, 478, 15 November 2017, pp175-178
5) Lunar and Planetary Institute “New NASA Study Shows Moon Once Had an Atmosphere” 5th October 2017 https://www.lpi.usra.edu/features/100517/moon-atmosphere/ with thanks to Mark
6) Jennifer Chu “Moon’s crust as fractured as can be: Study finds barrage of small asteroids shattered moon’s upper crust.” 10 September 2015 http://news.mit.edu/2015/moon-crust-fractured-0910
7) Rensselaer Polytechnic Institute. “Setting the stage for life: Scientists make key discovery about the atmosphere of early Earth”. 30th November 2011 https://news.rpi.edu/luwakkey/2953
8) Dustin Trail, E. Bruce Watson, Nicholas D. Tailby. The oxidation state of Hadean magmas and implications for early Earth’s atmosphere. Nature, 2011; 480 (7375) https://www.nature.com/articles/nature10655
9) Belinda Smith “Apollo 14 gems reveal moon is 4.51 billion years old” 12 January 2017 https://cosmosmagazine.com/geoscience/apollo-14-gems-reveal-moon-s-age-4-51-billion-years
10) A. Nemchin et al. “Record of the ancient Martian hydrosphere and atmosphere preserved in zircon from a Martian meteorite”. Nature Geoscience, 2014, 7: pp638-642, http://www.diva-portal.org/smash/get/diva2:757022/FULLTEXT01.pdf
Image 1 Credit: NASA MSFC
Image 2 credit: NRC, adapted from reference 2
Image 3 Credit: Andy Lloyd