The Origin of Ancient Xenon
I’ve often discussed the origin of various elements and compounds on Earth – most notably the isotopic ratio of water, and what that might tell us about the origin of terrestrial water (1). Data about this can help provide evidence for the Earth’s early history, and often the data is inconsistent with the general theories of oceanic origin, like the ‘late veneer theory’, for instance, where the bulk of terrestrial waters were supposed to have been supplied by comets. It turns out that the water was on this planet all along (2,3), raising questions about why the Sun’s heat had not driven this relatively volatile resource away from the primordial Earth during the early history of the solar system.
Despite such evidence, the ‘late veneer theory’ continues to hold ground for many scientists, and tends to go unchallenged within the science media. This is apparent within the following excerpt about a new paper on the mysterious presence of a particular isotope of the noble gas xenon found in ancient terrestrial water encased in rock:
“The scientists have been analysing tiny samples of ancient air trapped in water bubbles found in the mineral, quartz, which dates back more than three billion years. The team found that the air in the rocks is partly made up of an extremely rare form of the chemical element, xenon. It is known as U-Xe and what makes it so rare is that it isn’t usually found on Earth. The component is not present in the Earth’s mantle, nor is it found in meteorites.
“Therefore, the team believe that the U-Xe must have been added to the Earth after a primordial atmosphere had developed. Simply put, comets are the best candidates for carrying the U-Xe to the planet. Co-author, Prof Ray Burgess, from Manchester’s School of Earth and Environmental Sciences explains: “The Earth formed too close to the Sun for volatile elements, such as U-Xe, to easily condense and they would have rapidly boiled off the surface and been lost to space.
“”The reason that oceans and an atmosphere exist at all is because volatiles were still being added after the Earth formed. The puzzle is in identifying where the volatiles came from and what objects carried them to the early Earth. The difficulty is that many of the different volatile ingredients that were originally added have been thoroughly mixed together by geological processes during Earth’s long geological history.”” (4)
It turns out that xenon, in general, is mostly absent from the Earth’s atmosphere, particularly compared to other noble gases like argon. No one knows why. Perhaps the missing xenon is encapsulated within rocks buried deep within the Earth. Or perhaps, conversely, it has been driven off the Earth because it is not easily captured by rocks like perovskite (5). Xenon is missing from Mars, too, which may allude to its propensity for loss from a weak atmosphere.
This is a confusing picture, and the scientific explanations for what may be going on here don’t seem to me to entirely add up. Let’s work through what we know (or think we know):
Evidence suggests the ancient Earth already had its volatile aqueous oceans in place, rather than being delivered to a desiccated planet by comet bombardment over time (2).
The isotopic ratio of cometary water differs from that of the Earth, also suggesting that terrestrial waters do not originate from comets (6).
Data from ancient rocks now suggests that xenon was also present early on, but is now largely absent from the Earth’s atmosphere. What xenon is left on Earth has an isotopic distribution which bears little resemblance to those in asteroids, meteoritic chondrites and solar gases (7). This is itself anomalous – part of the ‘xenon paradox’ (5). These bizarre xenon isotopic distributions have led to the proposal of a theoretical xenon composition known as U-Xe (8), which represents the Earth’s original primordial component. It is argued that this theoretical isotopic composition is what has now been observed in the quartz rock – dated to 3.3 billion years ago, and representative of the Xenon composition in Earth’s atmosphere at that time (7).
The argument is put forward that the xenon, at least, was delivered by comets at a late stage in the Earth’s formation. In other words, xenon was not an original surviving component of the primordial Earth, but was delivered from the outer solar system late on by comets (perhaps during the late, heavy bombardment ~3.9 billion years ago)… and has now largely disappeared again.
Earth waters more closely resemble waters in the outer asteroid belt. It is thought that migration of the giant planets, particularly Jupiter and Saturn, may have created significant fluctuations in the orbits of asteroids, causing them to cross paths with the Earth. The outer ‘primitive’ component of the present asteroid belt would have originally formed in the outer solar system, but within a truncated primordial disk, and then delivered water to the Earth during the phase of planetary migration (9).
Given that both water and xenon are relatively volatile, it is unclear how they survived solar wind/radiation bombardment when the Earth was in its unprotected, primordial state. The mainstream mechanisms proposed for their appearance and disappearance over time seem inconsistent, and increasingly complex.
Into this heady mix, I’ll toss the following astounding possibility: The Earth began its life between Mars and Jupiter, in the position of the current outer asteroid belt, beyond 3AU. It wasn’t just the asteroids, Jupiter and Saturn that migrated about, but also our planet. It may have been catapulted into its present position through the intervention of a significant other, which caused all of this chaos in the first place, during the late, heavy bombardment. That significant other is the proposed ‘Dark Star’, sub-brown dwarf companion, Planet X, Nibiru, call it what you will… (10).
The recent observation of the theoretical U-Xe archaic composition of xenon in ancient rocks might provide some evidence for this alternative viewpoint that the Earth started its life further out, and could therefore hold onto volatiles like xenon more easily – at least initially. Only when Earth shifted inwards, closer to the Sun, did the planetary xenon gradually get stripped away, leaving only primordial traces trapped in ancient rocks.
This makes a lot of sense, doesn’t it? But it requires a huge leap of thinking, cutting through the ever-present assumption that the Earth’s place in the Cosmos is immutable.
Written by Andy Lloyd, 25th May 2017
1) Andy Lloyd “The Great Water Conundrum” from 2002 onwards http://www.darkstar1.co.uk/water.html
2) Andy Lloyd “Earth’s Primordial Waters” 24th November 2015, http://www.andylloyd.org/darkstarblog32.htm
3) L. J. Hallis et al. “Evidence for primordial water in Earth’s deep mantle”, Science, Vol. 350, 13th November 2015, p. 795, https://www.ncbi.nlm.nih.gov/pubmed/26564850
4) University of Manchester press release “Comets contributed to Earth’s atmosphere, says study of 3 billion-year-old minerals” 18th May 2017, http://www.manchester.ac.uk/discover/news/gases-trapped-in-three-billion-years-old-minerals-reveal-origins-of-the-earths-atmosphere/ with thanks to Lee
5) Svyatoslav Shcheka & Hans Keppler “The origin of the terrestrial noble-gas signature”, Nature, 490, 25th October 2012, pp531–534, http://www.nature.com/nature/journal/v490/n7421/abs/nature11506.html
6) Altwegg, K. et al. “67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio”, Science 347, 6220, 23rd January 2015, http://science.sciencemag.org/content/347/6220/1261952
7) Guillaume Avice, Bernard Marty & Ray Burgess “The origin and degassing history of the Earth’s atmosphere revealed by Archean xenon” Nature Communications 8, Article number: 15455, 18th May 2017, https://www.nature.com/articles/ncomms15455
8) R. Pepin “On the origin and early evolution of terrestrial planet atmospheres and meteoritic volatiles”, Icarus, 92, 2–79, July 1991, https://www.researchgate.net/publication/247043623_On_the_origin_and_early_evolution_of_terrestrial_planet_atmospheres_and_meteoritic_volatiles
9) D. O’Brien et al. “Water delivery and giant impacts in the ‘Grand Tack’ scenario”. Icarus 239, pp74–84, July 2014, https://arxiv.org/abs/1407.3290
10) Andy Lloyd “Dark Star: The Planet X Evidence” Timeless Voyager Press, 2005