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) Read More…
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.