‘Earth’s crust is still forming — its interior shapes surface habitability through outgassing and recycling of volatiles

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Sujoy Mukhopadhyay is professor of geochemistry and cosmochemistry at the University of California, Davis. Conversing with Times Evoke, he explains how Earth’s crust — and its deep interior — mould life on our planet:

I study how planets form, the creation of the early atmosphere and the degassing or release of volatiles likecarbon dioxide(CO2), hydrogen and sulphur dioxide from planetary interiors. In this process, noble gases are key. These are a group of inert elements they dont participate in chemical reactions. Carbon dioxide, nitrogen, hydrogen, etc., are chemically reactive but the inert nature of noble gases gives them an advantage in allowing these to keep a record of early events as no chemical reactions or biology alters them.

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These offer information about planet formation. Among these, and especially among the heavier noble gases like krypton or xenon, some have many different isotopes which belong to the same element but have different numbers of neutrons. Some of these isotopes are produced by radioactive decay while some are non-radiogenic or not produced by radioactivity. The non-radiogenic isotopes allow us to look at different sources of volatiles on Earth while the others hold information on timespans and decay. They act as tracers of how fast things might be happening. They are the culmination of figuring out where gases are coming from and the rate at which a planet is forming.

BOTH STATIS FLOW: Lavas (R) found in volcanic regions like Iceland s Odadahraun highlands (L) bring materials up from Earth s interior to the surface these include noble gases which bear unalterable evidence of planetary processes, their inert nature resisting any changes. Photo courtesy: iStock


Most noble gases are in the atmosphere while some are also located in the planets interior, typically trapped in the crystal lattices of minerals. For atmospheric gases, missions like the Curiosity rover have measured their composition on Mars. There are also some measurements from Venus and new missions are meant to follow this. For Earth, we can sample the atmosphere quite easily for the interior, we need to rely on lavas or melts which come up to Earths surface, bringing gases with them. When they solidify, these melts trap bubbles inside them noble gases are often found in these tiny bubbles. We study these by crushing samples apart in our lab and doing measurements using mass spectrometers. For other planetary materials like asteroids or meteorites, we often find gases trapped in the crystal lattices of minerals present there we use heating techniques to release those and mass spectrometers to measure them.

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Xenon is a noble gas it has nine separate isotopes. Of these, many are produced by the radioactive decay of three separate groups of elements, uranium, plutonium and iodine. Uranium is still around in the solar system and on Earth. Naturally occurring plutonium was present in the early solar system and Earth but has become extinct now, having all decayed away. The same is true of radioactive iodine, which was in the early solar system and our early planet but which hasnt completely decayed yet. So, xenons isotopes could only have been produced during certain times in Earths evolution whereas other isotopes are still being produced. Some are not even produced now they only come from stellar burning in stars. So, these different processes and their diverse timescales allow us to look at the history of planet formation. Examining a certain isotope tells us this had to have been produced maybe in the first 100 million years of Earths existence it couldnt have come later on. That puts a firm timestamp on when a certain process, like a melting event or the formation of the moon, might have happened.

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LUMINOUS:Isotopes indicate the moon's age. Photo courtesy: iStock


We also study feedbacks or linkages between Earths surface and its deep interior. We try to address how the rates of volatile exchange have changed through time this includes noble gases, water, CO2 and nitrogen. Water and CO2 play a vital role in making a planet surface habitable we now know that CO2 is very important in producing the greenhouse effect, too much of it is harmful while too little also makes a planet uninhabitable. We can study this not by directly looking at CO2 but rates of noble gases which are used as a proxy to tell us what other gases are doing. We look at how much interior degassing rates have changed as Earths interior cooled and how much of these gases were recycled back through time if there were no recycling, wed expect the surface inventory of these gases to keep growing through time. But if there is recycling, we see a balance between whats coming out of the interior and what is going back into Earth through plate tectonics. The balance between the outgassing and recycling of volatiles is important in keeping Earths surface habitable.

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Some vital aspects of Earths evolution start with the differentiation of the interior into the core and mantle as Earth was forming a critical aspect is the formation of Earths crust. It is an open question as to how much of the crust formed as a function of time. One hypothesis says it formed very early but was this early crust submerged under water? And when did most of it then emerge? The answers are important to understand the evolution of life.


Even today, Earth is evolving because this differentiation is an ongoing process currently, Earths interior is partially melting at mid-oceanic ridges where tectonic plates form and move apart. We have hotspots like Hawaii and the Galapagos where we can see the products of interior melting coming up. That transports certain materials from the interior to Earths surface. Subduction zones with these tectonic plates start going back, recycling some of these materials into the interior, also creating new continental crust. As all of us live on crust, its formation is an important milestone.


Many of us think of climate change as something that happens in Earths atmosphere. But Earths interior is key in this context too. There are two ways the atmospheric composition changes theres a short-term carbon cycle that affects the amount of CO2 in the atmosphere. There, Earths oceans play a very important role. Carbon exchange between the atmosphere and oceans control how much CO2 can be present in the atmosphere over a short timescale, covering around 1,000 to 10,000 years.


For the longer timescales, Earths interior affects how much potential CO2 there might be because of outgassing from the interior and recycling. As humans, we are messing around now with the carbon budget on a timescale which isnt usually associated with natural processes our impact has been to change atmospheric CO2 in an incredibly short time. Yet, Earths interior and crust are central in affecting the long-term composition of the atmosphere, relevant over millions of years. This long-term interaction is what makes a planet habitable in the first place the shorter-term cycles, where the ocean and biosphere come into play, then affect the amount of CO2 in the atmosphere as well. So, different cycles and entities impact the atmospheric composition that enables life we need to be more aware of the crust and interior of Earth in this regard.
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