Astronomy

Can a tectonically inactive planet retain a long-term atmosphere?

Can a tectonically inactive planet retain a long-term atmosphere?


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Can a planet be tectonically inactive and still retain a magnetosphere and protected atmosphere? How does that work?

How else could a planet retain a thick atmosphere like Earth's for extensive periods of time?

Note, I'm not asking on Earth science SE as, well, the question is not about Earth. I was hoping there was some sort of astronomical theory for other potential habitable planets.


Yes, a tectonically inactive planet can retain a long-term atmosphere.

You make the connection that a lack of plate tectonics on a planet indicates a "dead" core and thus said planet has no magnetosphere. As such, I'm going to interpret your question as, can a planet without a magnetosphere retain an atmosphere long-term? As proof, I offer up Venus.

Venus' Magnetic Field

Venus is a planet without a magnetosphere generated by a core. It is thought that the cause for this is Venus' slow rotation rate (of nearly 243 days) and a lack of convection, allowing for bulk motion in the core. As I'm sure you know, you need moving charge to make magnetic fields and Venus' core just isn't moving. As such, we see that Venus is a tectonically dead planet - it's surface is roughly 500 million years old, whereas Earth's surface gets recycled every 100 million years or less due to our plate tectonics.

Now, Venus is not entirely devoid of a magnetic field. Ironically enough, it's lack of a magnetosphere allows for the generation of a magnetic field by it's atmosphere. Because the Sun's radiation is more or less directly hitting the atmosphere, Venus has a strong ionosphere. When you get lots of charged particles moving around in an atmosphere, you get a magnetic field. But on the whole, this field is very, very weak compared to a true magnetosphere such as we have on Earth.

I found this source which talks a lot about this concept and why Venus doesn't have a magnetosphere. Check it out to get a lot more in depth detail.

Venus' Atmosphere

So, Venus has no appreciable magnetosphere (or plate tectonics). Why does it have an atmosphere? And boy does it have an atmosphere. The surface pressure on Venus is estimated to be $sim93:mathrm{atm}$.

In short, the answer is that the inundation of solar wind against an atmosphere is not necessarily the major contributing factor to atmospheric loss. It can be, but not always. For example, Mercury, another planet with a weak (but non-zero) magnetosphere, has no atmosphere (if it ever did) because it is so close to the Sun that the solar wind likely blew away that atmosphere long ago. Venus on the other hand is far enough away that the solar wind just can't strip the atmosphere. Here I'm going to quote wikipedia directly (emphasis mine).

A lack of magnetic field does not determine the fate of a planet's atmosphere. Venus, for instance, has no powerful magnetic field. Its close proximity to the Sun also increases the speed and number of particles, and would presumably cause the atmosphere to be stripped almost entirely, much like that of Mars. Despite this, the atmosphere of Venus is two orders of magnitudes denser than Earth's. Recent models indicate that stripping by solar wind accounts for less than 1/3 of total non-thermal loss processes.

Atmospheric Loss

If the solar wind isn't the contributing factor in atmospheric loss, what is? The answer to that is a process known as Jean's Escape. To put it simply, for gas particles in the atmosphere to escape into space, they need enough energy to climb out of the planet's gravity well. Some particles will have that energy and thus escape into space. Over time, the atmosphere bleeds off, little by little (this is happening for Earth too!).

The factors that contribute to the rate at which a planet loses its atmosphere are such things as the the planet's mass and radius, and the mass of the atmospheric particles. Let's look at Venus. It is comparable in mass and size to Earth and so has a reasonably appreciable gravity well. For something to escape Venus it must be traveling at $10.4:mathrm{km/s}$ (compared to the Earth's $11.2:mathrm{km/s}$). But, for Venus at least, the important factor there is that the atoms and molecules in its atmosphere are heavy. It is almost entirely ($sim97\%$) carbon dioxide which has a mass of $sim44:mathrm{amu}$. That means, the chances of such a massive particle getting the energy to escape is pretty small.

Out-gassing

Just one more point to add to this. One may argue that possibly the atmosphere is/can be replenished continuously, but that won't work here because we're assuming the planet is tectonically dead. You can't really have out-gassing on a planet with no active surface.

Conclusion

There are many factors that determine atmospheric escape. Different planets will lose their atmospherics for different reasons. However, it is entirely possible for a planet, under the right conditions, to maintain an atmosphere long-term, despite lacking a global magnetosphere. As we can see from Venus, the conditions are generally that the planet should be sufficiently far from the star, its atmosphere should be sufficiently dense and comprised of heavy particles, and the planet itself should be large enough to have an appreciable gravity well. If these conditions are all met, a planet may retain an atmosphere without having a magnetosphere to protect it.


Maybe Mars Didn’t Lose its Water After All. It’s Still Trapped on the Planet

Roughly 4 billion years ago, Mars looked a lot different than it does today. For starters, its atmosphere was thicker and warmer, and liquid water flowed across its surface. This included rivers, standing lakes, and even a deep ocean that covered much of the northern hemisphere. Evidence of this warm, watery past has been preserved all over the planet in the form of lakebeds, river valleys, and river deltas.

For some time, scientists have been trying to answer a simple question: where did all that water go? Did it escape into space after Mars lost its atmosphere, or retreat somewhere? According to new research from Caltech and the NASA Jet Propulsion Laboratory (JPL), between 30% and 90% of Mars’ water went underground. These findings contradict the widely-accepted theory that Mars lost its water to space over the course of eons.

The research was led by Eva Scheller, a Ph.D. candidate at the California Institute of Technology (Caltech). She was joined by Caltech Prof. Bethany Ehlmann, who is also the associate director for the Keck Institute for Space Studies Caltech Prof. Yuk Yung, a senior research scientist with NASA JPL Caltech graduate student Danica Adams and JPL research scientist Renyu Hu.

Artist’s impression of flowing water on Mars. Credit: Kevin M. Gill

In the past two decades, NASA and other space agencies have dispatched over a dozen robotic explorers to the Red Planet to characterize its geology, climate, surface, atmosphere, and evolution. In the process, they learned that Mars once had enough water on its surface to cover the entire planet in an ocean between 100 and 1,500 meters (330 to 4920 ft) in depth – a volume equal to half of the Atlantic Ocean.

By 3 billion years ago, Mars’ surface water had disappeared and the landscape became as it is today (freezing cold and desiccated). Given how much water once flowed there, scientists wondered how it could have disappeared so thoroughly. Until recently, scientists theorized that atmospheric escape was the key, where water is chemically disassociated and then lost to space.

This process is known as photodissociation, where exposure to solar radiation breaks down water molecules into hydrogen and oxygen. At this point, the theory goes, Mars’ low gravity allowed for it to be stripped from the atmosphere by solar wind. While this mechanism is sure to have played a role, scientists have concluded that it cannot account for the majority of Mars’ lost water.

Artist’s concept depicting the early Martian environment (right) versus the cold, dry environment seen at Mars today (left). Image Credit: NASA’s Goddard Space Flight Center

For the sake of their study, the team analyzed data from Martian meteorites, rover, and orbiter missions to determine how the ratio of deuterium to hydrogen (D/H) changed over time. They also analyzed the composition of Mars’ atmosphere and crust today, which allowed them to place constraints on how much water existed on Mars over time.

Deuterium (aka. “heavy water”) is a stable isotope of hydrogen that has both a proton and neutron in its nucleus, whereas normal hydrogen (protium) is made up of a single proton orbited by one electron. This heavier isotope accounts for a tiny fraction of hydrogen in the known Universe (about 0.02%) and has a harder time breaking free of a planet’s gravity and escaping into space.

Because of this, the loss of a planet’s water to space would leave a telltale signature in the atmosphere in the form of a larger-than-normal level of deuterium. However, this is inconsistent with the observed ratio of deuterium to protium in Mars’ atmosphere, hence why Scheller and her colleagues propose that much of the water was absorbed by minerals in the planet’s crust. As Ehlmann explained in a recent Caltech news release:

“Atmospheric escape clearly had a role in water loss, but findings from the last decade of Mars missions have pointed to the fact that there was this huge reservoir of ancient hydrated minerals whose formation certainly decreased water availability over time.”

Jezero Crater on Mars is the landing site for NASA’s Mars 2020 rover. Image Credit: NASA/JPL-Caltech/ASU

On Earth, flowing water weathers rocks to form clays and hydrous minerals, which contain water as part of their mineral structure. Since Earth is tectonically active, hydrated minerals are endlessly cycled between the mantle and the atmosphere (through volcanism). Clays and hydrated minerals have also been found on Mars, an indication that water once flowed there.

But since Mars is tectonically inactive (for the most part), its surface water was sequestered early on and never cycled back out. Thus, the features that indicate the past presence of water were preserved by the permanent drying of the surface. Meanwhile, a significant portion of that water was preserved by becoming absorbed beneath the surface.

This study not only addresses the question of how Mars’ water disappeared billions of years ago. It could also be good news for future crewed missions to Mars, which will depend on locally-harvested ice and water. Previously, co-authors Ehlmann, Huh, and Yung collaborated on research that traced the history of carbon on Mars – since carbon dioxide is the principle constituent of the Martian atmosphere.

In the future, the team plans to keep analyzing isotopic and mineral composition data to determine what became of nitrogen and sulfur-bearing minerals on Mars. In addition, Scheller plans to expand their research on what became of Mars’ water by conducting lab experiments that simulate Martian weathering processes and through observations of the ancient crust in the Jezero crater (where Perseverance is currently exploring).

Artist’s impression of the Perseverance rover on Mars. Credit: NASA/JPL-Caltech

Scheller and Ehlmann are also slated to assist with the operations of the Perseverance rover when it comes time for it to collect rock and drill samples. These will be returned to Earth by a subsequent NASA-ESA mission, where researchers will be able to examine them. For Scheller, Ehlmann, and their colleagues, this will allow them to test their theories about climate change on Mars and what drives it.

The study that describes their findings recently appeared in the journal Science, titled “Long-term Drying of Mars Caused by Sequestration of Ocean-scale Volumes of Water in the Crust,” and was presented on March 16 th during the Lunar and Planetary Science Conference (LPSC). Due to COVID restrictions, this year’s conference was virtual and took place from March 15 th to 19 th .


Probing the Martian subsurface

InSight touched down near the Martian equator in November 2018, kicking off a two-year, $850 million mission to probe the Red Planet's interior in unprecedented detail.

The stationary lander carries two main science instruments to do this work: a supersensitive suite of seismometers and a burrowing heat probe dubbed "the mole," which is designed to get at least 10 feet (3 meters) below the Red Planet's surface.

Analyses of marsquake and heat-transport measurements will allow the mission team to construct a detailed, 3D map of the Martian interior, NASA officials have said. In addition, InSight scientists are using radio signals beamed from the lander to track how much Mars wobbles on its axis over time. This information will help researchers determine how big and dense the planet's core is. (The mission's full name — Interior Exploration using Seismic Investigations, Geodesy and Heat Transport — references these various lines of investigation.)

Overall, InSight's observations will help scientists better understand how rocky planets such as Mars, Earth and Venus form and evolve, mission team members have said.

The mission's initial science returns, which were published today (Feb. 21) in six papers in the journals Nature Geoscience and Nature Communications, show that InSight is on track to meet that long-term goal, Banerdt said. (We have gotten a taste of these results over the past year or so, however, as mission team members have released some findings in dribs and drabs.)


Abstract

To better understand Earth's present tectonic style–plate tectonics–and how it may have evolved from single plate (stagnant lid) tectonics, it is instructive to consider how common it is among similar bodies in the Solar System. Plate tectonics is a style of convection for an active planetoid where lid fragment (plate) motions reflect sinking of dense lithosphere in subduction zones, causing upwelling of asthenosphere at divergent plate boundaries and accompanied by focused upwellings, or mantle plumes any other tectonic style is usefully called “stagnant lid” or “fragmented lid”. In 2015 humanity completed a 50+ year effort to survey the 30 largest planets, asteroids, satellites, and inner Kuiper Belt objects, which we informally call “planetoids” and use especially images of these bodies to infer their tectonic activity. The four largest planetoids are enveloped in gas and ice (Jupiter, Saturn, Uranus, and Neptune) and are not considered. The other 26 planetoids range in mass over 5 orders of magnitude and in diameter over 2 orders of magnitude, from massive Earth down to tiny Proteus these bodies also range widely in density, from 1000 to 5500 kg/m 3 . A gap separates 8 silicate planetoids with ρ = 3000 kg/m 3 or greater from 20 icy planetoids (including the gaseous and icy giant planets) with ρ = 2200 kg/m 3 or less. We define the “Tectonic Activity Index” (TAI), scoring each body from 0 to 3 based on evidence for recent volcanism, deformation, and resurfacing (inferred from impact crater density). Nine planetoids with TAI = 2 or greater are interpreted to be tectonically and convectively active whereas 17 with TAI <2 are inferred to be tectonically dead. We further infer that active planetoids have lithospheres or icy shells overlying asthenosphere or water/weak ice. TAI of silicate (rocky) planetoids positively correlates with their inferred Rayleigh number. We conclude that some type of stagnant lid tectonics is the dominant mode of heat loss and that plate tectonics is unusual. To make progress understanding Earth's tectonic history and the tectonic style of active exoplanets, we need to better understand the range and controls of active stagnant lid tectonics.


Un pianeta tettonicamente inattivo può conservare unɺtmosfera a lungo termine?

Un pianeta può essere tettonicamente inattivo e conservare ancora una magnetosfera e un'atmosfera protetta? Come funziona?

In quale altro modo un pianeta potrebbe conservare un'atmosfera densa come quella terrestre per lunghi periodi di tempo?

Nota, non sto chiedendo a Earth science SE, anche la domanda non riguarda la Terra. Speravo ci fosse una sorta di teoria astronomica per altri potenziali pianeti abitabili.

Sì, un pianeta tettonicamente inattivo può conservare un'atmosfera a lungo termine.

Fai il collegamento che la mancanza di tettonica a zolle su un pianeta indica un nucleo "morto" e quindi detto pianeta non ha magnetosfera. In quanto tale, interpreterò la tua domanda: un pianeta senza magnetosfera può conservare un'atmosfera a lungo termine? Come prova, offro Venere.

Campo magnetico di Venere

Venere è un pianeta senza una magnetosfera generata da un nucleo. Si ritiene che la causa di ciò sia la bassa velocità di rotazione di Venere (di quasi 243 giorni) e una mancanza di convezione, che consente il movimento di massa nel nucleo. Come sono sicuro che sai, devi creare una carica in movimento per creare campi magnetici e il nucleo di Venere non si muove. Come tale, vediamo che Venere è un pianeta tettonicamente morto - la sua superficie ha circa 500 milioni di anni, mentre la superficie della Terra viene riciclata ogni 100 milioni di anni o meno a causa della nostra tettonica a zolle.

Ora, Venere non è del tutto priva di un campo magnetico. Per ironia della sorte, la mancanza di una magnetosfera consente la generazione di un campo magnetico dalla sua atmosfera. Poiché la radiazione solare colpisce più o meno direttamente l'atmosfera, Venere ha una forte ionosfera. Quando si muovono molte particelle cariche in un'atmosfera, si ottiene un campo magnetico. Ma nel complesso, questo campo è molto, molto debole rispetto a una vera magnetosfera come quella che abbiamo sulla Terra.

Ho trovato questa fonte che parla molto di questo concetto e del perché Venere non ha una magnetosfera. Dai un'occhiata per avere molti più dettagli.

Atmosfera di Venere

In breve, la risposta è che l'inondazione del vento solare contro un'atmosfera non è necessariamente il principale fattore che contribuisce alla perdita atmosferica. Può essere, ma non sempre. Ad esempio, Mercurio, un altro pianeta con una magnetosfera debole (ma diversa da zero), non ha atmosfera (se mai lo ha fatto) perché è così vicino al Sole che probabilmente il vento solare ha spazzato via quell'atmosfera molto tempo fa. Venere, d'altra parte, è abbastanza lontana da impedire al vento solare di eliminare l'atmosfera. Qui citerò direttamente Wikipedia (enfasi sulla mia).

Una mancanza di campo magnetico non determina il destino dell'atmosfera di un pianeta. Venere, ad esempio, non ha un potente campo magnetico. La sua vicinanza al Sole aumenta anche la velocità e il numero di particelle, e presumibilmente causerebbe la distruzione quasi totale dell'atmosfera, proprio come quella di Marte. Nonostante ciò, l'atmosfera di Venere è di due ordini di grandezza più densa di quella terrestre. I modelli recenti indicano che lo stripping da eolico solare rappresenta meno di 1/3 dei processi totali di perdita non termica.

Perdita atmosferica

Se il vento solare non è il fattore che contribuisce alla perdita atmosferica, che cos'è? La risposta è un processo noto come Fuga di Jean . Per dirla semplicemente, affinché le particelle di gas nell'atmosfera fuggano nello spazio, hanno bisogno di energia sufficiente per uscire dal pozzo di gravità del pianeta. Alcune particelle avranno quell'energia e quindi fuggiranno nello spazio. Nel tempo, l'atmosfera si diffonde, a poco a poco (questo sta accadendo anche per la Terra!).

10.4 k m / s 11.2 k m / s ∼ 97 % ∼ 44 a m u

Out-fornire di gas

Solo un altro punto da aggiungere a questo. Si potrebbe sostenere che probabilmente l'atmosfera è / può essere riempita continuamente, ma che non funzionerà qui perché stiamo assumendo che il pianeta sia morto tettonicamente. Non si può davvero avere la degassificazione su un pianeta senza superficie attiva.

Conclusione

Ci sono molti fattori che determinano la fuga atmosferica. Pianeti diversi perderanno la loro atmosfera per diversi motivi. Tuttavia, è del tutto possibile per un pianeta, nelle giuste condizioni, mantenere un'atmosfera a lungo termine, nonostante manchi una magnetosfera globale. Come possiamo vedere da Venere, le condizioni sono generalmente che il pianeta dovrebbe essere sufficientemente lontano dalla stella, la sua atmosfera dovrebbe essere sufficientemente densa e composta da particelle pesanti, e il pianeta stesso dovrebbe essere abbastanza grande da avere un apprezzabile pozzo di gravità. Se tutte queste condizioni sono soddisfatte, un pianeta può conservare un'atmosfera senza avere una magnetosfera per proteggerla.


Earth: A Borderline Planet For Life?

Our planet is changing before our eyes, and as a result, many species are living on the edge. Yet Earth has been on the edge of habitability from the beginning. New work by astronomers at the Harvard-Smithsonian Center for Astrophysics shows that if Earth had been slightly smaller and less massive, it would not have plate tectonics - the forces that move continents and build mountains. And without plate tectonics, life might never have gained a foothold on our world.

"Plate tectonics are essential to life as we know it," said Diana Valencia of Harvard University. "Our calculations show that bigger is better when it comes to the habitability of rocky planets."

Plate tectonics involve the movement of huge chunks, or plates, of a planet's surface. Plates spread apart from each other, slide under one another, and even crash into each other, lifting gigantic mountain ranges like the Himalayas. Plate tectonics are powered by magma boiling beneath the surface, much like a bubbling pot of chocolate. The chocolate on top cools and forms a skin or crust, just as magma cools to form the planet's crust.

Plate tectonics are crucial to a planet's habitability because they enable complex chemistry and recycle substances like carbon dioxide, which acts as a thermostat and keeps Earth balmy. Carbon dioxide that was locked into rocks is released when those rocks melt, returning to the atmosphere from volcanoes and oceanic ridges.

"Recycling is important even on a planetary scale," Valencia explained.

Valencia and her colleagues, Richard O'Connell and Dimitar Sasselov (Harvard University), examined the extremes to determine whether plate tectonics would be more or less likely on different-sized rocky worlds. In particular, they studied so-called "super-Earths"-planets more than twice the size of Earth and up to 10 times as massive. (Any larger, and the planet would gather gas as it forms, becoming like Neptune or even Jupiter.)

The team found that super-Earths would be more geologically active than our planet, experiencing more vigorous plate tectonics due to thinner plates under more stress. Earth itself was found to be a borderline case, not surprisingly since the slightly smaller planet Venus is tectonically inactive.

"It might not be a coincidence that Earth is the largest rocky planet in our solar system, and also the only one with life," said Valencia.

Exoplanet searches have turned up five super-Earths already, although none have life-friendly temperatures. If super-Earths are as common as observations suggest, then it is inevitable that some will enjoy Earth-like orbits, making them excellent havens for life.

"There are not only more potentially habitable planets, but MANY more," stated Sasselov, who is director of the Harvard Origins of Life Initiative.

In fact, a super-Earth could prove to be a popular vacation destination to our far-future descendants. Volcanic "rings of fire" could span the globe while the equivalent of Yellowstone Park would bubble with hot springs and burst with hundreds of geysers. Even better, an Earth-like atmosphere would be possible, while the surface gravity would be up to three times that of Earth on the biggest super-Earths.

"If a human were to visit a super-Earth, they might experience a bit more back pain, but it would be worth it to visit such a great tourist spot," Sasselov suggested with a laugh.

He added that although a super-Earth would be twice the size of our home planet, it would have similar geography. Rapid plate tectonics would provide less time for mountains and ocean trenches to form before the surface was recycled, yielding mountains no taller and trenches no deeper than those on Earth. Even the weather might be comparable for a world in an Earth-like orbit.

"The landscape would be familiar. A super-Earth would feel very much like home," said Sasselov.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

This research was the subject of a press conference at the 211th meeting of the American Astronomical Society.


What happened to Mars's water? It is still trapped there

While it was previously suspected that most of Mars's water was lost to space, a significant portion—between 30 and 90 percent—has been lost to hydration of the crust, according to a new study. Some water was released from the interior via volcanism, but not enough to replenish the planet's once significant supply. Evidence for the water's fate was found in the ratio of deuterium to hydrogen in the planet's atmosphere and rocks. Credit: California Institute of Technology

Billions of years ago, the Red Planet was far more blue according to evidence still found on the surface, abundant water flowed across Mars and forming pools, lakes, and deep oceans. The question, then, is where did all that water go?

The answer: nowhere. According to new research from Caltech and JPL, a significant portion of Mars's water—between 30 and 99 percent—is trapped within minerals in the planet's crust. The research challenges the current theory that the Red Planet's water escaped into space.

The Caltech/JPL team found that around four billion years ago, Mars was home to enough water to have covered the whole planet in an ocean about 100 to 1,500 meters deep a volume roughly equivalent to half of Earth's Atlantic Ocean. But, by a billion years later, the planet was as dry as it is today. Previously, scientists seeking to explain what happened to the flowing water on Mars had suggested that it escaped into space, victim of Mars's low gravity. Though some water did indeed leave Mars this way, it now appears that such an escape cannot account for most of the water loss.

"Atmospheric escape doesn't fully explain the data that we have for how much water actually once existed on Mars," says Caltech Ph.D. candidate Eva Scheller (MS '20), lead author of a paper on the research that was published by the journal Science on March 16 and presented the same day at the Lunar and Planetary Science Conference (LPSC). Scheller's co-authors are Bethany Ehlmann, professor of planetary science and associate director for the Keck Institute for Space Studies Yuk Yung, professor of planetary science and JPL senior research scientist Caltech graduate student Danica Adams and Renyu Hu, JPL research scientist. Caltech manages JPL for NASA.

The team studied the quantity of water on Mars over time in all its forms (vapor, liquid, and ice) and the chemical composition of the planet's current atmosphere and crust through the analysis of meteorites as well as using data provided by Mars rovers and orbiters, looking in particular at the ratio of deuterium to hydrogen (D/H).

Water is made up of hydrogen and oxygen: H2O. Not all hydrogen atoms are created equal, however. There are two stable isotopes of hydrogen. The vast majority of hydrogen atoms have just one proton within the atomic nucleus, while a tiny fraction (about 0.02 percent) exist as deuterium, or so-called "heavy" hydrogen, which has a proton and a neutron in the nucleus.

The lighter-weight hydrogen (also known as protium) has an easier time escaping the planet's gravity into space than its heavier counterpart. Because of this, the escape of a planet's water via the upper atmosphere would leave a telltale signature on the ratio of deuterium to hydrogen in the planet's atmosphere: there would be an outsized portion of deuterium left behind.

However, the loss of water solely through the atmosphere cannot explain both the observed deuterium to hydrogen signal in the Martian atmosphere and large amounts of water in the past. Instead, the study proposes that a combination of two mechanisms—the trapping of water in minerals in the planet's crust and the loss of water to the atmosphere—can explain the observed deuterium-to-hydrogen signal within the Martian atmosphere.

When water interacts with rock, chemical weathering forms clays and other hydrous minerals that contain water as part of their mineral structure. This process occurs on Earth as well as on Mars. Because Earth is tectonically active, old crust continually melts into the mantle and forms new crust at plate boundaries, recycling water and other molecules back into the atmosphere through volcanism. Mars, however, is mostly tectonically inactive, and so the "drying" of the surface, once it occurs, is permanent.

"Atmospheric escape clearly had a role in water loss, but findings from the last decade of Mars missions have pointed to the fact that there was this huge reservoir of ancient hydrated minerals whose formation certainly decreased water availability over time," says Ehlmann.

"All of this water was sequestered fairly early on, and then never cycled back out," Scheller says. The research, which relied on data from meteorites, telescopes, satellite observations, and samples analyzed by rovers on Mars, illustrates the importance of having multiple ways of probing the Red Planet, she says.

Ehlmann, Hu, and Yung previously collaborated on research that seeks to understand the habitability of Mars by tracing the history of carbon, since carbon dioxide is the principal constituent of the atmosphere. Next, the team plans to continue to use isotopic and mineral composition data to determine the fate of nitrogen and sulfur-bearing minerals. In addition, Scheller plans to continue examining the processes by which Mars's surface water was lost to the crust using laboratory experiments that simulate Martian weathering processes, as well as through observations of ancient crust by the Perseverance rover. Scheller and Ehlmann will also aid in Mars 2020 operations to collect rock samples for return to Earth that will allow the researchers and their colleagues to test these hypotheses about the drivers of climate change on Mars.

The paper, titled "Long-term Drying of Mars Caused by Sequestration of Ocean-scale Volumes of Water in the Crust," published in Science on 16 March 2021.


What Happened to Mars's Water? It is Still Trapped There

Billions of years ago, the Red Planet was far more blue according to evidence still found on the surface, abundant water flowed across Mars and forming pools, lakes, and deep oceans. The question, then, is where did all that water go?

The answer: nowhere. According to new research from Caltech and JPL, a significant portion of Mars's water—between 30 and 99 percent—is trapped within minerals in the planet's crust. The research challenges the current theory that the Red Planet's water escaped into space.

The Caltech/JPL team found that around four billion years ago, Mars was home to enough water to have covered the whole planet in an ocean about 100 to 1,500 meters deep a volume roughly equivalent to half of Earth's Atlantic Ocean. But, by a billion years later, the planet was as dry as it is today. Previously, scientists seeking to explain what happened to the flowing water on Mars had suggested that it escaped into space, victim of Mars's low gravity. Though some water did indeed leave Mars this way, it now appears that such an escape cannot account for most of the water loss.

"Atmospheric escape doesn't fully explain the data that we have for how much water actually once existed on Mars," says Caltech PhD candidate Eva Scheller (MS ✠), lead author of a paper on the research that was published by the journal Science on March 16 and presented the same day at the Lunar and Planetary Science Conference (LPSC). Scheller's co-authors are Bethany Ehlmann, professor of planetary science and associate director for the Keck Institute for Space Studies Yuk Yung, professor of planetary science and JPL senior research scientist Caltech graduate student Danica Adams and Renyu Hu, JPL research scientist. Caltech manages JPL for NASA.

The team studied the quantity of water on Mars over time in all its forms (vapor, liquid, and ice) and the chemical composition of the planet's current atmosphere and crust through the analysis of meteorites as well as using data provided by Mars rovers and orbiters, looking in particular at the ratio of deuterium to hydrogen (D/H).

Water is made up of hydrogen and oxygen: H2O. Not all hydrogen atoms are created equal, however. There are two stable isotopes of hydrogen. The vast majority of hydrogen atoms have just one proton within the atomic nucleus, while a tiny fraction (about 0.02 percent) exist as deuterium, or so-called "heavy" hydrogen, which has a proton and a neutron in the nucleus.

The lighter-weight hydrogen (also known as protium) has an easier time escaping the planet's gravity into space than its heavier counterpart. Because of this, the escape of a planet's water via the upper atmosphere would leave a telltale signature on the ratio of deuterium to hydrogen in the planet's atmosphere: there would be an outsized portion of deuterium left behind.

However, the loss of water solely through the atmosphere cannot explain both the observed deuterium to hydrogen signal in the Martian atmosphere and large amounts of water in the past. Instead, the study proposes that a combination of two mechanisms—the trapping of water in minerals in the planet's crust and the loss of water to the atmosphere—can explain the observed deuterium-to-hydrogen signal within the Martian atmosphere.

When water interacts with rock, chemical weathering forms clays and other hydrous minerals that contain water as part of their mineral structure. This process occurs on Earth as well as on Mars. Because Earth is tectonically active, old crust continually melts into the mantle and forms new crust at plate boundaries, recycling water and other molecules back into the atmosphere through volcanism. Mars, however, is mostly tectonically inactive, and so the "drying" of the surface, once it occurs, is permanent.

"Atmospheric escape clearly had a role in water loss, but findings from the last decade of Mars missions have pointed to the fact that there was this huge reservoir of ancient hydrated minerals whose formation certainly decreased water availability over time," says Ehlmann.

"All of this water was sequestered fairly early on, and then never cycled back out," Scheller says. The research, which relied on data from meteorites, telescopes, satellite observations, and samples analyzed by rovers on Mars, illustrates the importance of having multiple ways of probing the Red Planet, she says.

Ehlmann, Hu, and Yung previously collaborated on research that seeks to understand the habitability of Mars by tracing the history of carbon, since carbon dioxide is the principal constituent of the atmosphere. Next, the team plans to continue to use isotopic and mineral composition data to determine the fate of nitrogen and sulfur-bearing minerals. In addition, Scheller plans to continue examining the processes by which Mars's surface water was lost to the crust using laboratory experiments that simulate Martian weathering processes, as well as through observations of ancient crust by the Perseverance rover. Scheller and Ehlmann will also aid in Mars 2020 operations to collect rock samples for return to Earth that will allow the researchers and their colleagues to test these hypotheses about the drivers of climate change on Mars.

The paper, titled "Long-term Drying of Mars Caused by Sequestration of Ocean-scale Volumes of Water in the Crust," published in Science on 16 March 2021. This work was supported by a NASA Habitable Worlds award, a NASA Earth and Space Science Fellowship (NESSF) award, and a NASA Future Investigator in NASA Earth and Space Science and Technology (FINESST) award.


Is Mars tectonically active like Earth? Or is Earth unique to our solar system in that aspect?

The consensus is "not any more" and that it may not ever have experienced major tectonic activity as you're probably imagining (plate tectonics).

There is however, some residual energy left in the lithosphere and faulting and small quakes are thought to occur, these are what the 2016 Insight Mission is hoping to measure.

Oh. That's interesting. I always just assumed (often a mistake) that Mars' Olympus Mons (Mount Olympus--the tallest mountain in the solar system) was formed by super violent tectonic activity. But according to Wikipedia, "Olympus Mons is the result of many thousands of highly fluid, basaltic lava flows that poured from volcanic vents over a long period of time. (The Hawaiian Islands exemplify similar shield volcanoes on a smaller scale – see Mauna Kea.) The extraordinary size of Olympus Mons is likely because Mars lacks mobile tectonic plates. Unlike on Earth, the crust of Mars remains fixed over a stationary hotspot, and a volcano can continue to discharge lava until it reaches an enormous height."


You can synthesize "everything", with enough energy

I put "everything" in quotes because there are some complex substances that we humans have not yet perfected creating. But for all the simple stuff, like Calcium Carbonate and Calcium Silicates that you need for making cement, you can synthesize, as long as you just have an abundance of energy.

So you bring your reactor, either plain old fission reactor running on Uranium or Thorium or discarded Plutonium, or a handy little fusion ditto like a Polywell running on Boron and Hydrogen. These are the first thing you then mine: fuel for your reactor.

Next up you want to sustain yourself. For that you need water, air and fertilizer. Using the energy from your handy reactor you reduce minerals to extract Oxygen, Nitrogen, Hydrogen, Phosphorous and Carbon Dioxide. From this you start your hydroponics. Then, using the plant matter from these, and the load of useful soil bacteria you brought along, you can start making real soil.

In the mean time you are also busy extracting all sorts of primary materials, Silicates, Calcium, Iron, Aluminium and so on, to be used for construction materials.

I am not saying this will be easy or efficient, but if you just hand-wave away the difficulties in producing energy — by for instance assuming that fusion is viable and works as well as we hope it will — then you will have all the starting material you need to get going.