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Geodynamics

Peculiar Planets

The geodynamics of Enceladus: exotic and familiar

Enceladus

This week, Gael Choblet, CNRS research associate in the Laboratoire de Planétologie et Géodynamique (University of Nantes and Angers), tells us everything about the interior of Enceladus, an interesting icy moon of Saturn!

Gael Choblet

Gaël Choblet

This is my first contribution in these pages. The choice of Saturn’s small moon Enceladus as a topic mostly results from my acquaintance with this planetary body. Yet, the reader probably remains to be convinced that this particular subject belongs to that specific section of EGU Geodynamics blog – I also do, although I noticed that some colleagues have proposed texts that are adjacent to the “geodynamics” central theme, albeit on far more important or at least far more urgent topics than Enceladus research. The ongoing extinction of living species is probably more pressing than the discovery of life outside the Earth, of which Enceladus could be one of the most accessible laboratories (probability not provided here – incidentally, this is not the subject of this text). Given this, the idea I propose in the following lines is to highlight the connection between progress in the knowledge on Enceladus’ interior and Geodynamics as a discipline, using a chronological approach. In the end, we (author, reader) will see whether this choice made sense: whether a dual perspective, exotic and familiar, ensues or whether the bridges built prove artificial and ephemeral.

Full disclosure: I have co-authored some of the work I mention and the following account certainly involves bias.

Enceladus, the moon, is known to humans since the middle of the french revolution although not because of it. Saturn being more distant than Jupiter, the discovery of its largest moon Titan waited 45 years after Galileo had spotted Jupiter’s four large moons of approximately the same size. In the case of Enceladus, almost precisely one order of magnitude smaller than Titan in radius, further instrumental developments (and a huge telescope, about one order of magnitude larger than Galileo’s) were necessary for german-born William Herschel, appointed Astronomer of the King of Great Britain at the time, to discover Enceladus. William’s younger sister Caroline probably helped: besides knowing how to make tea (illustration) and being also german-born, she participated in her brother’s astronomical research and conducted her own. For this reason, she won the Gold medal of the Royal Astronomical Society then became a member (first woman to achieve each of these distinctions).

Then, strangely enough, humankind lost interest in Enceladus.

Establishing a first connexion here with the Earth is hazardous since little is known by me about human’s discovery of its birth planet.

That Enceladus surface is made of water ice appeared likely in the 1970s, even before direct spectro- scopic evidence, owing to the bulk density of icy moons as well as the fact that Saturn’s rings spectra seem compatible with that of H2O ice. French engineers had already associated Enceladus to water when designing a fountain for the “Bosquet de l’Encelade” in the gardens of the Versailles castle (illus- tration). Yet, one has to admit that rather than a dazzling prescience (the moon was not known yet), their choice of water as a material was probably motivated by the inability to construct a magma foun- tain: Enceladus (the mythological figure), was one of these proto-gods, brothers and sisters of Saturn, whose access to fame mostly sums to their defeat against the (true) olympian deities. Early geodynam- icists often explained the known volcanic activity at the time by the fact these Giants/Titans were then buried beneath the ground and angry. For example, Giant Enceladus is told to lie beneath the ground of mount Etna after Athena gave him a beating. This volcanic character probably motivated the choice of a powerful water jet for the bosquet, which could also be fantasized as some form or foreknowledge, a concept not easily favored by geodynamicists.

So, water. Lots of water. Earth’s hydrosphere corresponds to a 2.5 km-thick shell at the surface. In fact, so little water that solid Earth’s topography is enough for continents to emerge and, strange thing (or not, you tell me), mountains are roughly as high as oceans are deep. Nothing of the sort for Enceladus: the hydrosphere thickness is 60 km (for a global radius of about 250 km) so that the surface of the rock component lies very deep beneath the surface. To say something slightly different in a slightly other way: water is half of Enceladus mass while the most extravagant hypotheses for Earth (if a number of water oceans are stored in the mantle and even more were present in the core) still provide a budget smaller than 2 %.

Quite early after I was born although not because of this, bold colleagues suggested that Enceladus’ location in the Saturn’s system, in the densest region of the otherwise very diffuse E-ring made of tiny ice particles, could be at the origin of this peculiar ring: as we will see below, this would prove a correct guess.

Enceladus viewed by NASA

Enceladus, an active little moon (NASA/JPL/Space Science Institute)

Because then occurred the real birth of Enceladus as a major body for planetary science (yes, knowledgeable reader, I intentionally mute the achievements of the Voyager spacecrafts for the sake of brevity): in the second half of 2004, the long planned Cassini-Huygens mission arrived in Saturn’s environment after a journey of about 7 years. Over the course of a remaining 13 years-long mission, this unique spacecraft with the mass of an elephant (launch mass: African bush elephant; dry mass: African forest elephant), carrying an impressive series of instruments developed in the 1990s fulfilled all the scientific expectations. Saturn itself and the architecture of the Saturn’s system were scrutinized. The planetary-size moon Titan (larger than Mercury) which, everything suggested, was certainly a fascinating body but whose orange glow formed by a dense atmosphere had removed the surface from the endeavors of previous space probes, turned out to be a fascinating body, arguably more so. For the reader more versed in serendipity, nevertheless, Enceladus’ activity is definitely the favorite pick in the list of Cassini’s top-ten discoveries. During the first months of Cassini’s Saturn trip, it occurred that a conductive cloud was located above Enceladus’ south pole. Further examinations by the multiple instruments revealed that this plume emerged from the moon (illustration), was composed of water (ice and vapor) and contaminants ejected from Enceladus interior through individual jets emanating from large parallel fractures in Enceladus’ icy crust (illustration) and, yes, Enceladus plume is feeding the E-ring (illustration). While Earth’s history of magmatism is known to significantly contribute to the evolution of its atmosphere, Enceladus’ gravity at the surface, a hundred times smaller, rather enables its volcanic activity to shape its orbital environment.

Still, such an activity remained puzzling given the moon’s dimension: infrared emission in the south polar region only (heat flowing through other terrains is too small to be detected) was soon estimated to be 10-15 GW. Equivalent to the geothermal power used by humans worldwide, this amount might seem modest to a non-geodynamicist as a negligible fraction of the Earth’s heat budget of internal origin (46 TW). Yet, the reader will have noticed that, given its size and rock content, Enceladus could be expected to expel only 50 times less energy if radiogenic decay were the sole provider of heat. This might not come as a surprise, though, for the planetary scientist aware of the peculiar heat budget of some icy moons: after all, Cassini’s older sibling devoted to the exploration of the Jupiter system, the Galileo spacecraft, had already estimated that 100 TW were transported through the volcanic surface of Io, the closest of the Galilean moons, and that the existence of Europa’s putative internal ocean also required another powerful heat source, identified even before the spacecraft observations to likely result from the dissipation of tidal deformation enabled by these two satellites’ orbits.

In the case of Enceladus, the localized emission at the South Pole, accompanied by complex tectonic features there confirming the ongoing activity witnessed by Cassini, led part of the research community to hypothesize the deep presence of a regional sea in the Southern hemisphere, beneath the ice, above the rocky core. Only such a liquid layer decoupling the motion of the ice shell and the rocky core would permit a sufficient deformation to dissipate large amounts of heat – viscous dissipation of tidal deformation was mostly envisioned in the ice layer as the supposedly cool rocks appeared too stiff, contrary to Io and its much hotter rocky mantle that most probably includes an asthenosphere much more developed than the Earth’s at present.

This was problematic, nevertheless. The poor knowledge of Saturn’s interior and possible dissipation mechanisms it may host implies that estimates of dissipation only rely on the observed motion of its natural satellites. At the time (second half of 2000s), this led to envision a necessarily episodic release of heat by Enceladus, in the fashion of Wilson cycles on the Earth where deviations of 25% around the long- term trend of the mean oceanic heat flux are postulated. Only, in the case of Enceladus, this episodic outburst appeared really exceptional (at best corresponding to maybe 1% of the duration of the moon’s duty cycle). A fairer comparison would thus be with the almost mythological catastrophic resurfacing event once postulated to explain the crater distribution on Venus if this one involved the recycling of a significant part of the lithosphere – a view that is probably in the process of being abandoned, anyway. At this stage of Enceladus research (early 2010s), the community was left with the difficult idea that it might be witnessing precisely an extraordinary phase of the evolution of the moon, a comforting notion possibly being that the sample of icy moons has a cardinal number larger than that of the terrestrial planets of the solar system (to the point where the use of statistics could almost begin to make sense).

Models of Enceladus

Interior models for Enceladus – left: before, right: after (NASA/JPL-Caltech)

Putting aside these uncertainties, a team composed Czech colleagues and people in our group in Nantes proposed an original way to constrain the interior of Enceladus. In the absence of seismology, a now classic approach for the study of planetary interiors relies on the use of the global shape (or topography) and gravity field, even if the results are more uncertain (more interior models satisfy a given observation). But in the case of Enceladus, a small moon, volcanic activity is so powerful that it could also reflect the global structure. Cassini has shown variations in the amplitude of this activity measured in terms of particle flux (the quantity of gas seems more or less constant) during the moon’s orbit. Never absent, it seems maximum at a precise point in the orbit (with a period of a little more than a day) which is repeated from one orbit to another. This seemed to indicate a control of the activity by tidal stresses that evolve with the position of Enceladus around Saturn and confirmed the prominent role played by tides. But this activity is delayed when compared to what would be expected of an elastic body: although other phenomena may contribute to this delay, we have bet that the viscous structure of the moon could be at the origin of it. We therefore built a method (certainly less precise than seismology, and with more uncertain foundations) to probe models of internal structure in terms of the rheological behavior that they oppose to tidal forcing. Several families of models could be suitable, some with a global ocean. But probably guided by a more or less conscious bias not unfamiliar to the geodynamicists that contributed to the debate on plate tectonics, our favorite candidates belonged to a group with a deep regional sea (illustration). Which proved wrong.

Pschiiit

Pschiitt (ina.fr)

Unlike in the case of plate tectonics, the paradigm shift was very rapid. The causes of this effectiveness would still have to be studied but rather than a change in the nature of scientific practice, I imagine (without any proof) that this shall be attributed to a much smaller research community as well as to the speed of change of opinions since the Cassini spacecraft mowed down the models as fast as new radar swaths were acquired. In fact, about a year before the end of the mission (2017), the meticulous evaluation of the motion of Enceladus’ surface from the compilation of numerous Cassini images indicated a very marked dynamic of the moon’s rotation (precisely, its physical libration, the motion that enables the visible side of the Moon from the Earth to represent a little more than half of the total surface). Unequivocally, such an amplitude made it necessary that the ice shell was free to move without being mechanically anchored to the rocky core. As a french president said to describe a completely different subject, the idea of a regional sea had made “pschitt”.

 

With this powerful estimate of the libration as well as chemical evidence that jets emerged from a salty water reservoir, Enceladus became the planetary body for which the existence of a global internal ocean is the least doubtful (illustration). It is now emblematic of an increasingly large family named since “Ocean worlds”.

In the meantime,

  • colleagues had demonstrated that dissipation in the Saturnian system could be much larger than anticipated so that the present-day activity of Enceladus might in the end correspond to a steady-state,
  • two independent analyses of the jets materials and ice particles in the E ring (tiny sand grains and endogenic hydrogen) revealed the likelihood of ongoing hydrothermal activity, pushing Enceladus in the short list of planetary bodies worth studying for the emergence of life (all the more so that the ocean delivers samples to spacecrafts flying through the plumes (as did Cassini several times) or would land on the surface, beneath the snow).

But this takes us away from geodynamics.

In the second and last episode, dear reader, I will show

  • how the principle of isostasy dear to geodynamicists since the 19th century was key to finally (?) understanding the interior of Enceladus,
  • how the thermal convection dear to geodynamicists from the beginning of the 20th century to describe the Earth’s mantle might also occur within the rocky core of Enceladus, with different modalities,
  • and perhaps how oceanic hotspots dear to geodynamicists from the second half of the 20th century might bear resemblance to Enceladus’ erupting centers (or not).

Join us next time, won’t you?

Iron volcanism on metallic asteroids?

Metallic asteroid

This week, Francis Nimmo, professor in the Department of Earth and Planetary Sciences (University of California Santa Cruz), tells us about volcanism on metallic asteroids! Around and after the formation of the solar system (4.5 billion years ago onwards), volcanoes on some of the gigantic bodies of the asteroid belt might have erupted … iron. Explanations.

Francis Nimmo

Francis Nimmo

One way in which we can learn about the insides of planetary bodies is by looking for signs of volcanism. Volcanism transports molten material from the interior to the surface of a body, where it solidifies. Something similar may happen on icy bodies, where cryovolcanism is thought to occur. Together with Coby Abrahams, one of my grad students, I recently wrote a paper proposing “ferrovolcanism”, which is the eruption of metallic iron. We suggested that this may have happened in the past on metallic asteroids.

 

What are metallic asteroids? Some asteroids, such as 16 Psyche are thought to be made mainly of iron and nickel, on the basis of their spectra and radar signatures. They probably originated from a catastrophic collision. The picture is that a proto-planet, which had already formed a dense metallic core, was broken apart so violently that its core was fragmented, and the fragments then cooled to form free-floating metallic bodies. Such an event would also explain the large number of iron meteorites in our collections. So far we have never seen a metallic asteroid up close, but NASA is planning to send an orbiter which will reach 16 Psyche in 2026.

Iron meteorite

Artistic view of an iron meteorite with ferrovolcanism (Elena Hartley)

How would “ferrovolcanism” work? Immediately before the catastrophic collision, the core would have been liquid (because the silicate mantle is a good insulator). After the collision, the surface of a free-floating iron blob would have initially cooled very rapidly. Analysis of iron meteorites tells us that, in some cases, the blob developed a strong, cold iron crust. The melt (liquid iron) beneath the crust would have been less dense than the crust – just like molten magma on Earth. And so, just like magma on Earth, the liquid iron would tend to rise to the surface – hence ferrovolcanism.

There are a few factors that make ferrovolcanism less likely than silicate volcanism. One is that, as the iron crust cools, it contracts, putting the whole body into compression and making it more difficult for melt-filled fractures to open up. Another is that iron is both ductile and strong, so that fracture propagation is harder. And iron is also much more thermally conductive than rock, so that melt-filled fractures will cool and solidify rapidly. Nonetheless, we don’t think any of these factors is fatal. For instance, Mercury also experiences global compression but still shows signs of volcanism. Iron eruptions might be aided by volatiles coming out of solution, just as with terrestrial eruptions, although the catastrophic impact may have removed some volatiles.

How do we test this hypothesis? One way is to wait for images of 16 Psyche! If we see things that look like iron volcanoes, that would be interesting. But identifying signs of ancient volcanism is notoriously hard – this paper documents several proposed volcanic or cryovolcanic features which turned out to be something else on further investigation. And our calculations show that the volume of erupted material will be only about 0.1 percent of the total volume of Psyche, so these features may be hard to spot – especially after 4 billion years of erosion by impacts.

Another way is to look at the iron meteorites in our collections, because ferrovolcanism should produce anomalous-looking meteorites. For instance, one might find vesicle-rich samples, or ones happening late in the crystallization sequence but showing extremely rapid cooling (because they were erupted to the surface).

How did we write the paper? It was more or less accidental. Coby was working on how iron asteroids might lose their volatiles early when one day he turned to me and said “I think they’ll erupt!”. I thought this was a neat idea, and so we started to look at how the eruptive process might work.

The strangest part of writing our paper was that at the LPSC conference someone came up to me and said “I just invented iron volcanism!”. It turned out that another group had been working on ferrovolcanism completely independently. They suggested that pallasite meteorites might be evidence of ferrovolcanism (something that I had completely missed). This simultaneous but independent development of ideas seems to be quite common in the Earth Sciences – but that is a topic for another day.

 

Geodynamics in Planetary Science

Geodynamics in Planetary Science

It is a question that humankind has been asking for thousands of years:

Are we alone in the Universe or are there other worlds like our own?

As of today, it is unknown whether or not inhabited planets exist outside of our own solar system. With the discovery of the extrasolar planet 51 Peg b in 1992, it was confirmed that our sun is not the only star that hosts planets and therefore the search for extraterrestrial life has expanded beyond our own solar system.
However, before we look for an inhabited exoplanet, we must understand what makes a planet habitable.
Of course, the best example of an inhabited (and hence habitable) planet is our Earth and therefore it is a reasonable approach to first look for Earth-like planets. So, the question we should ask is

What makes Earth habitable?

  • The planet should be in the so-called habitable zone: the zone where the planet contains liquid water on its surface. One usually calculates this zone assuming an Earth-like atmosphere.  [e.g. Lammer et al., 2009]
  • The planet also needs to have an atmosphere that protects it from radiation but also keeps the planet warm with greenhouse gases. [e.g. Seager, 2013]
  • The planet should be made of rock and should have a molten core. A convective outer core gives rise to a magnetic field that protects the planet from solar winds and cosmic rays. [e.g. Shahar et al., 2019]

Interestingly, we can couple all three points: greenhouse gases in the atmosphere can heat a planet that is too far away from its host star and therefore make it habitable. On the other hand, they can also heat a planet too much such that it becomes inhabitable.
The third point (a planet made of rock with a molten core) brings geodynamics into play: plate tectonics and volcanic outgassing contribute to burial and recycling of atmospheric gases [Seager, 2013].
In our solar system, Earth is the only inhabited planet, and it is also the only planet we know of that exhibits plate tectonics (including exoplanets).
For example, Venus, our neighbouring sister planet, is very similar to Earth in terms of size, mass and composition. Some studies even suggest that Venus might have been the first habitable planet of our solar system [Way et al., 2016].
But present-day Venus is an inhospitable planet with a very thick carbon dioxide atmosphere (90 times denser than that of Earth) and an extremely hot surface temperature (up to 750K) which is mainly because of runaway greenhouse gases. But why did Earth become habitable and Venus did not?
To explain their different evolutionary paths, plate tectonics might play a major role. Through plate tectonics, Earth can efficiently recycle carbon back into its surface (deep carbon cycle) and this may help to prevent a runaway Greenhouse effect.

The importance of plate tectonics on the habitability of a planet is still being studied, and it is not yet fully understood how efficient this recycling is.

Plate tectonics also influences the generation of a magnetic field. Plate tectonics efficiently cools the mantle by subducting cold slabs into the deep interior, which leads to high heat flow out of the core. Therefore, the style of mantle convection controls the convection in the outer core. This then generates the magnetic field of a planet. The magnetic field acts as a protective shield from the solar winds, which otherwise might erode the planet’s atmosphere. As discussed above, the atmosphere controls the climate mainly through greenhouse gases. The resulting climate influences the tectonic regime: cool climates are favourable for plate tectonics because they facilitate the formation of weak shear-zones in the lithosphere [Foley et al., 2016].
This coupling between the climate, mantle and the core is called the “whole planet coupling” [Foley et al., 2016] and as a whole, it might explain why Earth and Venus have evolved so differently.

Whole planet coupling“: The atmosphere controls the climate which influences the tectonic regime. Subducting slabs cool the mantle which leads to high heat flow out the core. Therefore, the mantle convection controls the type of convection in the outer core which can generate a magnetic field. The magnetic field protects the atmosphere from solar winds and cosmic rays.

To understand the habitability of exoplanets, we therefore need to investigate all the components of the whole planet coupling. Most interestingly for geodynamicists, it is the interior dynamics of a planet’s mantle that couples all these different components!

In the past years, astronomers have discovered many exoplanets, and we expect many more to join this list. For some of them, astronomers and astrophysicists can measure its size, mass, and sometimes even the atmospheric composition and/or surface temperature.
This is very different from studying the Earth, where we can gather a lot of information about the interior through, for example, seismology. Geophysicists, Astronomers, Astrophysicists and many other research disciplines have to collaborate such that they can understand an exoplanet’s whole planet coupling and potential habitability. For geodynamicists the challenge will be to infer the exoplanet’s interior dynamics from a limited amount of data only.

References:
Foley, B. J. and Driscoll, P. E.: Whole planet coupling between climate, mantle, and core: Implications for the evolution of rocky planets, Geochemistry, Geophysics, Geosystems, Vol. 17, 2016.
Lammer, H., et al.: What makes a planet habitable?, The Astronomy and Astrophysics Review, Vol. 17, 2009.
Seager, S.: Exoplanet Habitability. Science, Vol. 340, 2013.
Shahar, A., Driscoll, P., Weinberger, A. and Cody, G.: What makes a planet habitable?, Science, Vol. 364, 2019.
Way, M. J., et al.: Was Venus the first habitable world of our solar system?, Geophysical Research Letters, Vol. 43, 2016.

Oceans on Mars: the geodynamic record

Oceans on Mars: the geodynamic record

Apart from our own planet Earth, there are a lot of Peculiar Planets out there! In this series we take a look at a planetary body or system worthy of our geodynamic attention, and this week we are back to our own solar system, more precisely to our neighbour Mars. In this post, Robert Citron, PhD student at the University of California, Berkeley, writes about the links between oceans, shorelines, and volcanism on Mars. A David Bowie approved blog post!

Robert Citron

Whether Mars was warm enough in its early history to support large oceans remains controversial. Although today Mars is extremely cold and dry, several lines of geological evidence suggest early Mars was periodically warm and wet. Evidence for ancient liquid water includes river channels, deltas and alluvial fans, lakes, and even shorelines of an extensive ocean.

Such features are carved into Mars’ ancient crust, which contains a remarkable geologic record spanning from over 4 billion years ago to present. How and where fluvial erosion takes place is highly dependent on topography. However, Mars is a dynamic planet and the topography observed today does not necessarily represent the planetary surface billions of years ago. Geological markers that seem misaligned today, such as river flow directions and sea levels, may be more consistent with ancient topography. Geodynamic models of how the planet has changed shape over time can therefore be used to test and constrain evidence of water on early Mars.

Shorelines are one example where geodynamic models have helped interpret the geological record. Perhaps the most compelling evidence for ancient Martian oceans are the hypothetical palaeo-shorelines that border Mars’ northern lowland basin. However, the shorelines fail to follow an equipotential surface, or contours of constant elevation, which would be expected if they formed via a standing body of liquid water. One explanation is that the shorelines used to follow an equipotential surface, but subsequent changes to the planet’s shape warped them to their present-day elevations, which vary by up to several kilometers.

Two geodynamic processes that likely changed the global shape of Mars are surface loading and true polar wander. True polar wander occurs when a planet reorients relative to its spin axis, which reshapes the planet because it changes the location of the equatorial bulge produced by the planet’s rotation. Large scale true polar wander on Mars was examined by Perron et al. (2007), which found that it could have warped past shoreline profiles to their present-day topographic profiles.

Another possibility is that Martian shoreline markers were deflected by flexure associated with surface loads. The emplacement or removal of material on a planet’s surface can cause flexure and displacement of nearby crust. This is observed on Earth, where melting of glaciers has unburdened underlying crust, allowing for rebound and displacement of past shoreline markers. Similar processes could be at work on Mars, but on a global scale.

Mars topography: The massive Tharsis volcanic province (red) is situated on the boundary between the southern highlands (orange) and northern lowlands (blue). The lowlands may have been covered by one or more ancient oceans, and are bordered by palaeo-shorelines. Two of the most prominent shorelines are the older Arabia shoreline (dashed line) and younger Deuteronilus shoreline (solid line). Image constructed by R. Citron using MOLA data. Shoreline data from Ivanov et al. (2017) and Perron et al. (2007).

The largest load on Mars is the Tharsis rise, a volcanic province that dominates the topography and gravity of the planet. Tharsis was built by volcanic activity over hundreds of millions to billions of years. Its emplacement changed Mars’ shape on a global scale; in addition to the Tharsis rise, there is a circum-Tharsis depression and an antipodal bulge.

In recent work (Citron et al. 2018), we found that the present-day variations in shoreline elevations can be explained by flexure from Tharsis and its associated loading. Of the two most prominent Mars shorelines, the older (~ 4 billion years old) Arabia shoreline corresponds to pre-Tharsis topography, deformed by almost all of the flexure associated with Tharsis. The younger (~ 3.6 billion years old) Deuteronilus shoreline corresponds to late-Tharsis topography, requiring only ~17% of Tharsis loading to explain its variations in elevation. This suggests that the Arabia shoreline formed before or during the early stages of Tharsis, and the Deuteronilus shoreline formed during the late stages of Tharsis growth. The match between the present-day shoreline markers and ancient equipotential surfaces supports the hypothesis that the markers do indicate shorelines formed by an ancient ocean.

The timing of ancient Martian oceans is consistent with recent work by Bouley et al. (2016), which found that the Mars valley networks (ancient river channels) also better fit Mars’ pre-Tharsis topography. In the topography of Mars prior to Tharsis, the flow direction of the channels are more consistent with the topographic gradient, and the channels occur at latitudes and elevations where climate models predict water ice (resulting in ice melt) to form.

The timing of the shorelines and valley networks relative to Tharsis volcanism suggests a close link between the stability of water on Mars and volcanic activity. Atmospheric models predict a cold and icy early Mars, however it is possible that oceans may be more sustainable during periods of heightened volcanism. Tharsis activity has also been associated with outflow channels indicative of catastrophic flooding that may have inundated the northern plains with water. Further examination of the link between Tharsis volcanism and oceans could increase our understanding of early Mars habitability.

Further reading:

Bouley, S., Baratoux, D., Matsuyama, I., Forget, F., Séjourné, A., Turbet, M., & Costard, F. (2016). Late Tharsis formation and implications for early Mars. Nature531(7594), 344.

Citron, R. I., Manga, M., & Hemingway, D. J. (2018). Timing of oceans on Mars from shoreline deformation. Nature555(7698), 643.

Ivanov, M. A., Erkeling, G., Hiesinger, H., Bernhardt, H., & Reiss, D. (2017). Topography of the Deuteronilus contact on Mars: Evidence for an ancient water/mud ocean and long-wavelength topographic readjustments. Planetary and Space Science144, 49-70.

Matsuyama, I., & Manga, M. (2010). Mars without the equilibrium rotational figure, Tharsis, and the remnant rotational figure. Journal of Geophysical Research: Planets115(E12).

Perron, J. T., Mitrovica, J. X., Manga, M., Matsuyama, I., & Richards, M. A. (2007). Evidence for an ancient martian ocean in the topography of deformed shorelines. Nature447(7146), 840.

Ramirez, R. M., & Craddock, R. A. (2018). The geological and climatological case for a warmer and wetter early Mars. Nature Geoscience11(4), 230.