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Home , Climate of Mars, Gale (crater), Geology of Mars, Hesperian, Lucy (spacecraft), Main (Martian crater)

FROM WET TO PLANET Current and future exploration is shaping our understanding of how the of changed. Joel Davis deciphers the planet’s ancient, drying climate

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t has been an exciting for Mars exploration. 2020 saw three launches to the , each by diff erent space agencies—NASA, the Chinese INational Space Administration, and the (UAE) Space Agency. NASA’s latest rover, , is the fi rst step in a decade-long campaign for the eventual return of samples from Mars, which has the potential to truly transform our understanding of the still scientifi cally elusive Red Planet. On this side of the Atlantic, UK, European and Russian scientists are also getting ready for the launch of the (ESA) and Rosalind Franklin rover mission in 2022. The last 20 have been a golden for Mars exploration, with ever increasing amounts of data being returned from a variety of landed and orbital spacecraft. Such data help planetary like me to unravel the complicated yet fascinating history of our celestial neighbour. As planetary geologists, we can apply our understanding of to decipher the geological history of Mars, which is key to guiding future exploration. But why is planetary exploration so focused on Mars in particular? Until recently, the mantra of Mars exploration has been to follow the , which has played an important role in shaping the surface of Mars. Liquid water is also thought to be key to the formation and evolution of life. However, it is not the present environment that shows the richest evidence for water—it is the ancient geological past. Today, Mars is a , hyper-arid . Its surface is mostly covered by a thin layer of oxide , giving the Red Planet its characteristic red- hue, visible even with the most basic of from Earth. And we know that Mars is dry—the from its thin

CO2 is so low that liquid water at most would instantly boil to vapour. Towards the poles, water- is stable in the shallow sub- surface of and thick This illustration shows Crater—the landing site of the water-ice caps cover both the north and Perseverance rover—as it may have looked billions of years go on Mars, when it was a . An inlet and outlet are also visible on either side of poles. But liquid water, for the most the lake. Image Credit: NASA/JPL-Caltech. part, is not thought to be stable anywhere

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Figure 1: Geological time scale of Mars (and its inferred climate) compared to Earth

across the planet. The of Mars today tells us that in the ancient past, Mars was a different place. Water on Mars Nearly four billion years ago, in the early days of the Solar , the surface of Mars was being re-sculpted by and intense impact cratering. This on Mars is known as the Noachian, spanning from the planet’s formation around 4.5 billion years ago (Ga) to about 3.7 Ga (Fig. 1). Unlike on Earth, geological periods on Mars are determined Figure 2: (A) image of a network on Mars with multiple branching tributaries, a former from the of impact craters on system. Inferred flow direction is from the top to the bottom of the image. Credit: NASA/JPL/ASU. (B) Satellite surfaces (with more impact craters image of breaching into , likely the site of a palaeo-lake. A delta has formed at the accumulating over time). Surfaces are then breach. Credit: NASA/JPL/MSSS assigned absolute ages by calibrating to the , where surfaces have been dated The north pole of Mars. Elements of this image using the Apollo samples. were furnished by NASA. Around half of the is Noachian aged and across most of these surfaces we see strong evidence for liquid water. Cutting across many Noachian surfaces are what we as Mars geologists colloquially refer to as the “valley networks”. When viewed from , these valley networks are linear to branching erosional troughs that follow . They resemble river valleys on Earth and are thought to have formed in a similar manner, through and water (Fig. 2). The largest valley networks form systems up to 5,000 km long, equivalent to many continental- scale drainage basins on Earth. We have known about these valley networks since the Viking era of Mars exploration in the . Although there was initially some discussion about whether these features could have formed via other mechanisms (such as ), it is now generally accepted that they formed via water erosion. However, it is unclear how long these features took to form. The valley networks have not fully eroded the Noachian landscape (many ancient impact craters are still preserved), which suggests that they were only active for geologically brief periods of time, perhaps as little as a

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UNTIL RECENTLY, THE MANTRA OF MARS EXPLORATION HAS BEEN TO FOLLOW THE WATER, WHICH HAS PLAYED AN IMPORTANT ROLE IN SHAPING THE SURFACE OF MARS. LIQUID WATER IS ALSO THOUGHT TO BE KEY TO THE FORMATION AND EVOLUTION OF LIFE. HOWEVER, IT IS NOT THE PRESENT MARTIAN ENVIRONMENT THAT SHOWS Figure 3: (A) Image from the rover of a in crater, likely deposited by an ancient river. THE RICHEST EVIDENCE FOR Credit: NASA/JPL/MSSS. (B) False colour satellite image of the delta in Jezero crater, the future landing site of WATER—IT IS THE ANCIENT NASA’s Perseverance rover. The false colours indicate variations in composition. Credit: NASA/JPL/ASU GEOLOGICAL PAST.

few thousand years or maybe as long as ten million years—either way, a small fraction of the total time in the Noachian. Excitingly, valley networks have been observed to intersect impact craters, suggesting that in the Noachian water may have pooled in the craters to form a lake (Fig. 2). Gale crater, the exploration site for the Curiosity rover since 2012, is Figure 4: (A) 3D perspective view of a 100-km-long ridge system on Mars, interpreted as exhumed river one of these palaeo- (Fig. 3; although (an “inverted channel deposit”). Credit: NASA/JPL/MSS/Matt Balme. (B) Aerial view of Jurassic river it is unclear if Gale represents a Noachian channel deposits in Utah, USA, preserved as a ridge. Credit: Rebecca M.E. Williams lake or is younger in ). At least 200 large palaeo-lakes have been identified on deposits”. These features are perhaps even more than the valley the surface of Mars so far. In many, we amalgamations of river channel sediment networks. Many of these inverted channel find fan-shaped sedimentary deposits, that became resistant to erosion and are deposits are found adjacent to interpreted as ancient river deltas that now preserved as ridges in the landscape phyllosilicate-bearing rocks, which formed formed within crater lakes. These deltas instead of depressions (Fig. 4). River in the presence of water. Phyllosilicates are defined by the branching remains of channel deposits preserved as ridges and other aqueous are channels and meanders, and are seems unusual, but similar features exist particularly common on Noachian considered a prime target for the in on Earth: in Oman, Egypt, and surfaces; significant amounts of water detection of ancient due to the American southwest. Much of my would have been required to alter Mars’s the ability of deltas to concentrate own research is devoted to the basaltic and generate these minerals. potential organic matter. However, in investigation of these fascinating features, Taking a step back, when viewing Mars Earth’s record it is rare for such which cross the boundary between as a whole, we can see that chains of deltaic deposits to be preserved in geology and . Whilst the valley networks and palaeo-lake basins planform (that is, where their shape from valley networks represent upland, flow northwards, possibly representing above is still recognisable), so we have a erosional , we think that the inverted the remains of a planet-wide hydrological limited understanding of how much channel deposits were the lowland, cycle. These systems converge on the geological time these features represent. depositional rivers set within alluvial edge of Mars’s northern lowlands—a NASA’s Perseverance rover will begin to floodplains. Like many features on Mars, huge basin that makes up one third of the explore one of these ancient deltas at we are only beginning to understand the martian surface (Fig. 5). There is ample Jezero crater in 2021 (Fig. 3). formation of the inverted channel deposits, discussion about whether this Another unusual found on but they could potentially represent hemisphere-spanning basin once Noachian surfaces are “inverted channel millions of years of geological time, contained an ocean-sized body of water;

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Figure 5: of the surface of Mars. Most of the features formed by water are concentrated on the oldest, Noachian terrains, and converge on Mars’s northern lowland basin. Credit: NASA/JPL/GFSC

A commonly held assumption is that Mars had a thicker atmosphere in the Noachian than it does at present, and

that some sort of CO2-H2O greenhouse eff ect raised the surface above freezing. Hence, Noachian Mars is often referred to as being “warm and wet”. Measurements by NASA’s MAVEN spacecraft indicate that the Noachian atmosphere was about 1 pressure, similar to Earth today (Mars is 0.006 bars currently), and composed of

CO2, most of which has since been lost

to space. This loss of CO2 to space also explains why we do not see extensive Figure 6: Satellite image of an outfl ow channel on Mars, deposits of on Mars, the which likely caused huge mega-fl ooding. The inferred sedimentary sink for CO on Earth. fl ow direction is from left to right. Credit: NASA/JPL/ASU 2 Mars’s early atmosphere could have been shielded from removal by an

indeed, it is almost completely unknown northern ocean perhaps? Luckily for us, ancient magnetic fi eld, or perhaps CO2 whether there was enough water on the in 2023 the Rosalind Franklin rover will was continuously resupplied by surface of Mars to fi ll such a vast basin. at location called , on volcanic gases. However, our problems However, detailed analysis of Mars’s the margins of the northern , so in understanding the Noachian palaeo- geology can provide a framework to begin answering this question about the climate are just beginning. to address this fascinating question. My existence of an ancient Noachian ocean Although we can see extensive colleagues and I recently investigated an may not be too far off . evidence for liquid water in Mars’s ancient delta deposit located at the end of a Noachian rock record, we still do not large valley network system called The Mars climate problem fully understand the environment and Hypanis Vallis, at the edge of the northern There is abundant geological evidence climate that allowed it to exist. When lowlands basin. The geology of the from both orbiting spacecraft and climate modellers try to reconstruct the Hypanis delta suggests that it formed into rovers for water in the Noachian. But ancient Noachian palaeo-climate, the a large body of water and grew in response how was liquid water able to exist on models often fail to show mean annual to a falling water level—a receding ancient Mars when today it cannot? surface temperatures above freezing.

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This issue largely comes from the “faint was very slow. Correspondingly, we see a young paradox”, which is the idea lot less evidence for long-lived liquid that the Sun was much less luminous in water in the geologically younger THERE IS ABUNDANT its early history and so Mars received surfaces, which are divided into the GEOLOGICAL EVIDENCE FROM even less heat than it does today, and (~ 3.7-3.0 Ga) and BOTH ORBITING SPACECRAFT which the proposed greenhouse eff ect (~ 3.0-0 Ga) periods. The Hesperian and was unable to compensate for. So where Amazonian geological records suggest AND ROVERS FOR WATER IN does that leave liquid water in the that Mars became increasingly dry. This is THE NOACHIAN. BUT HOW WAS Noachian? Well, an alternative scenario is the case at Gale crater, where we see LIQUID WATER ABLE TO EXIST ON that water on Noachian Mars was mostly temporal transition in the rock record ANCIENT MARS WHEN TODAY frozen, locked up in thick ice caps in the from phyllosilicates (formed in the high- regions around the presence of water) to (formed IT CANNOT? . Melting would only occur due to anhydrously). One idea to explain this is localised and brief episodes of heating, that following the Noachian, the removal such as from volcanism or impact of Mars’s early thick atmosphere reached cratering. However, it is unclear whether a point where it was no longer able to outfl ow channels to a single source region, these brief melting episodes in this “icy sustain liquid water for long periods of which has led many to suggest that they highlands” scenario are suffi cient to time. However, that is not to say that formed by the rapid release of sub-surface explain Mars’s Noachian geological water was absent completely after the water or the melting of large volumes of record. Both the “warm and wet” and Noachian. ice in mega-fl ooding events, an order of “icy highlands” scenarios are end If we look at Hesperian surfaces, we see larger than the biggest fl oods members; the Noachian may have that many of them are cut by huge we know of on Earth. However, these actually been somewhere in between channel systems (Fig. 6). These features, mega-fl oods were probably brief events, these models—perhaps “occasionally known as outfl ow channels, strongly perhaps lasting as little as a few days, and warm and wet” is a better descriptor. contrast with the valley networks in that do not necessarily require a warm climate they are hundreds of kilometres wide and to sustain them. Instead, the mega-fl oods After the Noachian thousands of kilometres long— may have been triggered by large volcanic Another major question that planetary individually! The outfl ow channels eruptions or impact events. geologists like myself are interested in is typically branch around teardrop shaped Within Mars’s equatorial system, what happened to Mars after the Noachian? former islands, indicating that they had , we can see numerous, The fact that so much of this ancient immense erosive power and probably conical alluvial fans along the canyon walls geology is preserved for us to study today required huge volumes of water to form. (Fig. 7). On Earth (for example, in Death suggests that erosion after the Noachian We can often trace the origin of the Valley, USA), alluvial fans form sub-aerially

Figure 7: 3D perspective view of alluvial fans along the walls of Valles Marineris. Credit: NASA/JPL/MSSS

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in mountainous regions, driven by the Figure 8: False colour CaSSIS image of Oxia Planum, the landing site for the Rosalind Franklin rover in 2023. The dark material to the left is Corporate episodic fl ow of water and sediment down thought to be mafi c, volcanic material, whereas the orange and blue Supporters: Confirmed New Date a slope and into a dry basin, where it forms materials are hydrated phyllosilicate minerals, formed in the presence a fan. Interestingly, these alluvial fans are of water. Credit: ESA/University of Bern/UoA-Jason Perry believed to have formed after the mega- fl ooding events and it is not entirely clear Core Values: the Role of Core in 21st how the martian climate was able to sustain them. One idea is that local micro- may have existed in Valles Marineris and Century Reservoir Characterisation could sustain liquid water, leading to intermittent, geologically brief “wet 3-7 May 2021 episodes”, in an otherwise dry climate. There are many other features recorded in Convenors: Virtual Conference the Hesperian and Amazonian surfaces that Badley suggest that these intermittent “wet Adrian Neal Ashton - Chair episodes” aff ected other regions of Mars as one of the reasons why extensive, in situ hosted a lake or been on the margins of an well. Although we do not see the global rover investigations of the planet’s ocean-sized body of water. Oxia Planum is Lucy Williams networks of former rivers that we do on Noachian geology are necessary. Indeed, also rich in phyllosilicate-bearing rocks, Rockhopper Noachian surfaces, both Hesperian and both Perseverance and Rosalind Franklin which for their formation required Mike Ashton Amazonian terrain show some evidence for will explore Noachian-age sites at Jezero signifi cant amounts of water to alter the infl uence of liquid water, albeit a and Oxia Planum, respectively. Despite Mars’s basaltic crust. But, like many things Ashton Geology declining one. However, as we approach their age, the rocks at Jezero and Oxia about Mars, exactly how these Steve Dee the geologically recent past, we see more Planum may also record evidence that can phyllosilicates formed is far from clear. BP and more evidence that Mars became explain how the changed The UK is leading the development of Cliff Lovelock entirely frozen, paving the way for today’s after the Noachian, and whether liquid Rosalind Franklin’s Panoramic Camera, Shell hyper-arid surface environment. water continued to be available which will be able to determine the intermittently. composition and geological context of Emma Jude Future exploration Perseverance could reveal whether the these aqueous minerals. Detailed mapping BP Deciphering the ancient martian palaeo- Noachian-age lake at Jezero crater was using orbital images, such as from the Anton Padin climate and its evolution through time is covered by ice, a possibility under the “icy Colour and Stereo Imaging System Total and Krevor 2015 highlands” scenario. Perseverance will (CaSSIS; Fig. 8), of both sites is currently also be the fi rst spacecraft to investigate underway to provide important geological Keynote Speakers: Core has traditionally played a key role in the characterisation of conventional and unconventional examples of inverted channel deposits and context and to establish the locations of Mike Bowman hydrocarbon reservoirs, from exploration to mature production. It is the only means by which to observe and these deltaic in situ. Excitingly, target for investigation, prior Manchester University make measurements on actual reservoir rock. However, the recent oil industry downturn has driven many to found at Jezero (and rarely for Mars) to landing. Patricio Desjardins question the value of taking core, due to the associated increased costs and potential risks to well operations. are deposits of carbonate-bearing Mars exploration is now moving Shell UCs, Houston In tandem, advances in other reservoir visualisation techniques, such as seismic and borehole imaging, have rocks. Investigating how these towards the direct detection of ancient Haakon Fossen been used to give to the contention that coring is an increasingly redundant means of characterising formed and whether life, and this is the objective of both Bergen University reservoirs. they were sequestered from Perseverance and Rosalind Franklin. Bruce Levell Through four main themes this 5- conference will aim to redress the balance in this debate by an early, CO -rich However, establishing geological context 2 Oxford University exploring the role core can, or should, play in the exploration to production cycle: atmosphere will provide for the local, regional, and global • Is core critical to sound commercial decision making? key into the environment is imperative to enable us to Anna Matthews of the Noachian climate confi dently support any potential “life” BP • What are the challenges and benefits of integrating core-derived understanding across the geological, petrophysical and engineering spectrum? and its decline. One of the claim. Mars, originally believed to have Nordine Sabaou end goals of Perseverance been a barren, volcanic landscape, is now BHP, Houston • Integration of traditional core characterisation methods with new core, well and reservoir visualisation and mapping technologies - is the sum greater than its parts? is to cache various proving to have a complex , Kate Smout • How can the extensive network of global legacy core collections best be utilised to maximise their business samples for eventual rich in water-formed sedimentary rocks. Shell return to Earth. Having For me as , the exciting thing and research worth? Noachian samples of about both these upcoming rover Sponsors: known geological context missions is that they will be roving across fi nally in our hands will and sampling the Noachian. They will be signifi cantly advance our able to fi nally provide ground truth to For further information: understanding because we will our orbital observations and unravel the For more information, please contact Sarah Woodcock, [email protected] or visit be able to establish absolute ages four-billion-year-old mystery of the the conference website: https://www.geolsoc.org.uk/05-rescheduled-pg-core-values-2021 for events on Mars (as was the case Noachian climate and how it changed. with the Lunar Apollo samples). The geological environment at Oxia Dr Joel Davis is a postdoctoral research Planum is less clear: it is set within a assistant in the Department of Earth Sciences, Natural History Museum London At the forefront of petroleum geoscience Photo: NASA/JPL-Caltech/USGS topographic basin that may have once 20 DECEMBER 2020 | WWW.GEOLSOC.ORG.UK/GEOSCIENTIST www.geolsoc.org.uk/petroleum #EGCoreValues21