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the surface of Mars is dominated today by a thin atmosphere, very low temperatures, extreme dryness

and pervasive winds. However, spacecraft investigations during the past 50 years point to a geologically dynamic and water-rich ancient Mars. In particular, it has been suggested that a great ocean covered the martian lowlands in the northern hemisphere during the Noachian and Early Hesperian epochs, 4.6 to 3.6 billion years ago. Writing in Nature Geoscience, Di Achille and Hynek1 argue that the distribution of ancient delta deposits and river-valley termini lend support to the hypothesis that Mars once harboured a primeval, large and long-lasting ocean.

The most prominent geological feature on Mars is the topographic dichotomy that divides the planet into two provinces: the northern lowlands, which represent approximately one third of the planet’s surface, and the southern highlands. The northern lowlands lie on average 5.5 km lower than the southern highlands2. Images revealing the geomorphology and topography3 of Mars, obtained in the late 1970s, led to the hypothesis that the boundary between the highlands and lowlands coincides with an ancient palaeoshoreline for thousands of kilometres. Specifically, the nearly complete closure of the northern lowlands, and rates of coastal erosion similar to those reported for terrestrial palaeolakes, were argued as indicative of an ancient ocean.

High-resolution data of the martian surface, obtained by the Mars Global Surveyor in 1998, raised uncertainties regarding the presence of a palaeoshoreline coincident with the dichotomy boundary. The data showed substantial vertical variations, of up to 5.5 km, along the proposed shoreline, inconsistent with a fluid in hydrostatic equilibrium4. More recently, the deviation of the shoreline from an equipotential surface has been explained by subsequent vertical land motion triggered by tectonic activity and isostasy5–7. The absence of coastal morphologies formed by the action of water, including coastal erosion, was also revealed by new images8. Yet landforms

reminiscent of temperate-climate shorelines may be scarce if the planet harboured cold glacial oceans during the Noachian epoch5,9. Recent investigations lend additional support to the dichotomy palaeoshoreline hypothesis: the elevation of ancient river-valley outlets5 and water-level changes imprinted in terraced deltaic deposits10 coincide with the altitude of the proposed shoreline.

Now, Di Achille and Hynek1 suggest that the elevation of Noachian and Early Hesperian deltaic deposits and valley outlets delineates a planet-wide equipotential surface in the northern lowlands of Mars. The authors analysed 52 delta deposits scattered across a range of latitudes and elevations on the martian highlands. Seventeen of the deltas — located at or near the boundary between the lowlands and the highlands — sit at a similar elevation, deviating by no more than 200 metres from one another; the Earth’s geoid varies by the same amount. These deltas delineate the closest approximation to an equipotential palaeoshoreline proposed so far, and suggest that water was transported from the southern highlands to the northern lowlands of Mars. The newly proposed shoreline is broadly consistent with the termini of ancient river-valley networks, and with large portions of the previously proposed palaeoshoreline coincident with the dichotomy boundary3,5. Another 12 deltas — located in closed basins in the near-equatorial belt of the planet — are located at a similar elevation to those delineating the palaeoshoreline. Together, the findings suggest the presence of a globally uniform water level, in both equatorial lakes and the northern ocean.

The depositional geometry of the deltas suggests that the deposits were not formed by catastrophic pulses of water, but by long-lived runoff processes, perhaps spanning periods of up to one century11. Furthermore, the deltas are estimated to have formed between the Noachian and Early Hesperian. Such long-term deposition throughout different epochs is consistent with a long-lived primeval ocean on Mars. Stable oceans open prospects for the

emergence of life, as they require a complete and vigorous hydrological cycle, including evaporation, rain and/or snowfall, and fluvial transport. The longer the aqueous-driven depositional environment was active, the greater the likelihood that life could have bloomed.

The study by Di Achille and Hynek1 improves our understanding of martian hydrological history, but is unlikely to be the last word on the long-held debate regarding the presence of an ancient ocean on Mars. Clear proof of oceanic sedimentation — such as green marine

Planetary Science

refilling the oceans of early MarsThe northern plains of Mars are thought to have harboured an ocean more than 3.6 billion years ago. Delta deposits and river-valley termini ring the proposed seabed and define an equipotential palaeoshoreline.

alberto g. Fairén

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Figure 1 | Terraced delta in the mouth of a small valley located at the boundary between the highlands and lowlands in the Medusae Fossae formation on Mars. Di Achille and Hynek1 suggest that the elevation of ancient delta deposits and river-valley outlets delineates the boundaries of an ancient ocean, and provides evidence for a globally uniform water level on Mars more than 3.6 billion years ago. The elevation in the pictured valley outlet is −1,950 m, falling between the error bars of the level of the primeval ocean described by Di Achille and Hynek. The superimposed coloured observation shows the mineral composition of the deposit. An impact crater at the east end of the delta is exposing hydrated minerals (blue and purple), which probably comprised part of the buried sedimentary layers deposited by fluvial activity.

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clays, considered diagnostic of oceanic depositional environments on Earth12 — is needed to settle the debate. And, of course, the geomorphic examination of deltas and valley networks provides no information on the composition of the aqueous deposits that formed the deltas (Fig. 1). But the new vision of deltas and river networks conducting water from the highlands to the lowlands of Mars provides strong support for the hypothesis that the lowlands once harboured a vast ocean. ❐

Alberto G. Fairén is at the Carl Sagan Center for the Study of Life in the Universe, SETI Institute, 515 Whisman Road, Mountain View, California 94043, USA. e-mail: alberto.g.fairen@nasa.gov

references1. Di Achille, G. & Hynek, B. M. Nature Geosci. 3, 459–463 (2010).2. Watters, T., McGovern, P. J. & Irwin, R. P.

Annu. Rev. Earth Planet. Sci. 35, 621–652 (2007).3. Parker, T. J., Gorsline, D. S., Saunders, R. S., Pieri, D. C. &

Schneeberger, D. M. J. Geophys. Res. 98, 11061–11078 (1993).4. Head, J. W. et al. Science 286, 2134–2137 (1999).

5. Fairén, A. G. et al. Icarus 165, 53–67 (2003).6. Ruiz, J., Fairén, A. G., Dohm, J. M. & Tejero, R. Planet. Space Sci.

52, 1297–1301 (2004).7. Perron J. T., Mitrovica, J. X., Manga, M., Matsuyama, I. &

Richards, M. A. Nature 447, 840–843 (2007).8. Malin, M. C. & Edgett, K. S. Geophys. Res. Lett.

26, 3049–3052 (1999). 9. Fairén, A. G., Davila, A. F., Duport, L. G., Amils, R. & McKay, C.

Nature 459, 401–404 (2009).10. De Pablo, M. A. & Pacifici, A. Icarus 196, 667–671 (2008).11. Kleinhans, M. G., van de Kasteele, H. E. & Hauber, E.

Earth Planet. Sci. Lett. 294, 378–392 (2010).12. Odin, G. S. (ed.) Green Marine Clays (Developments in

Sedimentology 45, Elsevier Science, 1988).

the balance of chemical oxidizing and reducing agents in fluids, known as the redox state, plays an

important role in regulating biological activity on Earth today. In particular, redox conditions determine whether and where specific metabolic pathways are, thermodynamically, the most favourable. Biological activity, in turn, can have profound consequences on redox conditions, both locally and globally: in the most dramatic example, the evolution of oxygenic photosynthesis by cyanobacteria inexorably brought the world into a more oxidized state. But the transition to oxygen-rich oceans was interrupted by the appearance of anoxic and sulphide-rich (sulphidic) conditions about 1.8 billion years ago1–3. Writing in Nature Geoscience, Poulton and colleagues4 show that rather than reflecting a global shift in ocean chemistry, the sulphidic waters were confined between the oxygenated ocean surface and anoxic, iron-rich waters below.

Before the dawn of oxygenic photosynthesis, the oceans were anoxic and rich in dissolved iron (ferruginous). Oxygen was first produced by cyanobacteria at least 2.5 billion years ago, and for the past 542 million years, the oceans have been dominantly oxygenated. The sulphidic conditions 1.8 billion years ago2 were postulated to have arisen after an initial accumulation of atmospheric oxygen spurred oxidative weathering of the continents1. This led to the enhanced delivery of sulphate to the oceans and

an increase in the bacterial reduction of sulphate to hydrogen sulphide1. The widespread presence of sulphidic conditions is also supported by the discovery of organic biomarkers that reflect the presence of photosynthetic bacteria that utilize sulphide3.

However, as the record of oceanic conditions becomes better resolved, reports of coeval but conflicting redox conditions

have been published. For example, it has long been thought that the transition from anoxic to oxic conditions occurred during the Ediacaran period (~635–542 Myr)5, a time that also marks the first-known appearance of multicellular life. However, recent geochemical evidence indicates that, at least locally, ferruginous6 or even sulphidic7 conditions persisted throughout this period. Such different assessments

BiogeocheMiStry

earth’s redox evolutionThe timing and nature of changes in the chemistry of the early oceans are intensely debated. Geochemical analyses show that a prominent transition to sulphidic marine conditions 1.8 billion years ago may have been restricted to near-shore environments.

David Fike

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Figure 1 | Schematic of ocean redox conditions 1.8 billion years ago. In this scenario, the shallow waters are oxic (purple), reflecting contact with an, at least minimally, oxygenated atmosphere, whereas the deeper oceans are anoxic and ferruginous (red spots), reflecting the influx of dissolved iron into the deep oceans. Poulton and colleagues4 present data that suggest the presence of a thin wedge of sulphidic conditions between these layers (yellow spots), maintained in part by the delivery of organic matter from shallow waters and the microbial reduction of sulphate.

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