The Messinian Salinity Crisis and the Lost Oceans of Venus

Venus is the only other Earth-sized planet in our Solar System, volcanically active and rich in volatiles, but extremely hot, dry, and hostile to life. Models suggest either that Venus was always thus [1], or that in principle it could support oceans even today [2]. While the long history of the planet may be recorded in its ancient highlands, the lowland plains host a range of features suggestive of past water. Some canali are clearly lava channels [3], but others appear similar to fluvial [4] or submarine [5] channels, and our mapping of polygonal terrain implies a submarine sedimentary origin of them. The evidence is compelling that Venus once supported oceans and lost them.

Possibly the closest terrestrial analogue for these conditions is the Messinian Salinity Crisis (MSC), 5·97 to 5·33 Ma ago, during which the Mediterranean Sea became repeatedly restricted, and evaporated in part or whole [6,7]. Nearly 10⁶ km³ of gypsum and halite, in places several kilometres thick, were precipitated [8], with exposed salt flats covering most of the Mediterranean. With sea level lowered by 2 km or more, the major rivers—notably the Nile, Rhone, Ebron and Po—carved deep canyons into the continental margins, helping to maintain brine pools within the deepest basins. Remarkably, faunal exchanges took place across this inhospitable landscape [9].

Conditions on Venus were even more extreme. In the earliest stages of the runaway greenhouse, photochemical sulphur cycling in a hot steam‑rich atmosphere would have resulted in transient, intense episodes of sulphuric acid rainfall. Under these conditions, the subaerial uplands would have experienced extreme chemical weathering and flash flood erosion [10], rapidly depositing smectite-rich clays into saturated brine ocean basins, generating thick piles of salt-rich sediments. Extensive erosion of the Venus uplands and infilling of the plains basins with sediment and salt may in part explain the subdued topography of Venus and the distribution of certain features in the plains.

Polygonal terrains are usually located towards the margins of the lowlands, and nearly all the longer canali appear within these areas and terminate in the deeper basins, consistent with an origin as submarine density currents [5] or hypersaline channels [11]. Once the oceans were finally gone, desiccation of these salt-sediment piles under a superheated steam atmosphere [12] resulted in the smaller-scale polygonal fractures superposed on them. Continued heating first lithified and then metamorphosed these sediments.

Ignoring the effects of later topographic changes, and the likelihood of repeated cycles of flooding and evaporation, a lower bound for the mass of salt precipitated on Venus can be obtained by assuming that areas below 6051·3 km radius were filled with saturated brine (at approximately 36 wt% NaCl). The mass of anhydrous halite that results is ~2 x 10¹⁹ kg, equivalent to a salt layer 64 m thick in those basins. This mass is orders of magnitudes smaller than the salt content of Earth’s oceans, suggesting that the salt deposits on Venus may host far thicker salt deposits across its lowlands.

Venus coastlines were likely steep and dynamic, precluding the establishment of long-lived, well-defined shorelines. Nonetheless, a variety of observations suggest former marine environments and possible shorelines close to the 6053 km radius contour (Fig. 1). Streamlined islands and channels can be seen parallel to some coastlines and there are hints of shoreward drainage systems above them. Taking that sea level and modern topography means oceans covered close to 90% of the surface of Venus, and had a volume about 40% of Earth’s oceans, equating to an average ocean depth of 1400 m. Interestingly, dissolving the mass of halite calculated earlier into that ocean volume gives a salinity of 35‰, about the same as terrestrial seawater. Did those oceans also support life?

It is hard to comprehend Mediterranean drying out almost completely, and the environmental impact that must have had, let alone the world losing its oceans entirely. Yet the evidence suggests that Venus did once have oceans, perhaps teeming with life, and that it lost them within the last billion years. The implications of such an event for life outside the Solar System, and on our own planet, are profound indeed.

References

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8. K. J. Hsü, W. B. F. Ryan, and M. B. Cita, Nature 1973 242:5395 242, 240 (1973)

9. B. Haq, C. Gorini, J. Baur, J. Moneron, and J. L. Rubino, Glob. Planet. Change 184, (2020)

10. J. van der Made, J. Morales, and P. Montoya, Palaeogeogr. Palaeoclimatol. Palaeoecol. 238, 228 (2006)

11. M. Roveri, V. Manzi, A. Bergamasco, F. M. Falcieri, R. Gennari, S. Lugli, and B. C. Schreiber, Am. J. Sci. 314, 751 (2014)

12. J. F. Kasting, Icarus 74, 472 (1988)

Figures

Fig. 1 Features consistent with former marine environments: (A) shoreline and wavecut platform; (B) streamlined islands; (C) submarine channel; and (D) drainage channels. Each can be explained by other processes and therefore none is diagnostic of former marine environments. Left-looking Magellan image, color-coded with stereo-derived topography, Mercator conformal projection. The anomalously deep area near the image centre is an erroneous altimeter measurement in a stereo data gap.