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Discoveries made with the High Resolution Stereo Camera on the Mars Express orbiter show that, as recently as a few million years ago, the surface of Mars was being shaped by flowing water, lava and ice.

Several lines of evidence have recently reinforced the hypothesis that an ocean existed on early Mars. Carbonates are accordingly expected to have formed from oceanic sedimentation of carbon dioxide from the ancient martian atmosphere. But spectral imaging of the martian surface has revealed the presence of only a small amount of carbonate, widely distributed in the martian dust. Here we examine the feasibility of carbonate synthesis in ancient martian oceans using aqueous equilibrium calculations. We show that partial pressures of atmospheric carbon dioxide in the range 0.8-4 bar, in the presence of up to 13.5 mM sulphate and 0.8 mM iron in sea water, result in an acidic oceanic environment with a pH of less than 6.2. This precludes the formation of siderite, usually expected to be the first major carbonate mineral to precipitate. We conclude that extensive interaction between an atmosphere dominated by carbon dioxide and a lasting sulphate- and iron-enriched acidic ocean on early Mars is a plausible explanation for the observed absence of carbonates.

Over the past 30 years, the water-generated landforms and landscapes of Mars have been revealed in increasing detail by a succession of spacecraft missions. Recent data from the Mars Global Surveyor mission confirm the view that brief episodes of water-related activity, including glaciation, punctuated the geological history of Mars. The most recent of these episodes seems to have occurred within the past 10 million years. These new results are anomalous in regard to the prevailing view that the martian surface has been continuously extremely cold and dry, much as it is today, for the past 3.9 billion years. Interpretations of the new data are controversial, but explaining the anomalies in a consistent manner leads to potentially fruitful hypotheses for understanding the evolution of Mars in relation to Earth.

A variety of anomalous geomorphological features on Mars can be explained by a conceptual scheme involving episodic ocean and ice-sheet formation. The formation of valley networks early in Mars' history is evidence for a long-term hydrological cycle, which may have been associated with the existence of a persistent ocean. Cataclysmic flooding, triggered by extensive Tharsis volcanism, subsequently led to repeated ocean formation and then dissipation on the northern plains, and associated glaciation in the southern highlands until relatively late in Martian history.

The Meridiani Planum region on Mars is rich in minerals derived from evaporation, but lacks a topography consistent with standing water. Do the deposits stem from upwelling groundwater early in the planet's history?

Both Mars and Earth have experienced ice ages in geologically recent times. Coincidence of the phenomenon on two planets will further the scientific quest to answer the question of how ice ages originate.

IN this Article in the 15 August 1991 issue, an error in the Nature office led to the omission of a line from Table 3. In addition, some corrections noted by the authors were not made before publication. Corrected versions of the relevant passages appear below and in reprints. TABLE 3 Possible sources of C02 related to catastrophic outflow and ocean formation on Mars Source North polar cap Massive volcanism Adsorbed on regolith: ocean basin land Groundwater C02 clathrate C02 partial pressure (mbar) -20 -100 =£100 =£350 =£1,300 =£4,000 Fluvial history In first paragraph: The very high infiltration capacities of common martian sura-face rocks (lava flows and impact-brecciated regolith) would allow subsurface aquifers to be replenished easily, so that head differentials could be sustained and drive prolonged ground-water flow.

Hydrological conditions in arid regions are notoriously difficult to characterize in time and space. The inherent variability of rainfall and the resulting runoff poses problems for both economic development and palaeoenvironntental studies. Quantitative data on past floods are especially useful because the extreme rainfalls that generate desert floods are important environmental determinants in the world's arid environments. Unfortunately these are the environments that lack long-term hydrological records. We report here on the application of a palaeohydrological procedure1,2 that utilizes flood sediments as palaeostage indicators. By determining the ages of the responsible palaeofloods it is possible to evaluate their frequency of occurrence. The magnitude and frequency of floods on major desert stream or wadi systems provide critical palaeoclimatic information by delimiting areal and temporal shifts in rainfall-inducing weather systems. Central Australia is an ideal region in which to illustrate this technique because major phases of flooding correspond to the increased extratropical influence of the north Australia monsoons and/or tropical cyclones. We demonstrate here the use of palaeoflood hydrology to evaluate the recent phase of immense floods on the Finite River near Alice Springs in arid central Australia. The method approximates the magnitudes of the three great floods of 1967, 1972 and 1974, and it proves to be useful in other desert regions where conventional hydrological information is lacking.

Theoretical models of early Martian atmospheric evolution describe the maintenance of a dense CO2 atmosphere and a warm, wet climate until the end of the heavy-bombardment phase of impacting. However, the presence of very young, earthlike fluvial valleys on the northern flank of Alba Patera conflicts with this scenario. Whereas the widespread ancient Martian valleys generally have morphologies indicative of sapping erosion by the slow outflow of subsurface water, the local Alba valleys were probably formed by surface-runoff processes. Because subsurface water flow might be maintained by hydrothermal energy inputs and because surface-runoff valleys developed late in Martian history, it is not necessary to invoke drastically different planet-wide climatic conditions to explain valley development on Mars. The Alba fluvial valleys can be explained by hydrothermal activity or outflow-channel discharges that locally modified the atmosphere, including precipitation and local overland flow on low-permeability volcanic ash.