Our research concerns the potential for past lakes in the Martian highlands, with a goal of quantifying their characteristics. The presence of standing bodies of water on the Martian surface would tell us a great deal about the past climate. Lakes, more so than river channels, require a warmer climate with a water recharge system, and they may represent formerly habitable sites.
The best means for identifying paleolakes are deltas and continuous terraces at a constant elevation around a basin, or breached basins that have either a contributing valley, an outlet, or both, but no incision of the lower-lying basin floor1–3. Shore landforms, as exhibited by terrestrial lakes, are typically small and fine-grained, potentially limiting their observation and preservation on Mars4.
Ma'adim Vallis, shown in Figure 1, was carved by an overflowing paleolake up to ~1.1 million km2 in size, located in the Eridania basin south of the valley's source spillways1,2. This discovery demonstrates that large areas of the highlands were formerly flooded with water. The spillway, which represents the channel's elevation when it overflowed, provides the most compelling evidence that a former lake existed. Moreover, the Eridania basin's interior contains six highly degraded impact craters2 that have bowl-shaped rather than flat floors indicating that sediment accumulated mainly along their margins1,2 in standing bodies of water. It is hard to reconcile a large, long-lived lake at this high elevation without some means of recharging highland aquifers. The topography suggests that precipitation was the ultimate water source for the lake, and catastrophic flooding of this lake was the primary eroding agent of the large Ma'adim Vallis valley1,2.
1. Irwin et al. (2002), A Large Paleolake Basin at the Head of Ma'adim Vallis, Mars.
2. Irwin et al. (2004), Geomorphology of Ma'adim Vallis, Mars, and associated paleolake basins.
3. Irwin et al. (2005), An intense terminal epoch of widespread fluvial activity on early Mars: 2. Increased runoff and paleolake development.
4. Irwin and Zimbelman (2012), Morphometry of Great Basin pluvial shore landforms: Implications for paleolake basins on Mars.