Mars rover Curiosity landed on, or just beyond, the far end of an alluvial fan — rocks, gravel, and sand washed down by the Peace River from the north rim of Gale Crater. The rover has driven for 200 sols (Mars days) across a landscape that was shaped and mineralogically altered by water, and which lies near the lowest part of Gale Crater’s floor.
What are the odds that Gale Crater didn’t contain standing water — a lake — during its 3.8 billion year history?
A group of scientists led by William Dietrich (University of California, Berkeley) use new topographic data to argue that Gale shows evidence for lakes at three different elevation levels, plus a few possible shallow lakes near the landing site. The team reported its results (PDF) at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.
The geologists examined high-resolution digital elevation models to identify likely ancient shorelines, fluvial deltas, and other features of former lake levels. They write, “We describe the possible lake levels from highest to lowest, and hypothesize that this also records a time progression in lake levels because of preservation patterns.”
The highest lake level lies at about -2100 meters (-6900 feet) below Martian “sea level.” Images showing broad terrace-like features about the same height as Gale’s northern rim flank its giant mound, Mt. Sharp. These show features in HiRISE images consistent with being deposited underwater.
If so, the team notes, the lake level would have been higher than the northern rim, and very roughly coincident with a proposed northern ocean at a level of –1848 m (–6100 ft). In this model, Gale would have been an inlet or bay on the ocean’s shore.
The next lower lake level lies at about –3300 m (–10,800 ft). This roughly corresponds to the lower ends of channels on the south side of Mt. Sharp and to a distinct bench on the northeastern side of the crater. The scientists note that a lake at this level would have had an average depth of 650 m (2100 ft).
The third and lowest lake level (–3780 m / –12,000 ft) is the best defined through a bench that can be traced along the crater wall and on Mt. Sharp. This lake would have been confined to the northern and eastern sides of Mt. Sharp, and its average depth would have been about 170 m (560 ft).
As the lake shrank, it would have ended as shallow separate pools on the floor. The team notes that Peace Vallis built an alluvial fan that descends into a closed basin toward the Glenelg rover site, which is 670 m (2200 ft) below the third lake level. “It seems highly probable that shallow lakes may have formed there episodically,” they say.
“The simplest interpretation of this succession of lake levels,” explains the Dietrich team, “is that the crater filled with water and then the lake level progressively fell, perhaps stalling at two levels long enough to create a topographic record of the shoreline.”
The three high lake levels would have saturated deposits in Mt. Sharp and in the crater walls. Deposits in Mt. Sharp — at least the lower levels — are Curiosity’s prime geological targets.
“As lake levels in Gale decreased, the low area immediately adjacent to the Curiosity landing site would have been one of the last areas to dry out,” they note.