Mars Science Laboratory (MSL) Curiosity rover data are used to describe the morphology of desiccation cracks observed in ancient lacustrine strata at Gale crater, Mars, and to interpret their paleoenvironmental setting. The desiccation cracks indicate subaerial exposure of lacustrine facies in the Sutton Island member of the Murray formation. In association with ripple cross-stratification and possible eolian cross-bedding, these facies indicate a transition from longer-lived perennial lakes recorded by older strata to younger lakes characterized by intermittent exposure. The transition from perennial to episodically exposed lacustrine environments provides evidence for local to regional climate change that can help constrain Mars climate models.

During Sols 1555–1571, Curiosity investigated a series of distinctive centimeter-scale reticulate ridges on the surfaces of several slabs of rock that expose bedding planes in the Sutton Island member of the Murray formation. Their morphology and composition is characterized to determine if they formed via desiccation and to examine implications for the deposition of associated strata.

The kilometers-thick sedimentary succession in Gale crater provides an opportunity to observe changes in surface environments over extended periods in martian history. Studies of basal strata in the informally named Murray formation demonstrated the presence of long-lived perennial lakes in Gale crater at ca. 3.6–3.2 Ga ( Grotzinger et al., 2014 , 2015 ; Hurowitz et al., 2017 ). Recent facies observations at higher stratigraphic levels ( Fedo et al., 2017 ) may record an evolution of the environment over time. Here we present in situ evidence for lithified desiccation cracks in the Murray formation, indicating that the lakes may have partially dried in its younger history.

The focus of the investigation is an ∼80-cm-long, 40-cm-wide rock slab called “Old Soaker” (OS) that exposes a bedding plane with a red surface marked by a network of ridges that form polygons ( Fig. 2A ). The red mudstone is ∼1 cm thick and overlies a gray sandstone bed containing bedding-parallel seams of calcium sulfate (CaSO 4 ). OS and a similar nearby slab called “Squid Cove” (SC) were imaged with the Mast Camera (Mastcam) and the Mars Hand Lens Imager (MAHLI) to characterize the geometry and fill of the ridges. Their elemental compositions were examined with the rover’s ChemCam Laser Induced Breakdown Spectrometer (LIBS) and Alpha-Particle X-Ray Spectrometer (APXS).

The geometries of the polygonal ridges were determined using MAHLI images to evaluate whether their shape is consistent with desiccation. Images of ridges and their junctions were traced to calculate vertex angle distributions, widths of ridges and the polygons they form, and ridge surface area. A three-dimensional (3-D) model of OS was generated from 76 MAHLI images processed using photogrammetry software. The grain sizes of the red and gray beds were measured with ∼16 µm/pixel MAHLI images.

Some very fine sand grains and millimeter-scale concretions or embedded grains are visible in MAHLI images of the gray bed at OS ( Fig. 2F ). The ridges taper off within millimeter-scale depressions in the red mudstone at OS ( Fig. 2G ). Fractures associated with the ridges of the SC slab penetrate the red mudstone and terminate at the boundary with the underlying gray sandstone ( Fig. 4 ). The gray beds appear to lack ridges ( Figs. 2A and 2F ).

The red surfaces of OS and SC are covered by networks of arcuate ridges with up to 5 mm of positive relief that define predominantly four-sided and some five-sided, 0.5–3.5-cm-wide polygons ( Figs. 2B and 3A ). Red surfaces of adjacent slabs also show raised ridges spanning an area of a few square meters. The ridges range in length from a few centimeters to ∼0.3 m and mostly meet orthogonally, forming T-junctions ( Fig. 3B ). The ridges are made of red-to-gray sediment similar in color to the surrounding bed ( Figs. 2B and 2C ) and comprise ∼20% of OS’s surface. No grains in the ridges or surrounding surface are resolved in MAHLI images ( Fig. 2C ), indicating a maximum grain size of coarse silt. CaSO 4 veins distinct from ridge material follow most, but not all, of the ridges ( Figs. 2B and 2C ) and in some cases cross-cut the ridges (e.g., Fig. 2E ). Sub-millimeter-wide fractures occur within the polygons ( Fig. 2C ). Gray, semi-circular, millimeter-scale patches dot the red beds on OS and SC. They can show raised relief and in places are cross-cut by veins ( Figs. 2B and 2D ).

ChemCam observation points on the ridges validate that their composition is distinct from CaSO 4 vein fill and close to that of the gray bed, with lower Al 2 O 3 and SiO 2 , high H emission lines, and higher K 2 O abundances than the red bed. The presence of strong H lines on the ridges indicates the presence of a significant component of hydrous phases absent from the red layer. The dark patches (target “Gilley Field”; Table 1 ) in the red bed are enriched in FeO (up to 27 wt%) and MnO (0.7 wt%) relative to the surrounding rock. The bright veins are similar to CaSO 4 veins encountered since the beginning of the mission ( Table 1 ) ( Nachon et al., 2017 ).

ChemCam analysis of OS identified three distinct bed compositions ( Fig. 2A ): (1) a lowermost bright sandstone with no ridges and a composition consistent with cementation of sandstones by calcium sulfates; (2) a gray bed with comparatively high K 2 O abundance relative to the bright sandstone (1.5–2.5 wt%); and (3) an overlying red mudstone compositionally similar to other Murray mudstones ( Table 1 ) ( Mangold et al., 2017 ). APXS measurements of OS show that the red mudstone bed is similar in composition to average Murray bedrock, but is two to three times richer in Cl (2–3 wt%) and Br (1150–1430 ppm). The gray bed (target “Fresh Meadow”) is distinct from the overlying red bed, with relatively enriched K 2 O, SO 3 , Na 2 O, and FeO T and depleted TiO 2 , SiO 2 , and Al 2 O 3 ( Table 2 ).

Proposed formation mechanisms for the ridges must account for several observations: (1) ridges form polygonal networks with T-junctions and continuous arcuate shapes; (2) the ridges in the red mudstone beds correspond to fractures that penetrate those beds; (3) the fractures are restricted to the red beds and terminate at the boundary with coarser underlying material; (4) the fractures are filled with very fine-grained sediment; (5) CaSO 4 veins run along many but not all of the ridges, in some cases cross-cut the ridges, and, unlike the ridges, cut all beds in exposed cross sections; and (6) the ridges are compositionally similar to the underlying gray bed. The most likely fracturing mechanisms include desiccation, synaeresis, and hydraulic fracturing.

Shrinkage cracks form in response to tensile stresses within sediment that result from contraction due to moisture or heat loss (Shorlin et al., 2000). When stress exceeds local tensile strength, materials fracture and cracks begin to grow orthogonal to the direction of maximum tensile stress, typically resulting in a polygonal pattern (Sletten et al., 2003). In uniform material, new cracks will turn to converge with other cracks orthogonally, resulting in junctions mostly near 90° (Shorlin et al., 2000) as observed at OS and SC (Fig. 3). Abundant T-junctions show that sediments dried to completion, possibly in a single event, rather than undergoing multiple wetting and drying cycles that tend to form 120° junctions (Goehring et al., 2010).

Desiccation cracks form at the sediment-air interface and are preserved in the rock record through sediment infill from overlying strata (Plummer and Gostin, 1981). The compositional and color similarity of the ridges to the average Murray formation, which is predominantly comprised of silt-sized grains or smaller, suggests that the ridges are comprised of sediment. Ridge-forming sediment at OS and SC is indistinguishable from the surrounding bed based on grain size alone, so this observation is not definitive evidence for sediment infill from an overlying bed.