Recent exploration of Mars by robotic vehicles suggests that layers of sedimentary rock cover portions of the planet's surface. The layers contain cross bedding and relicts of gypsum crystals. At face value, what do these features suggest about depositional environments on Mars in the past? (D, E)

Imagine looking at the rippled patterns on a beach or the layers within shifting sand dunes; these are surface examples of what geologists call 'cross bedding'.

Through robotic explorations, we've glimpsed similar patterns on Mars, hinting at ancient processes. The layers of sedimentary rock on the Red Planet displaying cross bedding likely formed by materials like sand or silt being carried and deposited by fluid movements, be that liquid or gas. On Earth, cross bedding often occurs due to water currents in rivers or by wind in desert dunes; Martian cross bedding suggests similar forces at work, such as wind or possibly flowing water, painting a picture of a Mars with a more dynamic environment.

Glimmering amidst the Martian terrain, the remnants of gypsum crystals provide enticing clues about the planet's history.

Gypsum, a soft sulfate mineral, is typically found in thick beds or layers. On Earth, these crystals form in evaporite deposits as water evaporates and leaves behind sulfate minerals. The discovery of these crystal remnants on Mars implies the historical presence of water that, upon evaporating, left behind concentrated sulfate minerals. These gypsum relics are poignant markers indicating Mars once harbored water, perhaps in lakes or shallow seas, that underwent a cycle of evaporation and mineral deposition.

The term 'depositional environment' refers to the natural settings where sediments accumulate to form sedimentary rock layers.

These environments range from deserts to lakes, rivers, and seas, each with distinctive characteristics influencing the type of sediments deposited. For example, a river might carry finely ground particles, while a beach might collect sand and pebbles. On Mars, the presence of sedimentary rocks with cross bedding and gypsum crystals points towards past depositional environments that were quite dynamic. This insight allows scientists to speculate about historical water activity or windy conditions on the Martian surface resembling those that shape similar landforms on Earth.

Within the dry expanses of Mars, we find indicators of past moisture, some of which are 'evaporite deposits'.

Evaporites are minerals left behind when bodies of water undergo evaporation. Think of how sea salt is harvested from seawater; it's a simple comparison to how Martian evaporites like gypsum may have formed. Such deposits on Mars suggest that there were once standing bodies of water that evaporated under the planet’s thin atmosphere, leaving a salty, mineral-rich layer behind. This process of evaporation and deposition offers a significant signpost to understanding Mars' climatic past and piecing together the environmental conditions that once existed on the now arid planet.