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Potential Inadequacy of Liquid Water Duration on Mars for Life Formation

Mars has a history of liquid water presence on its surface, including lakes like the one that previously existed, which have since dried up. Ancient water carrying debris—and present-day melted water ice performing the same function—was believed to be the sole factor contributing to the formation of gullies scattered across the Martian terrain. However, recent findings suggest that dry ice might also play a role in shaping the landscape.

The Process of Sublimation

Previously, scientists attributed the formation of gullies on Mars solely to liquid water, drawing parallels with Earth’s geological processes. However, they overlooked the concept of sublimation, the direct transition of a substance from a solid to a gaseous state. Sublimation is the mechanism through which CO2 ice vanishes (and Mars experiences this phenomenon as well).

Frozen carbon dioxide, present abundantly on Mars, also exists in its gullies. When CO2 ice sublimates within these gullies, the resulting gas can displace debris downhill, contributing to the shaping of the landscape.

A team of scientists led by planetary researcher Lonneke Roelofs from Utrecht University in the Netherlands has discovered that the sublimation of CO2 ice could have been instrumental in forming Martian gullies. This finding suggests that the most recent occurrence of liquid water on Mars might have transpired further back in time than previously assumed. Consequently, the window for potential life emergence and sustenance on Mars could have been narrower than initially believed.

“Under Martian atmospheric conditions, the sublimation of CO2 ice can mobilize sediment and create landforms resembling those found on Mars,” stated Roelofs and her team in a recent study published in Communications Earth & Environment.

The Role of Sublimation

Both Earth and Martian gullies exhibit similar morphologies. The key distinction lies in the fact that liquid water is definitively responsible for the formation and ongoing evolution of Earth’s gullies. This process involves the continual carving of new channels and transportation of debris downstream.

While ancient Mars likely possessed adequate stable liquid water to facilitate such processes, the current Martian surface lacks sufficient liquid water to sustain comparable activities. This is where sublimation comes into play. The presence of CO2 ice on the Martian surface coincides with material movement.

Upon analyzing phenomena like these, the researchers postulated that gas-driven flows are initiated as frozen carbon dioxide sublimates. Due to the low pressure on Mars, sublimation results in a more substantial gas flux compared to Earth, providing enough force to enable the fluid motion of materials.

Two mechanisms can trigger sublimation and initiate these flows. Firstly, when a segment of a gully’s exposed area collapses, particularly on a steep incline, warmed sediment and debris can descend onto CO2 ice in a cooler, shaded region. The heat from the falling material could supply adequate energy for the frost to sublimate. Alternatively, CO2 ice and sediment may detach from the gully and fall onto warmer terrain, triggering sublimation.

Laboratory Simulation of Martian Conditions

An inherent challenge with these hypotheses is the absence of direct human observations on Mars, leading to reliance on spacecraft data and images. To address this limitation, the research team constructed a flume within a specialized environmental chamber to replicate a segment of Mars’ landscape. The chamber simulated the atmospheric pressure of Mars, maintaining a sufficient slope for downward material movement and cold temperatures to sustain CO2 ice stability. Additionally, warmer adjacent slopes were introduced to induce sublimation, driving debris displacement. Experimentation under scenarios mimicking potential Mars conditions—heat emanating from beneath CO2 ice and warm material deposition—yielded the anticipated flows.

Further experiments conducted under Earth-like pressures and in the absence of CO2 ice failed to produce comparable flows. The researchers concluded that these experiments provide direct proof that CO2 sublimation can induce and maintain granular flows under Martian atmospheric conditions.

By demonstrating that gullies and similar systems can be shaped by sublimation rather than solely by liquid water, these findings raise questions regarding Mars’ historical availability of liquid water to sustain any potential organisms. The habitable period of Mars might have been briefer than previously assumed. While this does not definitively negate the possibility of past life on Mars, Roelofs’ discoveries could influence future perspectives on planetary habitability.