Scientists have discovered a surprising mechanism that could explain how small amounts of liquid water move huge quantities of sediment on Mars. Under the planet’s low atmospheric pressure, boiling water can create a cushion of vapor that lifts wet sediment, allowing it to travel farther and potentially reshape the Martian landscape.
Mars is often described as a cold, dry desert, but its surface may be more dynamic than it appears. Scientists have observed gullies, dark streaks and channels that suggest material is moving down Martian slopes, sometimes under conditions where liquid water might briefly exist.
The mystery is that liquid water cannot remain stable for long on the planet’s exposed surface. Its thin atmosphere allows water to boil or evaporate rapidly, raising an important question: If liquid water disappears so quickly, how can it move enough soil and rock to create visible changes in the landscape?
A laboratory study led by planetary scientist Jan Raack and his research team offers a surprising explanation. Their experiments reveal that boiling water can lift wet sediment on a cushion of vapor, dramatically increasing how much material it transports.
The results suggest that some recent and potentially ongoing changes on Mars may require far less water than scientists previously estimated.
Why Liquid Water Behaves Differently on Mars
On Earth, water generally remains liquid under ordinary surface conditions, allowing rivers, streams and rainwater to carry sand, mud and rocks downhill. The amount of sediment transported depends on factors such as water speed, slope, grain size and the surrounding environment.
Mars presents a very different situation. Its atmospheric pressure is extremely low, making exposed liquid water unstable. Water can boil even when temperatures are far below its normal boiling point on Earth.
However, the Martian surface does not remain equally cold everywhere. Under favorable conditions, temperatures near the equator and at southern mid-latitudes can approach 300 kelvin, or approximately 27°C. Some observed locations associated with recurring slope lineae have reached temperatures around 296–298 kelvin.
These temperatures create the possibility of short-lived liquid water, particularly when melting occurs beneath protective layers of dust, soil or ice.
But the existence of water is only part of the puzzle. Scientists must also understand what happens when that water interacts with loose sediment under Martian atmospheric conditions.
The Surprising Discovery: Boiling Water Can Lift Soil
To investigate this problem, Raack and his colleagues conducted laboratory experiments using wet sediment under low-pressure conditions similar to those found on Mars.
They examined two temperature scenarios: relatively cool sediment at approximately 278 kelvin (5°C) and warmer sediment at approximately 297 kelvin (24°C).
The team discovered a previously recognized mechanism that becomes important when water boils beneath saturated sediment. As the water turns into vapor, it can create a thin cushion beneath portions of the wet material.
This vapor cushion temporarily lifts sediment from the underlying surface, allowing it to move more easily downhill.
The phenomenon resembles the Leidenfrost effect, in which a liquid can be supported above a hot surface by a layer of its own vapor. A familiar example is a water droplet skittering across a sufficiently hot pan.
On Mars, however, the important process is not simply a droplet floating above a hot surface. Boiling water beneath wet sediment generates vapor that helps lift and displace sediment particles.
Once lifted, sediment can move downslope and trigger additional movements as surrounding grains collapse into the disturbed area. These secondary movements create small, dry granular avalanches, further increasing the total amount of transported material.
In other words, boiling water does more than flow downhill. Under the right conditions, it can help loosen, lift and mobilize the ground itself.
Nine Times More Sediment With the Same Water
The most striking result of the experiments was the increase in sediment transport.
When vapor-driven levitation was active, the amount of sediment transported was approximately nine times greater than in experiments without this mechanism.
This means that moving a comparable volume of sediment could require only about 11% of the water otherwise needed under the tested conditions.
Put simply, a quantity of water that might ordinarily move one unit of sediment could, in these experiments, help move roughly nine units.
This does not mean that every Martian slope will experience the same increase. The effect depends on factors such as temperature, water supply, infiltration into the ground and the characteristics of the sediment.
Nevertheless, the findings challenge an important assumption behind some explanations of Martian surface activity: that large amounts of liquid water are necessary to transport substantial quantities of soil.
If boiling water can dramatically increase transport efficiency, even a brief release of a relatively small quantity of water could potentially produce noticeable changes.
Mars' Lower Gravity Could Make the Effect Even Stronger
The researchers also investigated how the mechanism might behave under Martian gravity.
Mars has only about 38% of Earth's surface gravity. This difference affects how sediment falls back to the ground, how water infiltrates the soil and how strongly gravity opposes the upward force generated by vapor.
Using numerical calculations, the team estimated that the levitation force could be approximately 6.8 times stronger relative to the weight of the sediment under Martian gravity.
Their calculations suggest that sediment pellets could remain airborne for as much as 48 times longer than under the corresponding terrestrial conditions.
That figure is a theoretical estimate, not a direct observation of sediment levitating on Mars. The researchers also acknowledge that the simplified calculations may overestimate the actual duration.
Even so, a much shorter extension of the levitation period could allow sediment to travel several metres or even tens of metres downslope, disturbing a considerably larger area than the initial water release might suggest.
The calculations also indicate that the mechanism could operate under cooler conditions than those required in the warmest experiments.
Could This Explain Martian Gullies and Dark Streaks?
Mars contains numerous gullies, channels and recurring slope lineae (RSL), which appear as narrow, dark markings on steep slopes. Some change seasonally, growing during warmer periods and fading as temperatures fall.
Scientists have proposed several explanations for these features, including liquid water, salty liquids called brines, carbon dioxide frost, dry avalanches and gas-driven sediment movement.
The new findings provide another mechanism to consider when evaluating water-related explanations.
For example, a sudden release of water from beneath an insulating layer could rapidly saturate loose sediment. If the conditions allowed boiling and vapor formation, the resulting levitation could help move sediment much more efficiently than ordinary water flow.
Similarly, some recurring slope lineae might potentially extend through brief episodes of enhanced sediment movement rather than through continuous, slow movement alone.
However, the study does not prove that liquid water currently flows on Mars or that boiling-water levitation is responsible for any particular gully or dark streak. The experiments investigate how unstable water can transport sediment, not where the water originates or how frequently it becomes available.
Further observations and experiments are needed to determine whether this mechanism operates on the Martian surface today.
Where Could the Water Come From?
Several possible sources could supply the temporary water needed for the process.
One possibility is the melting of shallow ice beneath dust or soil. Another is the sudden release of groundwater from underground reservoirs, although this explanation may not account for every feature, particularly those found near isolated hills and crater rims.
Climate conditions in the recent geological past may also have allowed near-surface ice to freeze and thaw repeatedly, creating opportunities for short-lived liquid water.
Under present-day conditions, protective layers could help water accumulate beneath the surface before being released rapidly.
The crucial requirement is that enough water reaches the sediment quickly enough for vapor production to outpace infiltration and sustain the levitation process. Exactly how often these conditions occur on Mars remains uncertain.
A New Way to Understand the Martian Landscape
The discovery has implications beyond Mars. Similar processes could potentially occur on other planetary bodies where liquids become unstable under local pressure and temperature conditions.
It also demonstrates why laboratory experiments are essential in planetary science. Scientists cannot routinely observe Martian sediment moving in real time, so they must combine spacecraft observations, physical experiments and mathematical models to understand how planetary landscapes evolve.
The central lesson is both simple and surprising: the amount of water required to reshape a landscape depends not only on how much water is present, but also on how efficiently it can move sediment.
Raack and his team's findings suggest that some Martian slope features may have formed with considerably less water than previously thought.
Mars may not need rivers of water to move its soil. Under the right conditions, a brief burst of boiling water could be enough to set an entire slope in motion.
Reference: Raack, J., Conway, S.J., Herny, C. et al. Water induced sediment levitation enhances downslope transport on Mars. Nat Commun 8, 1151 (2017). https://doi.org/10.1038/s41467-017-01213-z
Comments
Post a Comment