Martian landslip

High above the Atlantic Ocean, a team of planetary scientists is recreating the surface conditions of Mars inside an aircraft. Ralf Plechinger went with them to report on their experiments and experience weightlessness for himself.

Ralf Plechinger

Something strange happens on Mars during the planet’s warmer summer season. What appear to be stable inclines and slopes spontaneously begin to collapse, triggering landslides and small avalanches. These reshape the surface in ways that are now easily visible from the phalanx of orbiting spacecraft that continually monitor the Red Planet.

On Earth, such landslides are usually triggered by the infiltration of water into the soil or rock, which reduces the friction holding the slope together. As such, heavy rainfall, snowmelt or groundwater is the usual culprit. This simply cannot be the case at Mars.

The conditions on our neighbouring world are far colder and drier. Not to mention that the atmospheric pressure at the surface is less than one per cent of Earth’s, so any liquid water that did find itself on the surface would transform almost instantly into water vapour.

Without water to lubricate the grains, the Martian slope failures must therefore be driven by an entirely different process but what? This is the mystery that the high-flying team of scientists are investigating.

The experiment

The experiment took place in June 2024 aboard a research flight organised by the German Space Agency but the analysis has only just been published in The Planetary Science Journal. Even in 2024, this was not the team’s first flight. Led by the University of Duisburg-Essen’s Gerhard Wurm and Jens Teiser, they had flown twice before, testing how Martian sand behaves under the low gravity found on that planet. During the flights, three scientists monitored the desk-top recreation of the Martian surface. Two watched computers recording camera images from the apparatus, while a third controlled the measurement system.

Several theories have been proposed for the cause of the landslides. One proposed that despite the conditions, water was indeed present but hidden beneath the sand. After sunlight heats up the overlying material, the water evaporates and destabilises the slope. However, this is contradicted by the fact that the affected areas are located at the Martian equator, where the soils are known to be even drier than elsewhere.

Another suggestion was that the landslips might involve extremely salty water. Mars is known to contain salts such as perchlorates that can dramatically lower the freezing point of water, potentially allowing small amounts of liquid brine to form in the soil. Such brines could then lubricate loose sand on steep slopes.

This phenomenon is already known to give rise to dark streaks seen in orbiter images. These narrow markings appear from nowhere and extend downhill, only to fade again as dust slowly settles over them. So, the idea was floated that the landslips could simply be larger versions of this process. However, these features recur throughout the year, which is difficult to reconcile with the summer appearance of the landslips.

So, scientists have come to suspect that the landslips they are witnessing are an entirely dry phenomenon. Yet if that is true, it means there are even more mysteries to unpack.

On Earth, dry sand or dust typically begins to slide only when a slope reaches 30–35°, the so-called angle of repose. At this angle, Earth’s gravity finally overcomes the friction between the grains. Yet many Martian slips are seen on slopes as shallow as 10–15°. Explaining how such gentle slopes can fail, especially given the fact that the gravity on Mars is weaker than on Earth, has been a longstanding problem in planetary science.

Flying the parabola

A parabolic flight is the closest most scientists come to experiencing space. The downside is that the manoeuvres that create weightlessness can also upset the stomach. Even experienced researchers sometimes feel unwell during the flight, so those who ask are offered scopolamine beforehand to suppress nausea.

After take-off from Bordeaux, the aircraft headed for a carefully chosen region agreed in advance with air traffic control where turbulence was minimal. On our flight, this meant the Atlantic Ocean off the west coast of France. Calm air is essential if the aircraft is to trace the precise parabolic arcs needed to create weightlessness.

The first parabola serves as an acclimatisation exercise for both passengers and pilots. Scientists also use it to check and adjust their experiments before the real sequence begins. After that come 30 further parabolas in quick succession.

It was my first time on board, and I had the privilege of experiencing weightlessness during the first six parabolas in the free-float cage, a netted-off area of the cabin roughly two by three metres in size beside the experiments. The barrier is necessary: drifting uncontrollably into delicate equipment, or into a scientist at work, would be all too easy.

During the first parabola I was strapped to the floor. As the aircraft climbed steeply, the 1.8g acceleration pressed me firmly downward. The forces were enormous. Then, when the thrust levers were pulled back, I felt myself becoming lighter over the next three or four seconds until suddenly I weighed nothing at all. I was overcome by a feeling of being free from all worries. Twenty-two seconds later the experience ended abruptly as the aircraft pulled out of the dive and I was pressed back into the ground with 1.6G.

For the second parabola I was unbuckled and sitting on the floor. Weightless, I hovered just millimetres above it. Curious to float higher, I pushed gently with my fingertips but still rather too hard. I rose upwards while slowly rotating until I found myself ‘lying’ against the cabin ceiling. A truly impressive feeling.

I also noticed something astronauts often report: a puffy face. In weightlessness the heart continues to pump blood toward the head as usual, but without gravity pulling fluids downward, more blood accumulates in the upper body.

Around me, the scientists had little time to savour the experience. Most were strapped beside their instruments, focused on collecting data, while only occasionally did someone drift into the free-float cage during a short break. I was fortunate – I never felt nauseous. Two researchers who could not take medication for experimental reasons were less lucky and had to stop early. For everyone on board, though, that sequence of brief 22 seconds of weightlessness were unforgettable.

Thermal creep

Scientists have proposed that Mars possesses a mechanism for destabilising its slopes that does not operate on Earth. It involves a subtle interaction between sunlight, the planet’s thin atmosphere and the dusty structure of the Martian soil.

When researchers look closely at the sand on Mars, they see there are pores between the grains. Due to the low Martian atmospheric pressure, which is less than one percent that of Earth’s atmosphere, the size of these pores roughly corresponds to the ‘mean free path’ in the atmosphere. This means that the average distance between the molecules in the rarefied Martian atmosphere is approximately the diameter of these pores.

The upshot is that molecules can freely move between the pores. When the surface is warmed by the Sun, the uppermost grains heat up more quickly than the material beneath them. This creates a temperature gradient within the soil, with warmer layers above cooler ones. Under Martian conditions this temperature difference drives a phenomenon known as thermal creep, in which gas flows through the narrow pores from cooler regions toward warmer ones.

The result is the formation of a small pressure build-up just a few millimetres beneath thesurface. This effect is sometimes called a Knudsen compressor, after the Danish physicist Martin Knudsen who studied similar gas flows more than a century ago. Although the pressure generated in this way is tiny, it can nevertheless influence the stability of the soil.

the pilots first pull the aircraft steeply upwards to an angle of 50° and climb for 25 seconds. During this time, the force inside the aircraft is 1.8 times that of normal gravity.

By slightly supporting the uppermost layer of grains from below, it effectively reduces the weight pressing those grains together. With less weight holding them in place, the friction between grains decreases, making it easier for them to slide past one another.

In practical terms, this means that slopes on Mars could become unstable at angles much lower than would normally be required for dry sand on Earth.

Such a process cannot occur on our world due to the higher atmospheric pressure, which packs the air molecules much closer together, even though our sand has similarly sized pores to those on Mars.

To test the idea experimentally, the team needed to recreate Martian conditions as closely as possible. This is where the flights come into play.

Flying the experiment

The French company Novespace conducts several parabolic flight campaigns each year for the European Space Agency (ESA), the French space agency CNES, and the German Space Agency (Deutsche Raumfahrtagentur im DLR).

During such a flight, the aircraft follows a parabolic path. This means the pilots first pull the aircraft steeply upwards to an angle of 50° and climb for 25 seconds. During this time, the force inside the aircraft is 1.8 times that of normal gravity. Then, the engine power is reduced to idle. Simultaneously, the nose is slowly pushed downwards until a maximum angle of –42° is reached. Effectively the plane is in free-fall during this time and microgravity, or near weightlessness, prevails on board.

Unlike being in space, it is not absolute weightlessness, as minor air turbulence can lead to slight residual accelerations. However, this difference is barely perceptible to humans, and the accuracy is sufficient for many experiments. Each microgravity lasts for approximately 22 seconds. After which, the pilots pull up and exits the parabolic trajectory to avoid the aircraft exceeding its maximum speed.

During this pull-out phase, the force inside the aircraft rises again, this time to 1.6 times usual gravity until normal flight is resumed.

Typically, the various experiments remain on board for the entire duration of a parabolic flight campaign, which consists of three flight days, each with 30 parabolas flown. This gives the scientists a total of 33 minutes of weightless time. The advantage over the International Space Station (ISS), where exact weightlessness prevails, is that the scientists can participate in the flight itself and directly intervene in their experiments during the weightless phase.

The exact trajectory of the aircraft can also be adjusted to simulate Martian gravity on board. However, such flights are extremely rare. Therefore, the team had to experiment in microgravity and use additional technology to simulate Martian gravity. This involved using a centrifuge spinning at exactly the correct rate to recreate the acceleration due to gravity on Mars.

The experiment they devised was called CHARLIZE (Challenges in Angles of Repose with or without Light Interaction Zones in low-g Experiments – Particle movement at low slopes on Mars under solar radiation). The big answer they sought was when does a slope on Mars become unstable: at what angle of repose and at what level of solar radiation? Within this centrifuge sat two identical vacuum chambers, each containing a tray filled with Martian soil ‘simulant’. This was a basalticpowder with grain sizes similar to that of Martian sands. The chamber was pumped down to pressures as low as Mars’ atmosphere, and lamps provided illumination comparable to the Red Planet’s sunlight.

The centrifuge

The centrifuge containing the vacuum chambers. Two gondolas, each containing a vacuum chamber that can be tilted to create an incline, are suspended from the centrifuge.Each vacuum chamber contains a sample of sand. Attached to the vacuum chamber are a halogen lamp for illumination, a camera for observation, and an infrared camera for determining temperature differences. The vacuum is created by a vacuum pump. Three computers are integrated into the centrifuge. One records the measurements (pressure and temperature measurement, computer-controlled switching of the halogen lamp, etc.), while the other two store the camera images (the optical camera takes 20 photos per second). Each flight generates approximately 300 GB of data. The experiment uses approximately 300 metres of cable.

The aircraft

As the aircraft began its aerobatics, the team began the experiments.

During each short period of microgravity, the vacuum chambers were slowly tilted while artificial light heated the sand. Cameras recorded the motion of the grains. By gradually increasing the slope angle, the researchers could determine precisely when the material began to fall.

Crucially, only a small circular portion of the soil surface was illuminated by the lamp, while neighbouring regions remained in shadow. This allowed the team to compare the behaviour of illuminated and shadowed areas. The resulting images were then analysed frame by frame to detect when grains began to move.

Crucially, only a small circular portion of the soil surface was illuminated by the lamp, while neighbouring regions remained in shadow. This allowed the team to compare the behaviour of illuminated and shadowed areas.

The results showed a clear difference between the illuminated and shadowed regions. In the illuminated regions, the sand began to slip before the dark areas, indicating that light was indeed influencing the slope’s stability.

However, the short duration of each flight’s microgravity led the researchers to believe that they were not seeing the full effect. After all, in natural conditions on Mars, sunlight could warm a slope for minutes or hours. Back in the laboratory, they ran the experiments again. Although microgravity was absent, the samples could be illuminated for longer than just a dozen seconds or so. This revealed that the temperature gradient took minutes to build up. And so the results from the aircraft underestimated the angle of repose.

Once they folded all of their results together, they discovered that the shadowed soil behaved as expected, with avalanches beginning at slope angles of around 35°. However, in illuminated areas the slopes could fail at significantly lower angles. In some cases the onset of motion occurred at angles as low as about 10°. Such shallow slopes are strikingly similar to those observed in Martian slope streaks.

For the first time, they had shown that a Knudsen compressor can be established in Martian conditions and therefore thermal creep probably plays an important role in producing the Martian landslips seen from orbit.

And while this may be disappointing news for those who were hoping that it was another indication of present-day water on Mars, it serves as a reminder that Mars is not just an Earth-like desert. Instead, it is a fundamentally alien world with its own unique environment. And that makes it all the more fascinating.

Ralf Plechinger is a flight instructor, amateur astronomer and a freelance journalist in Germany.