Recurring Slope Lineae (RSL) are mysterious dark streaks that appear seasonally on steep Martian slopes, growing longer in warmer months and fading in colder ones—puzzling scientists since their discovery by NASA's Mars Reconnaissance Orbiter in 2011.
Cornice Avalanches on Mars
Our 'Cornice Avalanches on Mars' theory proposes that wind collects frost into cornices, which then collapse into small avalanches in the summer. As they undergo a granular flow down the cliff, they hydrate the soil, which then turns darker and stays that way for weeks or months.
Why this theory fits the evidence for RSL
Experiments with Ice Hydrating in a Vacuum
We tested in a vacuum that ice can hydrate a mixture of salt and iron sulfate. Although not an exact Mars simulant, this shows that solid H₂O can hydrate real Martian minerals under realistic conditions.
Recurrence Mechanism
The process is naturally recurring. Solar heating evaporates or sublimes water from hydrated soil, redepositing it as frost at higher elevations via atmospheric cycling. Only H₂O undergoes solid-gas phase changes at RSL temperatures, enabling this cycle for thousands or millions of years without depleting resources. Peak RSL activity is when temperatures are above where H₂O sublimates, hinting at H₂O. Note that CO₂ can phase change near the Martian poles but at the RSL locations the temperatures are too high.
Slope behavior
RSL trails sometimes cross one another—something liquid water cannot do, but successive avalanches could. RSL only appear on steep slopes, consistent with avalanches, not liquid flow.
Darkening Effect
Hydrated salts (e.g., perchlorates) are darker than dehydrated ones, matching RSL's darkened paths.
Persistence of Darkening
Hydrated salts dehydrate slowly because water is chemically bound, aligning with RSL fading that happens over weeks to months.
Possible Images of Cornices
Some HiRISE images show bright spots near RSL origins, potentially frost cornices before collapse. The expected cornice sizes are near the resolution of the current orbiting cameras.
No Debris Piles
Unlike rock or sand avalanches, RSL leave no permanent accumulations at the base—consistent with H₂O frost that sublimes or evaporates away.
Orbital Spectral Evidence
Orbital CRISM data detect hydrated salts at RSL sites during active seasons, though interpretations vary; this supports soil hydration.
Seasonality
Cornices would destabilize in the warm season when solar heating weakens the accumulated pile of frost. Activity would halt in cold periods when frost accumulates. This matches the observed RSL behavior.
Topography for Frost Collection
RSL tops are often sheltered alcoves or ledges, shaded in winter and ideal for wind-driven frost buildup. Nearby flat plains provide source areas for frost harvesting.
Plausible mass balance
The amount of H₂O required to hydrate and darken an RSL (we estimated 5–100 kg) seems consistent with quantities of frost wind could collect onto those slopes.
Why we don't like the other theories
While cornice avalanches explain the observations, competing theories face several challenges:
Dry Granular Flows
For this to continue for thousands or millions of years without slowing down or piling up material at the bottom of the RSL, the 'dry granular' material that is flowing has to be frost/ice that is recycled.
Transient Brine
Liquid brine that had enough H₂O to flow would boil. If it was flowing it would be moving salt down the hill and so in less than thousands of years run out of salt. Also, any fixed source of liquid H₂O would run out in less than thousands of years as well. There are many RSL sites and they do not seem to be slowing.
Hygroscopic Soil (Deliquescence)
Salts absorbing atmospheric moisture could darken soil but wouldn't create flow-like paths with sharp edges and branching—hallmarks of granular motion. If they absorbed enough H₂O to flow, then they would move down the hill and not last for thousands of years.
Summary
We suggest that H₂O-cornice avalanches may provide the best current explanation of RSL — although further observations and experiments are needed. This theory uses solar and wind energy to transport mass, enabling activity to continue for geologic timescales. The other theories lack credible mechanisms to explain the 'recurring' part of Recurring Slope Lineae. As the only RSL theory consistent with sustained, long-term recurrence, this should be the leading one.
Possible Modification
It might be possible for the cornice to absorb into the soil during the avalanche and then seep (or Water vapor diffusion) downhill after that without being liquid. The RSL can lengthen gradually and this might fit with that. For this part the soil might not be moving and the H₂O not "flowing" but moving down still. Some experiments have shown this can happen and we will probably try such an experiment.
Experiments done, planned, or desired
1) Have tested ice hydrating soil in a vacuum and it can. But this was stationary ice not a passing avalanche. Perhaps as the avalanche flows past, enough small particles from it are trapped in the surface of the soil that when they sublimate the soil can trap some of the H₂O. It would be good to try to test this.
2) We have tested that hydrated iron-sulfate takes a long time to dry out. This could be done with a more accurate Martian soil simulant and in a vacuum instead of just on a window sill in the sun.
3) Could test that a cornice shaped wedge of frost/snow could fall as an avalanche and not just sublimate. Maybe as the sun warms up the slope below the cornice some heat is conducted through the ground, or convective heat transfer as air movement along the heated surface (anabatic wind), to the lower edge of the cornice and melts that first (or "undercuts" it) so it falls over. We can try to test this in our clear 22 inch vacuum chamber and probably will.
4) Wondering if moisture can seep through the soil without flowing or losing too much. Could have some ice in a vacuum held at the top of a steep slope and as moisture goes into the soil we could see if it works its way down the slope.
5) It would be nice to simulate wind blowing frost into a cornice in Martian-like conditions. Experimenting with this probably takes a better setup than we have.
6) Go over thousands of images of RSL areas and see if there is a correlation between bright spots at the top of RSL going away and RSL trails becoming dark. If so it would be very convincing evidence that cornices caused the dark trails.
Evidence against this theory
So far we have not found any direct experimental evidence that clearly contradicts this theory. If anyone has such we would really like to hear of it.
However, there are models for frost and wind on Mars that might claim this theory can not work. That is not the same as experiments showing the theory wrong though. Those models could well be wrong.
Satellites don't always detect bright cornices (some estimate 30% of the time) or hydration. It can be that current satellites are just not sensitive enough. So it does not really contradict the theory either. Future satellites or rovers could show H₂O cornices and hydration trails beyond doubt.
Some claim that an H₂O cornice would just sublimate and not collapse. In the experimental section above we have ideas to test this.
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