Martian gullies are landforms consisting of an erosional alcove, a channel, and a depositional apron. A significant proportion of Martian gullies at the mid-latitudes is active today. The seasonal sublimation of CO2 ice has been suggested as a driver behind present-day gully activity. However, due to a lack of in-situ observations, the actual processes causing the observed changes remain unresolved. Here, we present results from flume experiments in environmental chambers in which we created CO2-driven granular flows under Martian atmospheric conditions. Our experiments show that under Martian atmospheric pressure, large amounts of granular material can be fluidized by the sublimation of small quantities of CO2 ice in the granular mixture (only 0.5% of the volume fraction of the flow) under slope angles as low as 10°. Dimensionless scaling of the CO2-driven granular flows shows that they are dynamically similar to terrestrial two-phase granular flows, i.e. debris flows and pyroclastic flows. The similarity in flow dynamics explains the similarity in deposit morphology with levees and lobes, supporting the hypothesis that CO2-driven granular flows on Mars are not merely modifying older landforms, but they are actively forming them. This has far-reaching implications for the processes thought to have formed these gullies over time. For other planetary bodies in our solar system, our experimental results suggest that the existence of gully-like landforms is not necessarily evidence for flowing liquids but that they could also be formed or modified by sublimation-driven flow processes.

Subtle mounds have been discovered in the source areas of martian kilometer-sized flows and on top of summit areas of domes. These features have been suggested to be related to subsurface sediment mobilization, opening questions regarding their formation mechanisms. Previous studies hypothesized that they mark the position of feeder vents through which mud was brought to the surface. Two theories have been proposed: a) ascent of more viscous mud during the late stage of eruption and b) expansion of mud within the conduit due to the instability of water under martian conditions. Here we present experiments performed inside a low-pressure chamber, designed to investigate whether the volume of mud changes when exposed to a reduced atmospheric pressure. Depending on the mud viscosity, we observe volumetric increase of up to 30% at the martian average pressure of ~6 mbar. This is because the low pressure causes instability of the water within the mud, leading to the formation of bubbles that increase the volume of the mixture. This mechanism bears resemblance to the volumetric changes associated with the degassing of terrestrial lavas or mud volcano eruptions caused by a rapid pressure drop. We conclude that the mounds associated with putative martian sedimentary volcanoes might indeed be explained by volumetric changes of the mud. We also show that mud flows on Mars and elsewhere in the Solar System could behave differently to those found on Earth, because mud dynamics are affected by the formation of bubbles in response to the low atmospheric pressure.

Mercury’s surface is dominated by tectonic landforms formed by compression. Other than within basins, extensional landforms are not well known and have been presumed to be much rarer, with only a handful reported [1]. To date, two types of extensional grabens associated with lobate scarps have been described in literature: pristine back-scarp grabens associated with small lobate scarps (10s of kms in length and 10s of metres in relief) [2] and crestal grabens found on Calypso Rupes (381km in length and ~1km in relief) [3], [4]. This study identifies that such extensional grabens found on lobate scarps are much more widespread than previously recognised. These form when thrusting produces a hanging wall anticline, and local tensional stresses along the anticlinal axis cause antithetic faults to form in the folded strata, parallel or sub-parallel along the hinge zone, producing a down-dropped fault block. These small-scale features (often less than 1km in width, 10s of kms in length and likely 10s to 100s of metres in depth) are not expected survive 100s of millions of years because of regolith formation and impact gardening masking their signature [1], [2]. Our discovery and documentation of more extensional grabens may indicate that significant movement on many of Mercury’s large lobate scarps persisted until geologically recent times. [1] P. K. Byrne, C. Klimczak, and A. M. C. Sengör, “The Tectonic Character of Mercury,” in Mercury : The View After MESSENGER, 1st Editio., S. C. Solomon, L. R. Nittler, and B. J. Anderson, Eds. Cambridge: Cambridge University Press, 2018, pp. 249–286. [2] T. R. Watters, K. Daud, M. E. Banks, M. M. Selvans, C. R. Chapman, and C. M. Ernst, “Recent tectonic activity on Mercury revealed by small thrust fault scarps,” Nat. Geosci., vol. 9, no. 10, pp. 743–747, 2016. [3] C. Klimczak, P. K. Byrne, A. M. C. Şengör, and S. C. Solomon, “Principles of structural geology on rocky planets,” Can. J. Earth Sci., vol. 56, no. 12, pp. 1437–1457, Dec. 2019. [4] M. E. Banks et al., “Duration of activity on lobate-scarp thrust faults on Mercury,” J. Geophys. Res. E Planets, vol. 120, no. 11, pp. 1751–1762, 2015.