Martian remote sensing
A decade of high-resolution monitoring has revealed extensive activity in fresh Martian gullies. Flows within the gullies are diverse: they can be relatively light, neutral or dark, colourful or bland, and range from superficial deposits to 10 m-scale topographic changes. We observed erosion and transport of material within gullies, new terraces, freshly eroded channel segments, migrating sinuous curves, channel abandonment, and lobate deposits. We also observed early stages of gully initiation, demonstrating that these processes are not merely modifying pre-existing landforms. The timing of activity closely correlates with the presence of seasonal CO2 frost, so the current changes must be part of ongoing gully formation that is driven largely by its presence. We suggest that the cumulative effect of many flows erodes alcoves and channels, and builds lobate aprons, with no involvement of liquid water. Instead, flows may be fluidized by sublimation of entrained CO2 ice or other mechanisms. The frequent activity is likely to have erased any features dating from high-obliquity periods, so fresh gully geomorphology at middle and high latitudes is not evidence for past liquid water. CO2 ice-driven processes may have been important throughout Martian geological history and their deposits could exist in the rock record, perhaps resembling debris-flow sediments.
Supplementary material: Figures, animations and a summary table describing details of known gully activity are available at https://doi.org/10.6084/m9.figshare.c.3936886
Figures & Tables
Martian Gullies and their Earth Analogues
CONTAINS OPEN ACCESS
Gullies on Mars resemble terrestrial gullies involved in the transport of abundant material down steep slopes by liquid water. However, liquid water should not be stable at the Martian surface. The articles in this volume present the two main opposing theories for Martian gully formation: climate-driven melting of surficial water-ice deposits and seasonal dry-ice sublimation. The evidence presented ranges from remote-sensing observations, to experimental simulations, to comparison with Earth analogues. The opposing hypotheses imply either that Mars has been unusually wet in the last few million years or that it has remained a cold dry desert – both with profound implications for understanding the water budget of Mars and its habitability. The debate questions the limits of remote-sensing data and how we interpret active processes on extra-terrestrial planetary surfaces, even beyond those on Mars, as summarized by the review paper at the beginning of the book.