A Migration Model for the Polar Spiral Troughs of Mars

Abstract

Mars' iconic polar spiral troughs are 400–1,000‐m‐deep depressions in the north polar layered deposits. As the north polar layered deposits accumulate, troughs migrate approximately poleward, anti‐parallel to the local wind patterns. Insolation is suspected to drive ice retreat through sublimation. Sublimation at the trough wall produces a growing sublimation lag that modulates further retreat; however, winds move material off the retreating slope faces, thinning the lag. Discontinuities in stratigraphy seen by radar highlight Trough Migration Paths (TMPs), which provide a record of the troughs' position, formation, and evolution to the present day. We investigate two adjacent troughs presently near 87°N to evaluate the mass balance conditions at those sites. We constrain the contribution of insolation‐induced sublimation to the migration in the observed TMPs. We present a phenomenological model that combines our simulations of the sublimation conditions at paleo‐trough surfaces with accumulation rates to create synthetic TMPs that are tunable to the observations. Models using nominal values of lag diffusivity, albedo, and atmospheric water vapor abundance and in which the trough walls have been covered in a lag on the order of millimeters thick and formed ~2.3 Myr ago match the observed trough migration and align with expectations of trough ages. Thicker lags, and/or older troughs, would generate TMPs of constant slope, which does not match the observed paths. We demonstrate the viability of our new theoretical model for predicting conditions that lead to trough migration, allowing us to connect observable TMPs to Martian climate processes.

North polar trough formation due to in-situ erosion as a source of young ice in mid-latitudinal mantles on Mars

“…The processes involved include solar insolation-driven sublimation of equatorward-facing ice, downslope transport of water vapor via katabatic winds, ice-vapor condensation onto the cold, poleward-facing slopes, and perhaps mechanical erosion and sedimentation of ice particles 14 , 16 , 17 . Simulations of the sublimation from the trough walls, combined with proposed NPLD accumulation rates recreate the trough-migration paths inferred to exist in the subsurface from radar-sounding data 20 . The trough migration process has also been invoked to explain the development of their spiral pattern as poleward migration rearranged systems of initially disorganized troughs 21 .…”

Modern Mars' geomorphological activity, driven by wind, frost, and gravity

“…However, most of these processes driven by both wind and sublimation are not directly observable due to very slow rates. Large-scale features like the chasmae/spiral troughs in the NPRC are thought to have formed into their present state over millions of years of erosion by katabatic winds and asymmetric insolation/sublimation (e.g., Bramson et al, 2019;Howard, 2000;Smith and Holt, 2010). Smaller-scale periodicities in the landscape may be related to sublimation dynamics of perennial ice layers interacting with the winds over thousands of years (e.g., Bordiec et al, 2020;Herny et al, 2014;Howard, 2000;Nguyen et al, 2020).…”

The Importance of the Climate Record in the Martian Polar Layered Deposits

“…The obliquity solutions used to model the formation of the NPLD are non-unique beyond 20 Ma, so the older age of the SPLD has only permitted estimates of its accumulation rate that are purely based on ratios of observed stratigraphic periodicities, and have a high degree of uncertainty [Becerra et al 2019]. Furthermore, although the NPLD models (cited above) mostly agree that the NPLD began accumulating ~4.2 Ma (roughly coinciding with a drop in Mars' mean obliquity), the age of the SPLD cratering record, as well as recent modeling, suggest that polar ice could survive higher mean obliquities with only thin covers of insulating dust [Bramson et al 2019]. In addition, the extent to which the much dustier SPLD built up a sublimation dust lag over single or multiple obliquity epochs, which could have major implications for our understanding of how buried ice survives on Mars, is not known.…”