Initiation and Flow Conditions of Contemporary Flows in Martian Gullies.

T De Haas,M G Kleinhans,F Salese,J N Mcelwaine,P M Grindrod,S J Conway,B W Mcardell

doi:10.1029/2018je005899

Abstract

Understanding the initial and flow conditions of contemporary flows in Martian gullies, generally believed to be triggered and fluidized by CO2 sublimation, is crucial for deciphering climate conditions needed to trigger and sustain them. We employ the RAMMS (RApid Mass Movement Simulation) debris flow and avalanche model to back calculate initial and flow conditions of recent flows in three gullies in Hale crater. We infer minimum release depths of 1.0–1.5 m and initial release volumes of 100–200 m3. Entrainment leads to final flow volumes that are ∼2.5–5.5 times larger than initially released, and entrainment is found necessary to match the observed flow deposits. Simulated mean cross‐channel flow velocities decrease from 3–4 m/s to ∼1 m/s from release area to flow terminus, while flow depths generally decrease from 0.5–1 to 0.1–0.2 m. The mean cross‐channel erosion depth and deposition thicknesses are ∼0.1–0.3 m. Back‐calculated dry‐Coulomb friction ranges from 0.1 to 0.25 and viscous‐turbulent friction between 100 and 200 m/s2, which are values similar to those of granular debris flows on Earth. These results suggest that recent flows in gullies are fluidized to a similar degree as are granular debris flows on Earth. Using a novel model for mass flow fluidization by CO2 sublimation we are able to show that under Martian atmospheric conditions very small volumetric fractions of CO2 of ≪1% within mass flows may indeed yield sufficiently large gas fluxes to cause fluidization and enhance flow mobility.

Highlights

Gullies are kilometer-scale alcove-channel-fan systems that are present on steep slopes in midlatitude to polar regions on Mars (e.g., Conway et al, 2017; Harrison et al, 2015; Malin & Edgett, 2000)
These results suggest that recent flows in gullies are fluidized to a similar degree as are granular debris flows on Earth
Initial and Flow Conditions Observations of the recent flows in Hale crater indicate that they are generated by restricted release areas, erode bed and bank sediments when traversing down the gully and form deposits with restricted thickness estimated to be in the order of a few decimeters at maximum (e.g., Kolb et al, 2010)

Summary

Introduction

Gullies are kilometer-scale alcove-channel-fan systems that are present on steep slopes in midlatitude to polar regions on Mars (e.g., Conway et al, 2017; Harrison et al, 2015; Malin & Edgett, 2000) They are geologically very young features that have been active over the last few million years (De Haas et al, 2015a; De Haas et al, 2017; Johnsson et al, 2014; Reiss et al, 2004; Schon et al, 2009). The seasonal frost may comprise both H2O and CO2, but the latter is generally more abundant and generally considered a more likely candidate for the triggering of the recent flows

Objectives

Methods

Results

Discussion

Conclusion