1. Measuring Mars sand flux seasonality and calibrating the threshold for sand mobility F. Ayoub, J-P. Avouac, C.E. Newman, M.I. Richardson, A. Lucas, S. Leprince, N.T. Bridges AGU Fall Meeting, December, 11, 2013 - fayoub@gps.caltech.edu

2. Nili Patera situation

3. 5.3km 14.8km Nili Patera dune field & HiRISE Time-series of 9 HiRISE images covering one Mars year 201020112012 85m Tracking ripple migration

4. Ripples displacement amplitude Bridges et al., 2012, Nature Measurement of ripple migration (amplitude and direction) Orthorectification, coregistration, and correlation with COSI-Corr

5. 2010 2011 2012 Estimating sand flux variability from sand ripple migration tracking Using the measured ripple migration, we estimate a sand flux Strong seasonal sand flux variation is observed

6. What could this sand flux measurement tell us about the shear stress threshold ? Shear stress threshold ( τ t ) is an important element for, e.g., simulation and prediction of sediment transport, climate and atmospheric simulations. But ( τ t ) is hard to estimate on Mars due to the challenge in transposing previously established values (earth analog, lab experiment, theoretical, empirical) to Mars AND at scale relevant to simulation (i.e., large scale) The approach to use the flux measurement to estimate a shear stress threshold is: Which sediment shear threshold would allow sediment transport simulations to reproduce the sand flux observed?

7. Atmospheric Circulation Model Wind Time Sand transport law τt ?τt ? Time Sand flux τ t : shear threshold Sand flux prediction at Nili Patera from atmospheric simulation Predicted Measured K * K: constant

8. Sand transport law τt ?τt ? Time Sand flux τ t : shear threshold Sand flux prediction at Nili Patera from atmospheric simulation MarsWRF GCM simulation (2° grid) Wind output every minute for one Mars year Final year prediction of a multi-year run Atmospheric Circulation Model Wind Time

9. Atmospheric Circulation Model Wind Time Sand flux prediction at Nili Patera from atmospheric simulation Sand transport law τt ?τt ? Time Sand flux τ t : shear threshold Lettau & Lettau Transport law: τ t range tested: 0.001 – 0.025 N/m 2

10. Time Sand flux (unitless) Q measured K * Q predicted (Lettau & Lettau) Only a small range of τ t allows to reproduce the seasonal flux variation observed Linear regression between predicted and measured fluxes for a range of τ t

11. 100 200 300 400 0.010 0.005 0.015 Shear stress threshold τ t (N/m 2 ) 0.02 0.06 0.1 Probability density χ 2 of linear regression τ t = 0.009 ± 0.0015 N/m 2 An ‘effective’ shear threshold is determined, that is relevant for wind at scale of few kilometers to few degrees Linear regression between predicted and measured fluxes for a range of τ This ‘effective’ threshold is approximately one order of magnitude lower than fluid threshold estimated traditionally.

12. Time Sand flux (unitless) Measured Lettau & Lettau Werner Durán Sørensen Different transport laws give approximately same optimal threshold Lettau & Lettau Werner Durán et al. Sørensen

13. 100 200 300 400 0.010 0.005 0.015 Shear stress threshold τ t (N/m 2 ) 0.02 0.06 0.1 Probability density χ 2 of linear regression τ t = 0.009-0.01 ± 0.0015 N/m 2 For all transport laws, only a small range of τ allows to reproduce the seasonal flux variation observed Different transport laws give approximately same optimal threshold

14. Conclusion 1)Measurement of sand flux, and its seasonal variation at Nili Patera dune field. Continuous sand flux throughout the year, with strong variation between summer and winter solstices. 2)Estimation of an ‘effective’ shear stress threshold relevant for simulation at scale from few km to few degrees. Next:Look at the annual variation of sand flux (on going work) Look at smaller scale simulation Estimation of specific parameters (e.g., grain size)