Reply to "Comment on `Minimal size of a barchan dune' "

TL;DR

This paper defends the self-consistency of the dune model used to determine minimal dune size and argues that Martian dunes can form under current conditions, considering the effects of nonerodible elements and wind speeds.

Contribution

It clarifies the dependence of minimal dune size on wind speed via saturation length and challenges previous assumptions about Martian dune formation conditions.

Findings

Dune model equations are self-consistent and show size dependence on wind speed.

Martian ripples are not reliable for grain size estimation due to nonerodible elements.

Martian dunes likely saltate with basaltic grains of 500 ± 100 μm during strong storms.

Abstract

We reply to the comment by Andreotti and Claudin (submitted to Phys. Rev. E, arXiv:0705.3525) on our paper ``Minimal Size of a Barchan Dune'' [Phys. Rev. E {\bf{75,}} 011301 (2007)]. We show that the equations of the dune model used in our calculations are self-consistent and effectively lead to a dependence of the minimal dune size on the wind speed through the saturation length. Furthermore, we show that Meridiani Planum ripples are probably not a good reference to estimate the grain size of Martian dune sands: the soil in the ripple troughs at the landing site is covered with nonerodible elements (``blueberries''), which increase the minimal threshold for saltation by a factor of 2.0. We conclude that, in the absence of large fragments as the ones found at the landing site, basaltic grains of diameter $d=500 \pm 100 {\mu}$m that compose the large, typical dark Martian dunes [K. S. Edgett and P. R. Christensen, J. Geophys. Res. {\bf{96,}} 22765 (1991)] probably saltate during the strongest storms on Mars. We also show that the wind friction speed $u_{\ast} \approx 3.0$ m/s that we found from the calculations of Martian dunes is within the values of maximum wind speeds that occur during Martian storms a few times a decade [R. E. Arvidson {\em{et al.}}, Science {\bf{222,}} 463 (1983); H. J. Moore, J. Geophys. Res. {\bf{90,}} 163 (1985); R. Sullivan {\em{et al.}}, Nature {\bf{436,}} 58 (2005); D. J. Jerolmack {\em{et al.}}, J. Geophys. Res. {\bf{111,}} E12S02 (2006)]. In this manner, the dune model predicts that Martian dunes can be formed under present Martian conditions, with no need to assume other conditions of wind and atmosphere that could have prevailed in the past.