Saltation Transport on Mars

We present the first calculation of saltation transport and dune formation on Mars and compare it to real dunes. We find that the rate at which grains are entrained into saltation on Mars is 1 order of magnitude higher than on Earth. With this fundamental novel ingredient, we reproduce the size and different shapes of Mars dunes, and give an estimate for the wind velocity on Mars.

Figure 1

Barchans in the Arkhangelsky crater, 41.0° S, 25.0° W on Mars: Mars orbiter camera images on the left (image courtesy of $\mathrm{NASA}/\mathrm{JPL}/\mathrm{MSSS}$) and calculated dunes on the right. Main plot: $L$ vs $W$ of the Arkhangelsky (circles) and north polar barchans at 77.6° N, 103.6° W (stars). Calculations of Arkhangelsky (north polar) dunes are represented by the continuous (dotted) line, obtained with $u_{*}/u_{*t}=1.45$ (1.80). The dashed line in the upper inset corresponds to terrestrial dunes obtained with $u_{*}/u_{*t}=1.45$. In the lower inset, we see $L/L_{\min }$ vs $H/H_{\min }$ from calculations of the Arkhangelsky (triangles) and terrestrial dunes (dashed line).

Figure 2

Dune velocity $v$ as function of dune length $L$. We see that Mars dunes (filled symbols) move typically 10 times faster than Earth dunes (empty symbols) of same $L$, obtained with similar values of $u_{*}/u_{*t}$ as on Mars.

Figure 3

MOC images on top (courtesy of $\mathrm{NASA}/\mathrm{JPL}/\mathrm{MSSS}$) and calculations obtained with bimodal wind regimes on bottom. A sketch showing the definition of the angle ${\theta }_w$ of the wind direction (arrows) is shown in (${\mathrm{a}}^{\prime }$). The wind changes its direction with frequency $1/T_w$. We chose $T_w=2.9$, 5.8, and 0.7 d to obtain dunes in (${\mathrm{a}}^{\prime }$), (${\mathrm{b}}^{\prime }$), and (${\mathrm{c}}^{\prime }$), respectively. The dune in (${\mathrm{a}}^{\prime }$) has been obtained with $|{\theta }_w|=50°$ and the dune in (${\mathrm{b}}^{\prime }$) with $|{\theta }_w|=70°$. In (${\mathrm{c}}^{\prime }$), a linear dune obtained with $|{\theta }_w|=60°$ decays into barchans after the angle $|{\theta }_w|$ is reduced to 40°. The dune in (${\mathrm{b}}^{\prime }$) elongates to the right further and further with time, which is not observed for dune in (${\mathrm{a}}^{\prime }$). The dune in (${\mathrm{c}}^{\prime }$) decays further until only a string of rounded barchans is visible.