Dune formation on the present Mars

We apply a model for sand dunes to calculate formation of dunes on Mars under the present Martian atmospheric conditions. We find that different dune shapes as those imaged by Mars Global Surveyor could have been formed by the action of sand-moving winds occuring on today’s Mars. Our calculations show, however, that Martian dunes could be only formed due to the higher efficiency of Martian winds in carrying grains into saltation. The model equations are solved to study saltation transport under different atmospheric conditions valid for Mars. We obtain an estimate for the wind speed and migration velocity of barchan dunes at different places on Mars. From comparison with the shape of bimodal sand dunes, we find an estimate for the time scale of the changes in Martian wind regimes.

Figure 1

Mars Global Surveyor (MGS), Mars Orbiter Camera (MOC) images of sand dunes on Mars (courtesy of NASA/JPL/MSSS). From the left to the right: (a) Barchan dunes at Arkhangelsky crater, near 41.0°S, 25.0°W; (b) north polar dunes near 77.6°N, 103.6°W; bimodal sand dunes near (c) 48.6°S, 25.5°W; (d) 49.6°S, 352.9°W, and (e) 76.4°N, 272.9°W.

Figure 2

(Color online) Ratio between the relative grain velocity $v_{\mathrm{f}}^{*}$ and the threshold friction speed for saltation, $u_{*\mathrm{ft}}$, calculated for Mars and for Earth as a function of the grain diameter $d$. The dashed line indicates the saltation-suspension transition at $v_{\mathrm{f}}^{*}∕u_{*\mathrm{ft}}=1.0$, and intercepts the Martian (terrestrial) continuous line at $d=110\mu \mathrm{m}$ $(d=45\mu \mathrm{m})$.

Figure 3

Main plot: Saturated sand flux $q_{\mathrm{s}}$ as a function of the relative shear velocity $u_{*}∕u_{*\mathrm{t}}-1$ for different values of atmospheric pressure—and therefore different values of $u_{*\mathrm{t}}$—obtained with a temperature $T=200\mathrm{K}$. The lower inset on the right shows the corresponding values of the average grain velocity $v_{\mathrm{s}}$. In the upper inset on the left, we show the saturated flux for $u_{*}=3.0\mathrm{m}∕\mathrm{s}$ calculated for different values of temperature valid on Mars.

Figure 4

(Color online) Sketch of a barchan dune showing the windward side, horns, and slip face. In the inset we see the definitions of dune width $W$ and length $L$.

Figure 5

Barchan dunes of width $W=650\mathrm{m}$ calculated using parameters for Mars, with $\gamma =10{\gamma }_{\text{Earth}}$, and different values of wind shear velocity $u_{*}∕u_{*\mathrm{t}}$.

Figure 6

(Color online) Main plot: characteristic length of flux saturation, ${\ell }_{\mathrm{s}}$, calculated with Eq. (10), as a function of $u_{*}∕u_{*\mathrm{t}}$. The inset shows the characteristic time $t_{\mathrm{s}}={\ell }_{\mathrm{s}}∕v_{\mathrm{s}}$ as a function of $u_{*}∕u_{*\mathrm{t}}$, where $v_{\mathrm{s}}$ is the average grain velocity [Eq. (B3)].

Figure 7

(Color online) Sand flux calculated over a flat sand bed on Mars and on Earth. Main plot: distance of flux saturation, ${\lambda }_{\mathrm{s}}$, normalized by the characteristic length ${\ell }_{\mathrm{s}}$, as a function of the influx $q_{\mathrm{in}}∕q_{\mathrm{s}}$ at the inlet. Inset: evolution of the normalized flux $q∕q_{\mathrm{s}}$ with downwind distance $x∕{\ell }_{\mathrm{s}}$ for $q_{\mathrm{in}}∕q_{\mathrm{s}}=0.20$.

Figure 8

Calculations of Martian north polar barchans near 77.6°N, 103.6°W [Fig. 1b]. We see MOC images of dunes of different sizes on the left, and on the right we see dunes calculated using $P=8.0\mathrm{mbar}$, $T=190\mathrm{K}$, $u_{*}=2.92\mathrm{m}∕\mathrm{s}$, and $q_{\mathrm{in}}∕q_{\mathrm{s}}=0.29$. Images courtesy of NASA/JPL/MSSS.

Figure 9

(Color online) (a) Main plot: length $L$ vs width $W$ of the Arkhangelsky barchans (circles) together with calculation results (straight line) obtained with parameters for that field. The dashed line shows $L$ vs $W$ for terrestrial dunes obtained with $u_{*}∕u_{*\mathrm{t}}=1.45$ and $q_{\mathrm{in}}∕q_{\mathrm{s}}=0.25$. In the inset we see $L$ vs $W$ of the north polar dunes (stars) together with calculations (straight line), while the dashed line corresponds to terrestrial dunes obtained with $u_{*}∕u_{*\mathrm{t}}=1.80$ and $q_{\mathrm{in}}∕q_{\mathrm{s}}=0.30$. (b) Main plot: aspect ratio $H∕W$ vs dune height $H$ of barchan dunes on Mars and on Earth. For each curve, the minimal dune is indicated by the filled symbol. All curves displayed in the main plot are shown again in the inset but in terms of the normalized height $h\equiv H∕H_{\min }$ and width $w\equiv W∕W_{\min }$.

Figure 10

(Color online) Dune velocity $v$ as a function of the length $L$ of the dunes calculated in Fig. 9b. In the inset, the dune velocity normalized with the velocity of the minimal dune, $v_0$, is shown as a function of the normalized dune volume, $V∕V_{\min }$, where $V_{\min }$ is the volume of the minimal dune. The straight line in the inset corresponds to the equation $v∕v_0={(V∕V_{\min })}^{-1∕3}$.

Figure 11

Calculation of the dunes at Wirtz crater [Fig. 1c]. In (a) we see one calculated dune, while in (b) we see an image of one dune at the Wirtz crater (image courtesy of NASA/JPL/MSSS). Arrows indicate the wind directions, with ${\theta }_{\mathrm{w}}=100°$ and $T_{\mathrm{w}}=2.9\text{days}$. Simulation snapshot in (a) corresponds to $t=15\text{years}$ after having started a calculation with a Gaussian hill with nearly the same volume of the bimodal dune shown in the figure. The Dune shape does not appear to change in time.

Figure 12

Dunes at 49.6°S, 352.9° W on Mars [Fig. 1d]. On the left we see snapshots of the calculation at $t=2.7$ (a), 6.6 (b), 14.5 (c), 24 (d), and $39\text{months}$ (e) after starting with a sand hill at $t=0$. ${\theta }_{\mathrm{w}}=140°$ and $T_{\mathrm{w}}=5.8\text{days}$. Wind directions are indicated by the arrows. An image of the dunes is shown on the right (f). Image courtesy of NASA/JPL/MSSS.

Figure 13

Calculation of Martian north polar dunes at 76.4°N, 272.9° W [Fig. 1e]. First, an elongated dune is formed from a Gaussian hill after $2.5\text{months}$ with ${\theta }_{\mathrm{w}}=120°$ and $T_{\mathrm{w}}=0.7\mathrm{day}$. Next, ${\theta }_{\mathrm{w}}$ is shifted to 80° $(t=0)$ giving rise to the dunes shown on the left, which correspond to snapshots at $t=5$ (a), 14 (b), 27 (c) and $45\text{days}$ (d). Arrows indicate the wind directions. Dunes are about $8\mathrm{m}$ high. On the right (e) we see an image of the north polar dunes (image courtesy of NASA/JPL/MSSS).