Mars and Its Shaking Orbit

If compared to the few watts change in the Earth energetic balance caused by human release of greenhouse gases, orbital cycles on Mars set a much higher standard of

Figure 3. Left panel: (a) Obliquity, (b) eccentricity, and (c) insolation at the north pole surface at the summer equinox over the last 20 Myr and for the next 10 Myr. Figure reprinted from Laskar et al. (2004). Right panel: (a) Subframe of a MGS/MOC image showing an example of the succesion of bright and dark layers down the trough walls of the north polar layered deposits. (b) DN profile extracted from the same MOC image along the white line traced in the image. Reprinted from Milkovich and Head (2005).

Figure 3. Left panel: (a) Obliquity, (b) eccentricity, and (c) insolation at the north pole surface at the summer equinox over the last 20 Myr and for the next 10 Myr. Figure reprinted from Laskar et al. (2004). Right panel: (a) Subframe of a MGS/MOC image showing an example of the succesion of bright and dark layers down the trough walls of the north polar layered deposits. (b) DN profile extracted from the same MOC image along the white line traced in the image. Reprinted from Milkovich and Head (2005).

climate perturbations. As computed by Laskar and Robutel (1993) from numerical integration of the motion of the solar system bodies, planetary secular perturbations cause the orbit of Mars to experience large variations, largely because no moon can stabilize it like it does for Earth. Over the last 100 million years (Myr henceforth), Mars' obliquity has evolved in a chaotic zone ranging from 0° to 60°. Obliquity oscillations can nevertheless be reconstructed for the last 20 Myr (Laskar et al., 2004; Figure 3, left panel). Changing inclination of the rotation axis has caused insolation at the poles to vary by more than a factor of three during this period. Five million years ago, when obliquity was oscillating around a higher value (35°) than today (25°), pole insolation was about 50% greater than today.

Mars orbit changes are generally represented by their impact on the summer insolation at the poles since the only reservoirs of water directly in contact with the atmosphere are located there. The sequence of bright and dark layers found within the scarps and troughs of the NPC indicate variation in erosional and de-positional rates of water ice, likely reflecting cyclic insolation conditions (Laskar et al., 2002; Milkovich and Head, 2005; Figure 3, right panel). In the polar regions, the solar zenith angle in summer decreases when obliquity increases and surface

Current iituation .

High Obliquily

Back to

Low Obliquity 1 IS"/.

Precession change

Figure 4. A summary of the recent (last millions of year) changes in the Mars orbital parameters and their consequences on the mobilization of water across the planet. In the first cartoon on the left, the blue arrow is reoriented to the north pole to indicate the seasonal sublimation and return of water to the permanent cap. All other cases with high, low obliquity and precession changes generate a permanent extraction of water from the cap towards locations indicated by the arrows.

temperatures are raised accordingly. It is estimated that the temperature at the North pole in summer (currently 240 K) would reach or exceed 270 K at obliquities greater than 45°. By virtue of the Clausius-Clapeyron law, this 30 K difference is exponentially translated into one or two orders of magnitude larger vapor pressure of water and thus into a sublimation process orders of magnitude stronger for the ice at the poles (Jakosky and Carr, 1985; Haberle et al., 2000; Richardson and Wilson, 2002a; Mischna et al., 2003; Levrard et al., 2004; Forget et al., 2006).

Obliquity alone has not paced Mars climate in the recent ages; eccentricity and precession changes have yielded substantial, though milder, solar forcing variations. These two parameters are intimately related as the effect of precession on climate depends on eccentricity. Precession refers to the circular motion of the rotation axis (Figure 4) which cycles on a 50 kyr timescale. While eccentricity makes the climate seasonally asymmetric, precession determines the timing of closest approach to the Sun and thus decides which summer hemisphere receives more sunlight than the other (currently the South). Many studies (Laskar et al., 2002; Hecht, 2003; Milkovich and Head, 2005) favor the precession factor, which has dominated the last 0.5 Myr cycle of insolation, as being one of the main controls on deposition of at least the first hundreds meters of the north polar terrains. This implies that even the smooth climatic changes caused by the precessing rotation axis of Mars have been sufficient to produce observable consequences on the geology. Presumably, changes due to obliquity may have been even more dramatic.

Was this article helpful?

0 0

Post a comment