Obliquity cycles and climate change

Long-term climate change likely results from perturbations induced by the planets and the Sun's non-spherical shape. These perturbations result in variations in Mars' orbital parameters (including eccentricity, inclination, and time of perihelion) and obliquity (Ward, 1992) and are analogous to the Milankovitch cycles which contribute to ice ages on Earth. Although the obliquity and orbital motion are strongly chaotic and numerical solutions are only accurate for the past 60 Ma, numerical solutions have been extended over longer time periods to estimate the range of values (Laskar and Robutel, 1993; Touma and Wisdom, 1993; Laskar et al., 2004). Those models indicate that eccentricity varies between and 0.12 with a most probable value of 0.068 and orbital inclination varies between 0° and 8°. Obliquity can vary from almost 0° to over 80° with a most probable value of 41.8°. These ranges are much greater than the variations experienced by the Earth, primarily because Phobos and Deimos are too small to provide a stabilizing influence on Mars. The obliquity and orbital variations lead to insolation changes which can influence the martian climate (Figure 7.8).

The poles receive more solar insolation as the obliquity increases, increasing their temperatures and causing sublimation of the CO2 and H2O ice in these regions. The addition of these gases increases the density of the atmosphere and the w 45

g 35

cr 3

O 15

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15 200

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Figure 7.8 Gravitational perturbations from the other planets cause Mars to experience variations in its obliquity (top), eccentricity (middle), and solar insolation (bottom). Numerical modeling shows how these parameters have varied over the past 10 Ma. (Reprinted by permission from Macmillan Publishers Ltd: Nature, Laskar et al. [2002], Copyright 2002.)

Figure 7.8 Gravitational perturbations from the other planets cause Mars to experience variations in its obliquity (top), eccentricity (middle), and solar insolation (bottom). Numerical modeling shows how these parameters have varied over the past 10 Ma. (Reprinted by permission from Macmillan Publishers Ltd: Nature, Laskar et al. [2002], Copyright 2002.)

accompanying surface pressure. Since H2O and CO2 are greenhouse gases, one expects that the surface temperature will rise and could allow liquid water to exist on the surface. Depending on the combination of orbital and obliquity values, liquid water can occur in regions that range from very localized, such as occurs today (Hecht, 2002), to a large fraction of the planet (Richardson and Mischna, 2005). Ice dominates over liquid on the surface at higher obliquities because of thermal blanketing produced by CO2 condensation in thick atmospheres and the greater extent of the winter polar cap (Haberle et al., 2003; Mischna et al., 2003). The stability zone for H2O ice moves towards the equator as the obliquity increases and is found preferentially at higher elevations and in regions of high thermal inertia.

The obliquity cycle has a period of about 2.5 Ma while eccentricity variations occur on timescales of ~1.7 Ma. Mars is currently moving from a higher to lower obliquity period and eccentricity is moving toward a high (Figure 7.8). Analysis of MOC, THEMIS, and HRSC imagery reveals features in near-equatorial zones which are interpreted as cold-based glaciers produced during the last high-obliquity

Figure 7.9 Flow lines within this deposit have been interpreted as a cold-based glacier emplaced during the last high-obliquity period. This deposit is located on the northwestern flank of Olympus Mons, near 22.5°N 222.0°E. Image is approximately 33km wide. (THEMIS image I16967012, NASA/JPL/ASU.)

period (Head and Marchant, 2003; Neukum et al., 2004; Head et al., 2005, 2006a) (Figure 7.9). Tongue-like deposits on crater walls at higher latitudes (Figure 5.47), the possible source of melting water to produce gullies (Christensen, 2003), may be debris-covered glaciers from the last high-obliquity period (Arfstrom and Hartmann, 2005). Ice-rich mantling deposits occur in the ~30°-60° latitude zone in both hemispheres (Mustard et al., 2001) and may have been deposited when obliquity last reached ~30°-35° (Head et al., 2003a). Layering in the polar layered deposits (Figure 5.44) may record depositional cycles of ice and dust associated with the obliquity cycles (Jakosky et al., 1993; Laskar et al., 2002).

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