As expected from general mechanical considerations, Mars has Milankovic cycles analogous to those of Earth. Mars' cycles differ in some key respects, because of the lack of a massive moon, and because of the proximity of Jupiter.
As for Earth, the precession angle of Mars increases at a nearly constant rate. However, because Mars does not have a moon as massive as Earth's, the precession is dominated by Solar gravity, and is slower. The Mars precessional cycle has a period of approximately 50,000 Earth years. The current precession phase is 145o, and will reach 180o in about 5000 years.
The obliquity and eccentricity variations are shown in Figure 7.15. Obliquity has short term variations with amplitude on the order of 20o. The period is not visible in the figure, but a finer scale examination of the data shows that the period is about 125,000 Earth years in recent times. The amplitude is markedly larger than that of Earth's obliquity cycle, but what is even more remarkable is that the obliquity drifts to values as large as 47o over 10 million years. The extreme obliquity variations are directly linked to the absence of Earth's massive moon, which can be shown to provide a considerable damping effect on obliquity. This raises the intriguing possibility that a massive moon may be a necessary condition for a planet to avoid extreme climate fluctuations that could compromise its habitability. Calculations of the Earth's obliquity have also been carried out for tens of millions of years, and do not yield any greater variations than have been encountered in the past million years.
Mars is close to its maximum eccentricity at present, though it can get somewhat larger. The eccentricity of Mars undergoes quasiperiodic large amplitude cycles with a period on the order of 3 million years. In addition, there are short period, lower amplitude eccentricity variations with a period on the order of 100,000 years, rather similar to Earth's. In contrast, the very long period variations are not found in Earth's eccentricity.
Mars has no ocean, little thermal inertia, and a thin atmosphere that has a relatively modest effect on the planet's surface temperature. These features should lead to a different, and perhaps simpler, response to orbital forcing on Mars as compared to Earth. The predicted climate changes have been simulated in detail using comprehensive climate models, but we will confine ourselves here to some general remarks. The main effect of Martian Milankovic cycles is likely to be the redistribution of water deposits, in the form of either glaciers or permafrost. There are two aspects to this redistribution. On the short precessional time scale, the asymmetry between the Northern and Southern polar ice caps should reverse. For example, about 25000 years ago, the Southern hemisphere should have had milder summers and winters, while the Northern had cold winters and hot summers; the default reasoning would imply that at such times, the Southern ice cap should be large and be composed mainly of water ice, whereas the Northern ice cap becomes smaller and experiences massive seasonal CO2 snow deposition. On the time scale of millions of
-400 -350 -300 -250 -200 -150 -100 -50 Thousands of years B.P
Figure 7.14: Comparison of Antarctic temperature reconstructed from Vostok ice core deuterium measurements, with the Earth's eccentricity cycle. The bottom panel shows the corresponding July insolation at 65N. Temperature is given as deviation from the mean modern value. Vostok temperature data was taken from Peteet et al. (2001).
Figure 7.15: Evolution of Mars' obliquity and eccentricity. Data taken from Laskar et al. (2003).
years, the obliquity of Mars becomes much greater, leading potentially to a situation where water my migrate from poles that are seasonally very hot, and re-deposit in the tropics. At times of much lower obliquity, permafrost ice may migrate to both poles. The migration of water deposits and changes in patterns of deposition of CO2 snow probably leaves some imprint on the surface geology of Mars, and the growth and decay of glaciers certainly does. These offer some prospects for reconstructing the consequences of Milankovic cycles on Mars. Even better information would be obtained through analyzing cores of the polar ice caps, much as is done in Antarctica and Greenland. It is a very exciting development that the technology for doing this robotically on Mars is already under development. With respect to Mars, we are more or less at the stage of Croll or Milankovic, who thought they found the key to Earth's ice ages. Data showed they were on the right track, but that the climate system is much more intricate than they imagined. Given that we do not yet have a satisfactory theory leading from orbital variations to climate response on Earth, one can look forward to many surprises, once data on the Martian climate response becomes available.
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