Habitability is usually believed to be a function of the size of a planet, the distance of the planet to its star (or the Sun for the Solar system), the brightness of its star, the absorption and reflection energy balance of the planet, the composition of its atmosphere, plate tectonics, presence of a magnetic field, gravitational interactions, protection from asteroids and comets, orbital and rotational dynamics, and more, as discussed in this book. Here we address rotation dynamics and their relationship with the magnetic field, as well as their relationship with atmospheric escape. In particular, we need to consider the influence of the solar wind on the atmosphere, as was discussed in the first part of this chapter. Solar wind is made up of high-energy protons and electrons that are emitted from the upper regions of the Sun and stream toward the Earth. Since protons and electrons are charged particles, they are deflected by a magnetic field. Living organisms on the Earth are protected from the effects of the solar wind because Earth's magnetic field deflects the charged particles away from the Earth or toward the poles. The rotation of a planet is an important mechanism often believed to be responsible for the enhancement of the magnetic field. Protection from the solar wind may therefore depend on the planetary rotation. Both magnetic fields and atmospheres protect living organisms from cosmic rays. A sufficiently dense atmosphere may also provide conditions favorable for the presence of liquid water on a planet, an essential prerequisite for the beginnings of life and its further evolution.

The interaction of the Sun's energy with the atmospheres of Earth and Mars has differences and similarities, not only related to the existence of a magnetic field, but also to their radii, rotation rates, gravitational accelerations, topography, surface pressure, and surface and internal compositions. The absence of a large intrinsic magnetic field on Mars is a current difference, but there was a magnetic field in the early history of Mars, as suggested by data of the MAG/ER (Magnetometer/Electron Reflectometer) instrument of Mars Global Surveyor (MGS) spacecraft (Connerney et al. 1999; Acuña et al. 2001). After the MGS orbit injection in 1997 and during the aerobraking phase and science-phasing orbit phase, it was possible to get magnetic measurements at periapses ranging from 85 to 170 km above Mars' surface. Connerney et al. (1999) (see also Purucker et al. 2000; Acuña et al. 2001) have discovered strongly magnetized regions in its crust, closely associated with the ancient, cratered terrain of the highlands in the southern hemisphere. There has been a great interest in the literature in the meaning of the spatial pattern of magnetization (see e.g. Jurdy and Stefanick 2004; Arkani-Hamed 2004), including possible lineations (these crustal magnetizations even suggest reversals) that were believed to suggest an analogy to plate tectonic (like the lineations derived from magnetization on Earth's ocean floor). Recent models favor thermal remanance acquired during the time these ancient rocks cooled, in conjunction with dike intrusions (Nimmo 2000) as the best way to explain the lineations. The MGS remnant magnetic field measurements show that the magnitude of the magnetic moment observed at satellite altitudes implies a source in the martian crust with a magnetic moment of ~1.6 x 1016 Am2 =1.6 x 1019 Gcm3 (Acuña et al. 1998): MGS has detected surface magnetic anomalies of up to 1500 nT. The magnetic sources are about an order of magnitude stronger than those of the Earth's continents (e.g., Toft and Arkani-Hamed 1992; Arkani-Hamed and Dyment 1996) and comparable in magnitude with the remanent magnetization of fresh extrusive basalt near the oceanic ridge axes of Earth (e.g., Bleil and Petersen 1983). These magnetic anomalies indicate the existence of a strong, ancient, intrinsic martian magnetic moment with a field strength 0.1 to 10 times of the present Earth field (Ness et al. 1999; Mitchell et al. 1999, 2001), first suspected from Viking probes data (Hargraves et al. 1977, 1979). The strength of the martian magnetic anomalies could in addition reflect a strong magnetic susceptibility due to the composition of the rocks.

Little is currently known about the interior of Mars. The only in situ observations important for the interior are those of the martian gravity field, the tidal effect on an orbiter, and the polar moment of inertia, which are derived from radio tracking of orbiting and landed spacecraft (e.g., Smith et al. 1998; Folkner et al. 1997; Konopliv et al. 2006). These observations are the main constraints for interior models additionally based on analysis of SNC meteorites or chondrite data and extrapolation of the Earth's internal structure to the lower pressures of Mars' interior (Sohl and Spohn 1997; Sanloup et al. 1999; Bertka and Fei 1998; Sohl et al. 2005; Verhoeven et al. 2005). The question of whether the core is presently liquid or solid is still an open question although the measured tidal effect on the orbits of the MGS, Mars Odyssey, and MEX suggests an at least partially liquid core (Yoder et al. 2003; Balmino et al. 2006; Konopliv et al. 2006; Duron et al. 2007).

The rotation and orientation of Mars at present are very similar to those of the Earth. But they may have been more different in their past, which can have other implications for planetary and atmosphere evolution. We know that the Earth has kept very similar orientation with a few degree differences, but as explained by Laskar et al. (2004b, see also 1994 1994, 1996, 1997; Laskar et al. 2002, 2004a), the obliquity of Mars may have changed considerably during its history. This issue will be addressed in Sect. 3, while Sect. 2 addresses the rotation of the planets and Sects. 4 and 5, the implications for the magnetic field and the associated effect on the magnetopause distances (atmosphere protection). The last section, Sect. 6 addresses the resulting atmosphere escape (see also the other two parts of the chapter).

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