Solar forcing affects the planets in the inner solar system in different ways, the most obvious being the solar gravitation force. But there are also other forcing terms affecting the planets: solar irradiation and the solar plasma outflow. The solar irradiation, with a spectrum from X-ray to infrared, provides the highest input power to the planetary environment, corresponding to a power between 490-720 W/m2 for Mars, with its elliptic orbit, for the Earth 1370 W/m2, and 2620 W/m2 for Venus. The power input from solar plasma outflow/the solar wind, is quite variable, but the average power is six orders of magnitude smaller (0.001-0.003 W/m2). Yet, one may argue that the solar plasma forcing has a more significant effect on a planetary atmosphere than solar irradiation alone. For instance, thermal escape (due to solar irradiation) primarily favours light atoms and molecules (e.g. hydrogen) while non-thermal escape processes (due to solar plasma forcing) are much less mass sensitive. Nonthermal escape in general results in an order of magnitude higher mass loss for the Earth-like planets. This apparent anomaly, with orders of magnitude difference in input power, demonstrates the non-linear behaviour of nature, i.e. sheer power is not sufficient to explain physical phenomena. It is the process that matters, such as in this case for the escape of planetary volatiles.
Thermal escape from an atmosphere is determined from a Maxwellian (thermalized) particle distribution, the escape rate given by the temperature of the distribution at the exobase and the escape velocity of the object at the exobase. Theoretically all particles within the Maxwellian distribution of particles having velocities above the escape velocity will be lost. Nonthermal escape may be defined as all other processes where the energization and escape of particles is related with (microscopic) nonthermal processes. Excluded are all (macroscopic) processes of catastrophic nature such as e.g. impact erosion by in falling objects from space. Non-thermal escape is not unrelated to thermal escape, because most non-thermal escape processes are based on the existence of photo ionization processes. The electromagnetic radiation from the Sun also determines the scale-height of the atmosphere, and correspondingly also the ionosphere and cross-sectional area for e.g. solar wind forcing. However, we separate the two mechanisms mainly because of their differences with respect to solar forcing. Thermal escape is (mainly) due to solar radiation, whereas non-thermal escape is related with a broader aspect of solar wind forcing, such as sputtering, ion pickup, ionospheric plasma energization etc. The basic argument is that solar wind energy and momentum, electromagnetic or corpuscular, defines the forcing regardless of individual processes inferred. Following the definitions by Chassefiere et al. (2007), there are two classes of thermal escape:
1. Jeans escape, driven by EUV and XUV heating of the upper atmosphere. Atmospheric atoms having velocities above escape velocity at the exobase level are free to escape into space.
2. Hydrodynamic escape, consisting of a bulk expansion of the upper atmosphere due to intense solar EUV/XUV fluxes, allowing atoms to overcome the gravitational binding force. Hydrodynamic escape plays an important role in low gravity environments (e.g. comets), but is also considered to have played a major role in outflow during early Noachian on Mars (e.g. Chassefiere and Leblanc 2004).
With regard to non-thermal escape we present a slightly modified definition as compared to that proposed by Chassefiere et al. (2007). The following four processes here identify non-thermal escape:
1. Photochemical escape, associated with dissociative recombination. Ions produced by photo ionization may reach higher temperatures than the neutral atmosphere in the ionosphere. Recombination/charge-exchange produce energetic neutrals, some of them have sufficient velocity to escape the planet (e.g. Luhmann et al. 1992; Lammer et al. 1996; Fox and Hac 1997; Kim et al. 1998; Chassefiere and Leblanc 2004).
2. Ion sputtering produced by ions impacting the upper atmosphere/corona leads to the ejection of neutral particles (e.g. Luhmann et al. 1992; Jakosky et al. 1994; Johnson et al. 2000; Leblanc and Johnson 2002).
3. Ionospheric plasma energization and escape driven by direct solar wind forcing (e.g. Pérez-de Tejada 1987, 1998; Lundin and Dubinin 1992). The process is more complex for a planet with an intrinsic magnetic dynamo such as the Earth (see e.g. Moore et al. 1999, for a review). Plasma waves are important for the transfer of energy and momentum from the solar wind to planetary magnetospheres (see e.g. Chaston et al. 2005). In a similar manner, waves observed in the shocked solar wind plasma are likely to take part in the energization of ionospheric ions near Mars (e.g. Espley et al. 2004; Winningham et al. 2006; Lundin et al. 2006b).
4. Ionospheric ion pickup, a process caused by the protrusion of the solar wind motional electric field into a planetary ionosphere. The combined solar wind electric and magnetic field (E x B) results in a cycloid motion of energized ionospheric ions (Luhmann and Kozyra 1991; Dubinin et al. 1993; Kallio et al. 1998; Kallio and Janhunen 2002; Ma et al. 2004; Nagy et al. 2004; Kallio et al. 2006; Dubinin et al. 2006; Luhmann et al. 2006). Mass loading leads to a local weakening of the motional electric field, and a correspondingly lower energization (Lundin et al. 1991; Kallio et al. 1998).
Notice that the first two non-thermal escape processes are associated with the escape of neutral atoms, while the last two processes are associated with the energization and escape of ionospheric plasma. In what follows we will focus on the energization and escape of ionospheric plasma. One important reason for this, obvious from the title of this report, is that a planetary magnetic field has implications for the ionospheric plasma escape and the corresponding atmospheric evolution. Another important reason is that ionospheric plasma escape is a topic where theory may be compared with numerous direct in situ observations. Photochemical escape and sputtering are processes that by and large lack adequate in situ measurements. Studies of the latter two processes are therefore based on models and simulations. In a similar way, quantitative results of thermal escape (Jeans escape and hydrodynamic escape) are based on models and simulations.
The magnetic field plays an important role in controlling ionized gases—plasmas. The solar wind, a wind of plasma escaping from the Sun, is in fact embedded in (frozen-in) the solar magnetic field (Alfven 1950; Parker 1958). In the same manner a plasma flow, like the solar wind, cannot easily protrude into a strong planetary magnetic field. For instance, the Earth's dipole magnetic field acts as a "magnetic umbrella" fending off the solar wind (Fig. 1). The standoff distance from the Earth is typically some 70000 km away in the sub solar region. Conversely, planets lacking strong intrinsic magnetic fields such as Mars and Venus have no "magnetic umbrella", and the solar wind can directly access the upper atmosphere. Recent measurements from Mars (Lundin et al. 2004b) show that the solar wind may impact as low as 270 km above the dayside surface of Mars. This illustrates the problem for planetary atmospheres without magnetic shielding. The relative erosion rate of ionospheric plasma is consequently lower for the Earth than for Mars, for example. The total outflow rate for the Earth, the mass flux dominated by O+ ions, is 1-3 kg/s (Chappell et al. 1987). However, recent data and arguments suggest that most of the outflowing ions
Fig. 1 The magnetic field of the Earth acting as a shield against direct solar wind forcing of the Earths atmosphere and ionosphere. The aurora in the close-up view of the Earth (upper right) illustrates that a limited amount of solar wind forcing occurs in the polar region
Fig. 2 The comet tail, an example of direct forcing by the Sun, the solar EUV/UV and the solar wind
Fig. 2 The comet tail, an example of direct forcing by the Sun, the solar EUV/UV and the solar wind
are recaptured by the Earth (Seki et al. 2001), i.e. less than 1 kg/s are lost to space. For Mars, the mass loss of O+ and O+ during solar maximum has been estimated to be ~ 1 kg/s (Lundin et al. 1989). Recent data from Mars Express imply a much lower escape during solar minimum (Barabash et al. 2007), with orders of magnitude variability connected with solar wind dynamic pressure changes (Lundin et al. 2006b). Mars has a very tenuous atmosphere, the average atmospheric pressure being two orders of magnitude lower than on the Earth. Moreover, the area of the solar wind obstacle (the solid planet) is about four times larger for the Earth compared to Mars. Therefore, considering the volatile inventory on both planets, Mars is losing atmospheric O+ and O+ much faster compared to the Earth.
Weakly magnetized planets like Mars and Venus, behave like comets, the nightside cavities forming elongated tails of escaping planetary plasma originating from the upper atmosphere/ionosphere. The planetary wind is stretching out in the antisolar direction in the same way as for comets (Fig. 2) yet at a rate lower than for a typical comet during solar approach. The main difference between a planet and a comet in terms of volatile escape is mass/gravity. Venus and Mars have much stronger gravity, which retains volatile substances for billions of years before they are significantly eroded away by the solar wind. The low gravity of comets means that their atmosphere builds up and expands while approaching the Sun, leading to a gradually expanding obstacle for the erosive solar wind. The loss from a km-size comet (e.g. Halley) is 1-10 x106 kg/day at 1 AU, while the loss from the Venus and Mars is typically 100 times lower. The heavy loss of matter is a reason why the tails of comets are visible and the plasma tails of planets are not.
The present rate of escape observed from a weakly magnetized planet such as Mars corresponds to significant losses of volatiles throughout the planetary lifetime. For instance,
Lammer et al. (1996, 2003) and Lundin andBarabash (2004a) estimated the loss of volatiles during the last 3.5 billion years; the water loss corresponds to a global equivalent layer GEL of 10-30 meters. These figures were obtained by considering the evolution of the Sun (Wood et al. 2002) and of the planetary atmosphere. On the other hand, morphological surface features suggest that more surface and near-surface water was present during Noachian and Hesperian times (GEL 100-500 m) (McKay and Stoker 1989; Baker 2001; Lunine et al. 2003; Bibring et al. 2004; Neukum 2005; Bertaux et al., this volume; Nisbet et al., this volume). In a similar manner Venus may have been subject to a heavy loss of water (Kulikov et al. 2006, 2007). The interaction of the solar wind with Venus (Russell et al. 2006) and the corresponding rate of outflow by ion pickup processes (Luhmann and Bauer 1992; Luhmann et al. 2006) based on Pioneer Venus Orbiter PVO measurements suggest a modest solar wind interaction with Venus. On the other hand, the comet-like ionospheric features found by Brace et al. (1987) imply a rather strong ionospheric response to solar forcing.
Under the assumption that all Earth-like planets accreted from matter of essentially the same chemical origin, the differences we observe today may be related with evolutionary processes. The evolution of volatiles is one particular aspect of that. The Earth is the only planet where a significant hydrosphere remains, while Mars and particularly Venus are strongly dehydrated in comparison.
In this report we discuss the evolution of planetary volatiles, with a focus on water, under varying solar forcing conditions with time. Our focus is on the acceleration and escape of ionospheric plasma, for reasons already mentioned, but also because we believe that plasma escape to a large extent couples to the other escape processes. We start by describing the variability of the solar radiation and the solar plasma environment. We continue by defining and describing the internal forcing that leads to the solar output and the external forcing that the Sun imposes on the planets. We then discuss why magnetic shielding plays such an important role in protecting a planetary atmosphere. Finally, we present a model and a scenario of the loss of water and CO2 from Mars, Venus and the Earth. We conclude on basis of this that a strong intrinsic magnetic field is important for maintaining water on a planet. A wealth of water is central for the evolution of life, for making a planet habitable for more advanced life-forms.
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