Surface-bound exospheres facilitate volatile migration across the surfaces of nearly airless bodies. However, such transport requires that the body can both form and retain an exosphere. To form a sublimation exosphere requires the surface of a body to be sufficiently warm for surface volatiles to sublime; to retain an exosphere, the ballistic escape and photodestruction rates and other loss mechanisms must be sufficiently low. Here we construct a simple free molecular model of exospheres formed by volatile desorption or sublimation. We consider the conditions for forming and retaining exospheres for common volatile species across the Solar System, and explore how three processes (desorption/sublimation, ballistic loss, and photodestruction) shape exospheric dynamics on airless bodies. Our model finds that the CO2 exosphere of Callisto is much too dense to be sustained by impact-delivered volatiles, but could be maintained by only-7 ha (-0.07 km(2)) of exposed CO2 ice distributed across Callisto (and refreshed through mass wasting). We use our model to predict the peak surface locations of Callisto's CO2 exosphere along with other Galilean moons, which could be tested by JUICE observations. Our model finds that to maintain Iapetus' two-tone appearance, its dark Cassini Regio likely has unresolved exposures of water ice, perhaps in sub-resolution impact craters, that amount to up to approximately-0.06% of its surface. In the Uranian system, we find that the CO2 deposits on Ariel, Umbriel, Titania, and Oberon are unlikely to have been delivered via impacts, but are consistent with both a magnetospheric origin, (as has been previously suggested) or sourced endogenously. We suggest that the leading/trailing CO2 asymmetries on these moons could result from exosphere-mediated volatile transport, and may be a seasonal equinox feature that could be largely erased by pole-to-pole volatile migration during the Uranian solstices. We calculate that-2.4-6.4 mm thick layer of CO2 (depending the moon) could migrate about the surface of Uranus' large moons during a seasonal cycle. Our model also confirms that water migration to Mercury's polar cold traps is inefficient without self-shield against photodestroying UV light, and that Callisto's bright spires could be formed/maintained by exospherically deposited H2O.

Ground-based telescopes and space exploration have provided outstanding observations of the complexity of icy planetary surfaces. This work presents our review of the varying nature of carbon dioxide (CO2) and carbon monoxide (CO) ices from the cold traps on the Moon to Pluto in the Kuiper Belt. This review is organized into five parts. First, we review the mineral physics (e.g., rheology) relevant to these environments. Next, we review the radiation-induced chemical processes and the current interpretation of spectral signatures. The third discusses the nature and distribution of CO2 in the giant planetary systems of Jupiter and Saturn, which are much better understood than the satellites of Uranus and Neptune, discussed in the subsequent section. The final sections focus on Pluto in comparison to Triton, having mainly CO, and a brief overview of cometary materials. We find that CO2 ices exist on many of these icy bodies by way of magnetospheric influence, while intermixing into solid ices with CH4 (methane) and N-2 (nitrogen) out to Triton and Pluto. Such radiative mechanisms or intermixing can provide a wide diversity of icy surfaces, though we conclude where further experimental research of these ices is still needed.

We develop an analytical model of the Alfven wings generated by the interaction between a moon's ionosphere and its sub-Alfvenic magnetospheric environment. Our approach takes into account a realistic representation of the ionospheric Pedersen conductance profile that typically reaches a local minimum above the moon's poles and maximizes along the bundle of magnetospheric field lines tangential to the surface. By solving the equation for the electrostatic potential, we obtain expressions for various quantities characterizing the interaction, such as the number flux and energy deposition of magnetospheric plasma onto the surface, the spatial distribution of currents within the Alfven wings and associated magnetic field perturbations, as well as the Poynting flux transmitted along the wings. Our major findings are: (a) Deflection of the magnetospheric plasma around the Alfven wings can reduce the number flux onto the surface by several orders of magnitude. However, the Alfvenic interaction alone does not alter the qualitative shape of the bullseye-like precipitation pattern formed without the plasma interaction. (b) Due to the deflection of the upstream plasma, the energy deposition onto the moon's exosphere achieves its minimum near the ramside apex and maximizes along the flanks of the interaction region. (c) Even when the ionospheric conductance profile is continuous, the currents along the Alfven wings exhibit several sharp jumps. These discontinuities generate spikes in the magnetic field that are still observable at large distances to the moon. (d) The magnitude and direction of the wing-aligned currents are determined by the slope of the ionospheric conductance profile.

Ocean worlds is the label given to objects in the solar system that host stable, globe-girdling bodies of liquid water-oceans. Of these the Earth is the only one to support its oceans on the surface, making it a model for habitable planets around other stars but not for habitable worlds elsewhere in the solar system. Elsewhere in the solar system, three objects Jupiter's moon Europa, and Saturn's moons Enceladus and Titan have subsurface oceans whose existence has been detected or inferred by two independent spacecraft techniques. A host of other bodies in the outer solar system are inferred by a single type of observation or by theoretical modeling to have subsurface oceans. This paper focusses on the three best-documented water oceans beyond Earth: those within Europa, Titan and Enceladus. Of these, Europa's is closest to the surface (less than 10 km and possibly less than 1 km in places), and hence potentially best suited for eventual direct exploration. Enceladus' ocean is deeper 5-40 km below its surface but fractures beneath the south pole of this moon allow ice and gas from the ocean to escape to space where it has been sampled by mass spectrometers aboard the Cassini Saturn Orbiter. Titan's ocean is the deepest perhaps 50-100 km-and no evidence for plumes or ice volcanism exist on the surface. In terms of the search for evidence of life within these oceans, the plume of ice and gas emanating from Enceladus makes this the moon of choice for a fast-track program to search for life. if plumes exist on Europa yet to be confirmed or places can be located where ocean water is extruded onto the surface, then the search for life on this lunar-sized body can also be accomplished quickly by the standards of outer solar system exploration.

Saturn is orbited by a half dozen ice rich middle-sized moons (MSMs) of diverse geology and composition. These comprise similar to 4.4% of Saturn's satellite mass; the rest is Titan, more massive per planet than Jupiter's satellites combined. Jupiter has no MSMs. Disk-based models to explain these differences exist, but have various challenges and assumptions. We introduce the hypothesis that Saturn originally had a 'galilean' system of moons comparable to Jupiter's, that collided and merged, ultimately forming Titan. Mergers liberate ice-rich spiral arms in our simulations, that self-gravitate into escaping clumps resembling Saturn's MSMs in size and compositional diversity. We reason that MSMs were spawned in a few such collisional mergers around Saturn, while Jupiter's original satellites stayed locked in resonance. (C) 2012 Elsevier Inc. All rights reserved.

The outer planets of our solar system Jupiter, Saturn, Uranus, and Neptune are fascinating objects on their own. Their intrinsic magnetic fields form magnetic environments (so called magnetospheres) in which charged and neutral particles and dust are produced, lost or being transported through the system. These magnetic environments of the gas giants can be envisaged as huge plasma laboratories in space in which electromagnetic waves, current systems, particle transport mechanisms, acceleration processes and other phenomena act and interact with the large number of moons in orbit around those massive planets. In general it is necessary to describe and study the global environments (magnetospheres) of the gas giants, its global configuration with its large-scale transport processes; and, in combination, to study the local environments of the moons as well, e.g. the interaction processes between the magnetospheric plasma and the exosphere/atmosphere/magnetosphere of the moon acting on time scales of seconds to days. These local exchange processes include also the gravity, shape, rotation, astrometric observations and orbital parameters of the icy moons in those huge systems. It is the purpose of this chapter of the book to describe the variety of the magnetic environments of the outer planets in a broad overview, globally and locally, and to show that those exchange processes can dramatically influence the surfaces and exospheres/atmospheres of the moons and they can also be used as a tool to study the overall physics of systems as a whole.

Saturn's diffuse E ring is the largest ring of the Solar System and extends from about 3.1 R-S (Saturn radius R-S = 60,330 km) to at least 8 RS encompassing the icy moons Mimas, Enceladus, Tethys, Dione, and Rhea. After Cassini's insertion into her saturnian orbit in July 2004, the spacecraft performed a number of equatorial as well as steep traversals through the E ring inside the orbit of the icy moon Dione. Here, we report about dust impact data we obtained during 2 shallow and 6 steep crossings of the orbit of the dominant ring source-the ice moon Enceladus. Based on impact data of grains exceeding 0.9 pm we conclude that Enceladus feeds a torus populated by grains of at least this size along its orbit. The vertical ring structure at 3.95 RS agrees well with a Gaussian with a full-width-half-maximum (FWHM) of similar to 4200 km. We show that the FWHM at 3.95 R-S is due to three-body interactions of dust grains ejected by Enceladus' recently discovered ice volcanoes with the moon during their first orbit. We find that particles with initial speeds between 225 and 235 m s(-1) relative to the moon's surface dominate the vertical distribution of dust. Particles with initial velocities exceeding the moon's escape speed of 207 m s(-1) but slower than 225 ms(-1) re-collide with Enceladus and do not contribute to the ring particle population. We find the peak number density to range between 16 x 10(-2) m(-3) and 21 x 10(-2) m(-3) for grains larger 0.9 mu m, and 2.1 x 10(-2) m(-3) and 7.6 x 10(-2) m(-3) for grains larger than 1.6 mu m. Our data imply that the densest point is displaced outwards by at least 0.05 R-S with respect of the Enceladus orbit. This finding provides direct evidence for plume particles dragged outwards by the ambient plasma. The differential size distribution n(s(d))ds(d) similar to s(d)(-qs)ds(d) for grains > 0.9 mu m is described best by a power law with slopes between 4 and 5. We also obtained dust data during ring plane crossings in the vicinity of the orbits of Mimas and Tethys. The vertical distribution of grains > 0.8 mu m at Mimas orbit is also well described by Gaussian with a FWHM of similar to 5400 km and displaced southwards by similar to 1200 km with respect to the geometrical equator. The vertical distribution of ring particles in the vicinity of Tethys, however, does not match a Gaussian. We use the FWHM values obtained from the vertical crossings to establish a 2-dimensional model for the ring particle distribution which matches our observations during vertical and equatorial traversals through the E ring. (c) 2007 Elsevier Inc. All rights reserved.