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Despite four decades of research, the origin of Valles Marineris on Mars, the longest trough system in the solar system, remains uncertain. Its formation mechanism has been variably related to rifting, strike-slip faulting, and subsurface mass removal. This study focuses on the structural geology of Ius and Coprates Chasmata in southern Valles Marineris using THEMIS (Thermal Emission Imaging System), Context Camera (CTX), and HiRISE (High Resolution Imaging Science Experiment) images. The main result of the work is that the troughs and their plateau margins have experienced left-slip transtensional deformation. Syntectonic soft-sediment deformation suggests the presence of surface water during the Late Amazonian left-slip tectonics in Valles Marineris. The total left-slip motion of the southern Valles Marineris fault zone is estimated to be 150-160 km, which may have been absorbed by east-west extension across Noctis Labyrinthus and Syria Planum in the west and across Capri and Eos Chasmata in the east. The discovery of a large-scale (>2000 km in length and >100 km in slip) and rather narrow (<50 km in width) strike-slip fault zone by this study begs the question of why such a structure, typically associated with plate tectonics on Earth, has developed on Mars.

The pressure dependence of thermal expansivity affects mineral density at pressure and is an extrapolator for calculating self-compression adiabats of a self-gravitating body. I review different models for the pressure dependence of expansivity and how to decide which performs best. A finite strain model, proposed here, performs better when used to calculate adiabatic temperature lapses in both the solid silicate and liquid metal parts of a planet than either an ad-hoc exponential dependence on pressure or a commonly used mineral physics model. Choosing a particular thermal expansivity pressure dependence leads to significantly different temperatures in planetary interiors, and to inferred subsolidus properties related to homologous melting temperature. In particular, thermal expansivity in liquid metal in planetary cores at pressures comparable to Earth's core is significantly affected. The universality of the parameterization provides a simple way to model rocky planet interiors in our solar system and exoplanet interiors.

A new tectonic model is proposed for the origin of the Tharsis rise on Mars, which occupies ∼25% of the planet. The model invokes initiation of plate subduction by a large impact during the Late Heavy Bombardment at ca. 4.0 Ga. The model explains migration of Tharsis volcanism by slab rollback and the lack of magnetized crust in the bulk of Tharsis by formation of juvenile crust after the Mars dynamo creased to operate. The model also explains (1) the formation of thrust systems as a result of impact-generated crustal thickening (i.e., Thaumasia thrust), retro-arc contraction (i.e., Solis-Lunae fold belt), and plate subduction (Lycus and Ulysses thrusts), (2) the development of dominantly NE-trending grabens and a major east-facing V-shaped conjugate strike-slip system across the Tharsis rise as a result of backarc extension, and (3) crustal thickening of the Tharsis rise as a result of magmatic accretion during protracted construction of arcs above an episodically stalled and thus stationary subducting slab. The model has several implications for the way in which a unified global plate-tectonic network may have been established on early Earth. First, large impacts were common during the Late Heavy Bombardment (ca. 4.2-3.9 Ma), and thus impact-induced plate subduction would have been highly likely in the Hadean period. Such subduction systems must be local in scale and associated only with trench retreat and slab rollback. Localized plate subduction permits other modes of tectonic processes to have occurred simultaneously on early Earth, reconciling conflicting observations for plate-tectonic and non-plate-tectonic processes. Second, the presence of water at the surface of Hadean Earth would have allowed rapid transformation of basaltic crust to eclogite, allowing a sustainable plate subduction process once it started. The Hadean and possibly Archean Earth may only have had localized subduction systems, all characterized by slab rollback and trench retreat. Trench advance and related shallow-angle plate subduction probably did not begin on Earth until Proterozoic time, when a single and united global plate-tectonic network was established. This may have been accomplished by gradual coalescence of formerly independent subduction systems over a significant period of geologic time (>1 b.y.). Incorporation of trench-advance and shallow-angle plate subduction in the Proterozoic may have been induced by complex interactions of multiple subduction systems in a single and kinematically linked global tectonic network. This in turn led to the beginning of the formation of the crustal structures and petrologic assemblages of modern Earth. Based on a simple conductive cooling model, it appears that the most critical factors that control whether plate subduction could have been initiated in a rocky planet during the Late Heavy Bombardment in the inner solar system are its initial crustal thickness and the cooling rate/thickening rate of the lithosphere.

Flood lavas are major geological features on all the major rocky planetary bodies. They provide important insight into the dynamics and chemistry of the interior of these bodies. On the Earth, they appear to be associated with major and mass extinction events. It is therefore not surprising that there has been significant research on flood lavas in recent years. Initial models suggested eruption durations of days and volumetric fluxes of order 107 m3 s-1 with flows moving as turbulent floods. However, our understanding of how lava flows can be emplaced under an insulating crust was revolutionized by the observations of actively inflating pahoehoe flows in Hawaii. These new ideas led to the hypothesis that flood lavas were emplaced over many years with eruption rates of the order of 104 m3 s-1. The field evidence indicates that flood lava flows in the Columbia River Basalts, Deccan Traps, Etendeka lavas, and the Kerguelen Plateau were emplaced as inflated pahoehoe sheet flows. This was reinforced by the observation of active lava flows of ≥100 km length on Io being formed as tube-fed flows fed by moderate eruption rates (102-103 m3 s-1). More recently it has been found that some flood lavas are also emplaced in a more rapid manner. New high-resolution images from Mars revealed \"platy-ridged\" flood lava flows, named after the large rafted plates and ridges formed by compression of the flow top. A search for appropriate terrestrial analogues found an excellent example in Iceland: the 1783-1784 Laki Flow Field. The brecciated Laki flow top consists of pieces of pahoehoe, not aa clinker, leading us to call this \"rubbly pahoehoe\". Similar flows have been found in the Columbia River Basalts and the Kerguelen Plateau. We hypothesize that these flows form with a thick, insulating, but mobile crust, which is disrupted when surges in the erupted flux are too large to maintain the normal pahoehoe mode of emplacement. Flood lavas emplaced in this manner could have (intermittently) reached effusion rates of the order of 106 m3 s-1.

Back lighting The Cassini spacecraft in orbit around Saturn has observed electron beams accelerating away from the planet in regions that map to the locations of Saturn's aurora. The conventional model for aurora creation involves electron beams that accelerate towards a planet's surface. A similar phenomenon was recently observed on Earth, suggesting that electrons going the ‘wrong’ way are a general property of aurora, and that this characteristic needs to be incorporated into any full theory of aurora formation. Strong discrete aurorae on Earth are excited by electrons, which are accelerated along magnetic field lines towards the planet 1 . Surprisingly, electrons accelerated in the opposite direction have been recently observed 2 , 3 , 4 , 5 , 6 . The mechanisms and significance of this anti-earthward acceleration are highly uncertain because only earthward acceleration was traditionally considered, and observations remain limited. It is also unclear whether upward acceleration of the electrons is a necessary part of the auroral process or simply a special feature of Earth's complex space environment. Here we report anti-planetward acceleration of electron beams in Saturn's magnetosphere along field lines that statistically map into regions of aurora. The energy spectrum of these beams is qualitatively similar to the ones observed at Earth, and the energy fluxes in the observed beams are comparable with the energies required to excite Saturn's aurora. These beams, along with the observations at Earth 2 , 3 , 4 , 5 , 6 and the barely understood electron beams in Jupiter's magnetosphere 7 , 8 , demonstrate that anti-planetward acceleration is a universal feature of aurorae. The energy contained in the beams shows that upward acceleration is an essential part of the overall auroral process.

Planetary magnetospheres are natural laboratories for many interesting plasma physical processes which are difficult to study under normal laboratory conditions. Among the major processes occurring in space plasmas are the reconnection phenomenon and field line resonances. This paper deals with the second of these processes. A field line resonance is the resonant coupling between an isotropic mode and an anisotropic mode in a magnetized plasma. Field line resonances allow us to understand many features of ultra-low frequency oscillations in the terrestrial magnetosphere, that is resonant mode coupling is the current paradigm to explain geomagnetic pulsations. A brief historical introduction as well as a physical description of the field line resonance is given. Resonant mode coupling is discussed for the terrestrial, Hermean (Mercury), and Kronian (Saturn) magnetospheres, which represent natural laboratories with different conditions such as size of the laboratory, the background plasma density and composition, and the strength of the magnetic field. This comparative approach allows a deeper insight into the critical coupling problem than an isolated study of the terrestrial field line resonance phenomenon. Finally, resonant mode coupling between elastic wave modes in the solid Earth is briefly tackled and compared with the magnetospheric situation.[PUBLICATION ABSTRACT]

Using line‐of‐sight measurements of horizontal plasma drift from the Super Dual Auroral Radar Network (SuperDARN) located in the Northern and Southern hemispheres over a period extending from 1998 to 2002, statistical models of the high‐latitude convection electric field are derived for various ranges of interplanetary magnetic field (IMF) magnitude and orientation and for several ranges of dipole tilt angle. Direct comparison of the corresponding convection patterns in each hemisphere shows that under neutral tilt conditions (dipole tilt angle magnitude <10°) the patterns are most similar. However, a strong dipole tilt angle dependence is observed under northward (Bz+) and By dominated IMF conditions. For IMF Bz+, reverse convection is observed to be much stronger during positive tilt than negative tilt. For IMF By dominated conditions (IMF Bz = 0), the round convection cell is more enhanced for positive tilt than for negative tilt, particularly for IMF By < 0 in both hemispheres. The presence of a lobe cell is a likely cause of this enhancement, although it is not entirely clear why it occurs preferentially under IMF By < 0. In addition, the crescent‐shaped cells are weakened as tilt angle progresses from negative to positive, most likely due to vastly different solar produced conductivities under different tilt angles. For IMF Bz−, asymmetric values of the cross‐polar cap potentials (ΦPC) are observed between hemispheres, with ΦPC in the south being systematically larger than ΦPC in the north. Although neutral tilt patterns are similar enough to be used interchangeably, convection has a strong dipole tilt dependence and a Northern Hemisphere convection model should not be applied to the Southern Hemisphere if dipole tilt angle is not taken into account. When dipole tilt is accounted for, ΦPC differs between hemispheres by less than 10% on average, but the strength of the convection in the individual cells differs by 15% to 20% on average.