High Obliquity Research Articles

Evidence for Amazonian mid-latitude glaciation on Mars from impact crater asymmetry

We find that crater slopes in the mid-latitudes of Mars have a marked north–south asymmetry, with the pole-facing slopes being shallower. We mapped impact craters in two southern hemisphere sites (Terra Cimmeria and Noachis Terra) and one northern hemisphere site (Acidalia Planitia) and used elevation data from the High Resolution Stereo Camera (HRSC) onboard Mars Express to find the maximum slope of impact crater walls in the four cardinal directions. Kreslavsky and Head (Kreslavsky, M.A., Head, J.W. [2003]. Geophys. Res. Lett. 30), using Mars Orbiter Laser Altimeter (MOLA) track data, also found that, in general, conjugate slopes are shallower in the pole-facing direction, but over a narrower (∼10°) and more constrained latitude band. They linked the asymmetry to active-layer formation (thaw) at high obliquity. However, Parsons and Nimmo (Parsons, R.A., Nimmo, F. [2009]. J. Geophys. Res. 114) studied crater asymmetry using MOLA gridded data and found no evidence of a relationship between crater asymmetry and latitude. Our work supports the observations of Kreslavsky and Head (Kreslavsky, M.A., Head, J.W. [2003]. Geophys. Res. Lett. 30), and shows that asymmetry is also found on conjugate crater slopes below the resolution of MOLA, over a wider latitude band than found in their work. We do not systematically find a sudden transition to asymmetric craters with latitude as expected for thaw-related processes, such as solifluction, gelifluction, or gully formation. The formation of gullies should produce the opposite sense of asymmetry to our observations, so cannot explain them despite the mid-latitude location and pole-facing preferences of gullies. We instead link this asymmetry to the deposition of ice-rich crater deposits, where the base of pole-facing slopes receive ten to hundreds of meters of additional net deposition, compared to equator-facing ones over the mid-latitudes. In support of this hypothesis we found that craters in Terra Cimmeria that have deposits on both their floor and pole-facing walls, occur preferentially at the mid-latitudes and have marked positive asymmetry. These deposits were likely laid down during high obliquity excursions (>45°) at least 5My ago and potentially over the whole Amazonian epoch. Their preservation to the present-day relies on the presence of a surface lag of debris, which inhibits sublimation.

The Icebreaker Life Mission to Mars: A Search for Biomolecular Evidence for Life

The search for evidence of life on Mars is the primary motivation for the exploration of that planet. The results from previous missions, and the Phoenix mission in particular, indicate that the ice-cemented ground in the north polar plains is likely to be the most recently habitable place that is currently known on Mars. The near-surface ice likely provided adequate water activity during periods of high obliquity, ≈ 5 Myr ago. Carbon dioxide and nitrogen are present in the atmosphere, and nitrates may be present in the soil. Perchlorate in the soil together with iron in basaltic rock provides a possible energy source for life. Furthermore, the presence of organics must once again be considered, as the results of the Viking GCMS are now suspect given the discovery of the thermally reactive perchlorate. Ground ice may provide a way to preserve organic molecules for extended periods of time, especially organic biomarkers. The Mars Icebreaker Life mission focuses on the following science goals: (1) Search for specific biomolecules that would be conclusive evidence of life. (2) Perform a general search for organic molecules in the ground ice. (3) Determine the processes of ground ice formation and the role of liquid water. (4) Understand the mechanical properties of the martian polar ice-cemented soil. (5) Assess the recent habitability of the environment with respect to required elements to support life, energy sources, and possible toxic elements. (6) Compare the elemental composition of the northern plains with midlatitude sites. The Icebreaker Life payload has been designed around the Phoenix spacecraft and is targeted to a site near the Phoenix landing site. However, the Icebreaker payload could be supported on other Mars landing systems. Preliminary studies of the SpaceX Dragon lander show that it could support the Icebreaker payload for a landing either at the Phoenix site or at midlatitudes. Duplicate samples could be cached as a target for possible return by a Mars Sample Return mission. If the samples were shown to contain organic biomarkers, interest in returning them to Earth would be high.

Subglacial Environments and the Search for Life Beyond the Earth

One of the most remarkable discoveries resulting from the robotic and remote sensing exploration of space is the inferred presence of bodies of liquid water under ice deposits on other planetary bodies: extraterrestrial subglacial environments. Most prominent among these are the ice-covered ocean of the Jovian moon, Europa, and the Saturnian moon, Enceladus. On Mars, although there is no current evidence for subglacial liquid water today, conditions may have been more favorable for liquid water during periods of higher obliquity. Data on these extraterrestrial environments show that while they share similarities with some subglacial environments on the Earth, they are very different in their combined physicochemical conditions. Extraterrestrial environments may provide three new types of subglacial settings for study: (1) uninhabitable environments that are more extreme and life-limiting than terrestrial subglacial environments, (2) environments that are habitable but are uninhabited, which can be compared to similar biotically influenced subglacial environments on the Earth, and (3) environments with examples of life, which will provide new opportunities to investigate the interactions between a biota and glacial environments.

Starspots and spin‐orbit alignment for Kepler cool host stars

AbstractThe angle between the spin axis of the host star and the orbit of its planets (i.e., the stellar obliquity) is precious information about the formation and evolution of exoplanetary systems. Measurements of the Rossiter‐McLaughlin effect revealed that many stars that host a hot‐Jupiter have high obliquities, suggesting that hot‐Jupiter formation involves excitation of orbital inclinations. In this contribution we show how the passage of the planet over starspots can be used to measure the obliquity of exoplanetary systems. This technique is used to obtain – for the first time – the obliquity of a system with several planets that lie in a disk, Kepler‐30, with the result that the star has an obliquity smaller than 10 degrees. The implications for the formation of exoplanetary systems, in particular the hot‐Jupiter population, are also discussed. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Patterned ground in martian high northern latitudes: Morphology and age constraints

We performed a systematic survey of typical surface patterns and small (5–50m) impact craters in a zone between 50° and 70° latitude in the Northern Lowlands of Mars. We found that small-scale (<10m) polygonal pattern formed by narrow fractures is typical in the northern half of the studied zone. We interpret this pattern to be formed by active thermal contraction polygons associated with the present-day shallow ground ice. These fresh small polygons are superposed over a larger-scale (∼15–30m) polygonal network of smooth gentle troughs and mounds. The latter pattern extends southward from the shallow ground ice boundary to the southern edge of the study zone. We interpret this pattern to be a relic from different climate conditions in the geologically recent past (possibly, to the most recent periods of high obliquity of Mars spin axis). The population of small craters is sparse and indicates intensive resurfacing in the recent geological past. The retention age of 5–10m diameter craters in the northern part of our study zone is surprisingly young, on the order of hundreds to thousands of years, which means that quick obliteration of such craters occurs under the present-day spin/orbit configuration of Mars. The crater obliteration time scale increases for larger craters and toward the southern edge of the study area.

INTERNAL GRAVITY WAVES MODULATE THE APPARENT MISALIGNMENT OF EXOPLANETS AROUND HOT STARS

We propose that the observed misalignment between extra-solar planets and their hot host stars can be explained by angular momentum transport within the host star. Observations have shown that this misalignment is preferentially around hot stars, which have convective cores and extended radiative envelopes. This situation is amenable to substantial angular momentum transport by internal gravity waves (IGW) generated at the convective-radiative interface. Here we present numerical simulations of this process and show that IGW can modulate the surface rotation of the star. With these two- dimensional simulations we show that IGW could explain the retrograde orbits observed in systems such as HAT-P-6 and HAT-P-7, however, extension to high obliquity objects will await future three- dimensional simulations. We note that these results also imply that individual massive stars should show temporal variations in their v sini measurements.

Reading the climate record of the martian polar layered deposits

The martian polar regions have layered deposits of ice and dust. The stratigraphy of these deposits is exposed within scarps and trough walls and is thought to have formed due to climate variations in the past. Insolation has varied significantly over time and caused dramatic changes in climate, but it has remained unclear whether insolation variations could be linked to the stratigraphic record. We present a model of layer formation based on physical processes that expresses polar deposition rates of ice and dust in terms of insolation. In this model, layer formation is controlled by the insolation record, and dust-rich layers form by two mechanisms: (1) increased summer sublimation during high obliquity, and (2) variations in the polar deposition of dust modulated by obliquity variations. The model is simple, yet physically plausible, and allows for investigations of the climate control of the polar layered deposits (PLD). We compare the model to a stratigraphic column obtained from the north polar layered deposits (NPLD) (Fishbaugh, K.E., Hvidberg, C.S., Byrne, S., Russel, P.S., Herkenhoff, K.E., Winstrup, M., Kirk, R. [2010a]. Geophys. Res. Lett., 37, L07201) and show that the model can be tuned to reproduce complex layer sequences. The comparison with observations cannot uniquely constrain the PLD chronology, and it is limited by our interpretation of the observed stratigraphic column as a proxy for NPLD composition. We identified, however, a set of parameters that provides a chronology of the NPLD tied to the insolation record and consistently explains layer formation in accordance with observations of NPLD stratigraphy. This model dates the top 500m of the NPLD back to ∼1millionyears with an average net deposition rate of ice and dust of 0.55mma−1. The model stratigraphy contains a quasi-periodic ∼30m cycle, similar to a previously suggested cycle in brightness profiles from the NPLD (Laskar, J., Levrard, B., Mustard, F. [2002]. Nature, 419, 375–377; Milkovich, S., Head, J.W. [2005]. J. Geophys. Res. 110), but here related to half of the obliquity cycles of 120 and 99kyr and resulting from a combination of the two layer formation mechanisms. Further investigations of the non-linear insolation control of PLD formation should consider data from other geographical locations and include radar data and other stratigraphic datasets that can constrain the composition and stratigraphy of the NPLD layers.

Alignment of the stellar spin with the orbits of a three-planet system

The Sun's equator and the planets' orbital planes are nearly aligned, which is presumably a consequence of their formation from a single spinning gaseous disk. For exoplanetary systems this well-aligned configuration is not guaranteed: dynamical interactions may tilt planetary orbits, or stars may be misaligned with the protoplanetary disk through chaotic accretion , magnetic interactions or torques from neighbouring stars. Indeed, isolated 'hot Jupiters' are often misaligned and even orbiting retrograde. Here we report an analysis of transits of planets over starspots on the Sun-like star Kepler-30 (ref. 8), and show that the orbits of its three planets are aligned with the stellar equator. Furthermore, the orbits are aligned with one another to within a few degrees. This configuration is similar to that of our Solar System, and contrasts with the isolated hot Jupiters. The orderly alignment seen in the Kepler-30 system suggests that high obliquities are confined to systems that experienced disruptive dynamical interactions. Should this be corroborated by observations of other coplanar multi-planet systems, then star-disk misalignments would be ruled out as the explanation for the high obliquities of hot Jupiters, and dynamical interactions would be implicated as the origin of hot Jupiters.

Doppler tomography of transiting exoplanets: a prograde, low-inclined orbit for the hot Jupiter CoRoT-11b

We report the detection of the Doppler shadow of the transiting hot Jupiter CoRoT-11b. Our analysis is based on line-profile tomography of time-series, Keck/HIRES high-resolution spectra acquired during the transit of the planet. We measured a sky-projected, spin-orbit angle of 0.1 +/- 2.6 degrees, which is consistent with a very low-inclined orbit with respect to the stellar rotation axis. We refined the physical parameters of the system using a Markov chain Monte Carlo simultaneous fitting of the available photometric and spectroscopic data. An analysis of the tidal evolution of the system shows how the currently measured obliquity and its uncertainty translate into an initial absolute value of less than about 10 degrees on the zero-age main sequence, for an expected average modified tidal quality factor of the star Q'* > 4 x 10^6. This is indicative of an inward migration scenario that would not have perturbed the primordial low obliquity of CoRoT-11b. Taking into account the effective temperature and mass of the planet host star (Teff=6440 K, M*=1.23 MSun), the system can be considered a new telling exception to the recently proposed trend, according to which relatively hot and massive stars (Teff>6250 K, M*>1.2 MSun) seem to be preferentially orbited by hot Jupiters with high obliquity.

The morphodynamics of rip channels on embayed beaches

We use a nonlinear morphodynamic model to examine the formation and nonlinear evolution of surfzone rip channels on embayed beaches. Starting from a range of embayed beach bathymetries characterized by different length and curvature, and under different time-invariant and time-varying wave conditions, the numerical model can reproduce the flow circulation and morphological characteristics observed on natural embayed beaches: (1) normal beach circulation, characterized by rips similar to non-embayed beaches and the presence of headland rips, (2) cellular circulation, with either headland rips only occurring at one or both ends of the embayment or a single rip at the centre of the beach and (3) transitional circulation, where both topography and currents influence rip location and behaviour. Time-invariant simulations show that, under oblique-wave forcing, rip spacing is systematically larger updrift than downdrift. Headland rips are preferably observed for straight beaches, with no clear dependence on wave angle. Wave shadowing and resulting alongshore gradients in wave height against the headland are the primary driving mechanism for headland rips. The formation of a single central rip is observed for short, curved embayed beaches, with no clear dependence on the wave angle as well. We use a novel non-dimensional embayment scaling parameter to quantify the degree of headland impact on beach circulation. Our simulations with shore-normal waves and initially alongshore-uniform embayed beaches show the parameter is consistent with observations. Our simulations also suggest that for high wave obliquity or time-varying wave angle to the shore, the influence of the headlands can progressively propagate into the whole domain. A time-varying wave angle results in persistent migration of rips towards the downcurrent headland rip, the splitting of shoals, an increase in merging of rip channels and more alongshore-variable rip spacing. The longshore variability of rip channel wavelength along embayed beaches is consistent with the hypothesis that rips are self-organized patterns and is consistent with recent field observations.

Modeling the ballistic response of the 14.5 mm BS41 projectile

This article presents computed results of the U. S. Army Research Laboratory (ARL) 14.5 mm BS41 projectile impacting steel targets with varying impact conditions. The ARL BS41 is a complex projectile that includes a soft metal jacket, lead filler, inert powder and a high-strength tungsten carbide (WC) core. The WC core includes a nominal 6 wt% cobalt binder (WC-.06Co) producing significant ductility in compression but very little in tension. Recent 3D numerical algorithm advancements and a more accurate material model for WC produce computed results that are in good agreement with experimental results. The computed results demonstrate the ability to reproduce several key observations: the stripping of the steel jacket, lead filler and inert powder when impacting thin and thick steel targets; rigid body penetration of the WC core into thin and thick steel targets for normal impact; severe fracture and fragmentation of the core when impacting obliquely; and core fracture due to a yaw angle at impact. The computed results may also explain why the core fractures at low obliquities and shatters at high obliquities. Experimental data are also discussed and compared to the computed results.

Past extreme warming events linked to massive carbon release from thawing permafrost

Between about 55.5 and 52 million years ago, Earth experienced a series of sudden and extreme global warming events (hyperthermals) superimposed on a long-term warming trend. The first and largest of these events, the Palaeocene-Eocene Thermal Maximum (PETM), is characterized by a massive input of carbon, ocean acidification and an increase in global temperature of about 5 °C within a few thousand years. Although various explanations for the PETM have been proposed, a satisfactory model that accounts for the source, magnitude and timing of carbon release at the PETM and successive hyperthermals remains elusive. Here we use a new astronomically calibrated cyclostratigraphic record from central Italy to show that the Early Eocene hyperthermals occurred during orbits with a combination of high eccentricity and high obliquity. Corresponding climate-ecosystem-soil simulations accounting for rising concentrations of background greenhouse gases and orbital forcing show that the magnitude and timing of the PETM and subsequent hyperthermals can be explained by the orbitally triggered decomposition of soil organic carbon in circum-Arctic and Antarctic terrestrial permafrost. This massive carbon reservoir had the potential to repeatedly release thousands of petagrams (10(15) grams) of carbon to the atmosphere-ocean system, once a long-term warming threshold had been reached just before the PETM. Replenishment of permafrost soil carbon stocks following peak warming probably contributed to the rapid recovery from each event, while providing a sensitive carbon reservoir for the next hyperthermal. As background temperatures continued to rise following the PETM, the areal extent of permafrost steadily declined, resulting in an incrementally smaller available carbon pool and smaller hyperthermals at each successive orbital forcing maximum. A mechanism linking Earth's orbital properties with release of soil carbon from permafrost provides a unifying model accounting for the salient features of the hyperthermals.

A HIGH STELLAR OBLIQUITY IN THE WASP-7 EXOPLANETARY SYSTEM

We measure a tilt of 86+-6 deg between the sky projections of the rotation axis of the WASP-7 star, and the orbital axis of its close-in giant planet. This measurement is based on observations of the Rossiter-McLaughlin (RM) effect with the Planet Finder Spectrograph on the Magellan II telescope. The result conforms with the previously noted pattern among hot-Jupiter hosts, namely, that the hosts lacking thick convective envelopes have high obliquities. Because the planet's trajectory crosses a wide range of stellar latitudes, observations of the RM effect can in principle reveal the stellar differential rotation profile; however, with the present data the signal of differential rotation could not be detected. The host star is found to exhibit radial-velocity noise (``stellar jitter') with an amplitude of ~30m/s over a timescale of days.

Stability of Mg-sulfates at −10°C and the rates of dehydration/rehydration processes under conditions relevant to Mars

[1] We report the results of low temperature (−10°C) experiments on the stability fields and phase transition pathways of five hydrous Mg-sulfates. A low temperature form of MgSO4·7H2O (LT-7w) was found to have a wide stability field that extends to low relative humidity (∼13% RH at −10°C). Using information on the timing of phase transitions, we extracted information on the reaction rates of five important dehydration and rehydration processes. We found that the temperature dependencies of rate constants for dehydration processes differ from those of rehydration, which reflect differences in reaction mechanisms. By extrapolating these rate constants versus T correlations into the T range relevant to Mars, we can evaluate the possibility of occurrence of specific processes and the presence of common Mg-sulfate species present on Mars in different periods and locations. We anticipate in a moderate obliquity period, starkeyite and LH-MgSO4·H2O should be two common Mg-sulfates at the surface, another polymorph MH-MgSO4·H2O can exist at the locations where hydrothermal processes may have occurred. In polar regions or within the subsurface of other regions, meridianiite (coexisting with water ice, near 100% RH) and LT-7w (over a large RH range) are the stable phases. During a high obliquity period, meridianiite and LT-7w should exhibit widespread occurrence. The correlations of reaction rates versus temperature found in this study imply that dehydration and rehydration of hydrous Mg-sulfates would always be slower than the sublimation and crystallization of water ice, which would be supported by mission observations from Odyssey and by Mars Exploration Rovers.

Combined obliquity and precession pacing of late Pleistocene deglaciations

Milankovitch proposed that Earth resides in an interglacial state when its spin axis both tilts to a high obliquity and precesses to align the Northern Hemisphere summer with Earth's nearest approach to the Sun. This general concept has been elaborated into hypotheses that precession, obliquity or combinations of both could pace deglaciations during the late Pleistocene. Earlier tests have shown that obliquity paces the late Pleistocene glacial cycles but have been inconclusive with regard to precession, whose shorter period of about 20,000 years makes phasing more sensitive to timing errors. No quantitative test has provided firm evidence for a dual effect. Here I show that both obliquity and precession pace late Pleistocene glacial cycles. Deficiencies in time control that have long stymied efforts to establish orbital effects on deglaciation are overcome using a new statistical test that focuses on maxima in orbital forcing. The results are fully consistent with Milankovitch's proposal but also admit the possibility that long Southern Hemisphere summers contribute to deglaciation.

High-latitude obliquity as a dominant forcing in the Agulhas current system

Abstract. The Agulhas Current transport of heat and salt from the Indian Ocean into the South Atlantic around South Africa (Agulhas leakage), can affect the Atlantic meridional overturning circulation (AMOC) and, thus, influence global climate. However, efforts to elucidate forcing mechanisms connecting the Agulhas leakage with the upstream dynamics of the current have been hampered by a lack of climate records extracted from the area where the Agulhas current originates. We determine 800-kyr sea surface temperature (SST) and salinity (SSS) records from the "precursor" region of the Agulhas current and show that these records contain strong 100-kyr and 41-kyr cycles. This latter obliquity-driven cycle is nearly in phase with changes in the annual mean insolation and air temperature at high southern latitudes. In contrast, our SST and SSS records did not reveal precession-driven cycles, which is surprising given the low-latitude location of the upstream Agulhas current. Together, this indicates that the dynamics of the Agulhas current system is mainly controlled by high latitude obliquity through its influence on the position of the Southern Hemisphere subtropical front (STF) and its associated westerlies. Our study demonstrates that obliquity may drive an important part of the 100 kyr cycles observed in the system rather than precession. Our results also suggest that a stronger Agulhas current, associated with a northward shift of the wind system during glacial periods, leads to reduced leakage, in accordance with the theory. We argue that during terminations, stronger Agulhas leakage of heat and salt was triggered by increased obliquity exerting a positive feedback on the global climate system through modulating long-term AMOC variations.

Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc

The northwestwards-directed Eocene propagation of the Western Alpine orogen is linked with (1) compressional structures in the basement and the Mesozoic sedimentary cover of the European foreland, well preserved in the External Zone (or Dauphiné Zone) of the Western Alps and (2) tectono-sedimentary features associated with the displacement of the early Tertiary foreland basin. Three major shortening episodes are identified: a pre-Priabonian deformation D1 (N-S shortening), supposedly linked with the Pyrenean-Provence orogeny, and two Alpine shortening events D2 (N- to NW-directed) and D3 (W-directed). The change from D2 to D3, which occurred during early Oligocene time in the Dauphiné zone, is demonstrated by a high obliquity between the trends of the D3 folds and thrusts, which follow the arcuate orogen, and of the D2 structures which are crosscut by them. This change is also recorded in the evolution of the Alpine foreland basins: the flexural basin propagating NW-wards from Eocene to earliest Oligocene shows thin-skinned compressional deformation, with syn-depositional basin-floor tilting and submarine removal of the basin infill above active structures. Locally, a steep submarine slope scar is overlain by kilometric-scale blocks slided NW-wards from the orogenic wedge. The deformations of the basin floor and the associated sedimentary and erosional features are kinematically consistent with D2 in the Dauphiné foreland. Since ∼32Ma, the previously subsiding areas were uplifted and the syntectonic sedimentation shifted westwards. Simultaneously, the paleo-accretionary prism, which developed during the previous, continental subduction stage, was rapidly exhumed during the Oligocene collision stage due to westward indentation by the Adriatic lithosphere, which likely enhanced the relief and erosion rate. The proposed palinspastic restoration takes into account this two-stage evolution, with important northward transport of the distal passive margin fragments (Briançonnais) involved in the accretionnary prism before the formation of the western arc, which now crosscuts the westward termination of the ancient orogen. By early Oligocene, the Ivrea body indentation, which was kinematically linked with the Insubric line activation, initiated the westward escape and the curvature of the arc was progressively acquired, as recorded by southward increasing counter-clockwise rotations in the internal nappes. We propose that the present N-S trend of the Ivrea lithospheric mantle indenter which appears roughly rectilinear at ∼15km depth could be a relict of the western transform boundary of Adria during its northward Eocene drift. The renewed Oligocene Alpine kinematics and the related change in the mode of accomodation of Africa–Europe convergence can be correlated with deep lithospheric causes, i.e. partial detachment of the Tethyan slab and/or a change in motion of the Adria plate, and was enhanced by the E-directed rollback of the eastern Ligurian oceanic domain and the incipient Ligurian rifting.

Equatorial glaciations on Mars revealed by gravitational collapse of Valles Marineris wallslopes

Martian global climate models that account for evidence of past glaciations reported in tropical to mid-latitude regions of Mars predict accumulation of water ice in Valles Marineris during past periods of high obliquity. Observational evidence for such glaciations is given here. Topographic basement ridges of tectonic origin are common in Valles Marineris, and display sackung features, an assemblage of tectonic patterns that are diagnostic of deep-seated gravitational slope deformation. This deformation is most easily explained by paraglacial ridge failure subsequent to ridge wall debuttressing and decohesion following the retreat of glaciers. This interpretation is supported by extensive bibliographic analysis of sackung triggers on Earth, by morphological evidence of subglacial erosion of the lower parts of Valles Marineris wallslopes, of periglacial erosion of their upper parts and by the presence of various types of glacial landforms on the floors of Valles Marineris troughs. The age of these equatorial glaciations is found to be older than 1.4 Gy and younger than 3.5 Gy.

WASP-50 b: a hot Jupiter transiting a moderately active solar-type star

We report the discovery by the WASP transit survey of a giant planet in a close orbit (0.0295+-0.0009 AU) around a moderately bright (V=11.6, K=10) G9 dwarf (0.89+-0.08 M_sun, 0.84+-0.03 R_sun) in the Southern constellation Eridanus. Thanks to high-precision follow-up photometry and spectroscopy obtained by the telescopes TRAPPIST and Euler, the mass and size of this planet, WASP-50b, are well constrained to 1.47+-0.09 M_jup and 1.15+-0.05 R_jup, respectively. The transit ephemeris is 2455558.6120 (+-0.0002) + N x 1.955096 (+-0.000005) HJD_UTC. The size of the planet is consistent with basic models of irradiated giant planets. The chromospheric activity (log R'_HK = -4.67) and rotational period (P_rot = 16.3+-0.5 days) of the host star suggest an age of 0.8+-0.4 Gy that is discrepant with a stellar-evolution estimate based on the measured stellar parameters (rho_star = 1.48+-0.10 rho_sun, Teff = 5400+-100 K, [Fe/H]= -0.12+-0.08) which favours an age of 7+-3.5 Gy. This discrepancy could be explained by the tidal and magnetic influence of the planet on the star, in good agreement with the observations that stars hosting hot Jupiters tend to show faster rotation and magnetic activity (Pont 2009; Hartman 2010). We measure a stellar inclination of 84 (-31,+6) deg, disfavouring a high stellar obliquity. Thanks to its large irradiation and the relatively small size of its host star, WASP-50b is a good target for occultation spectrophotometry, making it able to constrain the relationship between hot Jupiters' atmospheric thermal profiles and the chromospheric activity of their host stars proposed by Knutson et al. (2010).

Dielectric signatures of adsorbed and salty liquid water at the Phoenix landing site, Mars

The real part of the dielectric permittivity of the Martian regolith was measured by the Thermal and Electrical Conductivity Probe (TECP) on the Phoenix lander. We interpret these data using laboratory measurements of permittivity as a function of H2O and salt content, soil type, and temperature. Due to variability in sensor coupling, we focus on data taken at one locality (“Vestri”) three separate times, spanning multiple sols. A daytime increase in permittivity suggests progressive melting of a heterogeneous, disconnected, salty ice with a eutectic temperature of ∼239 K, which is close to the eutectic temperatures of NaClO4 or MgCl2. We found no evidence for Mg(ClO4)2. NaClO4 and MgCl2 are consistent with precipitation by freezing following a prior epoch of high obliquity. The evaporation of diurnal briny meltwater is inhibited by surface tension in small pores. An increase in permittivity occurred on the night of sol 70 that coincided with surface frost and measurement of a decrease in atmospheric water vapor. The permittivity jump can be matched by an increase in adsorbed H2O from ∼1 monolayer to 3 monolayers in an analog soil with a Viking‐like specific surface area (17 m2/g). However, the amount of adsorbed H2O is an order of magnitude larger than that inferred to have precipitated during the night. We suggest that the electrical signature of adsorbed water on Mars is stronger than we measured in the laboratory, possibly due to radiation damage of the regolith.