Jonathan M. Aurnou

A Reappraisal of The Habitability of Planets around M Dwarf Stars

Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.

A polar vortex in the Earth's core

Numerical dynamo models have been successful in explaining the origin of the Earths magnetic field and its secular variation by convection in the electrically conducting fluid outer core. An important component of the convection in the numerical dynamos are polar vortices beneath the core–mantle boundary in each hemisphere. These polar vortices in the outer core have been proposed as sources for both the anomalous rotation of the inner core and the toroidal part of the geomagnetic field. Here we use the observed structure of the Earths magnetic field and its variation since 1870 to infer the existence of an anticyclonic polar vortex with a polar upwelling in the northern hemisphere of the core, consistent with the polar vortices found in numerical dynamos.

Boundary layer control of rotating convection systems.

Turbulent rotating convection controls many observed features of stars and planets, such as magnetic fields, atmospheric jets and emitted heat flux patterns. It has long been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force. Here, however, we present results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. We formulate a predictive description of the transition between the two regimes on the basis of the competition between these two boundary layers. This transition scaling theory unifies the disparate results of an extensive array of previous experiments, and is broadly applicable to natural convection systems.

Experiments on convection in Earth’s core tangent cylinder

Abstract Results of thermal convection experiments in a rotating fluid with a geometry similar to the Earth’s core tangent cylinder are presented. We find four different states. In order of increasing Rayleigh number, these are: (1) subcritical (no convection), (2) helical plumes around the tangent cylinder, (3) helical plumes throughout the tangent cylinder, and (4) fully three-dimensional convection. The convection generates a retrograde (westward) azimuthal thermal wind flow below the outer spherical boundary, with maximum velocity on the tangent cylinder. The velocity of the thermal wind scales with convective buoyancy flux B = αgq / ρC p and Coriolis parameter f =2 Ω as U ≃2( B / f ) 1/2 . Tangent cylinder convection can explain the origin of polar vortices beneath the core–mantle boundary inferred from geomagnetic secular variation, if a large fraction of the buoyancy produced by inner core solidification remains within the tangent cylinder, and also if the inner core growth rate is high. According to our experiments, the tangent cylinder can act as a reservoir for products of inner core growth, although the effect is probably far too small to detect from surface observations.

Experiments on Rayleigh–Bénard convection, magnetoconvection and rotating magnetoconvection in liquid gallium

Thermal convection experiments in a liquid gallium layer subject to a uniform rotation and a uniform vertical magnetic field are carried out as a function of rotation rate and magnetic field strength. Our purpose is to measure heat transfer in a low-Prandtl-number ( Pr = 0.023), electrically conducting fluid as a function of the applied temperature difference, rotation rate, applied magnetic field strength and fluid-layer aspect ratio. For Rayleigh–Benard (non-rotating, non-magnetic) convection we obtain a Nusselt number–Rayleigh number law Nu = 0.129 Ra 0.272±0.006 over the range 3.0 × 10 3 Ra 4 . For non-rotating magnetoconvection, we find that the critical Rayleigh number Ra C increases linearly with magnetic energy density, and a heat transfer law of the form Nu ∼ Ra 1/2 . Coherent thermal oscillations are detected in magnetoconvection at ∼ 1.4 Ra C . For rotating magnetoconvection, we find that the convective heat transfer is inhibited by rotation, in general agreement with theoretical predictions. At low rotation rates, the critical Rayleigh number increases linearly with magnetic field intensity. At moderate rotation rates, coherent thermal oscillations are detected near the onset of convection. The oscillation frequencies are close to the frequency of rotation, indicating inertially driven, oscillatory convection. In nearly all of our experiments, no well-defined, steady convective regime is found. Instead, we detect unsteady or turbulent convection just after onset.

Sulfur's impact on core evolution and magnetic field generation on Ganymede

[1] Analysis of the melting relationships of potential core forming materials in Ganymede indicate that fluid motions, a requirement for a dynamo origin for the satellite’s magnetic field, may be driven, in part, either by iron (Fe) ‘‘snow’’ forming below the coremantle boundary or solid iron sulfide (FeS) floating upward from the deep core. Eutectic melting temperatures and eutectic sulfur contents in the binary Fe-FeS system decrease with increasing pressure within the interval of core pressures on Ganymede (<14 GPa). Comparison of melting temperatures to adiabatic temperature gradients in the core suggests that solid iron is thermodynamically stable at shallow levels for bulk core compositions more iron-rich than eutectic (i.e., <21 wt % S). Calculations based on highpressure solid-liquid phase relationships in the Fe-FeS system indicate that iron snow or floatation of solid iron sulfide, depending on whether the core composition is more or less iron-rich than eutectic, is an inevitable consequence of cooling Ganymede’s core. These results are robust over a wide range of plausible three-layer internal structures and thermal evolution scenarios. For precipitation regimes that include Fe-snow, we present scaling arguments that give typical Rossby and magnetic Reynolds numbers consistent with dynamo action occurring in Ganymede’s core. Furthermore, by applying recently derived scaling relationships relating magnetic field strength to buoyancy flux, we obtain estimates of surface magnetic field strength comparable with observed values.

Strong zonal winds from thermal convection in a rotating spherical shell

Zonal wind (ZW) generation by thermal convection in rotating spherical shells is studied using numerical calculations. Strong ZW accompany quasi-geostrophic, high Rayleigh number convection in shells with stress-free boundaries. In a thin shell (radius ratio 0.75) with stress-free boundaries, nearly 90% of the total kinetic energy is contained in the ZW at Rayleigh number 106 and Taylor number 4.4 × l07. The same parameters in a thicker shell produce weaker convection and weaker ZW. Rigid boundaries reduce the kinetic energy in the ZW to less than 20% of the total. The ZW are eastward (prograde) in the equatorial region and westward at higher latitudes, and are driven by Reynolds stresses associated with the convection. Episodes with strong ZW alternate with episodes of strong convection. Although far from the dynamical regime of Jupiter and Saturn, our results support the interpretation that the prograde equatorial jets on these planets originate from deep convection.

Axisymmetric simulations of libration-driven fluid dynamics in a spherical shell geometry

We report on axisymmetric numerical simulations of rapidly rotating spherical shells in which the axial rotation rate of the outer shell is modulated in time. This allows us to model planetary bodies undergoing forced longitudinal libration. In this study we systematically vary the Ekman number, 10−7≤E≲10−4, which characterizes the ratio of viscous to Coriolis forces in the fluid, and the libration amplitude, Δϕ. For libration amplitudes above a certain threshold, Taylor–Gortler vortices form near the outer librating boundary, in agreement with the previous laboratory experiments of Noir et al. [Phys. Earth Planet. Inter. 173, 141 (2009)]. At the lowest Ekman numbers investigated, we find that the instabilities remain spatially localized at onset in the equatorial region. In addition, nonzero time-averaged azimuthal (zonal) velocities are observed for all parameters studied. The zonal flow is characterized by predominantly retrograde flow in the interior, with a stronger prograde jet in the outer equatori...

Mechanics of inner core super-rotation

A mechanism is proposed to explain the seismologically-inferred prograde rotation of the Earths solid inner core in terms of the structure of convection in the fluid outer core. Numerical calculations of convection and dynamo action in the outer core exhibit excess temperatures inside the tangent cylinder surrounding the inner core. We show that this temperature difference generates a prograde thermal wind and a strong azimuthal magnetic field inside the tangent cylinder. Electromagnetic torques on the inner core derived from induced azimuthal magnetic fields and the ambient poloidal field equilibrate when the inner core angular velocity lags the nearby tangent cylinder fluid angular velocity by approximately 14%. The inferred prograde rotation of the inner core (1.1–3°/year relative to the mantle) can be produced by a very small (⋍ 0.001 K) temperature anomaly within the tangent cylinder and indicates strong toroidal magnetic fields with peak intensities of 24–66 mT in that region of the core.

Zonal flow regimes in rotating anelastic spherical shells: An application to giant planets

Abstract The surface zonal winds observed in the giant planets form a complex jet pattern with alternating prograde and retrograde direction. While the main equatorial band is prograde on the gas giants, both ice giants have a pronounced retrograde equatorial jet. We use three-dimensional numerical models of compressible convection in rotating spherical shells to explore the properties of zonal flows in different regimes where either rotation or buoyancy dominates the force balance. We conduct a systematic parameter study to quantify the dependence of zonal flows on the background density stratification and the driving of convection. In our numerical models, we find that the direction of the equatorial zonal wind is controlled by the ratio of the global-scale buoyancy force and the Coriolis force. The prograde equatorial band maintained by Reynolds stresses is found in the rotation-dominated regime. In cases where buoyancy dominates Coriolis force, the angular momentum per unit mass is homogenized and the equatorial band is retrograde, reminiscent to those observed in the ice giants. In this regime, the amplitude of the zonal jets depends on the background density contrast with strongly stratified models producing stronger jets than comparable weakly stratified cases. Furthermore, our results can help to explain the transition between solar-like (i.e. prograde at the equator) and the “anti-solar” differential rotations (i.e. retrograde at the equator) found in anelastic models of stellar convection zones. In the strongly stratified cases, we find that the leading order force balance can significantly vary with depth. While the flow in the deep interior is dominated by rotation, buoyancy can indeed become larger than Coriolis force in a thin region close to the surface. This so-called “transitional regime” has a visible signature in the main equatorial jet which shows a pronounced dimple where flow amplitudes notably decay towards the equator. A similar dimple is observed on Jupiter, which suggests that convection in the planet interior could possibly operate in this regime.