Current projects

• Mars climate evolution model - including multiple possible orbital histories, carbonate formation, and spatially-resolved liquid water availability.

• Turbulent dissipation in tiger stripes on Enceladus.

• Magma pools and the deep circulation of rocky extrasolar planets, with predictions for secondary eclipses and phase curves.

• Time variability in the comet-like disintegration of the hot rocky exoplanet orbiting KIC 12557548.

Papers

• Kite, Sustained eruptions on Enceladus explained by turbulent dissipation in tiger stripes, in prep. [ More]
  
  This paper is in preparation.

• Kite, Howard, Lucas, and Lewis, Resolving the era of river-forming climates on Mars using stratigraphic logs of river-deposit dimensions, in prep. [ More]
  
  This paper is in preparation.

• Kite, Howard, Lucas, Armstrong, Aharonson, and Lamb, Stratigraphy of Aeolis Dorsa, Mars: sequencing of the great river deposits, in review. [ More]
  
  Unraveling the stratigraphic record is the key to understanding ancient climate change on Mars. Stratigraphic records of river deposits hold particular promise because rain or snowmelt must exceed infiltration plus evaporation to allow sediment transport by rivers. Therefore, river deposits when placed in stratigraphic order could constrain the number, magnitudes, and durations of the wettest (and presumably most habitable) climates in Mars history. We use crosscutting relationships to establish the stratigraphic context of river and alluvial-fan deposits in the Aeolis Dorsa sedimentary basin, 10° E of Gale crater. At Aeolis Dorsa, wind erosion has exhumed a stratigraphic section of sedimentary rocks consisting of at least four unconformity-bounded rock packages, recording three or more distinct episodes of surface runoff. Early deposits (>700m thick) are embayed by river deposits (>400m thick), which are in turn unconformably draped by fan-shaped deposits (<100m thick) which we interpret as alluvial fans. Yardang-forming layered deposits (>900 m thick) unconformably drape all previous deposits.
  
  River deposits embay a dissected landscape formed of sedimentary rock. The river deposits are eroding out of at least two distinguishable units. There is evidence for pulses of erosion during the interval of river deposition. The total interval spanned by river deposits is >(1 x 10^6 - 2 x 10^7) yr, and this is extended if we include alluvial-fan deposits. Fan-shaped deposits unconformably postdate thrust faults which crosscut the river deposits. This relationship suggests a relatively dry interval of >4 x 10^7 yr after the river deposits formed and before the fan-shaped deposits formed, based on probability arguments. Yardang-forming layered deposits unconformably postdate all of the earlier deposits. They contain rhythmite and their induration suggests a damp or wet (near-)surface environment. The time gap between the end of river deposition and the onset of yardang-forming layered deposits is constrained to >1 x 10^8 yr by the high density of impact craters embedded at the unconformity. The time gap between the end of alluvial-fan deposition and the onset of yardang-forming layered deposits was at least long enough for wind-induced saltation abrasion to erode 20-30m into the alluvial-fan deposits. We correlate the yardang-forming layered deposits to the upper layers of Gale crater's mound (Mt. Sharp / Aeolis Mons), and the fan-shaped deposits to Peace Vallis fan in Gale crater. Alternations between periods of low mean obliquity and periods of high mean obliquity may have modulated erosion-deposition cycling in Aeolis.
  

• Kite, Williams, Lucas and Aharonson, Low palaeopressure of the martian atmosphere estimated from the size distribution of ancient craters, Nature Geoscience, 2014. [ More] [abstract] [Nature News] [Wired] [BBC News]
  
  The decay of the martian atmosphere - which is dominated by carbon dioxide - is a component of the long-term environmental change on Mars from a climate that once allowed rivers to flow to the cold and dry conditions of today. The minimum size of craters serves as a proxy for palaeopressure of planetary atmospheres, because thinner atmospheres permit smaller objects to reach the surface at high velocities and form craters. The Aeolis Dorsa region near Gale crater on Mars contains a high density of preserved ancient craters interbedded with river deposits and thus can provide constraints on atmospheric density at the time of fluvial activity. Here we use high-resolution images and digital terrain models from the Mars Reconnaissance Orbiter to identify ancient craters in deposits in Aeolis Dorsa that date to about 3.6 Gyr ago and compare their size distribution with models of atmospheric filtering of impactors. We obtain an upper limit of 0.9+/-0.1 bar for the martian atmospheric palaeopressure, rising to 1.9+/-0.2 bar if rimmed circular mesas - interpreted to be erosionally-resistant fills or floors of impact craters - are excluded. We assume target properties appropriate for desert alluvium: if sediment had rock-mass strength similar to bedrock at the time of impact, the paleopressure upper limit increases by a factor of up to two. If Mars did not have a stable multibar atmosphere at the time that the rivers were flowing - as suggested by our results - then a warm and wet CO2/H2O greenhouse is ruled out, and long-term average temperatures were most likely below freezing.
  

• Kite, Lewis, Lamb, Newman, and Richardson, Growth and form of the mound in Gale Crater, Mars: Slope-wind enhanced erosion and transport, Geology, 2013. [ More] [pdf] [supplementary information] [Red Planet Report] [news coverage in Science] [news coverage in Nature]
  

  Ancient sediments provide archives of climate and habitability on Mars. Gale Crater, the landing site for the Mars Science Laboratory (MSL), hosts a 5 km high sedimentary mound. Hypotheses for mound formation include evaporitic, lacustrine, fluviodeltaic, and aeolian processes, but the origin and original extent of Gale's mound is unknown. Here we show new measurements of sedimentary strata within the mound that indicate ~3° outward dips oriented radially away from the mound center, inconsistent with the first three hypotheses. Moreover, although mounds are widely considered to be erosional remnants of a once crater-filling unit, we find that the Gale mound's current form is close to its maximal extent. Instead we propose that the mound's structure, stratigraphy, and current shape can be explained by growth in place near the center of the crater mediated by wind-topography feedbacks. Our model shows how sediment can initially accrete near the crater center far from crater-wall katabatic winds, until the increasing relief of the resulting mound generates mound-flank slope-winds strong enough to erode the mound. Our results indicate mound formation by airfall-dominated deposition with a limited role for lacustrine and fluvial activity, and potentially limited organic carbon preservation. Morphodynamic feedbacks between wind and topography are widely applicable to a range of sedimentary mounds and ice mounds across the Martian surface, and possibly other planets.
  

• Kite, Halevy, Kahre, Wolff, and Manga, Seasonal melting and the formation of sedimentary rocks on Mars, with predictions for the Gale Crater mound, Icarus, 2013. [ More] [pdf] [journal version] [astrobites] [Red Planet Report] [Planetary Society]
  

  A model for the formation and distribution of sedimentary rocks on Mars is proposed. In this model (ISEE-Mars), the rate--limiting step is supply of liquid water from seasonal melting of snow or ice. The model is run for a 10^2 mbar pure CO2 atmosphere, dusty snow, and solar luminosity reduced by 23%. For these conditions snow melts only near the equator, when obliquity and eccentricity are high, and when perihelion occurs near equinox. These requirements for melting are satisfied by 0.01-20% of the probability distribution of Mars' past spin-orbit parameters. This fraction is small, consistent with the geologic record of metastable surface liquid water acting as a "wet-pass filter" of Mars climate history, only recording orbital conditions that permitted surface liquid water. Total melt production is sufficient to account for observed aqueous alteration of the sedimentary rocks. The pattern of seasonal snowmelt is integrated over all spin-orbit parameters and compared to the observed distribution of sedimentary rocks. The global distribution of snowmelt has maxima in Valles Marineris, Meridiani Planum and Gale Crater. These correspond to maxima in the sedimentary-rock distribution. Higher pressures and especially higher temperatures lead to melting over a broader range of spin-orbit parameters. The pattern of sedimentary rocks on Mars is most consistent with a model Mars paleoclimate that only rarely produced enough meltwater to precipitate aqueous cements (sulfates, carbonates, phyllosilicates and silica) and indurate sediment. This is consistent with observations suggesting that surface aqueous alteration on Mars was brief and at low water/rock ratio. The results suggest intermittency of snowmelt and long globally-dry intervals, unfavorable for past life on Mars. This model makes testable predictions for the Mars Science Laboratory Curiosity rover at Gale Crater's mound (Mount Sharp, Aeolis Mons). Gale Crater's mound is predicted to be a hemispheric maximum for snowmelt on Mars.
  

• Allen, ten Kate, Cody, Kite, Willacy, and Weibel, Cosmic pollution on Mars, submitted. [ More]

  In the same way as Earth, Mars is regularly bombarded by particles derived from asteroids and comets that contain organic material. Due to the Martian thin atmosphere, much of this organic material reaches the Martian surface relatively unprocessed. We have modeled the accumulation of this exogenous organic material - cosmic pollution in effect - on the surface and its burial beneath the surface, taking into account decomposition due to ultraviolet radiation and galactic cosmic rays. The computed abundance of exogenous organic material is a few parts per billion to a few parts per million by mass at the surface and for many meters beneath the surface. This material is predominantly insoluble polymeric organics. At these abundances, the meteoritic organics should be detectable by present and future landed experiments. As such, this amount of cosmic pollution can confound current and future plans to detect exogenous organic material that might be evidence for extinct or extant Martian life.
  

• Kite, Lucas, and Fassett, Pacing Early Mars river activity: Embedded craters in the Aeolis Dorsa region imply river activity spanned ≥(1-20) Myr, Icarus, 2013. [ More] [pdf] [supplementary table]
  

  The impactor flux early in Mars history was much higher than today, so sedimentary sequences include many buried craters. In combination with models for the impactor flux, observations of the number of buried craters can constrain sedimentation rates. Using the frequency of crater-river interactions, we find net sedimentation rate ≤ ~20-300 μm/yr at Aeolis Dorsa. This sets a lower bound of 1-15 Myr on the total interval spanned by fluvial activity around the Noachian-Hesperian transition. We predict that Gale Crater's mound (Aeolis Mons) took at least 10-100 Myr to accumulate, which is testable by the Mars Science Laboratory.

• Šrámek, McDonough, Kite, Lekić, Dye, and Zhong, Geophysical and geochemical constraints on geoneutrino fluxes from Earth's mantle, Earth and Planetary Science Letters, 2013. [ More] [pdf] [supplementary information] ["Research Highlight" in Nature]
  
  Knowledge of the amount and distribution of radiogenic heating in the mantle is crucial for understanding the dynamics of the Earth, including its thermal evolution, the style and planform of mantle convection, and the energetics of the core. Although the flux of heat from the surface of the planet is robustly estimated, the contributions of radiogenic heating and secular cooling remain poorly defined. Constraining the amount of heat-producing elements in the Earth will provide clues to understanding nebula condensation and planetary formation processes in early Solar System. Mantle radioactivity supplies power for mantle convection and plate tectonics, but estimates of mantle radiogenic heat production vary by a factor of more than 20. Recent experimental results demonstrate the potential for direct assessment of mantle radioactivity through observations of geoneutrinos, which are emitted by naturally occurring radionuclides. Predictions of the geoneutrino signal from the mantle exist for several established estimates of mantle composition. Here we present novel analyses, illustrating surface variations of the mantle geoneutrino signal for models of the deep mantle structure, including those based on seismic tomography. These variations have measurable differences for some models, allowing new and meaningful constraints on the dynamics of the planet. An ocean based geoneutrino detector deployed at several strategic locations will be able to discriminate between competing compositional models of the bulk silicate Earth.

• Kite and Howard, Commentary: Let's send the DoE to Alpha Centauri, Physics Today, September 2013. [pdf]

• Rappaport, Levine, Chiang, El Mellah, Jenkins, Kalomeni, Kite, Kotson, Nelson, Rousseau-Nepton, and Tran, Possible disintegrating short-period Super-Mercury orbiting KIC 12557548, Astrophysical Journal, 2012. [ More] [pdf] [htm]
  

  We report on the discovery of stellar occultations, observed with Kepler, which recur periodically at 15.685 hr intervals, but which vary in depth from a maximum of 1.3% to a minimum that can be less than 0.2%. The star that is apparently being occulted is KIC 12557548, a V = 16 mag K dwarf with T_eff = 4400 K. The out-of-occultation behavior shows no evidence for ellipsoidal light variations, indicating that the mass of the orbiting object is less than ~3 MJ (for an orbital period of 15.7 hr). Because the eclipse depths are highly variable, they cannot be due solely to transits of a single planet with a fixed size. We discuss but dismiss a scenario involving a binary giant planet whose mutual orbit plane precesses, bringing one of the planets into and out of a grazing transit. This scenario seems ruled out by the dynamical instability that would result from such a configuration. We also briefly consider an eclipsing binary, possibly containing an accretion disk, that either orbits KIC 12557548 in a hierarchical triple configuration or is nearby on the sky, but we find such a scenario inadequate to reproduce the observations. The much more likely explanation - but one which still requires more quantitative development - involves macroscopic particles escaping the atmosphere of a slowly disintegrating planet not much larger than Mercury in size. The particles could take the form of micron-sized pyroxene or aluminum oxide dust grains. The planetary surface is hot enough to sublimate and create a high-Z atmosphere; this atmosphere may be loaded with dust via cloud condensation or explosive volcanism. Atmospheric gas escapes the planet via a Parker-type thermal wind, dragging dust grains with it. We infer a mass-loss rate from the observations of order 1 Earth mass / Gyr , with a dust-to-gas ratio possibly of order unity. For our fiducial 0.1 M_Earth planet (twice the mass of Mercury), the evaporation timescale may be ~0.2 Gyr. Smaller mass planets are disfavored because they evaporate still more quickly, as are larger mass planets because they have surface gravities too strong to sustain outflows with the requisite mass-loss rates. The occultation profile evinces an ingress-egress asymmetry that could reflect a comet-like dust tail trailing the planet; we present simulations of such a tail.
  

• Kite, Gaidos and Manga, Climate instability on tidally locked exoplanets, Astrophysical Journal, 2011. [ More] [pdf] [htm] [Space.com]
  

  Feedbacks that can destabilize the climates of synchronously rotating rocky planets may arise on planets with strong day-night surface temperature contrasts. Earth-like habitable planets maintain stable surface liquid water over geologic time. This requires equilibrium between the temperature-dependent rate of greenhouse-gas consumption by weathering, and greenhouse-gas resupply by other processes. Detected small-radius exoplanets, and anticipated M-dwarf habitable-zone rocky planets, are expected to be in synchronous rotation (tidally locked). In this paper, we investigate two hypothetical feedbacks that can destabilize climate on planets in synchronous rotation. (1) If small changes in pressure alter the temperature distribution across a planetfls surface such that the weathering rate goes up when the pressure goes down, a runaway positive feedback occurs involving increasing weathering rate near the substellar point, decreasing pressure, and increasing substellar surface temperature. We call this feedback enhanced substellar weathering instability (ESWI). (2) When decreases in pressure increase the fraction of surface area above the melting point (through reduced advective cooling of the substellar point), and the corresponding increase in volume of liquid causes net dissolution of the atmosphere, a further decrease in pressure will occur. This substellar dissolution feedback can also cause a runaway climate shift. We use an idealized energy balance model to map out the conditions under which these instabilities may occur. In this simplified model, the weathering runaway can shrink the habitable zone and cause geologically rapid 10^3 - fold atmospheric pressure shifts within the habitable zone. Mars may have undergone a weathering runaway in the past. Substellar dissolution is usually a negative feedback or weak positive feedback on changes in atmospheric pressure. It can only cause runaway changes for small, deep oceans and highly soluble atmospheric gases. Both instabilities are suppressed if the atmosphere has a high radiative efficiency. Our results are most relevant for atmospheres that are thin, have low greenhouse-gas radiative efficiency, and have a principal greenhouse gas that is also the main constituent of the atmosphere. ESWI also requires land near the substellar point, and tectonic resurfacing (volcanism, mountain-building) is needed for large jumps in pressure. These results identify a new pathway by which habitable-zone planets can undergo rapid climate shifts and become uninhabitable.

• Mangold, Kite, Kleinhans, Newsom, Ansan, Hauber, Kraal, Quantin-Nataf and Tanaka, The origin and timing of fluvial activity at Eberswalde Crater, Mars, Icarus, 2012. [ More] [pdf]
  

  The fan deposit in Eberswalde crater has been interpreted as strong evidence for sustained liquid water on early Mars with a paleolake formed during the Noachian period (>3.7 Gy). This location became a key region for understanding the Mars paleo-environment. Eberswalde crater is located 50 km north of the rim of the 150 km diameter crater Holden. Stratigraphic relationships and chronology obtained using recent Mars Express High Resolution Stereo Camera and Mars Reconnaissance Orbiter Context Camera images show that Eberswalde fluvial activity crosscuts Holden ejecta and thus postdates Holden crater, whose formation age is estimated from crater counts as Late Hesperian (3.5 Gy, depending on models). fluvial modeling shows that short term activity (over several years to hundreds of years) involving dense flows (with sediment:water ratio between 0.01 and 0.3) may be as good an explanation of the fluvial landforms as dilute flow over longer durations. Modeling of the thermal effect of the Holden impact in the Eberswalde watershed is used to evaluate its potential role in aqueous activity. The relative timing of the Holden impact and Eberswalde's fan is a constraint for future studies about the origin of these landforms. Holden ejecta form a weak and porous substrate, which may be easy to erode by fluvial incision. In a cold climate scenario, impact heating could have produced runoff by melting snow or ground ice. Any attempt to model fluvial activity at Eberswalde should take into account that it may have formed as late as in the Late Hesperian, after the great majority of valley network formation and aqueous mineralization on Mars. This suggests that hypotheses for fan formation at Eberswalde by transient and/or localized processes (i.e. impact, volcanism, unusual orbital forcing) should be considered on a par with globally warmer climate.

• Kite, Michaels, Rafkin, Dietrich, and Manga, Chaos terrain, storms, and past climate on Mars, JGR-Planets, 2011.
  [ More] [pdf] [dynamic article] [Red Planet Report] ["Research Highlight" in Nature Geoscience]
  

  We model the atmospheric response to a chaos-forming event at Juventae Chasma, north of Valles Marineris, Mars, using the Mars Regional Atmospheric Modeling System (MRAMS). Interactions between lake-driven convergence, topography, and the regional wind field steer lake-induced precipitation to the southwest. Mean snowfall reaches a maximum of 0.9 mm/h water equivalent (peak snowfall 1.7 mm/h water equivalent) on the SW rim of the chasm. More than 80% of vapor released by the lake is trapped in or next to the lake as snow. Radiative effects of the thick cloud cover raise mean plateau surface temperature by up to 18 K locally. We find that the area of maximum modeled precipitation corresponds to the mapped Juventae plateau channel networks. At Echus Chasma, modeled precipitation maxima also correspond to mapped plateau channel networks. This is consistent with the earlier suggestion that Valles Marineris plateau layered deposits and interbedded channel networks result from localized precipitation. However, snowpack thermal modeling shows temperatures below freezing for the 12 mbar CO2 atmosphere used in our MRAMS simulations. This is true even for the most favorable orbital conditions, and whether or not the greenhouse effect of the lake storm is included. Moderately higher CO2 pressures, or non-CO2 greenhouse forcing, is very likely required for melting and plateau channel network formation under a faint young Sun. Required warming is ≤10 K: global temperatures need not be higher than today. In these localized precipitation scenarios, the rest of the planet remains dry.

• Kite and Lekić, Feasibility of mantle radiogenic power determination with geoneutrinos, in revision. [ More] [submitted ms]
  
  Geoneutrinos (gν) offer a new probe of deep Earth structure, and hold the potential for directly measuring Earth's mantle Urey number. Here, we use a 3D model of antineutrino emission from candidate structures to assess the feasibility of gν-based characterization of seismically-imaged lower mantle features. We focus on distinct classes of structures: (1) large, low shear velocity provinces (LLSVPs or superplumes) that may be compositionally distinct from the rest of the mantle; (2) ultra-low velocity zones (ULVZs) at the base of the mantle that may be partially molten. Both superplumes and ULVZs have been proposed to be reservoirs of radiogenic and other incompatible elements, as a means of resolving geochemical and heatflow paradoxes, yet determining their composition is difficult with only seismic constraints. We find that a sea-transportable gν detector could place constraints on the radiogenic power of the superplumes and on the total radiogenic-element budget of the mantle, provided that measurements are made from more than one site and that the radiogenic-element concentration of the continental crust is adequately measured by today's land-based gν detectors. However, constraining the radiogenic power of the ULVZs is more difficult, probably requiring a directional detector. A directional detector could also quickly determine the power density of the superplumes from a single oceanic location. We show that gν signal emanating from excess radiogenics within the LLSVPs and/or ULVZs can strongly bias mantle Urey number constraints from an arbitrarily situated non-directional detector, and map out locations that minimize this source of uncertainty. Conversely, locations at which gν counts are highly sensitive to the distribution of lower mantle radiogenics have the potential for constraining the relative radiogenic power of LLSVPs versus ULVZs and thus would offer a unique constraint on their composition.

• Manga, Patel, Dufek and Kite, Wet surface and dense atmosphere on early Mars inferred from the bomb sag at Home Plate, Mars, Geophysical Research Letters, 2011. [ More] [pdf] [htm]
  

  We use the Mars Exploration Rover Spirit observation of a bomb sag produced by an explosive volcanic eruption to infer the atmospheric density at the time of eruption. We performed analogue experiments to determine the relationship between the wetness of the substrate and the velocity and density of impacting clasts and 1) the formation (or not) of bomb sags, 2) the morphology of the impact crater, and 3) the penetration depth of the clast. The downward deflection of beds seen on Mars is consistent with watersaturated sediment in the laboratory experiments. Collision angles <20 degrees from vertical are needed to produce bomb sags. From the experiments we infer an impact velocity up to 40 m/s, lower than ejection velocities during phreatic and phreatomagmatic eruptions on Earth. If this velocity represents the terminal subaerial impact velocity, atmospheric density exceeded 0.4 kg/m^3 at the time of eruption, much higher than at present.

• Kite, Rafkin, Michaels, Manga, and Dietrich, Localized precipitation and runoff on Mars, JGR-Planets, 2011. [ More] [pdf] [dynamic article] [as reported by "New Scientist"]
  
  We use the Mars Regional Atmospheric Modeling System (MRAMS) to simulate lake storms on Mars, finding that intense localized precipitation will occur for lake size ≥10^3 km^2 . Mars has a low-density atmosphere, so deep convection can be triggered by small amounts of latent heat release. In our reference simulation, the buoyant plume lifts vapor above condensation level, forming a 20 km high optically thick cloud. Ice grains grow to 200μm radius and fall near (or in) the lake at mean rates up to 1.5 mm h^-1 water equivalent (maximum rates up to 6 mm h^-1 water equivalent). Because atmospheric temperatures outside the surface layer are always well below 273 K, supersaturation and condensation begin at low altitudes above lakes on Mars. In contrast to Earth lake-effect storms, lake storms on Mars involve continuous precipitation, and their vertical velocities and plume heights exceed those of tropical thunderstorms on Earth. For lake sizes 10^2.5 to 10^3.5 km, plume vertical velocity scales linearly with lake area. Convection does not reach above the planetary boundary layer for lakes <10^3 km^2 or for atmospheric pressure >O(10^2) mbar. Instead, vapor is advected downwind with little cloud formation. Precipitation occurs as snow, and the daytime radiative forcing at the land surface due to plume vapor and storm clouds is too small to melt snow directly (<+10 W m^-2). However, if orbital conditions are favorable, then the snow may be seasonally unstable to melting and produce runoff to form channels. We calculate the probability of melting by running thermal models over all possible orbital conditions and weighting their outcomes by probabilities given by long-term integrations of the chaotic diffusion of solar system orbital elements. With this approach, we determine that for an equatorial vapor source, sunlight 15% fainter than at present and snowpack with albedo 0.28 (0.35), melting may occur with 4% (0.1%) probability. This rises to 56% (12%) if the ancient greenhouse effect was modestly (6 K) greater than today.

• Kite, Manga and Gaidos, Geodynamics and rate of volcanism on massive Earth-like planets, Astrophysical Journal, 2009. [ More] [pdf] [htm]
  
  We provide estimates of volcanism versus time for planets with Earth-like composition and masses 0.25-25 M_Earth, as a step toward predicting atmospheric mass on extrasolar rocky planets. Volcanism requires melting of the silicate mantle. We use a thermal evolution model, calibrated against Earth, in combination with standard melting models, to explore the dependence of convection-driven decompression mantle melting on planet mass. We show that (1) volcanism is likely to proceed on massive planets with plate tectonics over the main-sequence lifetime of the parent star; (2) crustal thickness (and melting rate normalized to planet mass) is weakly dependent on planet mass; (3) stagnant lid planets live fast (they have higher rates of melting than their plate tectonic counterparts early in their thermal evolution), but die young (melting shuts down after a few Gyr); (4) plate tectonics may not operate on high-mass planets because of the production of buoyant crust which is difficult to subduct; and (5) melting is necessary but insufficient for efficient volcanic degassing - volatiles partition into the earliest, deepest melts, which may be denser than the residue and sink to the base of the mantle on young, massive planets. Magma must also crystallize at or near the surface, and the pressure of overlying volatiles must be fairly low, if volatiles are to reach the surface. If volcanism is detected in the 10 Gyr old Tau Ceti system, and tidal forcing can be shown to be weak, this would be evidence for plate tectonics.

• Chiang, Kite, Kalas, Graham and Clampin, Fomalhaut's debris disk and planet: Constraining the mass of Fomalhaut b using disk morphology, Astrophysical Journal, 2009. [ More] [pdf]
  
  Fomalhaut's debris disk is gravitationally shaped by a single interior planet. The model is simple, adaptable to other debris disks, and can be extended to accommodate multiple planets. If Fom b is the dominant perturber of the belt, then to produce the observed disk morphology it must have a mass M_pl < 3_MJ, an orbital semimajor axis a_pl > 101.5 AU, and an orbital eccentricity e_pl = 0.11-0.13. These conclusions are independent of Fom b's photometry. To not disrupt the disk, a greater mass for Fom b demands a smaller orbit farther removed from the disk; thus, future astrometric measurement of Fom b's orbit, combined with our model of planet-disk interaction, can be used to determine the mass more precisely. The inner edge of the debris disk at a ~ 133 AU lies at the periphery of Fom b's chaotic zone, and the mean disk eccentricity of e ~ 0.11 is secularly forced by the planet, supporting predictions made prior to the discovery of Fom b. However, previous mass constraints based on disk morphology rely on several oversimplifications. We explain why our constraint is more reliable. It is based on a global model of the disk that is not restricted to the planet's chaotic zone boundary. Moreover, we screen disk parent bodies for dynamical stability over the system age of ~ 100 Myr, and model them separately from their dust grain progeny; the latter's orbits are strongly affected by radiation pressure and their lifetimes are limited to ~ 0.1 Myr by destructive grain-grain collisions. The single planet model predicts that planet and disk orbits be apsidally aligned. Fomalhaut b's nominal space velocity does not bear this out, but the astrometric uncertainties may be large. If the apsidal misalignment proves real, our calculated upper mass limit of 3 MJ still holds. If the orbits are aligned, our model predicts M_pl = 0.5 MJ , a_pl = 115 AU, and e_pl = 0.12. Parent bodies are evacuated from mean-motion resonances with Fom b; these empty resonances are akin to the Kirkwood gaps opened by Jupiter. The belt contains at least 3 M_Earth of solids that are grinding down to dust, their velocity dispersions stirred so strongly by Fom b that collisions are destructive. Such a large mass in solids is consistent with Fom b having formed in situ.

• Kite, Matsuyama, Manga, Perron and Mitrovica, True polar wander driven by late-stage volcanism and the distribution of paleopolar deposits on Mars, Earth and Planetary Science Letters, 2009. [ More] [pdf] [som]
  
  The areal centroids of the youngest polar deposits on Mars are offset from those of adjacent paleopolar deposits by 5-10°. We test the hypothesis that the offset is the result of True Polar Wander (TPW), the motion of the solid surface with respect to the spin axis, caused by a mass redistribution within or on the surface of Mars. In particular, we consider the possibility that TPW is driven by late-stage volcanism during the Late Hesperian to Amazonian. There is observational and qualitative support for this hypothesis: in both north and south, observed offsets lie close to a great circle 90° from Tharsis, as expected for polar wander after Tharsis formed. We calculate the magnitude and direction of TPW produced by mapped late-stage lavas for a range of lithospheric thicknesses, lava thicknesses, eruption histories, and prior polar wander events. We find that if Tharsis formed close to the equator, the stabilizing effect of a fossil rotational bulge located close to the equator leads to predicted TPW of 2°, too small to account for observed offsets. If, however, Tharsis formed far from the equator, late-stage TPW driven by low-latitude, late-stage volcanism would be 6- 33°, similar to that inferred from the location of paleopolar deposits. A volume of 4.4 ± 1.3 x 10^19 kg of young erupted lava can account for the offset of the Dorsa Argentea Formation from the present-day south rotation pole. This volume is consistent with prior mapping-based estimates and would imply a mass release of CO2 by volcanic degassing similar to that in the atmosphere at the present time. The South Polar Layered Deposits are offset from the present rotation pole in a direction that is opposite to the other paleopolar deposits. This can be explained by either a sequential eruption of late-stage lavas, or an additional contribution from a plume beneath Elysium. We predict that significant volcanic activity occurred during the time interval represented by the Basal Unit/Planum Boreum unconformity; Planum Boreum postdates the Promethei Lingula Lobe; and that the north polar deposits span a substantial fraction of Solar System history. If the additional contribution to TPW from plumes is small, then we would also predict that Tharsis Montes Formation postdates the Promethei Lingula Lobe of the South Polar Layered Deposits. We conclude with a list of observational tests of the TPW hypothesis.

• Kalas, Graham, Chiang, Fitzgerald, Clampin, Kite, Stapelfeldt, Marois and Krist, Optical images of an exosolar planet 25 light years from Earth, Science, 2008 [ More] [abstract] [#2 Breakthrough of the Year]
  
  Fomalhaut, a bright star 7.7 parsecs (25 light-years) from Earth, harbors a belt of cold dust with a structure consistent with gravitational sculpting by an orbiting planet. Here, we present optical observations of an exoplanet candidate, Fomalhaut b. Fomalhaut b lies about 119 astronomical units (AU) from the star and 18 AU of the dust belt, matching predictions of its location. Hubble Space Telescope observations separated by 1.73 years reveal counterclockwise orbital motion. Dynamical models of the interaction between the planet and the belt indicate that the planet's mass is at most three times that of Jupiter; a higher mass would lead to gravitational disruption of the belt, matching predictions of its location. The flux detected at 0.8 μm is also consistent with that of a planet with mass no greater than a few times that of Jupiter. The brightness at 0.6 μm and the lack of detection at longer wavelengths suggest that the detected flux may include starlight reflected off a circumplanetary disk, with dimension comparable to the orbits of the Galilean satellites. We also observe variability of unknown origin at 0.6 μm.

• Kite and Hindmarsh, Did ice streams shape the largest channels on Mars?, Geophysical Research Letters, 2007. [ More] [pdf]
  
  The largest channels on Mars are the Northwestern Slope Valleys (NSVs) of Tharsis, which have previously been interpreted as the probable erosional trace of catastrophic flooding. It is argued here that ice-streaming within ancient ice sheets emplaced by atmospheric precipitation at high mean obliquity may instead account for these channels, explaining similarities between the region and terrestrial Pleistocene subglacial landscapes. An ice-sheet model shows extensive basal melting in and only in the NSV region, and ice streams which have significant erosive power.

Research interests

Mars. Once you start thinking about the Early Mars climate problem it is hard to think about anything else. My attack is to relate pre-modern, but still relatively young, geomorphic features to local inputs of water and energy. The hope is to provide insight into ancient features whose geological context is less clear.

Earth history & geobiology. Long-term climate stability. Geoneutrinos as a probe of deep-time thermal history. Planetary thermostats and planetary habitability.

Extrasolar planets. Thermal evolution of rocky planets; direct detections and phase curves. Radius anomalies. Substellar magma ponds. Climate feedbacks on tidally-locked planets.