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The initiation mechanism of Earth’s plate tectonic cooling system remains uncertain. A growing consensus suggests that multi-plate tectonics was preceded by cooling through a single-plate lithosphere, but models for how this lithosphere was first broken into plates have not converged on a mechanism or a typical early plate scale. A commonality among prior efforts is the use of continuum mechanics approximations to evaluate this solid mechanics problem. Here we use 3D spherical shell models to demonstrate a self-organized fracture mechanism analogous to thermal expansion-driven lithospheric uplift, in which globe-spanning rifting occurs as a consequence of horizontal extension. Resultant fracture spacing is a function of lithospheric thickness and rheology, wherein geometrically-regular, polygonal-shaped tessellation is an energetically favored solution because it minimizes total crack length. Therefore, warming of the early lithosphere itself—as anticipated by previous studies—should lead to failure, propagating fractures, and the conditions necessary for the onset of multi-plate tectonics., How Earth’s lithosphere first divided into tectonic plates remains uncertain. Here, the authors use 3D spherical shell models to demonstrate that anticipated warming of the early lithosphere should lead to thermal expansion and the initiation of a global network of rifts, dividing the lithosphere into tectonic plates.

The boundary between the solar wind (SW) and the Earth's magnetosphere, the magnetopause (MP), is highly dynamic. Its location and shape depend on SW dynamic pressure and interplanetary magnetic fi...

Thermal history models, that have been used to understand the geological history of Earth, are now being coupled to climate models to map conditions that allow planets to maintain surface water over geologic time - a criteria considered crucial for life. However, the lack of intrinsic uncertainty assessment has blurred guidelines for how thermal history models can be used toward this end. A model, as a representation of something real, is not expected to be complete. Unmodeled effects are assumed to be small enough that the model maintains utility for the issue(s) it was designed to address. The degree to which this holds depends on how unmodeled factors affect the certainty of model predictions. We quantify this intrinsic uncertainty for several parameterized thermal history models (a widely used subclass of planetary models). Single perturbation analysis is used to determine the reactance time of different models. This provides a metric for how long it takes low amplitude, unmodeled effects to decay or grow. Reactance time is shown to scale inversely with the strength of the dominant feedback (negative or positive) within a model. A perturbed physics analysis is then used to determine uncertainty shadows for model outputs. This provides probability distributions for model predictions and tests the structural stability of a model. That is, do model predictions remain qualitatively similar, and within assumed model limits, in the face of intrinsic uncertainty. Once intrinsic uncertainty is accounted for, model outputs/predictions and comparisons to observational data should be treated in a probabilistic way., 20 pages, 10 figures

The recent discoveries of telluric exoplanets in the habitable zone of different stars have led to questioning the nature of their atmosphere, which is required to determine their habitability. Atmospheric escape is one of the challenging problems to be solved: simply adapting what is currently observed in the solar system is doomed to fail due to the large variations in the conditions encountered around other stars. A better strategy is to review the different processes that shaped planetary atmospheres and to evaluate their importance depending upon the stellar conditions. This approach allowed us to show that processes like ion-pickup were a more important way to lose atmosphere at Mars in the past. We reviewed the different escape mechanisms and their magnitude in function of the different conditions. This led us to discover discrepancies in the current literature concerning problems such as the Xenon paradox or the importance of a magnetic field in protecting an atmosphere.This shows that one should be very careful before claiming the presence of an atmosphere on planets in the habitable zone of their M-dwarfs: new criteria such as the Alfven surface location with respect to the planet should be taken into account a-priori.Overall, the habitability of a planet should not be claimed only on by its location in the habitable zone but also after careful analysis of the interaction between its atmosphere and its parent star [Gronoff et al. 2020]. Gronoff, G., Arras, P., Baraka, S., Bell, J. M., Cessateur, G., Cohen, O., et al. ( 2020). Atmospheric Escape Processes and Planetary Atmospheric Evolution. Journal of Geophysical Research: Space Physics, 125, e2019JA027639. https://doi.org/10.1029/2019JA027639

Coastal regions have historically represented a significant challenge for air quality investigations because of water–land boundary transition characteristics and a paucity of measurements available over water. Prior studies have identified the formation of high levels of ozone over water bodies, such as the Chesapeake Bay, that can potentially recirculate back over land to significantly impact populated areas. Earth-observing satellites and forecast models face challenges in capturing the coastal transition zone where small-scale meteorological dynamics are complex and large changes in pollutants can occur on very short spatial and temporal scales. An observation strategy is presented to synchronously measure pollutants “over land” and “over water” to provide a more complete picture of chemical gradients across coastal boundaries for both the needs of state and local environmental management and new remote sensing platforms. Intensive vertical profile information from ozone lidar systems and ozonesondes, obtained at two main sites, one over land and the other over water, are complemented by remote sensing and in situ observations of air quality from ground-based, airborne (both personned and unpersonned), and shipborne platforms. These observations, coupled with reliable chemical transport simulations, such as the National Oceanic and Atmospheric Administration (NOAA) National Air Quality Forecast Capability (NAQFC), are expected to lead to a more fully characterized and complete land–water interaction observing system that can be used to assess future geostationary air quality instruments, such as the National Aeronautics and Space Administration (NASA) Tropospheric Emissions: Monitoring of Pollution (TEMPO), and current low-Earth-orbiting satellites, such as the European Space Agency’s Sentinel-5 Precursor (S5-P) with its Tropospheric Monitoring Instrument (TROPOMI).

The habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, chemical and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our Solar system: it is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H-dominated thermosphere for astrophysicists, or the Sun-like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth's atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so-called Xenon paradox.

Terrestrial planet mantles cannot transport the very high heat production in their early stages through subsolidus convection and instead produce voluminous melt that makes its way to the surface to transport the heat. This heat-pipe mode of heat transport implies a very different tectonics than either the rigid or mobile-lid tectonics driven by subsolidus convection. Although similar to rigid-lid convection in that there is relatively little horizontal motion, heat-pipe lithospheres are by no means stagnant. Vertical transport through the continuous eruption of new material on the surface reaches rates of several mm/year (with significant spatial and temporal variations). This strongly impacts the shape of the geotherm, producing a cold and strong lid (despite the high heat flow). In addition, this vertical transport produces global compressional stresses as old surfaces are buried and forced downward to smaller radii. The horizontal variations in burial rates will lead to stress concentrations and ultimately plastic failure and thrusting (see Io’s numerous tectonic uplifts as an example). The transition from the advectively dominated heat-pipe lithosphere to a thin conductive lithosphere reverses this process, resulting in a period of global extension (again with large horizontal variations) as global volcanism wanes. An additional aspect of vertical transport in the heat-pipe lithosphere is the cycling of water and other volatiles into the lithosphere and mantle as surface materials are buried. This material is available for metamorphic reactions and will interact with rocks at the wet solidus, producing evolved rock compositions and volatile by-products (e.g. methane) that will contribute to the early atmospheres of these planets. Evidence of vertical transport in ancient Earth rocks has generally been attributed to subduction but heat-pipe advection provides a more global opportunity for such cycling.

Fifty years after the main discovery period for plate tectonics, we still lack a consensus understanding of a critical question: how did the plate tectonic system initiate? For the period before initiation of plate tectonics, models increasingly call upon a stagnant lid (i.e., a single-plate lithosphere) atop a mantle which was hotter by a few hundred degrees than the present mantle. How was this lid first broken into plates? Various hypotheses suggest that the strength of the lid was overcome by (a) mantle convective forcing, potentially along locally pre-weakened zones, (b) lithospheric gravitational instabilities between oceanic lithosphere and either adjacent oceanic plateau lithosphere or adjacent overthickened (i.e., gravitationally collapsing) continental lithosphere, or (c) one or more large bolides. These models have not converged on a mechanism or a typical early plate scale. Here, we use a new solid-mechanics based approach to the problem of the origin of plate tectonics and the processes by which plate boundaries are initiated. Specifically, we employ 3D spherical shell models of a brittle lithosphere via the three-dimensional finite element code RFPA (Rock Failure Process Analysis code). The models are subjected to quasi-static, slowly increasing interior pressure in a displacement-controlled manner (e.g., induced by gradual thermal expansion). Brittle failure is implemented through a strength criterion representing a stress limit at which the strength drops and fracture occurs. To account for local randomness, each element is assigned a failure threshold obtained from a Weibull probability distribution which contains a parameter describing the degree of material homogeneity. Globe-spanning rifting occurs as a consequence of horizontal extension. Resultant fracture spacing is a function of lithospheric thickness and rheology, such that geometrically-regular, polygonal-shaped tessellation is energetically favored because it minimizes total crack length. Therefore, anticipated warming of the early lithosphere itself (as lithospheric chilling from downwards advection due to rapid volcanism wanes) should lead to failure, propagating fractures, and the conditions necessary for the onset of multi-plate tectonics.

Since their formation in the protosolar nebula some ∼4.5 billion years ago, comets are in storage in cold distant regions of the solar system, the Kuiper Belt/scattered disk or Oort Cloud. Therefore, they have been considered as mostly unaltered samples of the protosolar nebula. However, a significant dose of energy is deposited by galactic cosmic rays (GCRs) into the outermost tens of meters of cometary nuclei during their stay in the Oort Cloud or Kuiper Belt. We investigate the impact of energy deposition by GCRs on cometary nuclei. We use experimental results from laboratory experiments and the energy deposition by GCRs estimated by Gronoff et al. (2020), to discuss the depth down to which the cometary nucleus is altered by GCRs. We show that GCRs do not significantly change the isotopic composition of cometary material but modify the chemical composition and the ice structure in the outer layers of the nucleus, which cannot be considered as pristine solar nebula material. We discuss the effect of the collisional history of comets on the distribution of processed material inside the nucleus and its implication on the observation of comets.

Observations of the surfaces of all terrestrial bodies other than Earth reveal remarkable but unexplained similarities: endogenic resurfacing is dominated by plains-forming volcanism with few identifiable centers, magma compositions are highly magnesian (mafic to ultra-mafic), tectonic structures are dominantly contractional, and ancient topographic and gravity anomalies are preserved to the present. Here we show that cooling via volcanic heat pipes may explain these observations and provide a universal model of the way terrestrial bodies transition from a magma-ocean state into subsequent single-plate, stagnant-lid convection or plate tectonic phases. In the heat-pipe cooling mode, magma moves from a high melt-fraction asthenosphere through the lithosphere to erupt and cool at the surface via narrow channels. Despite high surface heat flow, the rapid volcanic resurfacing produces a thick, cold, and strong lithosphere which undergoes contractional strain forced by downward advection of the surface toward smaller radii. We hypothesize that heat-pipe cooling is the last significant endogenic resurfacing process experienced by most terrestrial bodies in the solar system, because subsequent stagnant-lid convection produces only weak tectonic deformation. Terrestrial exoplanets appreciably larger than Earth may remain in heat-pipe mode for much of the lifespan of a Sun-like star.

Interactions among tectonics, volcanism, and surface weathering are critical to the long-term climatic state of a terrestrial planet. Volcanism cycles greenhouse gasses into the atmosphere. Tectonics creates weatherable topography, and weathering reactions draw greenhouse gasses out of the atmosphere. Weathering depends on physical processes governed partly by surface temperature, which allows for the potential that climate-tectonic coupling can buffer the surface conditions of a planet in a manner that allows liquid water to exist over extended timescales (a condition that allows a planet to be habitable by life as we know it). We discuss modeling efforts to explore the level to which climate-tectonic coupling can or cannot regulate the surface temperature of a planet over geologic time. Thematically, we focus on how coupled climate-tectonic systems respond to the following: (1) changes in the mean pace of tectonics and associated variations in mantle melting and volcanism, (2) large-amplitude fluctuations about mean properties such as mantle temperature and surface plate velocities, and (3) changes in tectonic mode. We consider models that map the conditions under which plate tectonics can or cannot provide climate buffering as well as models that explore the potential that alternate tectonic modes can provide a level of climate buffering that allows liquid water to be present at a planet's surface over geological timescales. We also discuss the possibility that changes in the long-term climate state of a planet can feedback into the coupled system and initiate changes in tectonic mode.

We use a suite of 3-D numerical experiments to test and expand 2-D planar isoviscous scaling relationships of Moore (2008) for mixed heating convection in spherical geometry mantles over a range of Rayleigh numbers (Ra). The internal temperature scaling of Moore (2008), when modified to account for spherical geometry, matches our experimental results to a high degree of fit. The heat flux through the boundary layers scale as a linear combination of internal (Q) and basal heating, and the modified theory predictions match our experimental results. Our results indicate that boundary layer thickness and surface heat flux are not controlled by a local boundary layer stability condition (in agreement with the results of Moore (2008)) and are instead strongly influenced by boundary layer interactions. Subadiabatic mantle temperature gradients, in spherical 3-D, are well described by a vertical velocity scaling based on discrete drips as opposed to a scaling based on coherent sinking sheets, which was found to describe 2-D planar results. Root-mean-square (RMS) velocities are asymptotic for both low Q and high Q, with a region of rapid adjustment between asymptotes for moderate Q. RMS velocities are highest in the low Q asymptote and decrease as internal heating is applied. The scaling laws derived by Moore (2008), and extended here, are robust and highlight the importance of differing boundary layer processes acting over variable Q and moderate Ra.

Plate tectonics is a unique feature of Earth, and it plays a dominant role in transporting Earth's internally generated heat. It also governs the nature, shape, and the motion of the surface of Earth. The initiation of plate tectonics on Earth has been difficult to establish observationally, and modeling of the plate breaking process has not consistently accounted for the nature of the preplate tectonic Earth. We have performed numerical simulations of heat transport in the preplate tectonic Earth to understand the transition to plate tectonic behavior. This period of time is dominated by volcanic heat transport called the heat pipe mode of planetary cooling. These simulations of Earth's mantle include heat transport by melting and melt segregation (volcanism), Newtonian temperature-dependent viscosity, and internal heating. We show that when heat pipes are active, the lithosphere thickens and lithospheric isotherms are kept flat by the solidus. Both of these effects act to suppress plate tectonics. As volcanism wanes, conduction begins to control lithospheric thickness, and large slopes arise at the base of the lithosphere. This produces large lithospheric stress and focuses it on the thinner regions of the lithosphere resulting in plate breaking events.

Comets are small bodies thought to contain the most pristine material in the solar system. However, since their formation 4.5 Gy ago, they have been altered by different processes. While not exposed to much electromagnetic radiation, they experience intense particle radiation. Galactic cosmic rays and solar energetic particles have a broad spectrum of energies and interact with the cometary surface and subsurface; they are the main source of space weathering for a comet in the Kuiper Belt or in the Oort cloud; and also affect the ice prior to the comet agglomeration. While low energy particles interact only with the cometary surface, the most energetic ones deposit a significant amount of energy down to tens of meters. This interaction can modify the isotopic ratios in cometary ices and create secondary compounds through radiolysis, such as O2 and H2O2 (paper II: Maggiolo et al., 2020). In this paper, we model the energy deposition of energetic particles as a function of depth using a Geant4 application modified to account for the isotope creation process. We quantify the energy deposited in cometary nucleus by galactic cosmic rays and solar energetic particles. The consequences of the energy deposition on the isotopic and chemical composition of cometary ices and their implication on the interpretation of cometary observations, notably of 67P/Churyumov Gerasimenko by the ESA/Rosetta spacecraft, will be discussed in Paper II.

Remote sensing provides a unique perspective on our dynamic planet, tracking changes and revealing the course of complex interactions. Long term monitoring and targeted observation combine with modeling and mapping to provide increased awareness of hydro-meteorological and geological hazards. Disasters often follow hazards and the goal of NASA’s Disasters Program is to look at the earth as a highly coupled system to reduce risk and enable resilience. Remote sensing and geospatial science are used as tools to help answer critical questions that inform decisions. Data is not the same as information, nor does understanding of processes necessarily translate into decision support for disaster preparedness, response and recovery. Accordingly, NASA is engaging the scientific and decision-support communities to apply remote sensing, modeling, and related applications in Communities and Areas at Intensive Risk (CAIR).In 2017, NASA’s Applied Sciences Disasters Program hosted a regional workshop to explore these issues with particular focus on coastal Virginia and North Carolina. The workshop brought together partners in academia, emergency management, and scientists from NASA and partnering federal agencies to explore capabilities among the team that could improve understanding of the physical processes related to these hazards, their potential impact to changing communities, and to identify methodologies for supporting emergency response and risk mitigation. The resulting initiative, the mid-Atlantic CAIR project, demonstrates the ability to integrate satellite derived earth observations and physical models into actionable, trusted knowledge. Severe storms and associated storm surge, sea level rise, and land subsidence coupled with increasing populations and densely populated, aging critical infrastructure often leave coastal regions and their communities extremely vulnerable. The integration of observations and models allow for a comprehensive understanding of the compounding risk experienced in coastal regions and enables individuals in all positions make risk-informed decisions. This initiative uses a representative storm surge case as a baseline to produce flood inundation maps. These maps predict building level impacts at current day and for sea level rise (SLR) and subsidence scenarios of the future in order to inform critical decisions at both the tactical and strategic levels.To accomplish this analysis, the mid-Atlantic CAIR project brings together Federal research activities with academia to examine coastal hazards in multiple ways: 1) reanalysis of impacts from 2011 Hurricane Irene, using numerical weather modeling in combination with coastal surge and hydrodynamic, urban inundation modeling to evaluate combined impact scenarios considering SLR and subsidence, 2) remote sensing of flood extent from available optical imagery, 3) adding value to remotely sensed flood maps through depth predictions, and 4) examining coastal subsidence as measured through time-series analysis of synthetic aperture radar observations. Efforts and results are published via ArcGIS story maps to communicate neighborhoods and infrastructure most vulnerable to changing conditions. Story map features enable time-aware flood mapping using hydrodynamic models, photographic comparison of flooding following Hurricane Irene, as well as visualization of heightened risk in the future due to SLR and land subsidence.

We introduce a Bayesian method for guiding future directions for detection of life on exoplanets. We describe empirical and theoretical work necessary to place constraints on the relevant likelihoods, including those emerging from better understanding stellar environment, planetary climate and geophysics, geochemical cycling, the universalities of physics and chemistry, the contingencies of evolutionary history, the properties of life as an emergent complex system, and the mechanisms driving the emergence of life. We provide examples for how the Bayesian formalism could guide future search strategies, including determining observations to prioritize or deciding between targeted searches or larger lower resolution surveys to generate ensemble statistics and address how a Bayesian methodology could constrain the prior probability of life with or without a positive detection. Key Words: Exoplanets—Biosignatures—Life detection—Bayesian analysis. Astrobiology 18, 779–824., Table of Contents 1. Introduction 2. Setting the Stage: What Is Life? What Is a Biosignature? 3. Detecting Unknown Biology on Unknown Worlds: A Bayesian Framework 3.1. Habitability in the Bayesian framework for biosignatures 4. P(data|abiotic) 4.1. Stellar environment 4.2. Climate and geophysics 4.2.1. Coupled tectonic–climate models 4.2.2. Community GCM projects for generating ensemble statistics for P(data|abiotic) and P(data|life) 4.3. Geochemical environment 4.3.1. Anticipating the unexpected: statistical approaches to characterizing atmospheres of non-Earth-like worlds 5. P(data|life) 5.1. Black-box approaches to living processes 5.1.1. Type classification of Seager et al. (2013a) 5.1.1.1. Energy capture (type I) 5.1.1.2. Biomass capture (type II) 5.1.1.3. Other uses (type III) 5.1.1.4. Products of modification of gases (type IV) 5.1.2. Alternatives for type classification 5.1.2.1. Type I, energy capture 5.1.2.2. Type II, biomass capture 5.1.2.3. Type III, “other uses” 5.1.2.4. Type IV 5.1.3. When is it appropriate to deconstruct a black box? 5.2. Life as improbable chemistry 5.3. Life as an evolutionary process 5.3.1. Life as a coevolution with its planet: Earth as an example 5.3.2. Calculating conditional probabilities in biological evolution from past biogeochemical states 5.4. Insights from universal biology 5.4.1. Network biosignatures 5.4.2. Universal scaling laws, applicable to other worlds? 6. P(life) 6.1. P(emerge): constraining the probability of the origins of life 6.2. Biological innovations and the conditional probabilities for living processes 7. A Bayesian Framework Example: Detecting Atmospheric Oxygen 8. Tuning Search Strategies Based on the Bayesian Framework 9. Conclusions Acknowledgments Author Disclosure Statement References Abbreviations Used

We have produced the first series of spherical harmonic, numerical maps of the time-dependent surface perturbations in the Earth's magnetic field following the onset of substorms. Data from 124 ground magnetometer stations in the Northern Hemisphere at geomagnetic latitudes above 33° were used. Ground station data averaged over 5 min intervals covering 8 years (1998–2005) were used to construct pseudo auroral upper, auroral lower, and auroral electrojet (AU*, AL*, and AE*) indices. These indices were used to generate a list of substorms that extended from 1998 to 2005, through a combination of automated processing and visual checks. Events were sorted by interplanetary magnetic field (IMF) orientation (at the Advanced Composition Explorer (ACE) satellite), dipole tilt angle, and substorm magnitude. Within each category, the events were aligned on substorm onset. A spherical cap harmonic analysis was used to obtain a least error fit of the substorm disturbance patterns at 5 min intervals up to 90 min after onset. The fits obtained at onset time were subtracted from all subsequent fits, for each group of substorm events. Maps of the three vector components of the averaged magnetic perturbations were constructed to show the effects of substorm currents. These maps are produced for several specific ranges of values for the peak |AL*| index, IMF orientation, and dipole tilt angle. We demonstrate an influence of the dipole tilt angle on the response to substorms. Our results indicate that there are downward currents poleward and upward currents just equatorward of the peak in the substorms' westward electrojet.

As scientists, the terminology we choose influences our thinking as it carries our messages to colleagues and the public. In the face of pressure to turn science into clickbait, maintaining precision in the language we use is critical to dispel misinformation and, more broadly, to enable scientific progress.

The combination of the radio tracking of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft and Earth-based radar measurements of the planet's spin state gives three fundamental quantities for the determination of the interior structure of Mercury: mean density �� , moment of inertia C, and moment of inertia of the outer solid shell C m . This work focuses on the additional information that can be gained by a determination of the change in gravitational potential due to planetary tides, as parameterized by the tidal potential Love number k2. We investigate the tidal response for sets of interior models that are compatible with the available constraints (�� , C, and Cm). We show that the tidal response correlates with the size of the liquid core and the mean density of material below the outer solid shell and that it is affected by the rheology of the outer solid shell of the planet, which depends on its temperature and mineralogy. For a mantle grain size of 1 cm, we calculate that the tidal k2 of Mercury is in the range 0.45 to 0.52. Some of the current models for the interior structure of Mercury are compatible with the existence of a solid FeS layer at the top of the core. Such a layer, if present, would increase the tidal response of the planet.

– Larkman Nunatak (LAR) 06299 is a vesicular LL chondrite impact melt breccia that cooled rapidly (0.1–0.3 °C s−1) during crystallization. Ar-Ar data from the literature indicate that the impact event that formed this rock occurred approximately 1 Ga ago. About 30 vol% of the meteorite consists of a melt matrix containing faceted and intergrown mafic silicate grains (mainly 4–11 μm size olivine phenocrysts) partially to completely surrounded by 2–20 μm size patches of plagioclase. Suspended in the melt are 30–370 μm size ellipsoidal to spheroidal metal-sulfide nodules (several hundred per thin section), many connected to 8–600 μm size ellipsoidal to spheroidal vesicles. Most of the metal-sulfide nodules contain a large oblate metallic Fe-Ni bleb at one end of the nodule. For approximately 90% of the nodules, the metal blebs are aligned on the same side of the nodules; for approximately 80% of the nodules that are adjacent to vesicles, the vesicles are attached to the opposite end of the nodules from the oblate metal blebs. Most of the oblate metal blebs themselves are flattened in a direction perpendicular to the long axis of the nodule/vesicle. These features result from alignment in the gravitational field on the LL parent asteroid, making LAR 06299 the first known chondrite to indicate gravitational direction. Using reasonable estimates of the cooling rate, viscosity of the metal-sulfide melt, and asteroid density, as well as the observed sizes of constituent phases in LAR 06299, we obtain a lower limit of approximately 1.5 km for the radius of the LAR 06299 parent body. The body was probably substantially larger.

From a scientist’s viewpoint a web site is one tool used to conduct research. From an artist’s viewpoint web sites are a form of visual composition. From a developer’s point of view a web site is a type of application. While web sites are a relatively new medium with a particular set of constraints, they do adhere to the same basic design principles that apply to other art forms. These design principles are the basic assumptions that affect the arrangement of elements within a composition. A successful design uses the principles and elements to achieve a visual goal in the composition. A web site designed for scientists has unique properties which are not shared by many other types of web sites. These properties influence the overall visual design of the web sites. Recently at the Institute of Geophysics and Planetary Physics at UCLA undertook a re-design of a number of its websites. In the effort, the use of visual design principles combined with the properties of a science web site were put to the test. In all, six different web sites were designed each with a difference science focus. We describe the process used to design the web sites which involve forming teams of designers, scientists and developers. We present example pages from each design and conclude with a discussion of what was learned during the process.

The global shape of Europa is controlled by tidal and rotational potentials and possibly by lateral variations in ice shell thickness. We use limb proles from four Galileo images to determine the best-t hydrostatic shape, yielding a mean radius of 1560.8 ‐ 0.3 km and a radius difference a a c of 3.0 ‐ 0.9 km, consistent with previous determinations and inferences from gravity observations. Adding long-wavelength topography due to proposed lateral variations in shell thickness results in poorer ts to the limb proles. We conclude that lateral shell thickness variations and long-wavelength isostatically supported topographic variations do not exceed 7 and 0.7 km, respectively. For the range of rheologies investigated (basal viscosities from 10 14 to 10 15 Pa s) the maximum permissible (conductive) shell thickness is 35 km. The relative uniformity of Europais shell thickness is due to either a heat ux 7m W m a 2 from the silicate interior, lateral ice o w at the base of the shell, or convection within the shell.

Radio Doppler data, generated with NASA's Galileo spacecraft during its second encounter with Jupiter's moon Ganymede, are used to infer the locations and magnitudes of mass anomalies on Ganymede. We construct models for both surface and buried anomalies. With only one flyby and no global coverage, a solution for mass anomalies cannot be uniquely determined. However, we are able to constrain acceptable solutions for mass anomalies to four broad regions—a near polar region and three that are roughly equatorial. If the mass anomalies are constrained to lie at the surface, the centers of the regions are located near the coordinates (77° N, 333° W), (36° N, 0° W), (33° N, 130° W), and (7° N, 194° W). If the mass anomalies are located at the deep ice–rock interface 800 km below the surface, the regions' centers are approximately (65° N, 17° W), (32° N, 30° W), (37° N, 175° W), and (15° N, 211° W). For both models, the regions are up to a few thousand kilometers across. The magnitude of mass anomalies on the surface is on the order of 10 17 kg. Mass anomalies at the ice–rock interface are on average no more than an order of magnitude larger (10 18 kg). There are two positive and two negative mass anomalies in both the surface and ice–rock interface models. One of the positive mass anomalies at the surface is associated with Galileo Regio. The other positive surface mass anomaly is located at high northern latitudes with no obvious geological association. Negative surface mass anomalies lie near Uruk Sulcus and between Perrine Regio and Barnard Regio near Sicyan Sulcus and Phrygia Sulcus. The locations of the ice–rock interface mass anomalies lie approximately radially below the surface anomalies. Positive mass anomalies at the surface could be associated with the silicate-rich ice or accumulated silicate layers of the dark regions. Negative mass anomalies at the surface could be associated with the relatively clean, low-lying ice of sulci. Alternatively, Ganymede's mass anomalies could be associated with the topography or other mass concentrations at the deep ice–rock interface.

Models of tidal-convective equilibrium for Europa's ice shell are computed using a laboratory-derived composite flow law for ice. Volume diffusion creep is found to dominate the flow law at equilibrium, and thus the thickness of the shell is strongly dependent on the poorly known grain size of the ice. This grain size is, however, constrained to be less than a few millimeters if equilibrium is achieved at the current eccentricity. Europa's ice shell cannot be thinner than about 16 km in equilibrium at present, since tidal dissipation cannot generate enough heat in such a thin shell to balance the heat transport. No conductive equilibria are found; this is likely due to the fact that most of a conductive shell must be cold if temperature gradients are to be large enough to carry the heat. A minimum eccentricity of about 0.0025 (about 1/4 the present value) below which there are no equilibria is also found.

The dynamics of synchronous rotation and physical librations are revisited in order to establish a conceptually simple and general theoretical framework applicable to a variety of problems. Our motivation comes from disagreements between the results of numerical simulations and those of previous theoretical studies, and also because different theoretical studies disagree on basic features of the dynamics. We approach the problem by decomposing the orientation matrix of the body into perfectly synchronous rotation and deviation from the equilibrium state. The normal modes of the linearized equations are computed in the case of a circular satellite orbit, yielding both the periods and the eigenspaces of three librations. Libration in longitude decouples from the other two, vertical modes. There is a fast vertical mode with a period very close to the average rotational period. It corresponds to tilting the body around a horizontal axis while retaining nearly principal-axis rotation. In the inertial frame, this mode appears as nutation and free precession. The other vertical mode, a slow one, is the free wobble. The effects of the nodal precession of the orbit are investigated from the point of view of Cassini states. We test our theory using numerical simulations of the full equations of the dynamics and discuss the disagreements among our study and previous ones. The numerical simulations also reveal that in the case of eccentric orbits large departures from principal-axis rotation are possible due to a resonance between free precession and wobble. We also revisit the history of the Moon's rotational state and show that it switched from one Cassini state to another when it was at 46.2 Earth radii. This number disagrees with the value 34.2 derived in a previous study.

Calculations of the tidal responses of Ganymede and Callisto reveal that tidal amplitudes on these bodies may be as large as a few meters if a liquid ocean exists to decouple the surface ice from the interior. Tides on Ganymede's surface can exceed 7 m peak-to-peak variation, while on Callisto the tidal amplitude can exceed 5 m in the presence of a liquid ocean. Without an ocean, tidal amplitudes are less than 0.5 m on Ganymede and less than 0.3 m on Callisto. An orbiting spacecraft using an altimeter for crossover analysis and Doppler tracking from Earth should be able to achieve sufficient accuracy to identify the tidal amplitude to within about a meter over the course of a few months (observing tens of tidal cycles).

Doppler data generated with the Cassini spaceraft's radio carrier waves at X and Ka band can be used to determine the quadrupole moments of Rhea's gravitational field.

Long-term numerical simulations of the orbits of the Galilean satellites are carried out using a realistic physical model. The free libration of the Laplace angle, which plays a central role in the classical description of the tidal evolution of the Laplace resonance, is isolated from the simulated orbits. The amplitude and the period of the libration agree well with the values obtained by J. Lieske. In addition, we point out that tidal dissipation models need to take into account the large variations in Io's eccentricity due to Jupiter's oblateness.

Radio Doppler data generated by the Deep Space Network (DSN) from four encounters of the Galileo spacecraft with Io, Jupiter's innermost Galilean satellite, are used to infer Io's gravitational quadrupole moments. By combining the four flybys into a single solution for the gravity field, the response of Io to the second degree tidal and rotational potentials is accurately determined. This is characterized by the value of the second degree potential Love number k 2 = 1.2924 ± 0.0027. We construct interior models that satisfy constraints imposed by the mean radius R = 1821.6 ± 0.5 km, the mean density p = 3527.8 ± 2.9 kg/m 3 , and the normalized axial moment of inertia C/M R 2 = 0.37685 ± 0.00035. The gravitationally derived figure of Io has principal axes (c < b < a) a = 1830.0 ± 0.5 km, b = 1819.2 ± 0.5 km, and c = 1815.6 ± 0.5 km, consistent with the shape determined by imaging. Gravitational and other data strongly suggest that Io is in hydrostatic equilibrium. In this case, models of Io's interior density show that Io almost certainly has a metallic core with a radius between 550 and 900 km for an Fe-FeS core or between 350 and 650 km for an Fe core. Io is also likely to have a crust and a partially molten asthenosphere, but their thicknesses cannot be separately or uniquely determined from the gravitational data.

Radio Doppler data generated by the Deep Space Network (DSN) from five encounters of the Galileo spacecraft with Callisto, Jupiter's outermost Galilean satellite, have been used to determine the quadrupole moments of the satellite's external gravitational field.

It has been proposed that Jupiter's satellite Europa currently possesses a global subsurface ocean of liquid water. Galileo gravity data verify that the satellite is differentiated into an outer H2O layer about 100 km thick but cannot determine the current physical state of this layer (liquid or solid). Here we summarize the geological evidence regarding an extant subsurface ocean, concentrating on Galileo imaging data. We describe and assess nine pertinent lines of geological evidence: impact morphologies, lenticulae, cryovolcanic features, pull-apart bands, chaos, ridges, surface frosts, topography, and global tectonics. An internal ocean would be a simple and comprehensive explanation for a broad range of observations; however, we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm, soft ice with only localized or partial melting. Two different models of impact flux imply very different surface ages for Europa; the model favored here indicates an average age of ∼50 Myr. Searches for evidence of current geological activity on Europa, such as plumes or surface changes, have yielded negative results to date. The current existence of a global subsurface ocean, while attractive in explaining the observations, remains inconclusive. Future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within Europa today.

Lithospheric thinning by mantle plumes is an important planetary heat transfer process resulting in the broad topographic uplift that characterizes the volcanic rises on Venus, the Tharsis rise on Mars, and several hotspot swells on Earth. We present a suite of time-dependent, three-dimensional numerical calculations of a plume impinging upon the lithosphere in a temperature-dependent viscosity mantle. Efficient lithospheric thinning is found to depend on the formation of convective instabilities in the plume-lithosphere boundary layer. These instabilities are non-axisymmetric, time-dependent, and have horizontal scales of a few tens of kilometers. These instabilities depend on the temperature-dependence of viscosity and occur when the plume's viscosity is about an order of magnitude less than the background mantle, as predicted by boundary-layer theory. Thus, in planetary mantles, plumes with excess temperatures of 100 to 200 K will efficiently thin the lithosphere via small-scale convective instabilities.

The ice-rich surface of the jovian satellite Europa is sparsely cratered, suggesting that this moon might be geologically active today1. Moreover, models of the satellite's interior indicate that tidal interactions with Jupiter might produce enough heat to maintain a subsurface liquid water layer2,3,4,5. But the mechanisms of interior heat loss and resurfacing are currently unclear, as is the question of whether Europa has (or had at one time) a liquid water ocean6,7. Here we report on the morphology and geological interpretation of distinct surface features—pits, domes and spots—discovered in high-resolution images of Europa obtained by the Galileo spacecraft. The features are interpreted as the surface manifestation of diapirs, relatively warm localized ice masses that have risen buoyantly through the subsurface. We find that the formation of the features can be explained by thermally induced solid-state convection within an ice shell, possibly overlying a liquid water layer. Our results are consistent with the possibility that Europa has a liquid water ocean beneath a surface layer of ice, but further tests and observations are needed to demonstrate this conclusively.

Geoid and topography observations are used to constrain the density structure of the venusian lithosphere under the assumptions that convective stresses are small and local isostasy prevails. Results are presented for 13 venusian highlands, including Ishtar Terra. In general, Venus has a thick (200–400 km) thermal lithosphere which is thinned beneath volcanic highlands by as much as 80%, inducing slopes of 0.05 to 0.18 in the base of the lithosphere. From scalings of geoid/topography ratio and lithospheric basal slope as a function of internal Rayleigh number and viscosity contrast, we estimate lower bounds to these parameters of 10 7 and 10 5 , respectively. Several lines of evidence point to values near these lower limits, which puts Venus in the sluggish-lid regime of convection. There also exist several accumulations of crustal material, most notably Ovda Regio, where there may be as much as 90 km of crust.

The source of the buoyancy that supports the Hawaiian Swell is not well understood. The swell may be supported by replacement of negatively buoyant lithosphere with buoyant mantle or by emplacement of buoyant material beneath lithosphere of normal thickness. These mechanisms can be distinguished by examining the quadratic relationship between geoid height and bathymetry. At the Hawaiian Swell, the curvature of the geoid height vs. swell topography relationship is negative, indicating that the swell is supported by thinned lithosphere. The magnitude of the curvature suggests that the mantle filling in the region of thinned lithosphere is both thermally and chemically buoyant.

Doppler data generated with the Galileo spacecraft’s radio carrier wave during two Europa encounters on 19 December 1996 (E4) and 20 February 1997 (E6) were used to measure Europa’s external gravitational field. The measurements indicate that Europa has a predominantly water ice-liquid outer shell about 100 to 200 kilometers thick and a deep interior with a density in excess of about 4000 kilograms per cubic meter. The deep interior could be a mixture of metal and rock or it could consist of a metal core with a radius about 40 percent of Europa’s radius surrounded by a rock mantle with a density of 3000 to 3500 kilograms per cubic meter. The metallic core is favored if Europa has a magnetic field.

Radio Doppler data from the Galileo spacecraft's encounter with Amalthea, one of Jupiter's small inner moons, on 5 November 2002 yield a mass of (2.08 ± 0.15) × 10 18 kilograms. Images of Amalthea from two Voyager spacecraft in 1979 and Galileo imaging between November 1996 and June 1997 yield a volume of (2.43 ± 0.22) × 10 6 cubic kilometers. The satellite thus has a density of 857 ± 99 kilograms per cubic meter. We suggest that Amalthea is porous and composed of water ice, as well as rocky material, and thus formed in a cold region of the solar system, possibly not at its present location near Jupiter.

One of the most enticing targets for space exploration is Europa. It is widely believed that there is an ocean of liquid water, perhaps a hundred kilometers deep, a few kilometers beneath Europa’s icy surface. The conditions in this ocean are likely similar to Earth’s oceans. Furthermore, the ocean floor may have hydrothermal vents, providing energy and elements necessary for life. Therefore, Europa is considered one of the most likely locations beyond Earth and within the solar system for extant life. In this paper, detailed analysis of the primary engineering challenges of a mission to explore the ocean on Europa is presented. Candidate system designs and a timeline, or “roadmap,” for the development of these systems is then discussed. A range of mission architectures are presented and evaluated relative to the various science objectives that an Europan exploration mission might address. In order to compare the various architectures, a value system has been designed following an analytical hierarchy process. Lastly, a science traceability matrix for the proposed mission is evaluated.

The heat transport and lithospheric dynamics of early Earth are currently explained by plate tectonic and vertical tectonic models, but these do not offer a global synthesis consistent with the geologic record. Here we use numerical simulations and comparison with the geologic record to explore a heat-pipe model in which volcanism dominates surface heat transport. These simulations indicate that a cold and thick lithosphere developed as a result of frequent volcanic eruptions that advected surface materials downwards. Declining heat sources over time led to an abrupt transition to plate tectonics. Consistent with model predictions, the geologic record shows rapid volcanic resurfacing, contractional deformation, a low geothermal gradient across the bulk of the lithosphere and a rapid decrease in heat-pipe volcanism after initiation of plate tectonics. The heat-pipe Earth model therefore offers a coherent geodynamic framework in which to explore the evolution of our planet before the onset of plate tectonics. A heat-pipe model of Earth, whereby interior heat is brought to the surface through localized channels, yields predictions that agree with craton data and the detrital zircon record, and offers a global geodynamic framework in which to explore Earth’s evolution before the onset of plate tectonics. The Hadean era of Earth's history (4.5–3.8 billion years ago) is the dark age of geology, illuminated by occasional scraps of data. On the basis of a geodynamical model formerly applied to Jupiter's volcanically active moon Io, William Moore and Alexander Webb propose a 'heat-pipe' model to explain heat transport and lithospheric dynamics of an early Earth in transition between the early magma ocean and the era of plate tectonics. The model suggests that a cold, thick, single-plate lithosphere developed as a result of frequent volcanic eruptions that cycled surface materials downwards. The model's predictions compare well with the geological record, and indicate that declining heat sources over time could have precipitated an abrupt transition to plate tectonics.

The spatial variation of the geoid/topography ratio over the large Venusian volcanic highland Beta Regio is suggestive of thermal compensation, i.e., support of the highland's topography by lithospheric thinning. Both the thickness of the lithosphere and the density contrast at its base can be inferred from a quadratic regression of suitably filtered (600 km less than wavelength less than 4000 km) geoid vs. topography data. The regression yields a mean lithospheric thickness of 270 km and a density contrast of magnitude 2.5% to 3.0%. Simple isostatic balance of the long-wavelength topography at Beta Regio requires thinning of the lithosphere by 50-60% beneath the rise.

The global spherical harmonic model of Venus' gravity field MGNP60FSAAP, with horizontal resolution of about 600 km, shows that most coronae have little or no signature in the gravity field. Nevertheless, some coronae and some segments of chasmata are associated with distinct positive gravity anomalies. No corona has been found to have a negative gravity anomaly. The spatial coincidence of the gravity highs over four closely spaced 300- to 400-km-diameter coronae in Eastern Eistla Regio with the structures themselves is remarkable and argues for a near-surface or lithospheric origin of the gravity signals over such relatively small features. Apparent depths of compensation (ADCs) of the prominent gravity anomalies at Artemis, Latona, and Heng-o Coronae are about 150 to 200 km. The geoid/topography ratios (GTRs) at Artemis, Latona, and Heng-o Coronae lie in the range 32 to 35 m/km. The large ADCs and GTRs of Artemis, Latona, and Heng-o Coronae are consistent with topographically related gravity and a thick Venus lithosphere or shallowly compensated topography and deep positive mass anomalies due to subduction of underthrusting at these coronae. At arcuate segments of Hecate and Parga Chasmata ADCs are about 125 to 150 km, while those at Fauta Corona, four coronae in Eastern Eistla Regio, and an arcuate segment of Wester Parga Chasmata are about 75 km. The GTRs at Fauta Corona, the four coronae in eastern Eistla Regio, and the accurate segments of Hecate, Parga, and Western Parga Chasmata are about 12 to 21 m/km. By analogy with gravity anomalies of similar horizontal scale (600 km-several thousand kilometers) on the concave sides of terrestrial subduction zone arcs, which are due in large part to subducted lithosphere, it is inferred that the gravity anomalies on Venus are consistent with retrograde subduction at Artemis Chasma, along the northern and southern margins of Latona Coronam, and elsewhere along Parga and Hecate Chasmata.

A novel heat balance is proposed for Io's mantle in which heat produced by tidal dissipation is brought to the surface by rapid ascent of magma, rather than by convection. This is essentially a heat pipe mode of heat transport, with magma going up (taking latent heat) and solid mantle going down. Assuming latent heat dominates the heat transport, a simplified energy equation is coupled to the mass conservation equations and a Darcy flow law, resulting in two coupled, first-order equations for the melt velocity and melt fraction as functions of depth in the mantle. Tidal heating is modeled by uniformly distributing the observed surface heat flow of 2×1014 W over the partially molten region. Melt fractions less than 20% are required to remove this heat from the mantle via melt segregation.