We report the first measurements of the intrinsic strain fluctuations of living cells using a recentlydeveloped tracer correlation technique along with a theoretical framework for interpreting such data in heterogeneous media with non-thermal driving. The fluctuations' spatial and temporal correlations indicate that the cytoskeleton can be treated as a course-grained continuum with powerlaw rheology, driven by a spatially random stress tensor field. Combined with recent cell rheology results, our data imply that intracellular stress fluctuations have a nearly 1/ω 2 power spectrum, as expected for a continuum with a slowly evolving internal prestress.PACS numbers: 87.16. Ac, 87.15.Ya, 87.10.+e An accurate physical picture of the viscoelasticity and motion of the cytoskeleton is crucial for a complete understanding of processes such as intracellular transport [1], cell crawling [2], and mechano-chemical transduction [2]. Microrheology [3], based on the analysis of embedded tracer particle motion, has recently emerged as an experimental probe of cytoskeleton viscoelasticity and dynamics [4,5,6,7]. The viscoelastic properties of eucaryotic cells arise from an intricate network of protein filaments driven by specialized motor proteins and directional polymerization, that convert the chemical energy of adenosine triphosphate (ATP) to mechanical work and motion. A cell is thus a nonequilibrium soft material whose fluctuations are actively driven. Unlike the thermal fluctuations in an equilibrium material, the amplitude and spatial distribution of active fluctuations can be controlled via biochemical signaling pathways; perhaps allowing the cell to locally adjust its' mechanical properties to suit its' needs. Indeed, microscopic force generators play a central role in existing cell mechanics models such as the sol-gel [8], soft glassy rheology [4] and tensegrity [9] hypotheses.In this Letter, we extend a recently introduced method, termed two-point microrheology [10], and show that it can be used to characterize the activity of intracellular force generators by directly measuring a cell's intrinsic, random stress fluctuations. Our experimental data and theoretical framework show that a cell can be modelled as a coarse-grained viscoelastic continuum driven by a spatially random stress field having a 1/ω 2 power spectrum in our observable frequency range, 1 < ω < 60 rad/s.There are two distinct approaches to microrheology: the active approach measures the displacements of tracer particles induced by external forces and the passive approach measures fluctuations of particle positions in the absence of driving forces. The active approach provides a direct measure of the complex shear modulus µ(ω). In equilibrium systems the passive approach also measures µ(ω) because of the fluctuation-dissipation theorem (FDT) [11]. Literature results in cells using singleparticle versions of the two approaches yield shear moduli differing by orders of magnitude and exhibiting qualitatively different frequency dependencies [4,6]. These ...

[1] Galileo Near Infrared Mapping Spectrometer (NIMS) data of volcanic thermal emission are analyzed to determine the power output of a number of Ionian volcanoes, from which are calculated volumetric eruption rates. A two-temperature model is used to determine surface temperatures and power output. Portions of Prometheus and six other volcanoes are found to be in excess of 1100 K: areas of Prometheus are at temperatures close to 1500 K. These are minimum eruption temperatures and are consistent with basaltic silicate volcanism. For the 14 volcanoes in G1INNSPEC01, volumetric eruption rates (E) are constrained between 3 and 300 m 3 /s for basaltic and ultramafic compositions. Actual E values lie between these limits. The small crack fractions (hot area to warm area ratio) implied from these fits are indicative of a relatively quiescent eruption style, such as the emplacement of a pahoehoe or a-a flow field. The volumetric eruption rates derived from the NIMS data are greater than those observed in similar styles of volcanism on Earth. It is apparent that for similar eruption styles, the areal extent of activity is greater on Io than on Earth. Derived flow thicknesses range from 0.1 to 13 m.

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The incorporation of ammonia inside methane clathrate hydrate is of great interest to the hydrate chemistry community. We investigated the phase behavior of methane clathrate formed from aqueous ammonia solution....

Understanding lava flow processes is important for interpreting existing lavas and for hazard assessments. Although substantial progress has been made for basaltic lavas our understanding of silicic lava flows has seen limited recent advance. In particular, the formation of lava flow breakouts, which represent a characteristic process in coolinglimited basaltic lavas, but has not been described in established models of rhyolite emplacement. Using data from the 2011-2012 rhyolite eruption of Puyehue-Cordón Caulle, Chile, we develop the first conceptual framework to classify breakout types in silicic lavas, and to describe the processes involved in their progressive growth, inflation, and morphological change. By integrating multiscale satellite, field, and textural data from Cordón Caulle, we interpret breakout formation to be driven by a combination of pressure increase (from local vesiculation in the lava flow core, as well as from continued supply via extended thermally preferential pathways) and a weakening of the surface crust through lateral spreading and fracturing. Small breakouts, potentially resulting more from local vesiculation than from continued magma supply, show a domed morphology, developing into petaloid as inflation increasingly fractures the surface crust. Continued growth and fracturing results in a rubbly morphology, with the most inflated breakouts developing into a cleft-split morphology, reminiscent of tumulus inflation structures seen in basalts. These distinct morphological classes result from the evolving relative contributions of continued breakout advance and inflation. The extended nature of some breakouts highlights the role of lava supply under a stationary crust, a process ubiquitous in inflating basalt lava flows that reflects the presence of thermally preferential pathways. Textural analyses of the Cordón Caulle breakouts also emphasize the importance of late-stage volatile exsolution and vesiculation within the lava flow. Although breakouts occur across the compositional spectrum of lava flows, the greater magma viscosity is likely to make late-stage vesiculation much more important for breakout development in silicic lavas than in basalts. Such late-stage vesiculation has direct implications for hazards previously recognized from silicic lava flows, enhancing the likelihood of flow front collapse, and explosive decompression of the lava core.

The Autonomous Sciencecraft Experiment (ASE) will fly onboard the Air Force TechSat-21 constellation of three spacecraft scheduled for launch in 2004. ASE uses onboard continuous planning, robust task and goal-based execution, model-based mode identification and reconfiguration, and onboard machine learning and pattern recognition to radically increase science return by enabling intelligent downlink selection and autonomous retargeting. In this paper we discuss how these AI technologies are synergistically integrated in a hybrid multi-layer control architecture to enable a virtual spacecraft science agent. We also describe our working software prototype and preparations for flight.

Io, Jupiter’s innermost Galilean satellite, is the most volcanically active body in the solar system. Ashley Gerard Davies reviews the wealth of data returned by NASA's veteran spacecraft Galileo, that has led to a better understanding of the volcanic processes wracking Io. Jupiter’s moon Io is the only other body in the solar system known to have active, high‐temperature volcanism like that found on Earth. The Galileo spacecraft has been observing Io regularly since June 1996, and the data that it has returned have led to many new insights into the volcanic processes that have shaped not only Io, but Earth in its distant past.

Titan, Saturn's largest satellite, is the only icy moon with a dense atmosphere that is composed mainly of N 2 . Methane, the second most abundant constituent, would be depleted in only 30−100 million years by active photochemistry, suggesting replenishment from Titan's interior. Under Titan's near-surface conditions, clathrate hydrates are the stable form of methane and ice together, making them a likely methane reservoir. Cassini−Huygens observations suggest that ammonia is the main source of Titan's atmospheric N 2 . Ammonia is known to decrease the melting point of water ice and some clathrate hydrates, such as those of tetrahydrofuran. The present study investigates the interaction of ammonia with cyclopentane clathrate hydrates (atmospheric analogue for methane clathrates) via a detailed examination of phase behavior using micro-Raman spectroscopy, differential scanning calorimetry, and X-ray diffraction. The results show that ammonia has the same effect on the stability of the cyclopentane clathrate as on tetrahydrofuran clathrate and ice by lowering the dissociation temperature by several tens of degrees and inducing incongruent melting. Ammonia does not interact directly with cyclopentane and does not appear to be incorporated into the cyclopentane clathrate structure, whether in the lattice or within the cages. A similar effect could be expected for methane clathrates. The presence of ammonia in Titan's crust would thus destabilize methane clathrates, resulting in outgassing and replenishment of atmospheric methane.