250 research outputs found
Tidal Control of Jet Eruptions Observed by Cassini ISS
Observations by Cassini's Imaging Science Subsystem (ISS) of Enceladus' south polar region at high phase angles has revealed jets of material venting into space. Observations by Cassini's Composite Infrared Spectrometer (CIRS) have also shown that the south polar region is anomalously warm with hotspots associated with geological features called the Tiger Stripes. The Tiger Stripes are large rifts near the south pole of Enceladus, which are typically about 130 km in length, 2 km wide, with a trough 500 m deep, and are l1anked on each side by 100m tall ridges. Preliminary triangulation of jets as viewed at different times and with different viewing geometries in Cassini ISS images taken between 2005 and 2007 have constrained the locations of eight major eruptions of material and found all of them associated with the south polar fractures unofficially the 'Tiger Stripes', and found four of them coincident with the hotspots reported in 2006 by CIRS. While published ISS observations of jet activity suggest that individual eruption sites stay active on the timescale of years, any shorter temporal variability (on timescales of an orbital period, or 1.3 Earth days, for example) is more difficult to establish because of the spotty temporal coverage and the difficulty of visually isolating one jet from the forest of many seen in a typical image. Consequently, it is not known whether individual jets are continuously active, randomly active, or if they erupt on a predictable, periodic schedule. One mechanism that may control the timing of eruptions is diurnal tidal stress, which oscillates between compression/tension as well as right and left lateral shear at any given location throughout Enceladus' orbit and may allow the cracks to open and close regularly. We examine the stresses on the Tiger Stripe regions to see how well diurnal tidal stress caused by Enceladus' orbital eccentricity may possibly correlate with and thus control the observed eruptions. We then identify possible mechanisms by which tidal stress can provide access to the surface for volatile material and implications for observed jet activity
Tidal Control of Jet Eruptions on Enceladus as Observed by Cassini ISS between 2005 and 2007
Observations of Enceladus have revealed active jets of material erupting from cracks on its south polar surface. It has previously been proposed that diurnal tidal stress, driven by Enceladus' orbital eccentricity, may actively produce surface movement along these cracks daily and thus may regulate when eruptions occur. Our analysis of the stress on jet source regions identified in Cassini ISS images reveals tidal stress as a plausible controlling mechanism of jet activity. However, the evidence available in the published and preliminary observations of jet activity between 2005 and 2007 may not be able to solidify the link between tidal stress and eruptions from fissures. Ongoing, far more comprehensive analyses based on recent, much higher resolution jetting observations have the potential to prove otherwise
Using Geological Implications of a Physical Libration to Constrain Enceladus' Libration State
Observations of Enceladus' south pole revealed large rifts in the crust, called "tiger stripes", which exhibit higher temperatures than the surrounding terrain and are likely sources of observed eruptions. Tidal stress may periodically open the tiger stripe rifts, controlling the timing and location of eruptions. Moreover, shear motion along rifts may produce the heat to drive eruptions
Improved representation of phosphorus exchange on soil mineral surfaces reduces estimates of P limitation in temperate forest ecosystems
International audiencePhosphorus (P) availability affects the response of terrestrial ecosystems to environmental and climate change (e.g., elevated CO2), yet the magnitude of this effect remains uncertain. This uncertainty arises mainly from a lack of quantitative understanding of the soil biological and geochemical P cycling processes, particularly the P exchange with soil mineral surfaces, which is often described by a Langmuir sorption isotherm.We first conducted a literature review on P sorption experiments and terrestrial biosphere models (TBMs) using a Langmuir isotherm. We then developed a new algorithm to describe the inorganic P exchange between soil solution and soil matrix based on the double-surface Langmuir isotherm and extracted empirical equations to calculate the sorption capacity and Langmuir coefficient. We finally tested the conventional and new models of P sorption at five beech forest sites in Germany along a soil P stock gradient using the QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system) TBM.We found that the conventional (single-surface) Langmuir isotherm approach in most TBMs largely differed from P sorption experiments regarding the sorption capacities and Langmuir coefficients, and it simulated an overly low soil P-buffering capacity. Conversely, the double-surface Langmuir isotherm approach adequately reproduced the observed patterns of soil inorganic P pools. The better representation of inorganic P cycling using the double-surface Langmuir approach also improved simulated foliar N and P concentrations as well as the patterns of gross primary production and vegetation carbon across the soil P gradient. The novel model generally reduces the estimates of P limitation compared with the conventional model, particularly at the low-P site, as the model constraint of slow inorganic P exchange on plant productivity is reduced
Geological Implications of a Physical Libration on Enceladus
Given the non-spherical shape of Enceladus (Thomas et al., 2007), the satellite will experience gravitational torques that will cause it to physically librate as it orbits Saturn. Physical libration would produce a diurnal oscillation in the longitude of Enceladus tidal bulge which, could have a profound effect on the diurnal stresses experienced by the surface of the satellite. Although Cassini ISS has placed an observational upper limit on Enceladus libration amplitude of F < 1.5deg (Porco et al., 2006), smaller amplitudes can still have geologically significant consequences. Here we present the first detailed description of how physical libration affects tidal stresses and how those stresses then might affect geological processes including crack formation and propagation, south polar eruption activity, and tidal heating. Our goal is to provide a framework for testing the hypothesis that geologic features on Enceladus are produced by tidal stresses from diurnal physical and optical librations of the satellite
Soil solution phosphorus turnover: derivation, interpretation, and insights from a global compilation of isotope exchange kinetic studies
The exchange rate of inorganic phosphorus (P) between the
soil solution and solid phase, also known as soil solution P turnover, is
essential for describing the kinetics of bioavailable P. While soil solution
P turnover (Km) can be determined by tracing radioisotopes in a
soil–solution system, few studies have done so. We believe that this is due
to a lack of understanding on how to derive Km from isotopic exchange
kinetic (IEK) experiments, a common form of radioisotope dilution study.
Here, we provide a derivation of calculating Km using parameters
obtained from IEK experiments. We then calculated Km for 217 soils from
published IEK experiments in terrestrial ecosystems, and also that of 18
long-term P fertilizer field experiments. Analysis of the global compilation
data set revealed a negative relationship between concentrations of soil
solution P and Km. Furthermore, Km buffered isotopically
exchangeable P in soils with low concentrations of soil solution P. This
finding was supported by an analysis of long-term P fertilizer field
experiments, which revealed a negative relationship between Km and
phosphate-buffering capacity. Our study highlights the importance of
calculating Km for understanding the kinetics of P between the soil
solid and solution phases where it is bioavailable. We argue that our
derivation can also be used to calculate soil solution turnover of other
environmentally relevant and strongly sorbing elements that can be traced
with radioisotopes, such as zinc, cadmium, nickel, arsenic, and uranium
Leptospirosis followed by Kawasaki-like disease: case report from an adult Swiss patient and review of the literature
Kawasaki disease (KD) is a vasculitis that mostly occurs in children, but rare cases in adults have been reported. We describe the case of a 43-year-old Swiss male who developed symptoms compatible with KD 7 weeks after leptospirosis, which was presumably acquired after swimming in a creek in the Swiss Alps. We performed a literature review and identified 10 other cases (all in children), in which Kawasaki-like disease was diagnosed in the context of leptospirosis. Outcome was favourable in most cases, including our patient. This exceptional case demonstrates both the possibility of autochthonous cases of leptospirosis in Switzerland as well as a possible association of leptospirosis with Kawasaki-like disease
First imaging results from the Iapetus B/C flyby of the Cassini spacecraft
Cassini had a relatively close flyby at Iapetus on New Year's Eve 2005. The 288 ISS images set various constraints on the origin theories of the dark/bright dichotomy, as revealed multiple surface structures at up to 740 m/pxl size
Simulation of space weathering of planet-forming materials: Nanosecond pulse laser irradiation and proton implantation on olivine and pyroxene samples
Improved representation of phosphorus exchange on soil mineral surfaces reduces estimates of phosphorus limitation in temperate forest ecosystems
Phosphorus (P) availability affects the response of
terrestrial ecosystems to environmental and climate change (e.g., elevated
CO2), yet the magnitude of this effect remains uncertain. This
uncertainty arises mainly from a lack of quantitative understanding of the
soil biological and geochemical P cycling processes, particularly the P
exchange with soil mineral surfaces, which is often described by a Langmuir
sorption isotherm.
We first conducted a literature review on P sorption experiments and
terrestrial biosphere models (TBMs) using a Langmuir isotherm. We then
developed a new algorithm to describe the inorganic P exchange between soil
solution and soil matrix based on the double-surface Langmuir isotherm and
extracted empirical equations to calculate the sorption capacity and
Langmuir coefficient. We finally tested the conventional and new models of P
sorption at five beech forest sites in Germany along a soil P stock gradient
using the QUINCY (QUantifying Interactions between terrestrial Nutrient
CYcles and the climate system) TBM.
We found that the conventional (single-surface) Langmuir isotherm approach
in most TBMs largely differed from P sorption experiments regarding the
sorption capacities and Langmuir coefficients, and it simulated an overly low soil
P-buffering capacity. Conversely, the double-surface Langmuir isotherm
approach adequately reproduced the observed patterns of soil inorganic P
pools. The better representation of inorganic P cycling using the double-surface
Langmuir approach also improved simulated foliar N and P concentrations as well as
the patterns of gross primary production and vegetation carbon across the
soil P gradient. The novel model generally reduces the estimates of P
limitation compared with the conventional model, particularly at the low-P
site, as the model constraint of slow inorganic P exchange on plant
productivity is reduced.</p
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