Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating ...suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO
was 10
PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO
rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The subglacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms.
Display omitted
► We have performed a series of simulations of buoyant hydrothermal plumes within Europa’s ocean. ► Plumes in our model are 10–35km in diameter, have temperatures of 10–200μK, and ...velocities of 1–5cm/s. ► Plume parameters are controlled by a single nondimensional parameter, but differ in detail from previous theory.
The liquid water interiors of Europa and other icy moons of the outer Solar System are likely to be driven by geothermal heating from the sea floor, leading to the development of buoyant hydrothermal plumes. These plumes potentially control icy surface geomorphology, and are of interest to astrobiologists. We have performed a series of simulations of these plumes using the MIT GCM ocean circulation model. We assume here that Europa’s ocean is deep (of order 100km) and unstratified, and that plume buoyancy is controlled by temperature, not composition. Our experiments explore a limited region of parameter space, with ocean depth H ranging from 50 to 100km deep, source heat flux Q between 0.1 and 10GW, and Coriolis parameter f corresponding to Europa latitudes between 9° and 47°. As predicted by earlier work, the plumes in our simulations form narrow cylindrical chimneys (a few km across) under the influence of the Coriolis effect. These plumes broaden over time until they become baroclinically unstable, breaking up into cone-shaped eddies when they become 10–35km in diameter; the shed eddies are of a similar size. Large-scale currents in the region of the plume range between 1 and 5cm/s; temperature anomalies in the plume far from the seafloor are tiny, varying between 10 and 180μK. Variations in plume size, shape, speed, and temperature are in excellent agreement with previous laboratory tank experiments, and in rough agreement with theoretical predictions. Plume dynamics and geometry are controlled by a “natural Rossby number” which depends strongly on depth H and Coriolis parameter f, but only weakly on source heat flux Q. However, some specific theoretical predictions are not borne out by these simulations: this may occur because the plumes are “reingesting” their own emissions, a process not considered in our earlier theory.
Abstract
Jupiter's moon Europa likely hosts a saltwater ocean beneath its icy surface. Geothermal heating and rotating convection in the ocean may drive a global overturning circulation that ...redistributes heat vertically and meridionally, preferentially warming the ice shell at the equator. Here we assess the previously unconstrained influence of ocean‐ice coupling on Europa's ocean stratification and heat transport. We demonstrate that a relatively fresh layer can form at the ice‐ocean interface due to a meridional ice transport forced by the differential ice shell heating between the equator and the poles. We provide analytical and numerical solutions for the layer's characteristics, highlighting their sensitivity to critical ocean parameters. For a weakly turbulent and highly saline ocean, a strong buoyancy gradient at the base of the freshwater layer can suppress vertical tracer exchange with the deeper ocean. As a result, the freshwater layer permits relatively warm deep ocean temperatures.
Key Points
Coupling of Europa's ocean circulation and the ice shell impacts global stratification
A low‐latitude freshwater layer may suppress vertical heat and tracer transport
Parameter space is explored based on properties observed by future missions
Enceladus' south polar sea Collins, Geoffrey C.; Goodman, Jason C.
Icarus (New York, N.Y. 1962),
07/2007, Letnik:
189, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Recent observations of the south pole of Saturn's moon Enceladus by the Cassini spacecraft have revealed an active world, powered by internal heat. In this paper, we propose that localized subsurface ...melting on Enceladus has produced an internal south polar sea. Evidence for this localized sea comes from the shape of Enceladus, which does not match a differentiated body at its current orbital position. We show that melting induced by the observed heat flow at the south pole produces a large enough pit to match the shape of Enceladus with a differentiated rock and ice interior. Numerical modeling of melting and ice flow shows that the sea produced beneath the south pole is stable against inflow of ductile ice from its surroundings for the duration of the heating. The shape modification due to melting also produces a negative degree-two gravity anomaly, which can reorient the spin axis of Enceladus in order to place the sea at the pole.
Marine ice cover in the frigid Neoproterozoic climate system tends to isolate the ocean: the thickness of this ice, and thus the degree of isolation, is an important unknown in geochemical and ...biological arguments regarding the fully‐glaciated “Snowball Earth” hypothesis. The Pollard and Kasting (2005) coupled atmosphere/ice model has been modified to track ice of marine and atmospheric origin separately, and model their different optical properties. In contrast to Pollard and Kasting's results, a tropical region of thin ice is not stable in this model: ice is hundreds of meters thick everywhere. The overall pattern of the hydrological cycle in a Snowball climate is also discussed.
Micro-Abstract Our aim was to determine the utility of circulating micro RNA miR-141 as a potential biomarker of therapeutic response in prostate cancer (CaP) patients. We compared the values of ...miR-141 in plasma of 21 CaP patients to the levels of prostate specific antigen (PSA), circulating tumor cells (CTC) and lactate dehydrogenase (LDH). Data suggest a strong correlation of miR-141 values and clinical course.
Simulations of frigid Neoproterozoic climates have not considered the tendency of thick layers of floating marine ice to deform and spread laterally. We have constructed a simple model of the ...production and flow of marine ice on a planetary scale, and determined ice thickness and flow in two situations: when the ocean is globally ice‐covered (“hard snowball”) and when the tropical waters remain open (“soft snowball”). In both cases, ice flow strongly affects the distribution of marine ice. Flowing ice probably carries enough latent heat and freshwater to significantly affect the transition into a Snowball Earth climate. We speculate that flowing marine ice, rather than continental ice sheets, may be the erosive agent that created some Neoproterozoic glacial deposits.
An increasing number of network observatories have been established globally to collect long-term biogeochemical data at multiple spatial and temporal scales. Although many outstanding questions in ...biogeochemistry would benefit from network science, the ability of the earth- and environmental-sciences community to conduct synthesis studies within and across networks is limited and seldom done satisfactorily. We identify the ideal characteristics of networks, common problems with using data, and key improvements to strengthen intra- and internetwork compatibility. We suggest that targeted improvements to existing networks should include promoting standardization in data collection, developing incentives to promote rapid data release to the public, and increasing the ability of investigators to conduct their own studies across sites. Internetwork efforts should include identifying a standard measurement suite—we propose profiles of plant canopy and soil properties—and an online, searchable data portal that connects network, investigator-led, and citizen-science projects.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Hydrothermal plumes may be responsible for transmitting radiogenic or tidally generated heat from Europa's rocky interior through a liquid ocean to the base of its ice shell. This process has been ...implicated in the formation of chaos regions and lenticulae by melting or exciting convection in the ice layer. In contrast to earlier work, we argue that Europa's ocean should be treated as an unstratified fluid. We have adapted and expanded upon existing work describing buoyant plumes in a rotating, unstratified environment. We discuss the scaling laws governing the flow and geometry of plumes on Europa and perform a laboratory experiment to obtain scaling constants and to visualize plume behavior in a Europa‐like parameter regime. We predict that hydrothermal plumes on Europa are of a lateral scale (at least 25–50 km) comparable to large chaos regions; they are too broad to be responsible for the formation of individual lenticulae. Plume heat fluxes (0.1–10 W/m2) are too weak to allow complete melt‐through of the ice layer. Current speeds in the plume (3–8 mm/s) are much slower than indicated by previous studies. The observed movement of ice blocks in the Conamara Chaos region is unlikely to be driven by such weak flow.
A growing number of satellites in the outer solar system likely have global oceans beneath their outer icy shells. While the presence of liquid water makes these ocean worlds compelling ...astrobiological targets, the exchange of heat and materials between the deep interior and the surface also plays a critical role in promoting habitable environments. In this article, we combine geophysical, geochemical, and geological observations of the Jovian satellites Europa, Ganymede, and Callisto as well as the Saturnian satellites Enceladus and Titan to summarize our current state of understanding of their interiors and surface exchange processes. Potential mechanisms for driving exchange processes upward from the ocean floor and downward from the satellite surface are then reviewed, which are primarily based on numerical models of ice shell and ocean dynamics and complemented by terrestrial analog studies. Future missions to explore these exo-oceans will further revolutionize our understanding of ice-ocean exchange processes and their implications for the habitability of these worlds.
PDF
