Titan was once thought to have global oceans of light hydrocarbons on its surface, but after 40 close flybys of Titan by the Cassini spacecraft, it has become clear that no such oceans exist. There ...are, however, features similar to terrestrial lakes and seas, and widespread evidence for fluvial erosion, presumably driven by precipitation of liquid methane from Titan's dense, nitrogen-dominated atmosphere. Here we report infrared spectroscopic data, obtained by the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft, that strongly indicate that ethane, probably in liquid solution with methane, nitrogen and other low-molecular-mass hydrocarbons, is contained within Titan's Ontario Lacus.
Saturn's moon Enceladus emits a plume of water vapour and micrometre-sized ice particles from a series of warm fissures located near its south pole. This geological activity could be powered or ...controlled by variations in the tidal stresses experienced by Enceladus as it moves around its slightly eccentric orbit. The specific mechanisms by which these varying stresses are converted into heat, however, are still being debated. Furthermore, it has proved difficult to find a clear correlation between the predicted tidal forces and measured temporal variations in the plume's gas content or the particle flux from individual sources. Here we report that the plume's horizontally integrated brightness is several times greater when Enceladus is near the point in its eccentric orbit where it is furthest from Saturn (apocentre) than it is when near the point of closest approach to the planet (pericentre). More material therefore seems to be escaping from beneath Enceladus' surface at times when geophysical models predict its fissures should be under tension and therefore may be wider open.
Composition and Physical Properties of Enceladus' Surface Brown, Robert H; Clark, Roger N; Buratti, Bonnie J ...
Science (American Association for the Advancement of Science),
03/2006, Letnik:
311, Številka:
5766
Journal Article
Recenzirano
Observations of Saturn's satellite Enceladus using Cassini's Visual and Infrared Mapping Spectrometer instrument were obtained during three flybys of Enceladus in 2005. Enceladus' surface is composed ...mostly of nearly pure water ice except near its south pole, where there are light organics, CO₂, and amorphous and crystalline water ice, particularly in the region dubbed the "tiger stripes." An upper limit of 5 precipitable nanometers is derived for CO in the atmospheric column above Enceladus, and 2% for NH₃ in global surface deposits. Upper limits of 140 kelvin (for a filled pixel) are derived for the temperatures in the tiger stripes.
Evidence for a Polar Ethane Cloud on Titan Griffith, C.A; Penteado, P; Rannou, P ...
Science (American Association for the Advancement of Science),
09/2006, Letnik:
313, Številka:
5793
Journal Article
Recenzirano
Spectra from Cassini's Visual and Infrared Mapping Spectrometer reveal the presence of a vast tropospheric cloud on Titan at latitudes 51° to 68° north and all longitudes observed (10° to 190° west). ...The derived characteristics indicate that this cloud is composed of ethane and forms as a result of stratospheric subsidence and the particularly cool conditions near the moon's north pole. Preferential condensation of ethane, perhaps as ice, at Titan's poles during the winters may partially explain the lack of liquid ethane oceans on Titan's surface at middle and lower latitudes.
The radiant energy budget and internal heat are fundamental properties of giant planets, but precise determination of these properties remains a challenge. Here, we report measurements of Jupiter's ...radiant energy budget and internal heat based on Cassini multi-instrument observations. Our findings reveal that Jupiter's Bond albedo and internal heat, 0.503 ± 0.012 and 7.485 ± 0.160 W m
respectively, are significantly larger than 0.343 ± 0.032 and 5.444 ± 0.425 Wm
, the previous best estimates. The new results help constrain and improve the current evolutionary theories and models for Jupiter. Furthermore, the significant wavelength dependency of Jupiter's albedo implies that the radiant energy budgets and internal heat of the other giant planets in our solar system should be re-examined. Finally, the data sets of Jupiter's characteristics of reflective solar spectral irradiance provide an observational basis for the models of giant exoplanets.
Mutations of mtDNA are an important cause of human disease, but few animal models exist. Because mammalian mitochondria cannot be transfected, the development of mice with pathogenic mtDNA mutations ...has been challenging, and the main strategy has therefore been to introduce mutations found in cell lines into mouse embryos. Here, we describe a phenotype-driven strategy that is based on detecting clonal expansion of pathogenic mtDNA mutations in colonic crypts of founder mice derived from heterozygous mtDNA mutator mice. As proof of concept, we report the generation of a mouse line transmitting a heteroplasmic pathogenic mutation in the alanine tRNA gene of mtDNA displaying typical characteristics of classic mitochondrial disease. In summary, we describe a straightforward and technically simple strategy based on mouse breeding and histology to generate animal models of mtDNA-mutation disease, which will be of great importance for studies of disease pathophysiology and preclinical treatment trials.
Display omitted
•We present a method to isolate and identify pathogenic mtDNA mutations in mice•We describe a mouse with a pathogenic mutation in the mitochondrial tRNAALA gene•The mice display disrupted mitochondrial translation as a result of the mutation•The mice display molecular and histochemical symptoms of human mitochondrial disease
Kauppila et al. describe a phenotype-based screen in live mice to generate mouse models with pathogenic mtDNA mutations. As proof of concept, they present a mouse with a mutation in the mitochondrial tRNAALA gene that displays molecular and histochemical symptoms of human mitochondrial disease.
Saturn's ionosphere is produced when the otherwise neutral atmosphere is exposed to a flow of energetic charged particles or solar radiation. At low latitudes the solar radiation should result in a ...weak planet-wide glow in the infrared, corresponding to the planet's uniform illumination by the Sun. The observed electron density of the low-latitude ionosphere, however, is lower and its temperature higher than predicted by models. A planet-to-ring magnetic connection has been previously suggested, in which an influx of water from the rings could explain the lower-than-expected electron densities in Saturn's atmosphere. Here we report the detection of a pattern of features, extending across a broad latitude band from 25 to 60 degrees, that is superposed on the lower-latitude background glow, with peaks in emission that map along the planet's magnetic field lines to gaps in Saturn's rings. This pattern implies the transfer of charged species derived from water from the ring-plane to the ionosphere, an influx on a global scale, flooding between 30 to 43 per cent of the surface of Saturn's upper atmosphere. This ring 'rain' is important in modulating ionospheric emissions and suppressing electron densities.
► First infrared observation of Ligeia Mare and several small lakes in Titan’s Northern polar area. ► This paper demonstrates that the lakes have not evolved in size between 2007 and 2010. ► It ...provides new information on the clarity of Titan’s atmosphere at 5μm. ► It discusses a simple model of carbon cycle. ► It revisits the value of Titan’s non-synchronous rotation.
Since Titan entered Northern spring in August 2009, the North Pole has been illuminated allowing observations at optical wavelengths. On June 5, 2010 the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft observed the Northern Pole area with a pixel size from 3 to 7km. Since, as we demonstrate, little of the solar flux at 5μm is scattered by the atmosphere, these observations were obtained at relatively large incidence angles and allowed us to build a mosaic covering an area of more than 500,000km2 that overlaps and complements observations made by the Synthetic Aperture Radar (SAR) in 2007. We find that there is an excellent correlation between the shape of the radar dark area, known as Ligeia Mare and the VIMS 5-μm dark unit. Matching most of the radar shoreline, the 2010 VIMS observations suggest that the 125,000-km2 surface area of Ligeia Mare measured by RADAR in 2007 has not significantly changed. The VIMS observations complement the radar observations to the west of Ligeia Mare and suggest that Ligeia Mare is connected to Kraken Mare by either a diffuse network similar to a swamp area, or by well-defined, sub-pixel rivers. Considering the results of recent evaporation models of methane, our preferred interpretation of the relative constancy in surface area of Ligeia is that it is principally composed of ethane although we cannot rule out the possibility that methane evaporation is balanced with replenishment by either precipitation or underground seepage. There is also strong correlation between the location of the small radar lakes and the small VIMS 5-μm dark patches. The geographic location of the small lakes are within a VIMS pixel of the SAR location, suggesting that the non-synchronous component of Titan’s spin rate, if it exists, was less than 2.3×10−4deg/day between 2007 and 2010 in agreement with the recent T64 radar observations. These observations question the existence of non-synchronous rotation. Two radar-bright features appear dark at 5-μm. The simplest interpretation is that these are very shallow lakes, less than one meter deep. Three new small lakes, named Freeman, Cardiel, and Towada by the IAU, are found outside of the area mapped with the SAR. A single-scattering model describing reflection of sunlight at 5-μm suggests that the lake surface is mirror-like and that the albedo of the solid surfaces surrounding the lakes is about 8%. These observations together with information of the haze aerosols allow us to show that Titan’s lakes, atmospheric ethane and aerosol haze are smaller carbon reservoirs than Titan’s sand dunes and atmospheric methane. A simple model involving an outburst of methane a few hundreds of Myr ago followed by the dissociation of methane in the atmosphere leading to the formation of the haze particles that constitute the dune fields would be consistent with both the present observations and recent measurements of isotopic ratios in atmospheric methane (Mandt, K.E. et al. 2012. Astrophys. J. 749(160), 14).
A solar occultation by Titan's atmosphere has been observed through the solar port of the
Cassini/VIMS instrument on January 15th, 2006. Transmission spectra acquired during solar egress probe the ...atmosphere in the altitude range 70 to 900 km at the latitude of 71° S. Several molecular absorption bands of CH
4 and CO are visible in these data. A line-by-line radiative transfer calculation in spherical geometry is used to model three methane bands (1.7, 2.3, 3.3 μm) and the CO 4.7 μm band. Above 200 km, the methane 2.3 μm band is well fit with constant mixing ratio between 1.4 and 1.7%, in agreement with
in situ and other
Cassini measurements. Under 200 km, there are discrepancies between models and observations that are yet fully understood. Under 480 km, the 3.3 μm CH
4 band is mixed with a large and deep additional absorption. It corresponds to the C–H stretching mode of aliphatic hydrocarbon chains attached to large organic molecules. The CO 4.7 μm band is observed in the lower stratosphere (altitudes below 150 km) and is well fit with a model with constant mixing ratio of
33
±
10
ppm
. The continuum level of the observed transmission spectra provides new constraints on the aerosol content of the atmosphere. A model using fractal aggregates and optical properties of tholins produced by Khare et al. Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W., 1984. Icarus 60, 127–137 is developed. Fractal aggregates with more than 1000 spheres of radius 0.05 μm are needed to fit the data. Clear differences in the chemical composition are revealed between tholins and actual haze particles. Extinction and density profiles are also retrieved using an inversion of the continuum values. An exponential increase of the haze number density is observed under 420 km with a typical scale height of 60 km.
We present observations of significant dynamics within two UV auroral storms observed on Saturn using the Hubble Space Telescope in April/May 2013. Specifically, we discuss bursts of auroral emission ...observed at the poleward boundary of a solar wind‐induced auroral storm, propagating at ∼330% rigid corotation from near ∼01 h LT toward ∼08 h LT. We suggest that these are indicative of ongoing, bursty reconnection of lobe flux in the magnetotail, providing strong evidence that Saturn's auroral storms are caused by large‐scale flux closure. We also discuss the later evolution of a similar storm and show that the emission maps to the trailing region of an energetic neutral atom enhancement. We thus identify the auroral form with the upward field‐aligned continuity currents flowing into the associated partial ring current.
Key Points
Saturn's auroral storms exhibit fast‐propagating bursts on the poleward boundary
These are similar to terrestrial PBIs, indicating ongoing closure of lobe flux
Subsequent emission maps to the trailing Region 2 current of an ENA enhancement
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