154 research outputs found
Candidate regions on titan as promising landing sites for future in situ missions
The highly successful and still on-going Cassini-Huygens mission to the Saturnian system points to the need for a return mission, with both remote and in situ instrumentation. The surface of Saturnâs moon Titan, hosts a complex environment in which many processes occur shaping its landscape. Several of its geological features resemble terrestrial ones, albeit constructed from different material and reflecting the interiorsurface-atmosphere exchanges. The resulting observed morphotectonic features and cryovolcanic candidate regions could benefit from further extensive exploration by a return mission that would focus on these aspects with adapted state-of-the-art instrumentation affording higher spectral and spatial resolution and in situ capabilities. We suggest that some features on Titan are more promising candidate locations for future landing and we present the case for Tui Regio, Hotei Regio and Sotra Patera as to why they could provide a wealth of new scientific results
Supernovae-induced accretion and star formation in the inner kiloparsec of a gaseous disk
We consider the effects of supernovae (SNe) on accretion and star formation
in a massive gaseous disk in a large primeval galaxy. The gaseous disk we
envisage, roughly 1 kiloparsec (kpc) in size with >~ 10^8 M_Sun of gas, could
have formed as a result of galaxy mergers where tidal interactions removed
angular momentum from gas at larger radius and thereby concentrated it within
the central ~ 1 kpc region. We find that SNe lead to accretion in the disk at a
rate of roughly 0.1 - 1 M_Sun per year and induce star formation at a rate of ~
10 - 100 M_Sun per year which contributes to the formation of a bulge; a part
of the stellar velocity dispersion is due to the speed of SNa shells from which
stars are formed and a part due to the repeated action of the stochastic
gravitational field of the network of SNa remnants on stars. The rate of SNe in
the inner kpc is shown to be self- regulating, and it cycles through phases of
low and high activity. The supernova-assisted accretion transports gas from
about one kpc to within a few pc of the center. If this accretion were to
continue down to the central black hole (BH) then the resulting ratio of BH
mass to the stellar mass in the bulge would be of order ~ 10^-2 - 10^-3, in
line with the observed Magorrian relation.Comment: 16 pages, 1 figure; MNRAS in pres
Multiresolution analysis of active region magnetic structure and its correlation with the Mt. Wilson classification and flaring activity
Two different multi-resolution analyses are used to decompose the structure
of active region magnetic flux into concentrations of different size scales.
Lines separating these opposite polarity regions of flux at each size scale are
found. These lines are used as a mask on a map of the magnetic field gradient
to sample the local gradient between opposite polarity regions of given scale
sizes. It is shown that the maximum, average and standard deviation of the
magnetic flux gradient for alpha, beta, beta-gamma and beta-gamma-delta active
regions increase in the order listed, and that the order is maintained over all
length-scales. This study demonstrates that, on average, the Mt. Wilson
classification encodes the notion of activity over all length-scales in the
active region, and not just those length-scales at which the strongest flux
gradients are found. Further, it is also shown that the average gradients in
the field, and the average length-scale at which they occur, also increase in
the same order. Finally, there are significant differences in the gradient
distribution, between flaring and non-flaring active regions, which are
maintained over all length-scales. It is also shown that the average gradient
content of active regions that have large flares (GOES class 'M' and above) is
larger than that for active regions containing flares of all flare sizes; this
difference is also maintained at all length-scales.Comment: Accepted for publication in Solar Physic
Potentially active regions on Titan: New processing of Cassini/VIMS data
The Cassini Visual and Infrared Mapping Spectrometer (VIMS) obtained data of Titan's surface from flybys performed during the last seven years. In the 0.8-5.2 ”m range, these spectro-imaging data showed that the surface consists of a multivariable geological terrain hosting complex geological processes. The data from the seven narrow methane spectral "windows" centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.8 and 5 ”m provide some information on the lower atmospheric context and the surface parameters that we want to determine. Atmospheric scattering and absorption need to be clearly evaluated before we can extract the surface properties. We apply here a statistical method [1, 2] and a radiative transfer method [3, 1] on three potentially "active" regions on Titan, i.e. regions possibly subject to change over time (in brightness and/or in color etc) [4]: Tui Regio (20°S, 130°W) [5], a 1,500-km long flow-like figure, Hotei Regio (26°S, 78°W) [6], a 700-km wide volcanic-like terrain, and Sotra Facula (15°S, 42°W) [7], a 235-km in diameter area. With our method of Principal Component Analysis (PCA) we have managed to isolate specific regions of distinct and diverse chemical composition. We have tested this method on the previously studied Sinlap crater [8], delimitating compositional heterogeneous areas compatible with the published conclusions by Le Mouélic et al. (2008). Our follow-up method focuses on retrieving the surface albedo of the three areas and of the surrounding terrains with different spectral response by applying a radiative transfer (RT) code. We have used as input most of the Cassini HASI and DISR measurements, as well as new methane absorption coefficients [9], which are important to evaluate the atmospheric contribution and to allow us to better constrain the real surface alterations, by comparing the spectra of these regions. By superposing these results onto the PCA maps, we can correlate composition and morphology. As a test case, we used our RT code to verify the varying brightness of Hotei Regio reported by other investigators based on models lacking proper simulation of the atmospheric absorption [10]. Even though we have used exactly the same dataset, we did not detect any significant surface albedo variations over time; this led us to revise the definition of "active" regions: even if these regions have not visually changed over the course of the Cassini mission, the determination of the chemical composition and the correlation with the morphological structures [11] observed in these areas do not rule out that past and/or ongoing cryovolcanic processes are still a possible interpretation. [1] Solomonidou, A. et al. (2011). Potentially active regions on Titan: New processing of Cassini/VIMS data. In preparation. [2] Stephan, K. et al. (2008). Reduction of instrument-dependent noise in hyperspectral image data using the principal component analysis: Applications to Galileo NIMS data. Planetary and Space Science 56, 406-419. [3] Hirtzig, M. et al. (2011). Applications of a new methane linelist to Cassini/VIMS spectra of Titan in the 1.28-5.2 ”m range . In preparation. [4] Wall, s. D. et al. (2009). Cassini RADAR images at Hotei Arcus and western Xanadu, Titan: Evidence for geologically recent cryovolcanic activity. Journal of Geophysical Research 36, L04203, [5] Barnes, J.W. et al. (2006). Cassini observations of flow-like features in western Tui Regio, Titan. Geophysical Research Letters 33, L16204. [6] Soderblom, L.A. et al. (2009). The geology of Hotei Regio, Titan: Correlation of Cassini VIMS and RADAR. Icarus 204, 610-618. [7] Lopes, R.M.C. et al. (2010). Distribution and interplay of geologic processes on Titan from Cassini radar data. Icarus 205, 540-558. [8] Le Mouélic et al. (2008). Mapping and interpretation of Sinlap crater on Titan using Cassini VIMS and RADAR data. Journal of Geophysical Research 113, E04003. [9] Campargue, A. et al. (2011). An empirical line list for methane at 80 K and 296 K in the 1.26-1.71 ”m region for planetary investigations. Application to Titan. Icarus. Submitted. [10] Nelson, R. et al (2009). Saturn's Titan: Surface change, ammonia, and implications for atmospheric and tectonic activity. Icarus 199, 429-441. [11] Solomonidou, A. et al. (2011). Possible morphotectonic features on Titan and their origin. Planetary and Space Science. Submitted
The VMC survey - XV : The Small Magellanic Cloud-Bridge connection history as traced by their star cluster populations
Date of Acceptance: 19/03/2015We present results based on YJKs photometry of star clusters located in the outermost, eastern region of the Small Magellanic Cloud (SMC). We analysed a total of 51 catalogued clusters whose colour-magnitude diagrams (CMDs), having been cleaned from field-star contamination, were used to assess the clusters' reality and estimate ages of the genuine systems. Based on CMD analysis, 15 catalogued clusters were found to be possible non-genuine aggregates. We investigated the properties of 80 per cent of the catalogued clusters in this part of the SMC by enlarging our sample with previously obtained cluster ages, adopting a homogeneous scale for all. Their spatial distribution suggests that the oldest clusters, log(t/yr) â„ 9.6, are in general located at greater distances to the galaxy's centre than their younger counterparts - 9.0 †log(t/yr) †9.4 - while two excesses of clusters are seen at log(t/yr) ~9.2 and log(t yr-1) Ë 9.7. We found a trail of younger clusters which follow the wing/bridge components. This long spatial sequence does not only harbour very young clusters, log(t yr-1) ~7.3, but it also hosts some of intermediate ages, log(t/yr) ~9.1. The derived cluster and field-star formation frequencies as a function of age are different. The most surprising feature is an observed excess of clusters with ages of log(t/yr) < 9.0, which could have been induced by interactions with the LMC.Peer reviewedFinal Accepted Versio
Surface albedo changes with time on Titanâs possible cryovolcanic sites: Cassini/VIMS processing and geophysical implications
We present a study on Titanâs possibly cryovolcanic and varying regions as suggested from previous studies [e.g. 1;2;7]. These regions, which are potentially subject to change over time in brightness and are located close to the equator, are Tui Regio, Hotei Regio, and Sotra Patera. We apply two methods on Cassini/VIMS data in order to retrieve their surface properties and monitor any temporal variations. First, we apply a statistical method, the Principal Component Analysis (PCA) [3;4] where we manage to isolate regions of distinct and diverse chemical composition called âRegion of interest â RoIâ. Then, we focus on retrieving the spectral differences (with respect to the Huygens landing site albedo) among the RoIs by applying a radiative transfer code (RT) [5;3]. Hence, we are able to view the dynamical range and evaluate the differences in surface albedo within the RoIs of the three regions. In addition, using this double procedure, we study the temporal surface variations of the three regions witnessing albedo changes with time for Tui Regio from 2005-2009 (darkening) and Sotra Patera from 2005-2006 (brightening) at all wavelengths [3]. The surface albedo variations and the presence of volcanic-like features within the regions in addition to a recent study [6] that calculates Titan's tidal response are significant indications for the connection of the interior with the cryovolcanic candidate features with implications for the satelliteâs astrobiological potential
Looking at some equatorial regions on Titan using Cassini/VIMS and RADAR data: a case for changes in surface properties.
On the Properties of the HI shells in the Small Magellanic CLoud
There are 509 expanding neutral hydrogen shells catalogued in the Small
Magellanic Cloud (SMC), all apparently very young, with dynamical ages of a few
Myr. To examine their relationship with young stellar objects we
cross-correlated the shell catalogue with various catalogues of OB
associations, super giants, Cepheids, WR stars, supernova remnants, and star
clusters. The incidence of chance line-ups was estimated via Monte-Carlo
simulations, and found to be high. However, it is important that there are 1.5
times more shells that are {\em not} spatially correlated to an OB association,
than shells that are. Moreover, 59 of the 509 shells lie mainly in low stellar
density fields and have no young stellar objects associated with them, and
therefore no obvious energy source. It is shown that, on the whole, the
properties of these "empty" shells are very similar to the properties of the
rest of the shells, once selection biases are taken into account. Theoretical
implications of the results are discussed.Comment: 15 pages, 8 figures accepted for publication in Monthly Notices of
the Royal Astronomical Societ
The evolution of the atmosphere and surface of Titan from Cassini infrared observations
Saturnâs Earth-like satellite Titan has a thick and dense atmosphere consisting of nitrogen (98.4%), methane (1.6%) and trace gases such as hydrocarbons and nitriles [1]. The condensed organics are deposited on the surface and the atmosphere-surface-interior interactions shape the ground. In particular, Titanâs methane cycle, similarly to the Earthâs hydrologic cycle, plays an important role in these exchanges by transporting methane at all layers. By applying our radiative transfer code (ARTT) to Cassini/CIRS data taken during Titan flybys from 2004-2010 and to the 1980 Voyager 1 flyby values inferred from the reanalysis of the Infrared Radiometer Spectrometer (IRIS) spectra, as well as to the intervening ground- and space- based observations (such as with ISO), we study the stratospheric evolution over a Titanian year (V1 encounter Ls=9° was reached in mid-2010)
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