The effect of the Coriolis force on Kelvin-Helmholtz-driven mixing in protoplanetary disks

We study the stability of proto-planetary disks with vertical velocity gradients in their equilibrium rotation rates; such gradients are expected to develop when dust settles into the midplane. Using a linear stability analysis of a simple three-layer model, we show that the onset of instability occurs at a larger value of the Richardson number, and therefore for a thicker layer, when the effects of Coriolis forces are included. This analysis also shows that even-symmetry (midplane-crossing) modes develop faster than odd-symmetry ones. These conclusions are corroborated by a large number of nonlinear numerical simulations with two different parameterized prescriptions for the initial (continuous) dust distributions. Based on these numerical experiments, the Richardson number required for marginal stability is more than an order of magnitude larger than the traditional 1/4 value. The dominant modes that grow have horizontal wavelengths of several initial dust scale heights, and in nonlinear stages mix solids fairly homogeneously over a comparable vertical range. We conclude that gravitational instability may be more difficult to achieve than previously thought, and that the vertical distribution of matter within the dust layer is likely globally, rather than locally, determined.Comment: Accepted for publication in Ap

Probing the origin of the dark material on Iapetus

Among the icy satellites of Saturn, Iapetus shows a striking dichotomy between its leading and trailing hemispheres, the former being significantly darker than the latter. Thanks to the VIMS imaging spectrometer on-board Cassini, it is now possible to investigate the spectral features of the satellites in Saturn system within a wider spectral range and with an enhanced accuracy than with previously available data. In this work, we present an application of the G-mode method to the high resolution, visible and near infrared data of Phoebe, Iapetus and Hyperion collected by Cassini/VIMS, to search for compositional correlations. We also present the results of a dynamical study on the efficiency of Iapetus in capturing dust grains travelling inward in Saturn system to evaluate the viability of Poynting-Robertson drag as the physical mechanism transferring the dark material to the satellite. The results of spectroscopic classification are used jointly with the ones of the dynamical study to describe a plausible physical scenario for the origin of Iapetus' dichotomy. Our work shows that mass transfer from the outer Saturnian system is an efficient mechanism, particularly for the range of sizes hypothesised for the particles composing the newly discovered outer ring around Saturn. Both spectral and dynamical data indicate Phoebe as the main source of the dark material. However, we suggest a multi-source scenario where now extinct prograde satellites and the disruptive impacts that generated the putative collisional families played a significant role in supplying the original amount of dark material.Comment: 20 pages, 4 tables, 11 figures, major revision (manuscript extended and completed, figures added and corrected, new results added), minor revision and finalization of author list, moderate revision (update of the manuscript following reviewer's feedback and discovery of the new Saturnian outer ring

Merging of the USGS Atlas of Mercury 1:5,000,000 Geologic Series

After 30 years, the planet Mercury is going to give us new information. The NASA MESSENGER [1] already made its first successful flyby on December 2007 while the European Space Agency and the Japanese Space Agency ISAS/JAXA are preparing the upcoming mission BepiColombo [2]. In order to contribute to current and future analyses on the geology of Mercury, we have started to work on the production of a single digital geologic map of Mercury derived from the merging process of the geologic maps of the Atlas of Mercury, produced by the United States Geological Survey, based on Mariner 10 data. The aim of this work is to merge the nine maps so that the final product reflects as much as possible the original work. Herein we describe the data we used, the working environment and the steps made for producing the final map

Marco Polo: near Earth object sample return mission

Marco Polo is a joint European-Japanese mission of sample return from a Near Earth Object. The Marco Polo proposal was submitted to ESA on July 2007 in the framework of the Cosmic Vision 2015-2025 context, and on October 2007 passed the first evaluation process. The primary objectives of this mission is to visit a primitive NEO, belonging to a class that cannot be related to known meteorite types, to characterize it at multiple scales, and to bring samples back to Earth. Marco Polo will give us the first opportunity for detailed laboratory study of the most primitive materials that formed the planets. This will allow us to improve our knowledge on the processes which governed the origin and early evolution of the Solar System, and possibly of the life on Earth

Marco Polo: A near Earth object sample return mission

From Introduction: MARCO POLO is a joint European-Japanese sample return mission to a Near-Earth Object. In late 2007 this mission was selected by ESA, in the framework of COSMIC VISION 2015-2025, for an assessment scheduled to last until mid 2009. This Euro-Asian mission will go to a primitive Near-Earth Object (NEO), such as a C or D type asteroid. The spacecraft will rendezvous with the object, and over an extended period scientifically characterize it at multiple scales and bring samples back to Earth for detailed scientific investigation

Influência de estresse por déficit hídrico em acessos do gênero Paspalum.

Editado por Ana Rita de araújo Nogueira, Simone Cristina Méo Niciur

Avaliação da tolerância de acesso do gênero Paspalum ao sombreamento.

Editado por Ana Rita de Araújo Nogueira, Simone Cristina Meo Niciura

Weed flora in rice areas under distinct cropping systems, herbicide and irrigation managements

We aimed to evaluate the incidence of weeds in the pre-planting of the summer crop as a function of planting system, herbicide use and irrigation management. The experiment was installed in field conditions, in RBD and 3 x 2 factorial scheme with eight replications. Treatments consisted in submitting rice to three management factors: water management – continuously flooded or intermittend irrigation (Factor A), coupled to the application (traditional control) or not (semi-ecological system) of herbicides (Factor B), and planting system – conventional soil tillage, minimum tillage and no till systems (Factor C). One year after rice cultivation, preceding the planting of the next cropping season, phytosociological evaluations of the weed communities were carried out. We assessed the overall infestation and weed species composition, which were classified by their respective density, frequency and dominance. We also estimated the diversity coefficients of Simpson and Shannon Weiner, and the sustainability coefficient of Shannon; treatments were also grouped by similarity in weed composition. In flood-irrigated rice, no till provides the lowest levels of weed infestation and, together with the conventional cropping system, results in values closer to the ecological sustainability; The application of herbicides in flooded rice crops reduces weed infestation, increases diversity and equalizes the ecological sustainability, compared to areas without the application of weed management methods. However, chemical control leads to the selection of resistant or tolerant species to herbicides, such as Polypogon sp.; Both continuous and intermittent water management systems did not cause changes in the level of infestation, composition or diversity coefficients

Rosetta-Alice Observations of Exospheric Hydrogen and Oxygen on Mars

The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on 25 February 2007, closest approach being at 01:54UT. The Alice instrument on board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for in situ cometary spectroscopy in the 750-2000 A spectral band, was used to study the daytime Mars upper atmosphere including emissions from exospheric hydrogen and oxygen. Offset pointing, obtained five hours before closest approach, enabled us to detect and map the HI Lyman-alpha and Lyman-beta emissions from exospheric hydrogen out beyond 30,000 km from the planet's center. These data are fit with a Chamberlain exospheric model from which we derive the hydrogen density at the 200 km exobase and the H escape flux. The results are comparable to those found from the the Ultraviolet Spectrometer experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen emission at 1304 A is detected at altitudes of 400 to 1000 km above the limb during limb scans shortly after closest approach. However, the derived oxygen scale height is not consistent with recent models of oxygen escape based on the production of suprathermal oxygen atoms by the dissociative recombination of O2+.Comment: 17 pages, 8 figures, accepted for publication in Icaru