Mathematical and computational modelling of vegetated soil incorporating hydraulically-driven finite strain deformation

In this paper a new model for the hydro-mechanical behaviour of rooted soils is developed. It is a physically-based model that couples finite strain soil deformation with unsaturated water and air flow, while improving on existing cohesion-based approaches to mechanical root reinforcement and empirical soil water-uptake approaches typically used to deal with rooted slopes. The model is used to show that the dynamics of soil-water pressure and soil deformation depend strongly on the physics of the root-water uptake and the elasto-plastic soil mechanics. Root water uptake can cause suctions and corresponding soil shrinkage sufficiently large to necessitate a finite-strain approach. Although this deformation can change the intrinsic permeability, hydraulic conductivity remains dominated by the water content. The model incorporates simultaneous air-flow, but this is shown to be unimportant for soil-water dynamics under the conditions assumed in example simulations. The mechanical action of roots is incorporated via a root stress tensor and a simulation is used to show how root tension is mobilised within a swelling soil. The developed model may be used to simulate both laboratory experiments and full-scale vegetated slopes

ALMA spectral line and imaging survey of a low and a high mass-loss rate AGB star between 335 and 362 GHz

A spectral line and imaging survey of the low mass-loss rate AGB star R Dor (Mdot ~ 1e-7 Msun/yr) and the high mass-loss rate AGB star IK Tau (Mdot ~5e-6 Msun/yr) was made with ALMA between 335 and 362 GHz at a spatial resolution of ~150 mas, corresponding to the locus of the main dust formation region of both targets. Some 200 spectral features from 15 molecules (and their isotopologues) were observed, including rotational lines in both the ground and vibrationally excited states. Detected species include the gaseous precursors of dust grains such as SiO, AlO, AlOH, TiO, and TiO2. We present a spectral atlas for both stars and the parameters of all detected spectral features. A clear dichotomy for the sulphur chemistry is seen: while CS, SiS, SO, and SO2 are abundantly present in IK Tau, only SO and SO2 are detected in R Dor. Also other species such as NaCl, NS, AlO, and AlOH display a completely different behaviour. From some selected species, the minor isotopologues can be used to assess the isotopic ratios. The channel maps of many species prove that both large and small-scale inhomogeneities persist in the inner wind of both stars in the form of blobs, arcs, and/or a disk. The high sensitivity of ALMA allows us to spot the impact of these correlated density structures in the spectral line profiles. The spectral lines often display a half width at zero intensity much larger than expected from the terminal velocity, v_inf, previously derived for both objects (36 km/s versus v_inf ~17.7 km/s for IK Tau and 23 km/s versus v_inf ~5.5 km/s for R Dor). Both a more complex 3D morphology and a more forceful wind acceleration of the (underlying) isotropic wind can explain this trend. The formation of fractal grains in the region beyond ~400 mas can potentially account for the latter scenario. From the continuum map, we deduce a dust mass of ~3.7e-7 Msun for IK Tau and ~2e-8 Msun for R Dor.Comment: 39 pages, Astronomy & Astrophysics, in pres

Particle formation and interaction

A wide variety of experiments that involve the physics of small particles (μm to cm in size) of planetary significance can be conducted on the Space Station. Processes of interest include nucleation and condensation of particles from a gas, aggregation of small particles into larger ones, and low velocity collisions of particles. Only experiments relevant to planetary processes will be discussed in detail here

Experimental light scattering by fluffy aggregates of magnesiosilica, ferrosilica, and alumina cosmic dust analogs

Context: Fluffy aggregates are generally assumed to be important constituents of circumstellar and interplanetary environments as well as to be present among the solid debris ejected from active comets. Aims: We experimentally study light scattering properties of several fluffy aggregate samples. These cosmic dust analog aggregates are composed of coagulated magnesiosilica grains, ferrosilica grains, and alumina grains. The samples contain aggregates with different porosities. The individual grains have diameters of the order of a few tens of nanometers; the aggregates have diameters up to several micrometers. Methods: The samples were produced in a Condensation Flow Apparatus. Their light scattering properties were measured with the Amsterdam Light Scattering Facility at a wavelength of 632.8 nm. Results: We measured two scattering matrix elements as functions of the scattering angle, namely F11(theta) (phase function) and -F12(theta)/F11(theta) (degree of linear polarization for incident unpolarized light) for seven different samples of aggregates in random orientations in an aerosol jet. The samples consisted of fluffy aggregates with cosmic dust analog compositions. We provide detailed information about their production and nature. In addition, for four of these samples we measured F22(theta)/F11(theta). We covered an angle range of 5° to 174°, in small steps of 1° in the range from 5° to 10° and 170° to 174° and in steps of 5° for the rest of the angle range. Conclusions: The results for the analog samples show an extremely high -F12(theta)/F11(theta), with maxima between about 60% to almost 100%. This Rayleigh-like behavior has been demonstrated before for fluffy aggregates and suggests that the small-sized grains in the aggregates are the main cause. Measured results for phase functions are more scarce. The phase functions we measured show shapes that are similar to those of compact micron-sized particles, suggesting that it is the overall size of the aggregates that determines their shape. The modest negative branch of -F12(theta)/F11(theta) found for all seven samples seems to be mainly governed by aggregate structure. Thus, the unique combination of accurately measured phase functions and polarization functions over a fine mesh of scattering angles for cosmic dust analog aggregates enables the exploitation of the data as powerful diagnostic tools to constrain the different physical properties of dust in e.g. circumstellar clouds and in comet ejecta

Elevation and volume changes of seven Dickson Land glaciers, Svalbard, 1960–1990–2009

Melting Svalbard glaciers have been recognized as an early indicator of climate change. Large parts of Svalbard remain insufficiently investigated, including Dickson Land, in the quasi-continental interior of Svalbard. In this study, elevation and volume changes of seven glaciers located in the Pyramiden region are assessed by analysing contour lines from 1960 topographic maps and photogrammetrically derived 1990 and 2009 digital elevation models. Mass loss was documented for all seven glaciers. In the period 1960–1990, their average elevation change rate was −0.49 m a−1, while in the more recent period, 1990–2009, it was more negative at −0.78 m a−1, caused by a significant equilibrium line altitude shift with post-1990 rise in summer temperatures. Large variation in elevation change rates between individual glaciers was found and is attributed mainly to aspect and hypsometry. This highlights the importance of choosing a representative sample when investigating mass balance of whole regions. Evidence of a rapid increase in thinning rates in the upper parts of the studied glaciers, linked to decreasing albedo in former accumulation zones, was also found