How wet are gullies on Mars? Insights from high resolution topography

We have found that debris flows are the main process in forming two gullied crater slopes on Mars. We used 1 m/pix elevation models to derive three topographic indices: slope-area, CAD and DI-25. These indices allow the active slope processes to be identified by comparison to data from Earth analogues. We present data from Meteor Crater together with analogues previsouly presented by Conway et al. [1]. We also compare the signals from the martian gullied slopes with a non-gullied martian example: Zumba crater

Highly erosive glaciers on Mars - the role of water

International audiencePolewards of 30 • in each hemisphere, the surface of Mars hosts a suite of landforms reminiscent of glacial landscapes on Earth. Amongst these landforms are: 1) Viscous Flow Features (VFF), which resemble glaciers on Earth and are thought to contain large volumes of water ice, 2) martian gullies which are km-scale features resembling water-eroded gullies on Earth and 3) arcuate ridges thought to be moraines from previous glaciations. Gullies have been long-associated with a surface unit originally called "pasted-on terrain" and now often called the "latitude dependant mantle". Arcuate ridges are often found at the base of hillslopes with gullies, but are also found on hillslopes with pasted-on terrain and no gullies. We have found a systematic lowering of the slope of the bedrock exposure located topographically above the pasted-on terrain whether that same slope hosts gullies or not. The lowered bedrock exposures display a different surface texture from bedrock exposed on other parts of the crater wall and from fresh crater walls-it appears fragmented and has reduced relief. Using 1-m-digital elevation models from the High Resolution Imaging Science Experiment (HiRISE) we compared the slopes of eight "eroded" craters and seven unmodified craters. We estimated their age using the crater size-frequency distribution of small craters on their ejecta blankets. From this information we calculated bedrock retreat rates for the eroded craters and found they were up to ∼103 m Myr-1-equivalent to erosion rates of wet-based glaciers on Earth. This is several orders of magnitude higher than previous estimates of erosion by VFF (10-2-101 m Myr-1), which themselves are roughly equivalent to cold-based glaciers on Earth. Such erosion rates are sufficient to erase previously existing landforms, such as martian gullies. We hypothesise, therefore, that the pasted-on terrain is a glacial deposit, overturning its previous interpretation as an airfall deposit of ice nucleated on dust. We maintain the interpretation of the arcuate ridges as moraines, but further conclude that they are likely the result of glaciotectonic deformation of sub-marginal and proglacial sediment in the presence of sediment pore-water. We do not support the generation of large quantities of glacial meltwater because it would have broken-up and degraded the arcuate ridges and pasted-on terrain an produced a suite of landforms (e.g., hummocky moraine, lacustrine forms, outwash plains, eskers) which are not observed

A Markov model for inferring flows in directed contact networks

Directed contact networks (DCNs) are a particularly flexible and convenient class of temporal networks, useful for modeling and analyzing the transfer of discrete quantities in communications, transportation, epidemiology, etc. Transfers modeled by contacts typically underlie flows that associate multiple contacts based on their spatiotemporal relationships. To infer these flows, we introduce a simple inhomogeneous Markov model associated to a DCN and show how it can be effectively used for data reduction and anomaly detection through an example of kernel-level information transfers within a computer.Comment: 12 page

A nonlinear random walk approach to concentration-dependent contaminant transport in porous media

We propose a nonlinear random walk model to describe the dynamics of dense contaminant plumes in porous media. A coupling between concentration and velocity fields is found, so that transport displays non-Fickian features. The qualitative behavior of the pollutant spatial profiles and moments is explored with the help of Monte Carlo simulation, within a Continuous Time Random Walk approach. Model outcomes are then compared with experimental measurements of variable-density contaminant transport in homogeneous and saturated vertical columns.Comment: 8 pages, 9 figure

A renormalisation approach to excitable reaction-diffusion waves in fractal media

Of fundamental importance to wave propagation in a wide range of physical phenomena is the structural geometry of the supporting medium. Recently, there have been several investigations on wave propagation in fractal media. We present here a renormalization approach to the study of reaction-diffusion (RD) wave propagation on finitely ramified fractal structures. In particular we will study a Rinzel-Keller (RK) type model, supporting travelling waves on a Sierpinski gasket (SG), lattice

Why drug shortages are an ethical issue

Drug shortages are a growing problem in developed countries. To some extent they are the result of technical and organisational failures, but to view drug shortages simply as technical and economic phenomena is to miss the fact that they are also ethical and political issues. This observation is important because it highlights both the moral and political imperative to respond to drug shortages as vigorously as possible, and the need for those addressing shortages to do so in ethically and politically sophisticated ways. This brief article outlines the ethical issues that need to be considered by anyone attempting to understand or address drug shortages

Two-dimensional individual clustering model

10 pagesInternational audienceThis paper is devoted to study a model of individual clustering with two specific reproduction rates in two space dimensions. Given q > 2 and an initial condition in W 1,q (Ω), the local existence and uniqueness of solution have been shown in [6]. In this paper we give a detailed proof of existence of global solution

Well-posedness for a model of individual clustering

25 pagesInternational audienceWe study the well-posedness of a model of individual clustering. Given p > N ≥ 1 and an initial condition in W 1,p (Ω), the local existence and uniqueness of a strong solution is proved. We next consider two specific reproduction rates and show global existence if N = 1, as well as, the convergence to steady states for one of these rates