Variations in planetary convection via the effect of climate on damage

A new model for the generation of plate tectonics suggests an important interaction between a planet's climate and its lithospheric damage behavior; and thus provides a simple explanation for the tectonic difference between Earth and Venus. We propose that high surface temperatures will lead to higher healing rates (e.g. grain growth) in the lithosphere that will act to suppress localization, plate boundary formation, and subduction. This leads to episodic or stagnant lid convection on Venus because of its hotter climate. In contrast, Earth's cooler climate promotes damage and plate boundary formation. The damage rheology presented in this paper attempts to describe the evolution of grain size by allowing for grain reduction via deformational work input and grain growth via surface tension-driven coarsening. We explore the interaction of damage and healing in two-dimensional numerical convection simulations. We also develop a simple "drip-instability" model to test the hypothesis that the competition between damage and healing controls convective and plate tectonic style by modulating episodicity at subduction zones. At small values of damage, f A , (or large values of healing, k A ) the lithosphere remains strong enough to resist subduction on time scales of billions of years. At intermediate values of f A and k A the lithosphere may become mobilized and allow for short bursts of tectonic behavior followed by periods of quiescence. At large (small) values of f A (k A ) the fineness is increased so that the viscosity of the plate boundary is reduced to allow for continuous, unimpeded subduction of lithosphere and plate-like deformation. The results suggest the feasibility of our proposed hypothesis that the interplay of climate and damage control the mode of tectonics on a planet

Dynamic contrast-enhanced and diffusion-weighted MRI for early detection of tumoral changes in single-dose and fractionated radiotherapy: evaluation in a rat rhabdomyosarcoma model

We aimed to examine different intratumoral changes after single-dose and fractionated radiotherapy, using diffusion-weighted (DW) and dynamic contrast-enhanced (DCE) MRI in a rat rhabdomyosarcoma model. Four WAG/Rij rats with rhabdomyosarcomas in the flanks received single-dose radiotherapy of 8 Gy, and four others underwent fractionated radiotherapy (five times 3 Gy). In rats receiving single-dose radiotherapy, a significant perfusion decrease was found in the first 2 days post-treatment, with slow recuperation afterwards. No substantial diffusion changes could be seen; tumor growth delay was 12 days. The rats undergoing fractionated radiotherapy showed a similar perfusion decrease early after the treatment. However, a very strong increase in apparent diffusion coefficient occurred in the first 10 days; growth delay was 18 days. DW-MRI and DCE-MRI can be used to show early tumoral changes induced by radiotherapy. Single-dose and fractionated radiotherapy induce an immediate perfusion effect, while the latter induces more intratumoral necrosis