An unexpected cooling effect in Saturn's upper atmosphere

The upper atmospheres of the four Solar System giant planets exhibit high temperatures that cannot be explained by the absorption of sunlight. In the case of Saturn the temperatures predicted by models of solar heating are approximately 200 K, compared to temperatures of approximately 400 K observed independently in the polar regions and at 30 degrees latitude. This unexplained 'energy crisis' represents a major gap in our understanding of these planets' atmospheres. An important candidate for the source of the missing energy is the magnetosphere, which injects energy mostly in the polar regions of the planet. This polar energy input is believed to be sufficient to explain the observed temperatures, provided that it is efficiently redistributed globally by winds, a process that is not well understood. Here we show, using a numerical model, that the net effect of the winds driven by the polar energy inputs is not to heat but to cool the low-latitude thermosphere. This surprising result allows us to rule out known polar energy inputs as the solution to the energy crisis at Saturn. There is either an unknown--and large--source of polar energy, or, more probably, some other process heats low latitudes directly.     

Against defaultism and towards localism in the contingency/inevitability conversation: Or,why we should shut up about putting-up

Philosophers and historians of science have for some time now debated whether the results of current science are ‘contingent’ or ‘inevitable’. Scholars have noted that inevitabilism often enjoys the status of a presumptive default position. Consequently, contingentists are, from the outset, lumbered with the burden of proof. This is evident in the case of the inevitabilist demand that the contingentist “put up or shut up” (PUSU). This paper adds to the existing case which says that inevitabilism's default-status is unjustified. However, whilst some have suggested that contingentism should replace inevitabilism as the default position, I argue that the contingency/inevitability (C/I) conversation should proceed sans default. This move is motivated largely by my claim that the C/I issue is best conceived as a ‘local’, rather than a global or universal one. The main problem with taking inevitabilism or contingentism as the default is the globalist nature of such a tack. Whilst localism is arguably an emergent reality of the growing C/I literature, its implications have not been fully realised. I suggest that fully and explicitly embracing localism, including the closely related move of doing away with defaults, represents the most promising way forward for the C/I conversation. In addition, I will show how these moves entail that we stop worrying about the inevitabilist PUSU demand, or more bluntly, that we shut up about putting-up.     

Consensus supertrees: The synthesis of rooted trees containing overlapping sets of labeled leaves

Given two dendrograms (rooted tree diagrams) which have some but not all of their base points in common, a supertree is a dendrogram from which each of the original trees can be regarded as samples The distinction is made between inconsistent and consistent sample trees, defined by whether or not the samples provide contradictory information about the supertree An algorithm for obtaining the strict consensus supertree of two consistent sample trees is presented, as are procedures for merging two inconsistent sample trees Some suggestions for future work are made     

A stability limit for the atmospheres of giant extrasolar planets

Recent observations of the planet HD209458b indicate that it is surrounded by an expanded atmosphere of atomic hydrogen that is escaping hydrodynamically. Theoretically, it has been shown that such escape is possible at least inside an orbit of 0.1 au (refs 4 and 5), and also that H3+ ions play a crucial role in cooling the upper atmosphere. Jupiter's atmosphere is stable, so somewhere between 5 and 0.1 au there must be a crossover between stability and instability. Here we show that there is a sharp breakdown in atmospheric stability between 0.14 and 0.16 au for a Jupiter-like planet orbiting a solar-type star. These results are in contrast to earlier modelling that implied much higher thermospheric temperatures and more significant evaporation farther from the star. (We use a three-dimensional, time-dependent coupled thermosphere-ionosphere model and properly include cooling by H3+ ions, allowing us to model globally the redistribution of heat and changes in molecular composition.) Between 0.2 and 0.16 au cooling by H3+ ions balances heating by the star, but inside 0.16 au molecular hydrogen dissociates thermally, suppressing the formation of H3+ and effectively shutting down that mode of cooling.