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.     

A reversible wet/dry adhesive inspired by mussels and geckos

The adhesive strategy of the gecko relies on foot pads composed of specialized keratinous foot-hairs called setae, which are subdivided into terminal spatulae of approximately 200 nm (ref. 1). Contact between the gecko foot and an opposing surface generates adhesive forces that are sufficient to allow the gecko to cling onto vertical and even inverted surfaces. Although strong, the adhesion is temporary, permitting rapid detachment and reattachment of the gecko foot during locomotion. Researchers have attempted to capture these properties of gecko adhesive in synthetic mimics with nanoscale surface features reminiscent of setae; however, maintenance of adhesive performance over many cycles has been elusive, and gecko adhesion is greatly diminished upon full immersion in water. Here we report a hybrid biologically inspired adhesive consisting of an array of nanofabricated polymer pillars coated with a thin layer of a synthetic polymer that mimics the wet adhesive proteins found in mussel holdfasts. Wet adhesion of the nanostructured polymer pillar arrays increased nearly 15-fold when coated with mussel-mimetic polymer. The system maintains its adhesive performance for over a thousand contact cycles in both dry and wet environments. This hybrid adhesive, which combines the salient design elements of both gecko and mussel adhesives, should be useful for reversible attachment to a variety of surfaces in any environment.     

Spin-based logic in semiconductors for reconfigurable large-scale circuits

Research in semiconductor spintronics aims to extend the scope of conventional electronics by using the spin degree of freedom of an electron in addition to its charge. Significant scientific advances in this area have been reported, such as the development of diluted ferromagnetic semiconductors, spin injection into semiconductors from ferromagnetic metals and discoveries of new physical phenomena involving electron spin. Yet no viable means of developing spintronics in semiconductors has been presented. Here we report a theoretical design that is a conceptual step forward-spin accumulation is used as the basis of a semiconductor computer circuit. Although the giant magnetoresistance effect in metals has already been commercially exploited, it does not extend to semiconductor/ferromagnet systems, because the effect is too weak for logic operations. We overcome this obstacle by using spin accumulation rather than spin flow. The basic element in our design is a logic gate that consists of a semiconductor structure with multiple magnetic contacts; this serves to perform fast and reprogrammable logic operations in a noisy, room-temperature environment. We then introduce a method to interconnect a large number of these gates to form a 'spin computer'. As the shrinking of conventional complementary metal-oxide-semiconductor (CMOS) transistors reaches its intrinsic limit, greater computational capability will mean an increase in both circuit area and power dissipation. Our spin-based approach may provide wide margins for further scaling and also greater computational capability per gate.     

Evolution of cooperation in a finite homogeneous graph

Recent theoretical studies of selection in finite structured populations have worked with one of two measures of selective advantage of an allele: fixation probability and inclusive fitness. Each approach has its own analytical strengths, but given certain assumptions they provide equivalent results. In most instances the structure of the population can be specified by a network of nodes connected by edges (that is, a graph), and much of the work here has focused on a continuous-time model of evolution, first described by ref. 11. Working in this context, we provide an inclusive fitness analysis to derive a surprisingly simple analytical condition for the selective advantage of a cooperative allele in any graph for which the structure satisfies a general symmetry condition ('bi-transitivity'). Our results hold for a broad class of population structures, including most of those analysed previously, as well as some for which a direct calculation of fixation probability has appeared intractable. Notably, under some forms of population regulation, the ability of a cooperative allele to invade is seen to be independent of the nature of population structure (and in particular of how game partnerships are specified) and is identical to that for an unstructured population. For other types of population regulation our results reveal that cooperation can invade if players choose partners along relatively 'high-weight' edges.     

The development of a protoplanetary disk from its natal envelope

Class 0 protostars, the youngest type of young stellar objects, show many signs of rapid development from their initial, spheroidal configurations, and therefore are studied intensively for details of the formation of protoplanetary disks within protostellar envelopes. At millimetre wavelengths, kinematic signatures of collapse have been observed in several such protostars, through observations of molecular lines that probe their outer envelopes. It has been suggested that one or more components of the proto-multiple system NGC 1333-IRAS 4 (refs 1, 2) may display signs of an embedded region that is warmer and denser than the bulk of the envelope. Here we report observations that reveal details of the core on Solar System dimensions. We detect in NGC 1333-IRAS 4B a rich emission spectrum of H2O, at wavelengths 20-37 microm, which indicates an origin in extremely dense, warm gas. We can model the emission as infall from a protostellar envelope onto the surface of a deeply embedded, dense disk, and therefore see the development of a protoplanetary disk. This is the only example of mid-infrared water emission from a sample of 30 class 0 objects, perhaps arising from a favourable orientation; alternatively, this may be an early and short-lived stage in the evolution of a protoplanetary disk.     

The structural basis of yeast prion strain variants

Among the many surprises to arise from studies of prion biology, perhaps the most unexpected is the strain phenomenon whereby a single protein can misfold into structurally distinct, infectious states that cause distinguishable phenotypes. Similarly, proteins can adopt a spectrum of conformations in non-infectious diseases of protein folding; some are toxic and others are well tolerated. However, our understanding of the structural differences underlying prion strains and how these differences alter their physiological impact remains limited. Here we use a combination of solution NMR, amide hydrogen/deuterium (H/D) exchange and mutagenesis to study the structural differences between two strain conformations, termed Sc4 and Sc37 (ref. 5), of the yeast Sup35 prion. We find that these two strains have an overlapping amyloid core spanning most of the Gln/Asn-rich first 40 amino acids that is highly protected from H/D exchange and very sensitive to mutation. These features indicate that the cores are composed of tightly packed beta-sheets possibly resembling 'steric zipper' structures revealed by X-ray crystallography of Sup35-derived peptides. The stable structure is greatly expanded in the Sc37 conformation to encompass the first 70 amino acids, revealing why this strain shows increased fibre stability and decreased ability to undergo chaperone-mediated replication. Our findings establish that prion strains involve large-scale conformational differences and provide a structural basis for understanding a broad range of functional studies, including how conformational changes alter the physiological impact of prion strains.     

'Rejuvenation' protects neurons in mouse models of Parkinson's disease

Why dopamine-containing neurons of the brain's substantia nigra pars compacta die in Parkinson's disease has been an enduring mystery. Our studies suggest that the unusual reliance of these neurons on L-type Ca(v)1.3 Ca2+ channels to drive their maintained, rhythmic pacemaking renders them vulnerable to stressors thought to contribute to disease progression. The reliance on these channels increases with age, as juvenile dopamine-containing neurons in the substantia nigra pars compacta use pacemaking mechanisms common to neurons not affected in Parkinson's disease. These mechanisms remain latent in adulthood, and blocking Ca(v)1.3 Ca2+ channels in adult neurons induces a reversion to the juvenile form of pacemaking. Such blocking ('rejuvenation') protects these neurons in both in vitro and in vivo models of Parkinson's disease, pointing to a new strategy that could slow or stop the progression of the disease.     

A link between large mantle melting events and continent growth seen in osmium isotopes

Although Earth's continental crust is thought to have been derived from the mantle, the timing and mode of crust formation have proven to be elusive issues. The area of preserved crust diminishes markedly with age, and this can be interpreted as being the result of either the progressive accumulation of new crust or the tectonic recycling of old crust. However, there is a disproportionate amount of crust of certain ages, with the main peaks being 1.2, 1.9, 2.7 and 3.3 billion years old; this has led to a third model in which the crust has grown through time in pulses, although peaks in continental crust ages could also record preferential preservation. The 187Re-187Os decay system is unique in its ability to track melt depletion events within the mantle and could therefore potentially link the crust and mantle differentiation records. Here we employ a laser ablation technique to analyse large numbers of osmium alloy grains to quantify the distribution of depletion ages in the Earth's upper mantle. Statistical analysis of these data, combined with other samples of the upper mantle, show that depletion ages are not evenly distributed but cluster in distinct periods, around 1.2, 1.9 and 2.7 billion years. These mantle depletion events coincide with peaks in the generation of continental crust and so provide evidence of coupled, global and pulsed mantle-crust differentiation, lending strong support to pulsed models of continental growth by means of large-scale mantle melting events.     

Epithelial-cell-intrinsic IKK-beta expression regulates intestinal immune homeostasis

Intestinal epithelial cells (IECs) provide a primary physical barrier against commensal and pathogenic microorganisms in the gastrointestinal (GI) tract, but the influence of IECs on the development and regulation of immunity to infection is unknown. Here we show that IEC-intrinsic IkappaB kinase (IKK)-beta-dependent gene expression is a critical regulator of responses of dendritic cells and CD4+ T cells in the GI tract. Mice with an IEC-specific deletion of IKK-beta show a reduced expression of the epithelial-cell-restricted cytokine thymic stromal lymphopoietin in the intestine and, after infection with the gut-dwelling parasite Trichuris, fail to develop a pathogen-specific CD4+ T helper type 2 (T(H)2) response and are unable to eradicate infection. Further, these animals show exacerbated production of dendritic-cell-derived interleukin-12/23p40 and tumour necrosis factor-alpha, increased levels of CD4+ T-cell-derived interferon-gamma and interleukin-17, and develop severe intestinal inflammation. Blockade of proinflammatory cytokines during Trichuris infection ablates the requirement for IKK-beta in IECs to promote CD4+ T(H)2 cell-dependent immunity, identifying an essential function for IECs in tissue-specific conditioning of dendritic cells and limiting type 1 cytokine production in the GI tract. These results indicate that the balance of IKK-beta-dependent gene expression in the intestinal epithelium is crucial in intestinal immune homeostasis by promoting mucosal immunity and limiting chronic inflammation.     

Potential release of hydrogen fluoride from domestic coal in endemic fluorosis area in Guizhou,China

Almost half of the total rural area of Guizhou Province and many regions within the 11 adjacent provinces in southwestern China have a long history(at least 70 years) of endemic fluorosis,including dental fluorosis and osteofluorosis along with its associated deformities and disabilities.Over decades of research,this specific type of endemic fluorosis has been defined as coal-burning fluorosis,which is distinct from drinking-water fluorosis.It is generally acknowledged that indoor burning and combustion of high-fluorine coal leads to food contamination,and fluorine then enters the human body.However,the exact chemical form of fluorine during its release and transfer to the body is still unknown.In the present study,21 domestic coal samples from outcrop and semi-outcrop coal collected in five villages with fluorosis were analyzed by time-of-flight secondary ion mass spectrometry(TOF-SIMS).The total mass fraction of sulfur in the samples ranged from 0.24%-5.58% and total fluorine content ranged 90.2-149.2 mg/kg.H3O+,H2SO4+ and HSO4-were detected in the samples by TOF-SIMS,which indicated that sulfuric acid hydrate(H2SO4.H2O) was present in the samples.F-was detected in all of these,which suggested the samples contain ionic fluorine compounds.Under certain circumstances,such as heating or burning,the prevalence and coexistence of the acid(H2SO4.H2O) and base(F-) would lead to a neutralization reaction producing volatile hydrogen fluoride(HF,bp = 19.5℃).This would be the chemical form of fluorine released from the coal.Further studies using HF and SO2 test tubes on headspace gas over coal samples heated to 200℃ in the laboratory and on headspace gas over stoves or chimney tops at rural residences confirmed the release of HF.