An Earth-like correspondence between Saturn's auroral features and radio emission

Saturn is a source of intense kilometre-wavelength radio emissions that are believed to be associated with its polar aurorae, and which provide an important remote diagnostic of its magnetospheric activity. Previous observations implied that the radio emission originated in the polar regions, and indicated a strong correlation with solar wind dynamic pressure. The radio source also appeared to be fixed near local noon and at the latitude of the ultraviolet aurora. There have, however, been no observations relating the radio emissions to detailed auroral structures. Here we report measurements of the radio emissions, which, along with high-resolution images of Saturn's ultraviolet auroral emissions, suggest that although there are differences in the global morphology of the aurorae, Saturn's radio emissions exhibit an Earth-like correspondence between bright auroral features and the radio emissions. This demonstrates the universality of the mechanism that results in emissions near the electron cyclotron frequency narrowly beamed at large angles to the magnetic field.     

Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter

It has often been stated that Saturn's magnetosphere and aurorae are intermediate between those of Earth, where the dominant processes are solar wind driven, and those of Jupiter, where processes are driven by a large source of internal plasma. But this view is based on information about Saturn that is far inferior to what is now available. Here we report ultraviolet images of Saturn, which, when combined with simultaneous Cassini measurements of the solar wind and Saturn kilometric radio emission, demonstrate that its aurorae differ morphologically from those of both Earth and Jupiter. Saturn's auroral emissions vary slowly; some features appear in partial corotation whereas others are fixed to the solar wind direction; the auroral oval shifts quickly in latitude; and the aurora is often not centred on the magnetic pole nor closed on itself. In response to a large increase in solar wind dynamic pressure Saturn's aurora brightened dramatically, the brightest auroral emissions moved to higher latitudes, and the dawn side polar regions were filled with intense emissions. The brightening is reminiscent of terrestrial aurorae, but the other two variations are not. Rather than being intermediate between the Earth and Jupiter, Saturn's auroral emissions behave fundamentally differently from those at the other planets.     

Discovery of the triple asteroidal system 87 Sylvia

After decades of speculation, the existence of binary asteroids has been observationally confirmed, with examples in all minor planet populations. However, no triple systems have hitherto been discovered. Here we report the unambiguous detection of a triple asteroidal system in the main belt, composed of a 280-km primary (87 Sylvia) and two small moonlets orbiting at 710 and 1,360 km. We estimate their orbital elements and use them to refine the shape of the primary body. Both orbits are equatorial, circular and prograde, suggesting a common origin. Using the orbital information to estimate its mass and density, 87 Sylvia appears to have a rubble-pile structure with a porosity of 25-60 per cent. The system was most probably formed through the disruptive collision of a parent asteroid, with the new primary resulting from accretion of fragments, while the moonlets are formed from the debris, as has been predicted previously.     

Extraterrestrial meteors: a martian meteor and its parent comet

Regular meteor showers occur when a planet approaches the orbit of a periodic comet--for example, the Leonid shower is evident around 17 November every year as Earth skims past the dusty trail of comet Tempel-Tuttle. Such showers are expected to occur on Mars as well, and on 7 March last year, the panoramic camera of Spirit, the Mars Exploration Rover, revealed a curious streak across the martian sky. Here we show that the timing and orientation of this streak, and the shape of its light curve, are consistent with the existence of a regular meteor shower associated with the comet Wiseman-Skiff, which could be characterized as martian Cepheids.     

Suppression of Polycomb group proteins by JNK signalling induces transdetermination in Drosophila imaginal discs

During the regeneration of Drosophila imaginal discs, cellular identities can switch fate in a process known as transdetermination. For leg-to-wing transdetermination, the underlying mechanism involves morphogens such as Wingless that, when activated outside their normal context, induce ectopic expression of the wing-specific selector gene vestigial. Polycomb group (PcG) proteins maintain cellular fates by controlling the expression patterns of homeotic genes and other developmental regulators. Here we report that transdetermination events are coupled to PcG regulation. We show that the frequency of transdetermination is enhanced in PcG mutant flies. Downregulation of PcG function, as monitored by the reactivation of a silent PcG-regulated reporter gene, is observed in transdetermined cells. This downregulation is directly controlled by the Jun amino-terminal kinase (JNK) signalling pathway, which is activated in cells undergoing regeneration. Accordingly, transdetermination frequency is reduced in a JNK mutant background. This regulatory interaction also occurs in mammalian cells, indicating that the role of this signalling cascade in remodelling cellular fates may be conserved.     

Voltage-dependent electrogenic chloride/proton exchange by endosomal CLC proteins

Eukaryotic members of the CLC gene family function as plasma membrane chloride channels, or may provide neutralizing anion currents for V-type H(+)-ATPases that acidify compartments of the endosomal/lysosomal pathway. Loss-of-function mutations in the endosomal protein ClC-5 impair renal endocytosis and lead to kidney stones, whereas loss of function of the endosomal/lysosomal protein ClC-7 entails osteopetrosis and lysosomal storage disease. Vesicular CLCs have been thought to be Cl- channels, in particular because ClC-4 and ClC-5 mediate plasma membrane Cl- currents upon heterologous expression. Here we show that these two mainly endosomal CLC proteins instead function as electrogenic Cl-/H+ exchangers (also called antiporters), resembling the transport activity of the bacterial protein ClC-e1, the crystal structure of which has already been determined. Neutralization of a critical glutamate residue not only abolished the steep voltage-dependence of transport, but also eliminated the coupling of anion flux to proton counter-transport. ClC-4 and ClC-5 may still compensate the charge accumulation by endosomal proton pumps, but are expected to couple directly vesicular pH gradients to Cl- gradients.     

Disruption of Bardet-Biedl syndrome ciliary proteins perturbs planar cell polarity in vertebrates

The evolutionarily conserved planar cell polarity (PCP) pathway (or noncanonical Wnt pathway) drives several important cellular processes, including epithelial cell polarization, cell migration and mitotic spindle orientation. In vertebrates, PCP genes have a vital role in polarized convergent extension movements during gastrulation and neurulation. Here we show that mice with mutations in genes involved in Bardet-Biedl syndrome (BBS), a disorder associated with ciliary dysfunction, share phenotypes with PCP mutants including open eyelids, neural tube defects and disrupted cochlear stereociliary bundles. Furthermore, we identify genetic interactions between BBS genes and a PCP gene in both mouse (Ltap, also called Vangl2) and zebrafish (vangl2). In zebrafish, the augmented phenotype results from enhanced defective convergent extension movements. We also show that Vangl2 localizes to the basal body and axoneme of ciliated cells, a pattern reminiscent of that of the BBS proteins. These data suggest that cilia are intrinsically involved in PCP processes.     

Phyllosilicates on Mars and implications for early martian climate

The recent identification of large deposits of sulphates by remote sensing and in situ observations has been considered evidence of the past presence of liquid water on Mars. Here we report the unambiguous detection of diverse phyllosilicates, a family of aqueous alteration products, on the basis of observations by the OMEGA imaging spectrometer on board the Mars Express spacecraft. These minerals are mainly associated with Noachian outcrops, which is consistent with an early active hydrological system, sustaining the long-term contact of igneous minerals with liquid water. We infer that the two main families of hydrated alteration products detected-phyllosilicates and sulphates--result from different formation processes. These occurred during two distinct climatic episodes: an early Noachian Mars, resulting in the formation of hydrated silicates, followed by a more acidic environment, in which sulphates formed.     

Two pathways converge at CED-10 to mediate actin rearrangement and corpse removal in C. elegans

The removal of apoptotic cells is essential for the physiological well being of the organism. In Caenorhabditis elegans, two conserved, partially redundant genetic pathways regulate this process. In the first pathway, the proteins CED-2, CED-5 and CED-12 (mammalian homologues CrkII, Dock180 and ELMO, respectively) function to activate CED-10 (Rac1). In the second group, the candidate receptor CED-1 (CD91/LRP/SREC) probably recognizes an unknown ligand on the apoptotic cell and signals via its cytoplasmic tail to the adaptor protein CED-6 (hCED-6/GULP), whereas CED-7 (ABCA1) is thought to play a role in membrane dynamics. Molecular understanding of how the second pathway promotes engulfment of the apoptotic cell is lacking. Here, we show that CED-1, CED-6 and CED-7 are required for actin reorganization around the apoptotic cell corpse, and that CED-1 and CED-6 colocalize with each other and with actin around the dead cell. Furthermore, we find that the CED-10(Rac) GTPase acts genetically downstream of these proteins to mediate corpse removal, functionally linking the two engulfment pathways and identifying the CED-1, -6 and -7 signalling module as upstream regulators of Rac activation.