Energetic neutral atoms from a trans-Europa gas torus at Jupiter

The space environments--or magnetospheres--of magnetized planets emit copious quantities of energetic neutral atoms (ENAs) at energies between tens of electron volts to hundreds of kiloelectron volts (keV). These energetic atoms result from charge exchange between magnetically trapped energetic ions and cold neutral atoms, and they carry significant amounts of energy and mass from the magnetospheres. Imaging their distribution allows us to investigate the structure of planetary magnetospheres. Here we report the analysis of 50-80 keV ENA images of Jupiter's magnetosphere, where two distinct emission regions dominate: the upper atmosphere of Jupiter itself, and a torus of emission residing just outside the orbit of Jupiter's satellite Europa. The trans-Europa component shows that, unexpectedly, Europa generates a gas cloud comparable in gas content to that associated with the volcanic moon Io. The quantity of gas found indicates that Europa has a much greater impact than hitherto believed on the structure of, and the energy flow within, Jupiter's magnetosphere.     

An essential role for postsynaptic calmodulin and protein kinase activity in long-term potentiation

The phenomenon of long-term potentiation (LTP), a long lasting increase in the strength of synaptic transmission which is due to brief, repetitive activation of excitatory afferent fibres, is one of the most striking examples of synaptic plasticity in the mammalian brain. In the CA1 region of the hippocampus, the induction of LTP requires activation of NMDA (N-methyl-D-aspartate) receptors by synaptically released glutamate with concomitant postsynaptic membrane depolarization. This relieves the voltage-dependent magnesium block of the NMDA-receptor ion channel, allowing calcium to flow into the dendritic spine. Although calcium has been shown to be a necessary trigger for LTP (refs 11, 12), little is known about the immediate biochemical processes that are activated by calcium and are responsible for LTP. The most attractive candidates have been calcium/calmodulin-dependent protein kinase II (CaM-KII) (refs 13-16), protein kinase C (refs 17-19), and the calcium-dependent protease, calpain. Extracellular application of protein kinase inhibitors to the hippocampal slice preparation blocks the induction of LTP (refs 21-23) but it is unclear whether this is due to a pre- and/or postsynaptic action. We have found that intracellular injection into CA1 pyramidal cells of the protein kinase inhibitor H-7, or of the calmodulin antagonist calmidazolium, blocks LTP. Furthermore, LTP is blocked by the injection of synthetic peptides that are potent calmodulin antagonists and inhibit CaM-KII auto- and substrate phosphorylation. These findings demonstrate that in the postsynaptic cell both activation of calmodulin and kinase activity are required for the generation of LTP, and focus further attention on the potential role of CaM-KII in LTP.     

Transient aurora on Jupiter from injections of magnetospheric electrons

Energetic electrons and ions that are trapped in Earth's magnetosphere can suddenly be accelerated towards the planet. Some dynamic features of Earth's aurora (the northern and southern lights) are created by the fraction of these injected particles that travels along magnetic field lines and hits the upper atmosphere. Jupiter's aurora appears similar to Earth's in some respects; both appear as large ovals circling the poles and both show transient events. But the magnetospheres of Jupiter and Earth are so different---particularly in the way they are powered---that it is not known whether the magnetospheric drivers of Earth's aurora also cause them on Jupiter. Here we show a direct relationship between Earth-like injections of electrons in Jupiter's magnetosphere and a transient auroral feature in Jupiter's polar region. This relationship is remarkably similar to what happens at Earth, and therefore suggests that despite the large differences between planetary magnetospheres, some processes that generate aurorae are the same throughout the Solar System.     

The auroral footprint of Enceladus on Saturn

Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus could lead to electrodynamic coupling between Enceladus and Saturn like that which links Jupiter with Io, Europa and Ganymede. Powerful field-aligned electron beams associated with the Io-Jupiter coupling, for example, create an auroral footprint in Jupiter's ionosphere. Auroral ultraviolet emission associated with Enceladus-Saturn coupling is anticipated to be just a few tenths of a kilorayleigh (ref. 12), about an order of magnitude dimmer than Io's footprint and below the observable threshold, consistent with its non-detection. Here we report the detection of magnetic-field-aligned ion and electron beams (offset several moon radii downstream from Enceladus) with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in a few per cent of the scans of the moon's footprint. The footprint varies in emission magnitude more than can plausibly be explained by changes in magnetospheric parameters--and as such is probably indicative of variable plume activity.     

Anti-planetward auroral electron beams at Saturn

Strong discrete aurorae on Earth are excited by electrons, which are accelerated along magnetic field lines towards the planet. Surprisingly, electrons accelerated in the opposite direction have been recently observed. The mechanisms and significance of this anti-earthward acceleration are highly uncertain because only earthward acceleration was traditionally considered, and observations remain limited. It is also unclear whether upward acceleration of the electrons is a necessary part of the auroral process or simply a special feature of Earth's complex space environment. Here we report anti-planetward acceleration of electron beams in Saturn's magnetosphere along field lines that statistically map into regions of aurora. The energy spectrum of these beams is qualitatively similar to the ones observed at Earth, and the energy fluxes in the observed beams are comparable with the energies required to excite Saturn's aurora. These beams, along with the observations at Earth and the barely understood electron beams in Jupiter's magnetosphere, demonstrate that anti-planetward acceleration is a universal feature of aurorae. The energy contained in the beams shows that upward acceleration is an essential part of the overall auroral process.     

Site-directed mutagenesis of cytochrome c shows that an invariant Phe is not essential for function

Phenylalanine 87 of yeast iso-1-cytochrome c (Phe 82 in horse heart and bonito) is phylogenetically conserved and occurs near the surface of the protein. It has been suggested that this residue is directly involved in electron transfer between cytochrome c and cytochrome c peroxidase (CCP) and may also control the polarity of the haem environment. Because Phe residues are not susceptible to chemical modification, no direct means of studying the functional role of Phe 87 has been available, so we have chosen Phe 87 as our initial target here to test the feasibility of using site-directed mutagenesis as a means of studying structure-function relationships in cytochrome c. We have changed the codon for Phe 87 to that of either a Ser, a Tyr or a Gly. The mutated genes have been introduced into a yeast strain lacking both isozymes of cytochrome c. Unlike the recipient strain, transformants grow on a non-fermentable carbon source, indicating that the mutant proteins can reduce cytochrome oxidase. The purified mutant proteins are similar to wild type with respect to their visible spectra, 20-70% as active as wild-type protein in the CCP assay, and their reduction potentials are lowered by as much as 50 mV. Thus Phe 87 is not essential for cytochrome c to transfer electrons but is involved in determining the reduction potential.     

Inhibition of climbing fibres is a signal for the extinction of conditioned eyelid responses

A fundamental tenet of cerebellar learning theories asserts that climbing fibre afferents from the inferior olive provide a teaching signal that promotes the gradual adaptation of movements. Data from several forms of motor learning provide support for this tenet. In pavlovian eyelid conditioning, for example, where a tone is repeatedly paired with a reinforcing unconditioned stimulus like periorbital stimulation, the unconditioned stimulus promotes acquisition of conditioned eyelid responses by activating climbing fibres. Climbing fibre activity elicited by an unconditioned stimulus is inhibited during the expression of conditioned responses-consistent with the inhibitory projection from the cerebellum to inferior olive. Here, we show that inhibition of climbing fibres serves as a teaching signal for extinction, where learning not to respond is signalled by presenting a tone without the unconditioned stimulus. We used reversible infusion of synaptic receptor antagonists to show that blocking inhibitory input to the climbing fibres prevents extinction of the conditioned response, whereas blocking excitatory input induces extinction. These results, combined with analysis of climbing fibre activity in a computer simulation of the cerebellar-olivary system, suggest that transient inhibition of climbing fibres below their background level is the signal that drives extinction.     

A nebula of gases from Io surrounding Jupiter

Several planetary missions have reported the presence of substantial numbers of energetic ions and electrons surrounding Jupiter; relativistic electrons are observable up to several astronomical units (au) from the planet. A population of energetic (>30[?]keV) neutral particles also has been reported, but the instrumentation was not able to determine the mass or charge state of the particles, which were subsequently labelled energetic neutral atoms. Although images showing the presence of the trace element sodium were obtained, the source and identity of the neutral atoms---and their overall significance relative to the loss of charged particles from Jupiter's magnetosphere---were unknown. Here we report the discovery by the Cassini spacecraft of a fast (>103[?]km[?]s-1) and hot magnetospheric neutral wind extending more than 0.5[?]au from Jupiter, and the presence of energetic neutral atoms (both hot and cold) that have been accelerated by the electric field in the solar wind. We suggest that these atoms originate in volcanic gases from Io, undergo significant evolution through various electromagnetic interactions, escape Jupiter's magnetosphere and then populate the environment around the planet. Thus a 'nebula' is created that extends outwards over hundreds of jovian radii.