西亚试剂：Curtain eruptions from Enceladus’ south-polar terrain

Observations of the south pole of the Saturnian moon Enceladus revealed large rifts in the south-polar terrain, informally called ‘tiger stripes’, named Alexandria, Baghdad, Cairo and Damascus Sulci. These fractures have been shown to be the sources of the observed jets of water vapour and icy particles1, 2, 3, 4 and to exhibit higher temperatures than the surrounding terrain5, 6. Subsequent observations have focused on obtaining close-up imaging of this region to better characterize these emissions. Recent work7 examined those newer data sets and used triangulation of discrete jets3 to produce maps of jetting activity at various times. Here we show that much of the eruptive activity can be explained by broad, curtain-like eruptions. Optical illusions in the curtain eruptions resulting from a combination of viewing direction and local fracture geometry produce image features that were probably misinterpreted previously as discrete jets. We present maps of the total emission along the fractures, rather than just the jet-like component, for five times during an approximately one-year period in 2009 and 2010. An accurate picture of the style, timing and spatial distribution of the south-polar eruptions is crucial to evaluating theories for the mechanism controlling the eruptions.

Despite much progress in semiconductor integrated circuit technology, the extreme complexity of the human cerebral cortex1, with its approximately 1014 synapses, makes the hardware implementation of neuromorphic networks with a comparable number of devices exceptionally challenging. To provide comparable complexity while operating much faster and with manageable power dissipation, networks2 based on circuits3, 4 combining complementary metal-oxide-semiconductors (CMOSs) and adjustable two-terminal resistive devices (memristors) have been developed. In such circuits, the usual CMOS stack is augmented with one3 or several4 crossbar layers, with memristors at each crosspoint. There have recently been notable improvements in the fabrication of such memristive crossbars and their integration with CMOS circuits5, 6, 7, 8, 9, 10, 11, 12, including first demonstrations5, 6, 12 of their vertical integration. Separately, discrete memristors have been used as artificial synapses in neuromorphic networks13, 14, 15, 16, 17, 18. Very recently, such experiments have been extended19 to crossbar arrays of phase-change memristive devices. The adjustment of such devices, however, requires an additional transistor at each crosspoint, and hence these devices are much harder to scale than metal-oxide memristors11, 20, 21, whose nonlinear current–voltage curves enable transistor-free operation. Here we report the experimental implementation of transistor-free metal-oxide memristor crossbars, with device variability sufficiently low to allow operation of integrated neural networks, in a simple network: a single-layer perceptron (an algorithm for linear classification). The network can be taught in situ using a coarse-grain variety of the delta rule algorithm22 to perform the perfect classification of 3 × 3-pixel black/white images into three classes (representing letters). This demonstration is an important step towards much larger and more complex memristive neuromorphic networks.

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