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There is a north-south divide on Mars. The southern highlands covering about 60% of the planet, are heavily cratered, while the northern lowlands are lightly cratered and geologically younger, with a thinner underlying crust. Favoured explanations for this 'hemispheric dichotomy' are mantle convection or a giant impact, but there is little evidence to distinguish between the theories. Three Letters in this issue provide support for the giant impact model. Marinova et al. present dynamical simulations of dichotomy-forming impacts that demonstrate the feasibility of a giant impact origin. A simulation with favoured impact conditions is shown on the cover: the snapshot is about 30 min after impact and the colours code for internal energy. (Cover image: S Lombeyda, Caltech Center for Advanced Computing Research/ M Marinova & O Aharonson, Caltech.) Andrews-Hanna et al. use gravity and topography data to map the dichotomy boundary beneath the Tharsis volcanic province, and find an elliptical boundary, consistent with an oblique giant impact origin. Nimmo et al. use numerical modelling to show that a vertical impact, as well as excavating a crustal cavity of the right size, can explain the observed crustal disruption and the formation of the northern lowlands crust. In an accompanying News & Views , Walter Kiefer sums up the evidence for the impact model.
The state of California is clamping down on companies that offer direct-to-consumer genetic testing in a move that threatens the burgeoning industry. Meredith Wadman looks at a grey area in US regulation.
The UK government has invested heavily in science. Now it's looking for a return, and some worry that the research councils are being pressured to deliver, possibly at the expense of 'blue skies' research. Geoff Brumfiel looks at the changing landscape of science funding in Britain.
Fewer people are searching for near-Earth asteroids, astronomer David Morrison said in the 1990s, than work a shift in a small McDonalds. But that group — a little larger now — has over the past two decades discovered a host of happily harmless rocks, and in doing so reduced the risk of an unknown asteroid blighting civilization (see page 1178). David Chandler puts together the story in the words of those who watched, and those who watched the watchers.
From a 5-millimetre dent on a satellite to a 3-kilometre pit in the surface of Mars, the scars of impact events can be seen at every scale. We present a gallery of some particularly appealing ones from Earth and beyond.
A survey of large objects near Earth has shown that there is little risk of a cataclysmic impact in the next century. Alan Harris asks if such cataloguing efforts should continue.
A spotlight on the historic US fishing port of Gloucester fails to capture the complexity of the fisheries collapse caused by overexploitation and regulation, says Daniel Pauly.
Watercolour artist and explorer Tony Foster paints in some extreme places. He has climbed mountains, sketched erupting volcanoes and drawn underwater. As an exhibition of his works of Mount Everest and the Grand Canyon opens in London, he tells Nature why he goes to such extraordinary efforts.
Oliver Morton recalls how the first major science fiction novel to depict an impact event conjured the thrill and the horror of natural cataclysm — and even inspired some researchers.
Discerning the meaning behind Maurizio Cattelan's violent, provocative and now highly valuable sculpture of Pope John Paul II felled by a meteorite raises many questions for viewers, explains Martin Kemp.
The way performers shape notes brings music to life. Nicholas Cook argues that measuring these subtle changes can help us appreciate and replicate the performer's art.
Thanks to a fateful letter, the theory of evolution by natural selection was unveiled 150 years ago this week. Andrew Berry and Janet Browne celebrate the letter's writer, Alfred Russel Wallace.
Early in its history, Mars suffered a convulsion that left a lasting geological and topographical scar. The latest work adds to evidence that the cause was external — a massive impact.
Flies are cleverer than previously thought. They can remember their original destination even if distracted en route by another landmark. This behaviour depends on a specific group of neurons.
When a blood vessel becomes blocked, the ideal treatment would be a drug that induces new vessel formation in the damaged tissue, without affecting healthy tissues. With the chemical nitrite, we might be on to a winner.
Halogens are known to decrease the levels of stratospheric ozone. The latest measurements show that something similar occurs in the lower atmosphere over tropical oceans — and probably above most other oceans, too.
Advances in DNA-sequencing technology provide unprecedented insight into the entire collection of four genomes' transcribed sequences. They herald a new era in the study of gene regulation and genome function.
The transition from water to land during the late Devonian is marked by the early tetrapods Ichthyostega and Acanthostega, and the tetrapod-like fish Tiktaalik. An analysis of recently discovered material shows that Ventastega curonica might be seen as a simple intermediate between the Tiktaalik and Acanthostega. However, the picture is more complicated than this due to the unexpected morphological diversity of early tetrapods.
In Xenopus, the bone morphogenetic protein (BMP) activity gradient is defined by a 'shuttling-based' mechanism, whereby the BMP ligands are translocated ventrally through their association with the BMP inhibitor Chordin. This shuttling, with feedback repression of the BMP ligand Admp, offers a quantitative explanation to earlier observations, and accounts naturally for the scaling of embryo pattern with its size.
The hemispheric dichotomy between the southern highlands and northern lowlands on Mars is characterized by a change in crustal thickness along an apparently irregular boundary, which can be traced around the planet, except where it is presumably buried beneath the Tharsis volcanic rise. The gravity and topography of Mars is used to constrain the location of the dichotomy boundary beneath Tharsis, and finds that the dichotomy boundary along its entire path around the planet is accurately fit by an ellipse measuring about 10,600 by 8,500 km.
The Martian 'hemispheric dichotomy' is expressed as a dramatic difference in elevation and crustal thickness between the southern highlands and northern lowlands. A set of single impact initial conditions by which a large impactor can produce features that are consistent with the observed dichotomy's crustal structure have been found. Using 3D hydrodynamic simulations, the models produce large variations in post-impact states depending on impact energy, velocity and, importantly, impact angle.
A high-resolution 2D hydrocode is used to model vertical impacts over a range of parameters appropriate to early Mars. It is proposed that the impact model, in addition to excavating a crustal cavity of the correct size, explains two additional observations: crustal disruption at the impact antipode, which it is inferred is probably responsible for the observed antipodal decline in magnetic field strength; and the impact-generated melt forming the northern lowlands crust.
The observation of a single gap in the superconductor SmFeAsO0.85F0.15 is reported. The gap value, its decrease with temperature and vanishing at Tc are all consistent with the Bardeen–Cooper–Schrieffer predictions, but dramatically different from that of the pseudogap behaviour in the copper oxide superconductors. The results are not compatible with models involving antiferromagnetic fluctuations, strong correlations, t-J model, and the like, originally designed for the high-Tc copper oxides.
Double catalytic enantioselective transformations are powerful synthetic methods that can facilitate the construction of stereochemically-complex molecules in a single operation. In this paper, the use of a double stereoablative enantioselective alkylation reaction in a concise synthesis of the marine diterpenoid (−)-cyanthiwigin F is reported. The key step involves the conversion of a complicated mixture of racemic and meso diastereomers into a synthetically useful intermediate with exceptional enantiomeric excess.
This paper reports eight months of spectroscopic measurements at the Cape Verde Observatory indicative of the ubiquitous daytime presence of bromine monoxide and iodine monoxide in the tropical marine boundary layer. A year-round data set of co-located in situ surface trace gas measurements made in conjunction with low level aircraft observations show that the mean daily observed ozone loss is ∼50 per cent greater than that simulated by a global chemistry model using a classical photochemistry scheme that excludes halogen chemistry.
New deep submergence technologies are used to obtain photographic images of a 'zero-age' volcanic terrain on the ultra-slow spreading Gakkel Ridge in the Arctic Basin. The imagery reveals that the axial valley at 4,000m water depth is blanketed with unconsolidated pyroclastic deposits, raising important questions regarding the accumulation and discharge of magmatic volatiles on such ridges and demonstrating that large-scale pyroclastic activity is possible along even the deepest portions of the global mid-ocean ridge volcanic system.
Using recently developed DNA sequencing technologies, nucleic acid transcripts are characterized in unprecedented detail from the yeast Schizosaccharomyces pombe. The sequences definitively demonstrate that 90% of more of the genome is transcribed into RNA, and show a previously unseen link between transcription and splicing efficiency at different points in the cell's growth.
Visual orientation in a complex environment requires a memory for targets' spatial position, in case they become temporarily out of sight, a faculty known as 'spatial working memory' in vertebrates. Use of a virtual-reality arena to present visual targets to walking fruit flies shows that insects share the faculty. Cell-specific gene rescue in learning mutants, ablation of brain areas and neuron-specific silencing experiments revealed that a distinct subset of neurons in the central brain is required for a spatial working memory in flies.
In the brain, both rate and temporal codes are critical for information storage. Theta phase precession is a change in action potential timing in the hippocampus where place cells fire at progressively earlier phases of the theta rhythm as the animal moves across the firing field of the neuron. This paper explores the circuitry of theta phase precession and shows that phase precession is expressed independently of the hippocampus in spatially modulated grid cells in parts of the entorhinal cortex.
Some people earn rewards more successfully when performing goal-directed tasks, but the neuronal changes that could mediate this reward-directed learning are not well understood. Rats were trained to self-administer a sucrose reward, and it was shown that reward learning depends on increased activity and synaptic strength in the amygdala, a brain region important for emotional learning. The level of learning attained by individual animals correlated well with the degree of synaptic strength enhancement.
Antiviral drugs are seen as essential requirements for control of initial influenza outbreaks caused by a new virus. This paper presents enzymatic properties and crystal structures of neuraminidase mutants from H5N1 infected patients that explain the molecular basis of observed resistance against the neuraminidase inhibitor Oseltamivir.
On the basis of the known crystallographic structures of 5000 'homo-oligomeric' complexes, this study derives plausible pathways for the emergence of ever more complex such assemblies during evolution. Using electrospray mass spectrometry, it is observed that the same pathways are followed on the shorter time-scale of protein assembly in vitro.
Hydrogen bonds have been widely assumed to be strongly stabilizing in membrane proteins. But in this work, interaction free energies were measured between eight hydrogen bonded side chains in bacteriorhodopsin and it was found that most make only modest stabilizing contributions. This suggests that views of membrane protein folding, evolution and function should reflect such weak polar side chain interactions.
Drosophila Argos is an antagonist of EGF receptor signalling that functions by binding and sequestering EGFR ligands. The structure of Argos bound to the EGFR ligand Spitz reveals that Argos engulfs the ligand using three related domains with structural resemblance to receptors for TGF-β and urokinase plasminogen activator.
The 5′ and 3′ ends of eukaryotic mRNAs are brought together to form a closed loop, which helps stabilize the mRNA and facilitates translation. There are two distinct closed loops formed in yeast, which differ in the form of the ribosome that is bound. The form containing the full ribosome is also surprisingly associated with termination factors, even though termination is not required for loop formation.