The New MDS Website

Ventastega curonica and the origin of tetrapod morphology

Nature 453, 1199 (2008). doi:10.1038/nature06991

Authors: Per E. Ahlberg, Jennifer A. Clack, Ervīns Lukševičs, Henning Blom & Ivars Zupiņš

The gap in our understanding of the evolutionary transition from fish to tetrapod is beginning to close thanks to the discovery of new intermediate forms such as Tiktaalik roseae. Here we narrow it further by presenting the skull, exceptionally preserved braincase, shoulder girdle and

Scaling of the BMP activation gradient in Xenopus embryos

Nature 453, 1205 (2008). doi:10.1038/nature07059

Authors: Danny Ben-Zvi, Ben-Zion Shilo, Abraham Fainsod & Naama Barkai

In groundbreaking experiments, Hans Spemann demonstrated that the dorsal part of the amphibian embryo can generate a well-proportioned tadpole, and that a small group of dorsal cells, the ‘organizer’, can induce a complete and well-proportioned twinned axis when transplanted into a host embryo. Key to

The Borealis basin and the origin of the martian crustal dichotomy

Nature 453, 1212 (2008). doi:10.1038/nature07011

Authors: Jeffrey C. Andrews-Hanna, Maria T. Zuber & W. Bruce Banerdt

The most prominent feature on the surface of Mars is the near-hemispheric dichotomy between the southern highlands and northern lowlands. The root of this dichotomy is 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 isostatic compensation of these distinct provinces and the ancient population of impact craters buried beneath the young lowlands surface suggest that the dichotomy is one of the most ancient features on the planet. However, the origin of this dichotomy has remained uncertain, with little evidence to distinguish between the suggested causes: a giant impact or mantle convection/overturn. Here we use the gravity and topography of Mars to constrain the location of the dichotomy boundary beneath Tharsis, taking advantage of the different modes of compensation for Tharsis and the dichotomy to separate their effects. We find that the dichotomy boundary along its entire path around the planet is accurately fitted by an ellipse measuring approximately 10,600 by 8,500 km, centred at 67° N, 208° E. We suggest that the elliptical nature of the crustal dichotomy is most simply explained by a giant impact, representing the largest such structure thus far identified in the Solar System.

Mega-impact formation of the Mars hemispheric dichotomy

Nature 453, 1216 (2008). doi:10.1038/nature07070

Authors: Margarita M. Marinova, Oded Aharonson & Erik Asphaug

The Mars hemispheric dichotomy is expressed as a dramatic difference in elevation, crustal thickness and crater density between the southern highlands and northern lowlands (which cover ∼42% of the surface). Despite the prominence of the dichotomy, its origin has remained enigmatic and models for its formation largely untested. Endogenic degree-1 convection models with north–south asymmetry are incomplete in that they are restricted to simulating only mantle dynamics and they neglect crustal evolution, whereas exogenic multiple impact events are statistically unlikely to concentrate in one hemisphere. A single mega-impact of the requisite size has not previously been modelled. However, it has been hypothesized that such an event could obliterate the evidence of its occurrence by completely covering the surface with melt or catastrophically disrupting the planet. Here we present a set of single-impact initial conditions by which a large impactor can produce features consistent with the observed dichotomy’s crustal structure and persistence. Using three-dimensional hydrodynamic simulations, large variations are predicted in post-impact states depending on impact energy, velocity and, importantly, impact angle, with trends more pronounced or unseen in commonly studied smaller impacts. For impact energies of ∼(3–6) × 1029 J, at low impact velocities (6–10 km s-1) and oblique impact angles (30–60°), the resulting crustal removal boundary is similar in size and ellipticity to the observed characteristics of the lowlands basin. Under these conditions, the melt distribution is largely contained within the area of impact and thus does not erase the evidence of the impact’s occurrence. The antiquity of the dichotomy is consistent with the contemporaneous presence of impactors of diameter 1,600–2,700 km in Mars-crossing orbits, and the impact angle is consistent with the expected distribution.

Implications of an impact origin for the martian hemispheric dichotomy

Nature 453, 1220 (2008). doi:10.1038/nature07025

Authors: F. Nimmo, S. D. Hart, D. G. Korycansky & C. B. Agnor

The observation that one hemisphere of Mars is lower and has a thinner crust than the other (the ‘martian hemispheric dichotomy’) has been a puzzle for 30 years. The dichotomy may have arisen as a result of internal mechanisms such as convection. Alternatively, it may have been caused by one or several giant impacts, but quantitative tests of the impact hypothesis have not been published. Here we use a high-resolution, two-dimensional, axially symmetric hydrocode to model vertical impacts over a range of parameters appropriate to early Mars. We propose that the impact model, in addition to excavating a crustal cavity of the correct size, explains two other observations. First, crustal disruption at the impact antipode is probably responsible for the observed antipodal decline in magnetic field strength. Second, the impact-generated melt forming the northern lowlands crust is predicted to derive from a deep, depleted mantle source. This prediction is consistent with characteristics of martian shergottite meteorites and suggests a dichotomy formation time ∼100 Myr after martian accretion, comparable to that of the Moon-forming impact on Earth.

The total synthesis of (-)-cyanthiwigin F by means of double catalytic enantioselective alkylation

Nature 453, 1228 (2008). doi:10.1038/nature07046

Authors: John A. Enquist Jr & Brian M. Stoltz

Double catalytic enantioselective transformations are powerful synthetic methods that can facilitate the construction of stereochemically complex molecules in a single operation. In addition to generating two or more stereocentres in a single reaction, multiple asymmetric reactions also impart increased enantiomeric excess to the final product in comparison with the analogous single transformation. Furthermore, multiple asymmetric operations have the potential to independently construct several stereocentres at remote points within the same molecular scaffold, rather than relying on pre-existing chiral centres that are proximal to the reactive site. Despite the inherent benefits of multiple catalytic enantioselective reactions, their application to natural product total synthesis remains largely underutilized. Here we report the use of a double stereoablative enantioselective alkylation reaction in a concise synthesis of the marine diterpenoid (-)-cyanthiwigin F (ref. 8). By employing a technique for independent, selective formation of two stereocentres in a single stereoconvergent operation, we demonstrate that a complicated mixture of racemic and meso diastereomers may be smoothly converted to a synthetically useful intermediate with exceptional enantiomeric excess. The stereochemical information generated by means of this catalytic transformation facilitates the easy and rapid completion of the total synthesis of this marine natural product.

Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean

Nature 453, 1232 (2008). doi:10.1038/nature07035

Authors: Katie A. Read, Anoop S. Mahajan, Lucy J. Carpenter, Mathew J. Evans, Bruno V. E. Faria, Dwayne E. Heard, James R. Hopkins, James D. Lee, Sarah J. Moller, Alastair C. Lewis, Luis Mendes, James B. McQuaid, Hilke Oetjen, Alfonso Saiz-Lopez, Michael J. Pilling & John M. C. Plane

Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation; the prediction of future trends in tropospheric ozone will require a full understanding of both its precursor emissions and its destruction processes. A large proportion of tropospheric ozone loss occurs in the tropical marine boundary layer and is thought to be driven primarily by high ozone photolysis rates in the presence of high concentrations of water vapour. A further reduction in the tropospheric ozone burden through bromine and iodine emitted from open-ocean marine sources has been postulated by numerical models, but thus far has not been verified by observations. Here we report 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 shows 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. We perform box model calculations that indicate that the observed halogen concentrations induce the extra ozone loss required for the models to match observations. Our results show that halogen chemistry has a significant and extensive influence on photochemical ozone loss in the tropical Atlantic Ocean boundary layer. The omission of halogen sources and their chemistry in atmospheric models may lead to significant errors in calculations of global ozone budgets, tropospheric oxidizing capacity and methane oxidation rates, both historically and in the future.

Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean

Nature 453, 1236 (2008). doi:10.1038/nature07075

Authors: Robert A. Sohn, Claire Willis, Susan Humphris, Timothy M. Shank, Hanumant Singh, Henrietta N. Edmonds, Clayton Kunz, Ulf Hedman, Elisabeth Helmke, Michael Jakuba, Bengt Liljebladh, Julia Linder, Christopher Murphy, Ko-ichi Nakamura, Taichi Sato, Vera Schlindwein, Christian Stranne, Maria Tausenfreund, Lucia Upchurch, Peter Winsor, Martin Jakobsson & Adam Soule

Roughly 60% of the Earth’s outer surface is composed of oceanic crust formed by volcanic processes at mid-ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid-ocean ridges, particularly at depths below the critical point for seawater (3,000 m). A pyroclastic deposit has never been observed on the sea floor below 3,000 m, presumably because the volatile content of mid-ocean-ridge basalts is generally too low to produce the gas fractions required for fragmenting a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel ridge in the Arctic Basin at 85° E, to acquire photographic and video images of ‘zero-age’ volcanic terrain on this remote, ice-covered ridge. Here we present images revealing that the axial valley at 4,000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele), covering a large (>10 km2) area. At least 13.5 wt% CO2 is necessary to fragment magma at these depths, which is about tenfold the highest values previously measured in a mid-ocean-ridge basalt. These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global mid-ocean ridge volcanic system.

Part II of McChesney's tour de force

Many Philadelphians rudely reject the premise meticulously detailed in the new book by veteran journalist J. Patrick O'Connor: police and prosecutors framed Mumia Abu-Jamal placing an innocent man on death row.... "From the beginning of this case, it was corrupt. It was a railroad job," O'Connor said recently during a reading/book signing at a small venue on Baltimore Ave in West Philadelphia sponsored by the organization, Journalists for Abu-Jamal. "I wrote the book to show not only that Mumia did not kill Officer Faulkner but to show how and why they framed Mumia," said O'Connor who lived in the Philadelphia area at the time of the brutal December 1981 crime at the heart of this controversial case.

Preventative health care positions are being threatened by state budget cuts and may soon disappear since they are not part of mandated nurse-patient ratios. San Francisco is looking to lose 30 percent of its home health care program for Medi-Cal patients. In less heavily populated areas, including Monterey and Humboldt counties, one home health care nurse makes the difference between care or no care for people isolated by lack of public transportation.

In preparation for the upcoming Climate Convergence, a public forum in Corvallis, OR on (7/8) will discuss the risks of locating LNG terminals along seismically active Cascadian subduction zone of coastal Oregon

Our representatives have turned on us. It is very clear, with the passage of HR6304 in the House of Representatives, that it will be up to us to make a difference in this country.

This news show airs weekly on Freak Radio, Friday mornings 10-12 noon, broadcasting news and opinion from independent media worldwide, focused on indymedia sites but also drawing from other websites. This week we heard audio from KBOO, KBCS, Radio4All, AudioPort, and indymedia sites indybay, santa cruz IMC, indymedia UK, san diego IMC, and more

The Justice for Javon Dawson Committee is calling on all honest and freedom-loving people to support the demands for justice and reparations in the St. Petersburg police murder of this 17-year-old African who was attending a high school graduation party and unarmed when he was shot twice in the back and killed. (read full story: http://uhurunews.com)