On the horizon

Do mixed wild grasses make the best biofuel?

Scientists at the University of Minnesota have found that native wild grasses, shrubs, and tree seedlings may provide more energy as a biofuel than crops such as corn, oil seeds, sugar cane, or switch grass. Native species also store more atmospheric carbon dioxide in soils and lead to less chemical pollution than crops cultivated for biofuels.

The researchers seeded 152 plots of land that were so sandy and nutrient-poor that farmers had abandoned them long ago. They planted from one to 16 species of grassland perennials on each plot. Most were grasses and shrubs. Some plots hosted a few oak seedlings. The land was unfertilized during the experiment, and the team watered the plots only at the outset.

The experiment ran from 1994 to 2005. In the final three years, researchers found that the greater the diversity of plants on a plot, the greater the energy content of the plant material. Computer simulations showed that, when used in synthetic gasoline or diesel fuel, these plants would produce 51 percent more energy per hectare from degraded land than corn does from fertile soils. Moreover, corn ethanol and soybean biodiesel are carbon-positive sources after taking into account the CO2 emitted in growing and burning them. The wild grasses so enriched the soils with carbon and required so little human attention that they were carbon-neutral – even after they were burned as an additive to coal or turned into ethanol and burned. The research appears in the current issue of the journal Science.

Mystery of the Martian crust

A team of scientists has found that the crust under Mars' northern lowlands is far older than many believed. That's an important clue for scientists trying to solve one of the planet's enduring mysteries: Why the stark difference in terrain between the northern and southern hemisphere?

The boundary between the two is abrupt – the highlands vault from 2 km (1.2 miles) to 5 km above the lightly cratered lowlands. This suggests to many researchers that the lowland crust must be geologically young. How did this younger crust form? Perhaps giant impacts did the deed, or some form of plate tectonics, or maybe the lowland crust is the cold remnant of an ancient magma ocean.

Enter Thomas Watter of the Smithsonian Institution's National Air and Space Museum and his colleagues. A unique radar aboard the European Space Agency's Mars Express orbiter uncovered 11 impact craters ranging from 130 to 470 km across. Given the amount of surface they covered (14 percent of the lowlands) and the number of craters wider than 200 km they found, the team estimates that the lowland crust is 4 billion years old. The results appear in Thursday's issue of the journal Nature.

Arctic summer ice may disappear by 2040

US and Canadian scientists calculate that late-summer sea ice in the Arctic could vanish by 2040, due to the warming effect of carbon dioxide from burning oil, coal, and gas. The calculations assume that greenhouse gases will continue to build in the atmosphere at their current rates.

The researchers note that summer sea ice already has declined dramatically. The losses, they say, are likely to accelerate. Open water absorbs more heat from the sun, and changing ocean-circulation patterns are driving warmer water into the Arctic Ocean.

Researchers used an advanced climate model to test the effects of three possible greenhouse-gas emissions paths on warming and Arctic sea ice. They initially simulated conditions from 1870 to 2005 and found that the model tracked observed conditions well – including the dramatic shrinkage of ice over the past 26 years. Then they looked at seven simulations of future ice trends under the emissions scenarios.

The simulations showed an abrupt decline in ice cover within 30 to 50 years. In comparing their results with those from 15 other simulations using different models and different rates of increase in greenhouse gases, the team concludes that if emissions slowed, the retreat of summer ice also would slow. The work appears in the current issue of Geophysical Review Letters.

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