- Moon Crater Map Reveals Early Solar System History
- Clouds Are Shaped by Where They’re From
- How Mass Migration Might Have Evolved
- Hubble Captures Cosmic Ice Sculptures
Posted: 16 Sep 2010 01:51 PM PDT
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The first complete topographic map of the moon and its craters has revealed details of billions of years of bombardment by asteroids, and the early history of our solar system. Among other things, the map confirms theories of an onslaught of massive asteroids around 3.9 billion years ago that likely evaporated any water present on Earth at the time.
"Ever since the surface of the moon could be photographed, scientists have counted craters on the moon and tried to decipher the projectile-bombardment rate and the geological history of the moon," said geologist James Head of Brown University, lead author of the study in Science Sept. 16. "But until now we've had uneven or low-resolution coverage."
The map was created using data from NASA's Lunar Reconnaissance Orbiter that has been circling the moon since June 2009. The orbiter measured the height of the surface by sending billions of laser pulses towards the surface and measuring the time it took for the pulses to return. The method is precise enough it would have been able to detect a small house if there were one, Head said.
Once the map was made, Head and his team cataloged all the craters bigger than 12.5 miles across, more than 5,000 craters in total.
With the catalog of craters, the scientists were able to confirm there have been two different eras of asteroid-pummeling in the solar system. In the most ancient regions of the moon, the craters are all different sizes, large and small. But in the younger regions of the moon that were resurfaced by volcanic activity, the craters sizes are much smaller.
"The evidence we have is that the shift happened before the dominant mare [volcanic flow regions that appear as dark spots on the moon] were created 3.6 billion years ago, and probably before that," said geologist Caleb Fassett of Brown University, co-author of the study.
Planetary surface geologist Robert Strom, who first proposed the theory of the shift in asteroid types in 2005, argues the shift was a result of of a repositioning of Saturn and Jupiter around 3.9 billion years ago. The gravitational pull of the planets causes there to be regions of the meteor belt where anything that enters gets ejected. A shift in the positioning of the planets would have changed the location of these regions and caused a relatively sudden expulsion of meteors of all sizes that happened to be in those, now vacant, areas.
"The intense bombardment that happened around 3.9 billion years ago, the Earth didn't escape that," said Strom. "Earth was impacted so much and by such big objects that it's likely that if there was any water on Earth at that time it would have been evaporated. And any life would have been terminated."
Since this recalibration of the asteroid belt, only relatively small asteroids have drifted into these vacant regions and been booted out, Strom said. Small asteroids drift over time because they are affected by solar energy, whereas large ones have so much mass they are relatively motionless.
Mapping the topography of Mars and Mercury in the future will also help to confirm this theory, Strom said.
The accurate map of the moon was also used to confirm that the oldest regions on the moon are the southern near side and the north-central far side. The moon was volcanic for about half of its history, until it cooled to the point where the volcanism shut down around 2 billion years ago.
Posted: 16 Sep 2010 12:21 PM PDT
Scientists' view of clouds is clearing up. Two new studies show that cloud-forming particles in the atmosphere, called aerosols, look different and make different clouds depending on their origins.
One study found that in one of the most pristine environments on Earth — above the treetops of the Amazon Rainforest — clouds mostly come from gas emitted by the plants. The entire rainforest is a self-sustaining engine in which "the plants cause the rainfall, and the rainfall causes the plants," said Harvard environmental chemist Scot Martin, a coauthor of the study.
The other, which was compiled from 15 years of data from airplanes flying through clouds, found that aerosols with human origins are larger, more numerous and contribute more to haze than biogenic particles.
This paper "demonstrates the importance of combustion-produced aerosols for controlling cloud-forming particles," commented cloud scientist Robert Wood of the University of Washington, who was not involved in either study.
Both studies, which appear in the September 17 Science, fill in "areas that so far have been nearly white spots on the landscape of aerosol information," said Urs Baltensperger of the Paul Scherrer Institute in Switzerland, who wrote a Perspectives article in Science. By providing a baseline to compare modern aerosol conditions against and a comprehensive look at what those conditions are, the two studies may provide a more complete and accurate way to investigate climate change.
Studying the air above the Amazon is about as close as you can get to studying the pre-industrial atmosphere, Martin and his colleagues argue, especially during the rainy season when human influence is minimal.
"That establishes a baseline so we can understand how human activities are changing things now," Martin said. "The Amazon is a laboratory for understanding the way things were, and therefore for measuring how humans have changed things."
Martin and his team built a 130-foot-high research tower deep in the forest north of Manus, Brazil, and collected aerosols from the atmosphere over a period of 10 days in March 2008. Using a wide range of techniques, some of which had never been used in the Amazon before, the team analyzed the samples both on site and back in their labs to determine the particles' size, concentration and origin.
"This is definitely an important paper," commented atmospheric chemist Joel Thornton of the University of Washington, who was not involved in the study. "It's the first study of its kind to make a set of comprehensive measurements of all the aerosol properties we can measure in the Amazon."
The team found that the aerosol concentration — the number of particles in a small volume of space — was 10 to 100 times lower above the Amazon than above more populated areas, even rural regions that are generally considered clean.
The data also show that the number of cloud droplets above the Amazon depends directly on the number of aerosols. This is in contrast to more polluted areas, where the number of cloud droplets depends on how quickly hot particles from burning fires or fossil fuels ascend into the atmosphere. In the cool regions of the upper atmosphere, water droplets condense onto hot aerosols like fog on a window.
These different limiting factors form different clouds, says atmospheric scientist Ulrich Poschl of the Max Planck Institute for Chemistry in Germany, lead author of the study. "Depending on aerosol particles, you form clouds with different properties," he said. "With different cloud properties, you can suppress rainfall so you have less frequent but more intense rain, especially under heavy pollution areas."
What exactly this difference means for the global climate is yet to be seen. But "we now have a clean background scenario to which we can compare our polluted environment," he said. "It's a key for just benchmarking or validating climate models from which we want to learn how we are already influencing the environment, and how that will evolve in the future."
The team also found that clouds and rain in the region mostly came from the plants. Plants emit gases from their leaves and sap, which is one reason why they have distinctive smells. When those gases interact with sunlight, their chemistry changes such that they condense from diffuse gas to liquid droplets less than one micrometer — a thousandth of a millimeter — in size. These droplets then serve as the nucleus of a cloud.
Particles larger than one micrometer, which are important in forming ice crystals, also came from plant matter like pollen, fungus spores and bits of crumpled up leaf.
"The tight coupling that we're able to show between emissions from plants and hydrological cycle shows one area that could end up being quite sensitive to unintended consequences," Martin said.
The second study compiled data from 12 separate experiments conducted since 1995, in which atmospheric scientist Anthony Clarke of the University of Hawaii and colleagues collected atmosphere samples from airplanes over the Pacific Ocean.
"This paper impresses by its wealth of data, many of which stem from areas where very little data has been obtained so far," Baltensperger commented.
The study found that aerosol particles that result from human activity, from crop clearing to combustion engines, are just the right size to interact with light and build clouds. This means that, even if two regions have the same number of aerosols, the man-made aerosols will have a bigger impact.
"In cloud formation and in radiation transfer, larger particles that are sourced from combustion can play a more important role," Clarke said.
Particles of this size — a few hundred nanometers — are around the same size as the wavelength as visible light, which means light bounces off the particles and doesn't make it to the ground. The clouds they form also tend to be a brighter white, meaning they are more reflective. These properties could mean that combustion-based clouds could have a cooling effect on climate, even while greenhouse gases from the same combustion processes heat the planet.
"Of course everybody wants to know, what's the effects of all this?" Clarke said. "Unfortunately that's not easily done without very complex models. But there is data out there now for modelers."
All that data may be the ultimate legacy of these two studies.
"In that sense they are highly complimentary," Poschl said. These studies represent "a major benchmark in advancing the models in this direction, that we really can understand these processes in cloud and rainfall formation."
Images: 1) View of the clouds from a research airplane. Anthony Clarke. 2) The research tower in the Amazon rainforest where atmospheric chemists collected samples. Science/AAAS. 3) Science/AAAS 4) Anthony Clarke
Posted: 16 Sep 2010 08:43 AM PDT
Just a few small changes in the social behaviors of even solitary animals may set in motion an evolutionary cascade ending in massive, globe-spanning migrations, suggests a study of migration's origins.
Such migrations — caribou across the Arctic and wildebeest across the Serengeti, birds and butterflies over oceans — are among nature's most beautiful and mystifying phenomena. Many models suggest how migration works now, in terms of individual actions producing collective behavior; but how it could have started in the first place is far harder to explain.
"Despite the ubiquity of collective migration, and the key function it plays in the ecology of many species, it is still unclear what role social interactions play in the evolution of migratory strategies," wrote Princeton University evolutionary biologists Iain Couzin and Vishwesha Guttal in a study published Sept. 14 in the Proceedings of the National Academy of Sciences.
In their evolutionary model, Couzin and Guttal assumed two fundamental traits. First, the digital animals needed the ability to respond to a direction-linked environmental cue, of the sort provided in reality by temperature, geomagnetism, wind and chemical gradients. The second required trait was sociability, or an ability to be attracted toward moving neighbors and physically align with them.
Each adaptation came with a cost, reflecting the energy required to follow a cue and the dangers of disease associated with group exposure. The evolutionary benefit was scored according to how far organisms migrated. They ran the model again and again, for a wide range of population densities and migration costs and benefits. Over and over, the same pattern emerged. Evolution tossed up two distinct types of individuals: "leaders," who followed environmental cues and ignored everyone else, and "sociable" individuals, who were attracted to others but themselves oblivious to the cues.
Extrapolation from computer models to real-world behavior is always tricky, but the findings do correlate with observed patterns in migrations; a swarm of bees, for example, can follow a few scouts to a new hive. The findings also raise some interesting hypotheses. In migrating populations, a few individuals often and inexplicably fail to migrate; maybe they're just not getting the message from their leaders. In the models, it was also possible for migration to evolve when individuals were widely scattered — which fits with the existence of migration in such insects as dragonflies and Monarch butterflies, which live independently.
"Guttal and Couzin add evolutionary dynamics to the mix and set the scene for a new generation of experimental tests and applications," wrote University of Sydney biologists Stephen Simpson and Gregory Sword in a commentary on the experiment.
Some of the the study's implications are, however, troubling. Over time, the simulated populations tended to settle into a ratio with far more followers than leaders. If leader-producing mutations are rare in the wild, then lost leaders could be very difficult to replace, and migration easily compromised. This lesson is reflected in the traditions of Inuit hunters, who once allowed lead animals to pass, hunting only from the middle of the pack.
In the models, migrations lost to habitat fragmentation were also difficult to rekindle. Even after restoring the habitat, "a population's migratory ability does not recover at the same habitat recovery at which it declined," wrote Guttal and Couzin. Migration could disappear in a few generations, and take many more to come back, if at all. Indeed, bison in North America no longer seem able to migrate, a fate that may soon be shared by wildebeest in the Serengeti.
Migration may vanish at a scale measured in human years, and recover at time scales measured in planetary cycles.
Image: Caribou./Flickr, Sami Keinanen.
Citation: "Social interactions, information use, and the evolution of collective migration." By Vishwesha Guttal and Iain D. Couzin, Proceedings of the National Academy of Sciences, Vol 107. No. 37, September 14, 2010.
"Evolving migration." By Stephen J. Simpson1 and Gregory A. Sword. Proceedings of the National Academy of Sciences, Vol 107. No. 37, September 14, 2010.
Posted: 16 Sep 2010 08:31 AM PDT
This water-color-esque image captures hot stellar winds carving away at pillars of cold gas, like ice sculptors wielding torches.
These one-light-year-tall pillars of cold hydrogen and dust are located 7,500 light-years away in the Carina Nebula. Violent stellar winds and powerful radiation from massive stars are sculpting the surrounding nebula. Inside the dense structures, new stars may be born.
This image is a composite of two images taken by the Hubble Space Telescope's Advanced Camera for Surveys, one in July 2005 and one in February 2010. The 2005 observations captured light emitted by hydrogen atoms, which shows up here in blue and cyan. The 2010 image is of oxygen light, which appears yellow and gold.
Image: NASA, ESA, and the Hubble Heritage Project (STScI/AURA)
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