How Aerosols Contribute To Climate Change

SOURCE

ScienceDaily (June 23, 2009) — What happens in Vegas may stay in Vegas, but what happens on the way there is a different story.

As imaged by Lynn Russell, a professor of atmospheric chemistry at Scripps Institution of Oceanography at UC San Diego, and her team, air blown by winds between San Diego and Las Vegas gives the road to Sin City a distinctive look.
The team has sampled air from the tip of the Scripps Pier since last year, creating a near real-time record of what kinds of particles — from sea salt to car exhaust — are floating around at any given time. Add data about wind speed and direction and the scientists can tell where particles came from and can map their pathways around Southern California.
When Russell and her students put it all together, the atmosphere of greater San Diego comes alive in colors representing the presence of different airborne chemical compounds in aerosol form. One streak of deep red draws a distinct line from the pier that sometimes extends all the way to Las Vegas. The red denotes organic mass, a carbon-based component of vehicular and industrial emissions that pops up on Russell’s readouts frequently. Plot the streak on a road atlas and it reveals the daily life of pollution in Southern California. For one stretch of time, it neatly traced Interstate 15 all the way past the California-Nevada border.
“We were really surprised,” said Russell. “We did not expect to have such consistent winds for the selected study days.”
The hunt for various types of aerosols is helping Russell draw new kinds of global maps, ones that depict what organic compounds—whether natural or from sources such as Southern California traffic and industries—could do to affect rainfall, snowfall, atmospheric warming and cooling, and a host of other climate phenomena. Russell is part of an effort that involves several researchers at Scripps and UCSD and around the world. Collectively they are attempting to address a human-caused phenomenon in the Earth system scarcely considered before the last decade.
Aerosol research is considered one of the most critical frontiers of climate change science, much of which is devoted to the creation of accurate projections of future climate. These projections are generated by computer models — simulations of phenomena such as warming patterns, sea level fluctuations, or drought trends. The raw data for the models can come from historical records of climate basics like temperature and precipitation, but scientists often must rely on incomplete data and best guesses to represent more complex phenomena. The more such uncertainty goes into a model, the greater its margin of error becomes, making it less reliable as a guide for forecasts and adaptive actions.
Among these complex phenomena, the actions of aerosols are what some researchers consider the field’s holy grail, representing the biggest barrier to producing accurate representations of climate. In fact, the Intergovernmental Panel on Climate Change in 2007 specifically listed the effect of aerosols on cloud formation as the largest source of uncertainty in present-day climate models.
Bits of dust, sea salt, the remnants of burned wood, and even living things like bacteria all add to the mix of aerosols that create the skeletons on which clouds form. Around these particles, water and ice condense and cluster into cloud masses. The size and number of each of these droplets determine whether the clouds can produce rain or snow.
The aerosols are also influencing climate in other ways. Diesel exhaust, industrial emissions, and the smoke from burning wood and brush eject myriad bits of black carbon, usually in the form of soot, into the sky and form so-called “brown clouds” of smog. This haze has a dual heating and cooling effect. The particles absorb heat and make the air warmer at the altitudes to which they tend to rise but they also deflect sunlight back into space. This shading effect cools the planet at ground level.
The Arctic Circle is one of the places in the world most sensitive to changes in the mix of aerosols. Since the beginnings of the Industrial Revolution, scientists and explorers have noted the presence of the Arctic haze, a swirl of pollution that appears when sunlight returns after a winter of darkness. The presence of smog over a mostly uninhabited region leads many scientists to believe it is the reason the Arctic is experiencing the most rapid climate-related changes in the world. The haze now lingers for a longer period of time every year. It may be contributing to the forces now causing a meltdown of Arctic ice, a release of methane once stored in permafrost, and a host of ecological changes affecting the spectrum of organisms from mosquitoes to polar bears.
Russell has taken part in two recent analyses of polar air to understand where its imported aerosols come from and how the chemical components of those aerosols could be affecting temperature and cloud formation. From a research vessel in the Norwegian Sea and via continuous measurements from a ground station in Barrow, Alaska, Russell’s team is analyzing particles likely to have been blown to the Arctic from Europe and Asia. Her group has just compiled a full season of air samples fed through intake valves onto filters collected at Barrow.
With it, she believes she has proven what colleagues have previously theorized about where the particles are coming from. She is especially interested in organic particles—aerosols containing carbon supplied either by natural sources such as ocean or land plants or by human sources. Work in her group has shown that organics in the spring haze carry a signature consistent with dust and biomass burning taking place most likely in Siberia. The chemical signature changes in other seasons, revealing itself in infrared spectroscopy readings to be the product of aerosols from natural sources.
The aerosols could be influencing how much snowfall the Arctic gets and keeps. Human-produced aerosols are thought to stifle precipitation in some areas but may provide the impetus for torrential rain in others depending on their chemical make-up. Even if the Asian aerosols are not affecting precipitation, however, Russell said they appear to cool the Arctic atmosphere by deflecting light into space. At the same time, there is strong evidence that they are accelerating ice melt in the Arctic by darkening and heating ice once they fall to the ground the way a dark sweater makes its wearer hotter on a sunny day than does a white sweater.
Russell has been part of another collaborative effort launched in 2008, the International Polar Year, that created chemical profiles of relatively untainted air off the Norwegian west coast, which is only occasionally tinged by European smog. She has also teamed with collaborators at Scripps, NOAA and other universities to profile aerosols around Houston, Texas, and Mexico City.
In the latter two projects, she has provided evidence that agriculture adds more to the aerosol mix in an oil town like Houston than previously thought and that organic particles in Mexico City, rising from the smoke of street vendors and exhaust of cars driving on gas formulated differently than in the United States, glom on to dust in a different manner than American pollution to create aerosols with distinct chemical structures. Figuring out what they do locally and regionally is the next step.
Russell collaborates with a number of other faculty at UCSD whose research also focuses on aerosols, such as Kim Prather, an atmospheric chemistry professor with joint appointments at Scripps and the UCSD Department of Chemistry and Biochemistry. Russell and Prather are comparing their results form Mexico City in an effort to better understand the sources of aerosols in the atmosphere.
“We are trying to understand the major sources of aerosols in our atmosphere and how they affect the overall temperature of our planet; as opposed to greenhouse gases which we know are warming, aerosols can cool or warm depending on their composition and where they are located in the atmosphere," said Prather. Like Russell, Prather also studies long-range transport of aerosols from terrestrial and marine sources. Prather and Russell have worked together on several other projects and recently helped form the Aerosol Chemistry and Climate Institute, a collaboration between Scripps and the Department of Energy’s Pacific Northwest National Laboratory.
For her most comprehensive study, Russell need only to make her shortest journey to the end of Scripps Pier. It is possible that aerosol journeys of a thousand miles or more might be explained by shorter commutes between Southern California counties. Complete analysis of the Interstate 15 data suggests Vegas might not be a source of dirtiness after all. Using data collected over longer time periods, Russell’s pollution map of local counties now suggests organic human-made aerosols might just be blowing toward Nevada from San Bernardino and Riverside then back toward San Diego as winds shift. Russell employs a suite of complementary measurements at the pier to characterize short- and long-term aerosol trends. Those are combined with particle profiles made by Prather’s group and collaborators whose numbers are growing out of necessity.
“Understanding the big picture is the only way we’re going to be able to reduce the uncertainty associated with aerosol particles and their effects on climate,” said Russell. “There are so many parameters, there’s no one instrument or even one person who can do all of it at once.”
Adapted from materials provided by University of California, San Diego, via Newswise.

Changes In The Sun Are Not Causing Global Warming, New Study Shows

SOURCE

ScienceDaily (May 12, 2009) — With the U.S. Congress beginning to consider regulations on greenhouse gases, a troubling hypothesis about how the sun may impact global warming is finally laid to rest.

Carnegie Mellon University's Peter Adams along with Jeff Pierce from Dalhousie University in Halifax, Canada, have developed a model to test a controversial hypothesis that says changes in the sun are causing global warming.
The hypothesis they tested was that increased solar activity reduces cloudiness by changing cosmic rays. So, when clouds decrease, more sunlight is let in, causing the earth to warm. Some climate change skeptics have tried to use this hypothesis to suggest that greenhouse gases may not be the global warming culprits that most scientists agree they are.
In research published in Geophysical Research Letters, and highlighted in the May 1 edition of Science, Adams and Pierce report the first atmospheric simulations of changes in atmospheric ions and particle formation resulting from variations in the sun and cosmic rays. They find that changes in the concentration of particles that affect clouds are 100 times too small to affect the climate.
"Until now, proponents of this hypothesis could assert that the sun may be causing global warming because no one had a computer model to really test the claims," said Adams, a professor of civil and environmental engineering at Carnegie Mellon.
"The basic problem with the hypothesis is that solar variations probably change new particle formation rates by less than 30 percent in the atmosphere. Also, these particles are extremely small and need to grow before they can affect clouds. Most do not survive to do so," Adams said.
Despite remaining questions, Adams and Pierce feel confident that this hypothesis should be laid to rest. "No computer simulation of something as complex as the atmosphere will ever be perfect," Adams said. "Proponents of the cosmic ray hypothesis will probably try to question these results, but the effect is so weak in our model that it is hard for us to see this basic result changing."
Journal references:
J. R. Pierce and P. J. Adams. Can cosmic rays affect cloud condensation nuclei by altering new particle formation rates? Geophys. Res. Lett., 2009; (in press) DOI: 10.1029/2009GL037946
Richard A. Kerr. Study Challenges Cosmic Ray-Climate Link. Science, 2009; 324 (5927): 576 DOI: 10.1126/science.324_576b
Adapted from materials provided by Carnegie Mellon University.

As Earth's air slowly trickles away into space, will our planet come to look like Venus?

SOURCE

By Kevin J. Zahnle and David C. Catling

Many of the gases that make up Earth’s atmosphere and those of the other planets are slowly leaking into space. Hot gases, especially light ones, evaporate away; chemical reactions and particle collisions eject atoms and molecules; and asteroids and comets occasionally blast out chunks of atmosphere.
This leakage explains many of the solar system’s mysteries. For instance, Mars is red because its water vapor got broken down into hydrogen and oxygen, the hydrogen drifted away, and the surplus oxygen oxidized—in essence, rusted—the rocks. A similar process on Venus let carbon dioxide build up into a thick ocean of air; ironically, Venus’s huge atmosphere is the result of the loss of gases.

One of the most remarkable features of the solar system is the variety of planetary atmospheres. Earth and Venus are of comparable size and mass, yet the surface of Venus bakes at 460 degrees Celsius under an ocean of carbon dioxide that bears down with the weight of a kilometer of water. Callisto and Titan—planet-size moons of Jupiter and Saturn, respectively—are nearly the same size, yet Titan has a nitrogen-rich atmosphere thicker than our own, whereas Callisto is essentially airless. What causes such extremes? If we knew, it would help explain why Earth teems with life while its planetary siblings appear to be dead. Knowing how atmospheres evolve is also essential to determining which planets beyond our solar system might be habitable.
A planet can acquire a gaseous cloak in many ways: it can release vapors from its interior, it can capture volatile materials from comets and asteroids when they strike, and its gravity can pull in gases from interplanetary space. But planetary scientists have begun to appreciate that the escape of gases plays as big a role as the supply. Although Earth’s atmosphere may seem as permanent as the rocks, it gradually leaks back into space. The loss rate is currently tiny, only about three kilograms of hydrogen and 50 grams of helium (the two lightest gases) per second, but even that trickle can be significant over geologic time, and the rate was probably once much higher. As Benjamin Franklin wrote, “A small leak can sink a great ship.” The atmospheres of terrestrial planets and outer-planet satellites we see today are like the ruins of medieval castles—remnants of riches that have been subject to histories of plunder and decay. The atmospheres of smaller bodies are more like crude forts, poorly defended and extremely vulnerable.
Recognizing the importance of atmospheric escape changes our perspective on the solar system. For decades, scientists have pondered why Mars has such a thin atmosphere, but now we wonder: Why does it have any atmosphere left at all? Is the difference between Titan and Callisto a consequence of Callisto’s losing its atmosphere, rather than of Titan having been born of airier stuff? Was Titan’s atmosphere once even thicker than it is today? How did Venus steadfastly cling to its nitrogen and carbon dioxide yet thoroughly lose its water? Did escape of hydrogen help to set the stage for complex life on Earth? Will it one day turn our planet into another Venus?
When the Heat Is OnA spaceship that reaches escape velocity is moving fast enough to break free of a planet’s gravity. The same is true of atoms and molecules, although they usually reach escape velocity less purposefully. In thermal escape, gases get too hot to hold on to. In nonthermal processes, chemical or charged-particle reactions hurl out atoms and molecules. And in a third process, asteroid and comet impacts blast away the air.
Thermal escape is, in some ways, the most common and straightforward of the three. All bodies in the solar system are heated by sunlight. They rid themselves of this heat in two ways: by emitting infrared radiation and by shedding matter. In long-lived bodies such as Earth, the former process prevails; for others, such as comets, the latter dominates. Even a body the size of Earth can heat up quickly if absorption and radiation get out of balance, and its atmosphere—which typically has very little mass compared with the rest of the planet—can slough off in a cosmic instant. Our solar system is littered with airless bodies, and thermal escape seems to be a common culprit. Airless bodies stand out as those where solar heating exceeds a certain threshold, which depends on the strength of the body’s gravity [Purchase the digital edition to see related sidebar].
Thermal escape occurs in two ways. In the first, called Jeans escape, after James Jeans, the English astronomer who described it in the early 20th century, air literally evaporates atom by atom, molecule by molecule, off the top of the atmosphere. At lower altitudes, collisions confine particles, but above a certain altitude, known as the exobase, which on Earth is about 500 kilometers above the surface, air is so tenuous that gas particles hardly ever collide. Nothing stops an atom or molecule with sufficient velocity from flying away into space.

As the lightest gas, hydrogen is the one that most easily overcomes a planet’s gravity. But first it must reach the exobase, and on Earth that is a slow process. Hydrogen-bearing molecules tend not to rise above the lowest layer of atmosphere: water vapor (H2O) condenses out and rains back down, and methane (CH4) is oxidized to form carbon dioxide (CO2). Some water and methane molecules reach the stratosphere and decompose, releasing hydrogen, which slowly diffuses upward until it reaches the exobase. A small amount clearly makes it out because ultraviolet images reveal a halo of hydrogen atoms surrounding our planet [Purchase the digital edition to see related sidebar].
The temperature at Earth’s exobase oscillates but is typically about 1,000 kelvins, implying that hydrogen atoms have an average speed of five kilometers per second. That is less than Earth’s escape velocity at that altitude, 10.8 kilometers per second, but the average conceals a wide range, so some hydrogen atoms still manage to break free of our planet’s gravity. This loss of particles from the energetic tail of the speed distribution explains about 10 to 40 percent of Earth’s hydrogen loss today. Jeans escape also partly explains why our moon is airless. Gases released from the lunar surface easily evaporate off into space.
A second type of thermal escape is far more dramatic. Whereas Jeans escape occurs when a gas evaporates molecule by molecule, heated air can also flow en masse. The upper atmosphere can absorb ultraviolet sunlight, warm up and expand, pushing air upward. As the air rises, it accelerates smoothly through the speed of sound and then attains the escape velocity. This form of thermal escape is called hydrodynamic escape or, more evocatively, the planetary wind—the latter by analogy to the solar wind, the stream of charged particles blown from the sun into interplanetary space.
Dust in the WindAtmospheres rich with hydrogen are the most vulnerable to hydrodynamic escape. As hydrogen flows outward, it can pick up and drag along heavier molecules and atoms with it. Much as the desert wind blows dust across an ocean and sand grains from dune to dune, while leaving cobbles and boulders behind, the hydrogen wind carries off molecules and atoms at a rate that diminishes with their weight. Thus, the present composition of an atmosphere can reveal whether this process has ever occurred.
In fact, astronomers have seen the telltale signs of hydrodynamic escape outside the solar system, on the Jupiter-like planet HD 209458b. Using the Hubble Space Telescope, Alfred Vidal-Madjar of the Paris Astrophysics Institute and his colleagues reported in 2003 that the planet has a puffed-up atmosphere of hydrogen. Subsequent measurements discovered carbon and oxygen in this inflated atmosphere. These atoms are too heavy to escape on their own, so they must have been dragged there by hydrogen. Hydrodynamic loss would also explain why astronomers find no large planets much closer to their stars than HD 209458b is. For planets that orbit within three million kilometers or so of their stars (about half the orbital radius of HD 209458b), hydrodynamic escape strips away the entire atmosphere within a few billion years, leaving behind only a scorched remnant.
This evidence for planetary winds lends credence to ideas put forth in the 1980s about hydrodynamic escape from ancient Venus, Earth and Mars. Three clues suggest this process once operated on these worlds. The first concerns noble gases. Were it not for escape, chemically unreactive gases such as neon or argon would remain in an atmosphere indefinitely. The abundances of their different isotopes would be similar to their original values, which in turn are similar to that of the sun, given their common origin in the solar nebula. Yet the abundances differ.
Second, youthful stars are strong sources of ultraviolet light, and our sun was probably no exception. This radiation could have driven hydrodynamic escape.

Third, the early terrestrial planets may have had hydrogen-rich atmospheres. The hydrogen could have come from chemical reactions of water with iron, from nebular gases or from water molecules broken apart by solar ultraviolet radiation. In those primeval days, asteroids and comets hit more frequently, and whenever they smacked into an ocean, they filled the atmosphere with steam. Over thousands of years the steam condensed and rained back onto the surface, but Venus is close enough to the sun that water vapor may have persisted in the atmosphere, where solar radiation could break it down.
Under such conditions, hydrodynamic escape would readily operate. In the 1980s James F. Kasting, now at Pennsylvania State University, showed that hydrodynamic escape on Venus could have carried away an ocean’s worth of hydrogen within a few tens of millions of years [see “How Climate Evolved on the Terrestrial Planets,” by James F. Kasting, Owen B. Toon and James B. Pollack; Scientific American, February 1988]. Kasting and one of us (Zahnle) subsequently showed that escaping hydrogen would have dragged along much of the oxygen but left carbon dioxide behind. Without water to mediate the chemical reactions that turn carbon dioxide into carbonate minerals such as limestone, the carbon dioxide built up in the atmosphere and created the hellish Venus we see today.
To a lesser degree, Mars and Earth, too, appear to have suffered hydrodynamic losses. The telltale signature is a deficit of lighter isotopes, which are more easily lost. In the atmospheres of Earth and Mars, the ratio of neon 20 to neon 22 is 25 percent smaller than the solar ratio. On Mars, argon 36 is similarly depleted relative to argon 38. Even the isotopes of xenon—the heaviest gas in Earth’s atmosphere apart from pollutants—show the imprint of hydrodynamic escape. If hydrodynamic escape were vigorous enough to sweep up xenon, why did it not sweep up everything else in the atmosphere along with it? To solve this puzzle, we may need to construct a different history for xenon than for the other gases now in the atmosphere.
Hydrodynamic escape may have stripped Titan of much of its air, too. When it descended through Titan’s atmosphere in 2005, the European Space Agency’s Huygens probe found that the ratio of nitrogen 14 to nitrogen 15 is 70 percent of that on Earth. That is a huge disparity given that the two isotopes differ only slightly in their tendency to escape. If Titan’s atmosphere started with the same nitrogen isotopic composition as Earth’s, it must have lost a huge amount of nitrogen—several times the substantial amount it currently has—to bring the ratio down to its present value. In short, Titan’s atmosphere might once have been even thicker than it is today, which only heightens its mystery.
Better Escaping through ChemistryOn some planets, including modern Earth, thermal escape is less important than nonthermal escape. In nonthermal escape, chemical reactions or particle-particle collisions catapult atoms to escape velocity. What nonthermal escape mechanisms have in common is that an atom or molecule reaches a very high velocity as the outcome of a single event that takes place above the exobase, so that bumping into something does not thwart the escapee. Many types of nonthermal escape involve ions. Ordinarily these charged particles are tethered to a planet by its magnetic field, either the global (internally generated) magnetic field—if there is one—or the localized fields induced by the passage of the solar wind. But they find ways to slip out.
In one type of event, known as charge exchange, a fast hydrogen ion collides with a neutral hydrogen atom and captures its electron. The result is a fast neutral atom, which is immune to the magnetic field. This process accounts for 60 to 90 percent of the present loss of hydrogen from Earth and most of the hydrogen loss from Venus.

Another way out exploits a weak spot—dare we say a loophole—in the planet’s magnetic trap. Most magnetic field lines loop from one magnetic pole to the other, but the widest field lines are dragged outward by the solar wind and do not loop back; they remain open to interplanetary space. Through this opening, ions can escape. To be sure, the ions must still overcome gravity, and only the lightest ions such as hydrogen and helium make it. The resulting stream of charged particles, called the polar wind (not to be confused with the planetary wind), accounts for 10 to 15 percent of Earth’s hydrogen loss and almost its entire helium leak.
In some cases, these light ions can sweep up heavier ions with them. This process may explain the xenon puzzle: if the polar wind was more vigorous in the past, it could have dragged out xenon ions. One piece of evidence is that krypton does not have the same isotopic pattern as xenon does, even though it is a lighter gas and, all else being equal, ought to be more prone to escape. The difference is that krypton, unlike xenon, resists ionization, so even a strong polar wind would have left it unaffected.
A third nonthermal process known as photochemical escape operates on Mars and possibly on Titan. Oxygen, nitrogen and carbon monoxide molecules drift into the upper atmosphere, where solar radiation ionizes them. When the ionized molecules recombine with electrons or collide with one another, the energy released splits the molecules into atoms with enough speed to escape.
Mars, Titan and Venus lack global magnetic fields, so they are also vulnerable to a fourth nonthermal process known as sputtering. Without a planetary field to shield it, the upper atmosphere of each of these worlds is exposed to the full brunt of the solar wind. The wind picks up ions, which then undergo charge exchange and escape. Mars’s atmosphere is enriched in heavy nitrogen and carbon isotopes, suggesting that it has lost as much as 90 percent of an earlier atmosphere. Sputtering and photochemical escape are the most likely culprits. In 2013 NASA plans to launch the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission to measure escaping ions and neutral atoms and reconstruct the planet’s atmospheric history.
Inescapable ConsequencesBoth thermal and nonthermal escape are like tiny trickles compared with the huge splash when comets or asteroids crash into planets. If projectiles are sufficiently big and fast, they vaporize both themselves and a similar mass of the surface. The ensuing hot gas plume can expand faster than the escape velocity and drive off the overlying air. The larger the impact energy, the wider the cone of atmosphere ejected. For the asteroid that killed off the dinosaurs 65 million years ago, the cone was about 80 degrees wide from the vertical and contained a hundred-thousandth of the atmosphere. An even more energetic impact can carry away the entire atmosphere above a plane that is tangent to the planet.
Another factor determining the width of the cone is the atmospheric density. The thinner the air, the greater the fraction of the atmosphere that gets lost. The implication is gloomy: once a vulnerable atmosphere starts wearing away, impact erosion becomes ever easier until the atmosphere vanishes altogether. Unfortunately, Mars spent its youth in a bad neighborhood near the asteroid belt and, being small, was especially susceptible. Given the expected size distribution of impactors early in a solar system’s history, the planet should have been stripped of its entire atmosphere in less than 100 million years.
The large moons of Jupiter also live in a dangerous neighborhood—namely, deep in the giant planet’s gravitational field, which accelerates incoming asteroids and comets. Impacts would have denuded these moons of any atmospheres they ever had. In contrast, Titan orbits comparatively far from Saturn, where impact velocities are slower and an atmosphere can survive.

In all these ways, escape accounts for much of the diversity of atmospheres, from the lack of air on Callisto and Ganymede to the absence of water on Venus. A more subtle consequence is that escape tends to oxidize planets, because hydrogen is lost more easily than oxygen. Hydrogen escape is the ultimate reason why Mars, Venus and even Earth are red. Most people do not think of Earth as a red planet, but much of the continental crust is red. Soil and vegetation hide this native hue. All three worlds started out the gray-black color of volcanic rock and reddened as the original minerals oxidized to iron oxides (similar to rust). To account for its color, Mars must have lost an ocean of water equivalent to a global layer meters to tens of meters deep.
On Earth, most researchers attribute the accumulation of oxygen 2.4 billion years ago to photosynthetic organisms, but in 2001 we suggested that the escape of hydrogen also played an important role. Microbes break apart water molecules in photosynthesis, and the hydrogen can pass like a baton from organic matter to methane and eventually reach space. The expected amount of hydrogen loss matches the net excess of oxidized material on Earth today.
Escape helps to solve the mystery of why Mars has such a thin atmosphere. Scientists have long hypothesized that chemical reactions among water, carbon dioxide and rock turned the original thick atmosphere into carbonate minerals. The carbonates were never recycled back into carbon dioxide gas because Mars, being so small, cooled quickly and its volcanoes stopped erupting. The trouble with this scenario is that spacecraft have so far found only a single small area on Mars with carbonate rock, and this outcrop probably formed in warm subsurface waters. Moreover, the carbonate theory offers no explanation for why Mars has so little nitrogen or noble gases. Escape provides a better answer. The atmosphere did not get locked away as rock; it dissipated into space.
A nagging problem is that impact erosion ought to have removed Mars’s atmosphere altogether. What stopped it? One answer is simple chance. Large impacts are inherently rare, and their frequency fell off rapidly about 3.8 billion years ago, so Mars may have been spared the final devastating blow. A large impact of an icy asteroid or comet could have deposited more volatiles than subsequent impacts could remove. Alternatively, remnants of Mars’s atmosphere may have survived underground and leaked out after the bombardment had subsided.
Although Earth seems comparatively unscathed by escape, that will change. Today hydrogen escape is limited to a trickle because the principal hydrogen-bearing gas, water vapor, condenses in the lower atmosphere and rains back to the surface. But our sun is slowly brightening at about 10 percent every billion years. That is imperceptibly slow on a human timescale but will be devastating over geologic time. As the sun brightens and our atmosphere warms, the atmosphere will get wetter, and the trickle of hydrogen escape will become a torrent.
This process is expected to become important when the sun is 10 percent brighter—that is, in a billion years—and it will take another billion years or so to desiccate our planet’s oceans. Earth will become a desert planet, with at most a shrunken polar cap and only traces of precious liquid. After another two billion years, the sun will beat down on our planet so mercilessly even the polar oases will fail, the last liquid water will evaporate and the greenhouse effect will grow strong enough to melt rock. Earth will have followed Venus into a barren lifelessness.
This story was originally printed with the title "The Planetary Air Leak"
ABOUT THE AUTHOR(S)Planetary scientist David C. Catling studies the coupled evolution of planetary surfaces and atmospheres. Formerly at the NASA Ames Research Center, he joined the faculty at the University of Washington in 2001. He is a co- investigator for NASAs Phoenix lander, which completed its mission last December. Kevin J. Zahnle has been a research scientist at the NASA Ames center since 1989. Even by the eclectic standards of planetary science, he has an unusually wide range of interests, from planetary interiors to surfaces to atmospheres. In 1996 Zahnle received the NASA Exceptional Achievement Medal for his work on the impact of Comet Shoemaker-Levy 9 into Jupiter.

Origin Of 'Breathable' Atmosphere Half A Billion Years Ago Discovered

Source:

http://www.sciencedaily.com/releases/2007/10/071029094320.htm

ScienceDaily (Oct. 30, 2007) — Ohio State University geologists and their colleagues have uncovered evidence of when Earth may have first supported an oxygen-rich atmosphere similar to the one we breathe today.

The study suggests that upheavals in the earth's crust initiated a kind of reverse-greenhouse effect 500 million years ago that cooled the world's oceans, spawned giant plankton blooms, and sent a burst of oxygen into the atmosphere.
That oxygen may have helped trigger one of the largest growths of biodiversity in Earth's history.
Matthew Saltzman, associate professor of earth sciences at Ohio State, reported the findings October 28 at the meeting of the Geological Society of America in Denver.
For a decade, he and his team have been assembling evidence of climate change that occurred 500 million years ago, during the late Cambrian period. They measured the amounts of different chemicals in rock cores taken from around the world, to piece together a complex chain of events from the period.
Their latest measurements, taken in cores from the central United States and the Australian outback, revealed new evidence of a geologic event called the Steptoean Positive Carbon Isotope Excursion (SPICE).
Amounts of carbon and sulfur in the rocks suggest that the event dramatically cooled Earth's climate over two million years -- a very short time by geologic standards. Before the event, the Earth was a hothouse, with up to 20 times more carbon dioxide in the atmosphere compared to the present day. Afterward, the planet had cooled and the carbon dioxide had been replaced with oxygen. The climate and atmospheric composition would have been similar to today.
“If we could go back in time and walk around in the late Cambrian, this seems to be the first time we would have felt at home,” Saltzman said. “Of course, there was no life on land at the time, so it wouldn't have been all that comfortable.”
The land was devoid of plants and animals, but there was life in the ocean, mainly in the form of plankton, sea sponges, and trilobites. Most of the early ancestors of the plants and animals we know today existed during the Cambrian, but life wasn't very diverse.
Then, during the Ordovician period, which began around 490 million years ago, many new species sprang into being. The first coral reefs formed during that time, and the first true fish swam among them. New plants evolved and began colonizing land.
“If you picture the evolutionary ‘tree of life,' most of the main branches existed during the Cambrian, but most of the smaller branches didn't get filled in until the Ordovician,” Saltzman said. “That's when animal life really began to develop at the family and genus level.” Researchers call this diversification the “Ordovician radiation.”
The composition of the atmosphere has changed many times since, but the pace of change during the Cambrian is remarkable. That's why Saltzman and his colleagues refer to this sudden influx of oxygen during the SPICE event as a “pulse” or “burst.”
“After this pulse of oxygen, the world remained in an essentially stable, warm climate, until late in the Ordovician,” Saltzman said.
He stopped short of saying that the oxygen-rich atmosphere caused the Ordovician radiation.
“We know that oxygen was released during the SPICE event, and we know that it persisted in the atmosphere for millions of years -- during the time of the Ordovician radiation -- so the timelines appear to match up. But to say that the SPICE event triggered the diversification is tricky, because it's hard to tell exactly when the diversification started,” he said.
“We would need to work with paleobiologists who understand how increased oxygen levels could have led to a diversification. Linking the two events precisely in time is always going to be difficult, but if we could link them conceptually, then it would become a more convincing story.”
Researchers have been trying to understand the sudden climate change during the Cambrian period ever since Saltzman found the first evidence of the SPICE event in rock in the American west in 1998. Later, rock from a site in Europe bolstered his hypothesis, but these latest finds in central Iowa and Queensland, Australia, prove that the SPICE event occurred worldwide.
During the Cambrian period, most of the continents as we know them today were either underwater or part of the Gondwana supercontinent, Saltzman explained. Tectonic activity was pushing new rock to the surface, where it was immediately eaten away by acid rain. Such chemical weathering pulls carbon dioxide from the air, traps the carbon in sediments, and releases oxygen -- a kind of greenhouse effect in reverse.
“From our previous work, we knew that carbon was captured and oxygen was released during the SPICE event, but we didn't know for sure that the oxygen stayed in the atmosphere,” Saltzman said.
They compared measurements of inorganic carbon -- captured during weathering -- with organic carbon -- produced by plankton during photosynthesis. And because plankton contain different ratios of the isotopes of carbon depending on the amount of oxygen in the air, the geologists were able to double-check their estimates of how much oxygen was released during the period, and how long it stayed in the atmosphere.
They also studied isotopes of sulfur, to determine whether much of the oxygen being produced was re-captured by sediments.
It wasn't.
Saltzman explained the chain of events this way: Tectonic activity led to increased weathering, which pulled carbon dioxide from the air and cooled the climate. Then, as the oceans cooled to more hospitable temperatures, the plankton prospered -- and in turn created more oxygen through photosynthesis.
“It was a double whammy,” he said. “There's really no way around it when we combine the carbon and sulfur isotope data -- oxygen levels dramatically rose during that time.”
What can this event tell us about climate change today? “Oxygen levels have been stable for the last 50 million years, but they have fluctuated over the last 500 million,” Saltzman said. “We showed that the oxygen burst in the late Cambrian happened over only two million years, so that is an indication of the sensitivity of the carbon cycle and how fast things can change.”
Global cooling may have boosted life early in the Ordovician period, but around 450 million years ago, more tectonic activity -- most likely, the rise of the Appalachian Mountains -- brought on a deadly ice age. So while most of the world's plant and animal species were born during the Ordovician period, by the end of it, more than half of them had gone extinct.
Coauthors on this study included Seth Young, a graduate student in earth sciences at Ohio State; Ben Gill, a graduate student, and Tim Lyons, professor of earth sciences, both at the University of California, Riverside; Lee Kump, professor of geosciences at Penn State University; and Bruce Runnegar, professor of paleontology at the University of California, Los Angeles.
Adapted from materials provided by Ohio State University.

Fausto Intilla

www.oloscience.com

Secrets Of Red Tide Revealed

Source:

http://www.sciencedaily.com/releases/2007/08/070830150106.htm

Science Daily — In work that could one day help prevent millions of dollars in economic losses for seaside communities, MIT chemists have demonstrated how tiny marine organisms likely produce the red tide toxin that periodically shuts down U.S. beaches and shellfish beds.

In the Aug. 31 cover story of Science, the MIT team describes an elegant method for synthesizing the lethal components of red tides. The researchers believe their method approximates the synthesis used by algae, a reaction that chemists have tried for decades to replicate, without success.
Understanding how and why red tides occur could help scientists figure out how to prevent the blooms, which cause significant ecological and economic damage. The New England shellfish industry, for example, lost tens of millions of dollars during a 2005 outbreak, and red tide killed 30 endangered manatees off the coast of Florida this spring.
The discovery by MIT Associate Professor Timothy Jamison and graduate student Ivan Vilotijevic not only could shed light on how algae known as dinoflagellates generate red tides, but could also help speed up efforts to develop cystic fibrosis drugs from a compound closely related to the toxin. Red tides, also known as algal blooms, strike unpredictably and poison shellfish, making them dangerous for humans to eat. It is unknown what causes dinoflagellates to produce the red tide toxins, but it may be a defense mechanism, possibly provoked by changes in the tides, temperature shifts or other environmental stresses.
One of the primary toxic components of red tide is brevetoxin, a large and complex molecule that is very difficult to synthesize.
Twenty-two years ago, chemist Koji Nakanishi of Columbia University proposed a cascade, or series of chemical steps, that dinoflagellates could use to produce brevetoxin and other red tide toxins. However, chemists have been unable to demonstrate such a cascade in the laboratory, and many came to believe that the "Nakanishi Hypothesis" would never be proven.
"A lot of people thought that this type of cascade may be impossible," said Jamison. "Because Nakanishi's hypothesis accounts for so much of the complexity in these toxins, it makes a lot of sense, but there hasn't really been any evidence for it since it was first proposed."
Jamison and Vilotijevic's work offers the first evidence that Nakanishi's hypothesis is feasible. Their work could also help accelerate drug discovery efforts. Brevenal, another dinoflagellate product related to the red tide toxins, has shown potential as a powerful treatment for cystic fibrosis (CF). It can also protect against the effects of the toxins.
"Now that we can make these complex molecules quickly, we can hopefully facilitate the search for even better protective agents and even more effective CF therapies," said Jamison.
Until now, synthesizing just a few milligrams of red tide toxin or related compounds, using a non-cascade method, required dozens of person-years of effort.
The new synthesis depends on two critical factors-giving the reaction a jump start and conducting the reaction in water.
Many red tide toxins possess a long chain of six-membered rings. However, the starting materials for the cascades, epoxy alcohols, tend to form five-membered rings. To overcome that, the researchers attached a "template" six-membered ring to one end of the epoxy alcohol. That simple step effectively launches the cascade of reactions that leads to the toxin chain, known as a ladder polyether.
"The trick is to give it a little push in the right direction and get it running smoothly," said Jamison.
The researchers speculate that in dinoflagellates, the initial jump start is provided by an enzyme instead of a template.
Conducting the reaction in water is also key to a successful synthesis. Water is normally considered a poor solvent for organic reactions, so most laboratory reactions are performed in organic solvents. However, when Vilotijevic introduced water into the reaction, he noticed that it proceeded much more quickly and selectively.
Although it could be a coincidence that these cascades work best in water and that dinoflagellates are marine organisms, water may nevertheless be directly involved in the biosynthesis of the toxins or emulating an important part of it, said Jamison. Because of this result, the researchers now believe that organic chemists should routinely try certain reactions in water as well as organic solvents.
The research was funded by the National Institute of General Medical Sciences, Merck Research Laboratories, Boehringer Ingelheim, and MIT.
"This is an elegant piece of work with multiple levels of impact," said John Schwab, who manages organic chemistry research for the National Institute of General Medical Sciences. "Not only will it allow chemists to synthesize this important class of complex molecules much more easily, but it also provides key insights into how nature may make these same molecules. This is terrific bang for the taxpayers' buck!"
Note: This story has been adapted from a news release issued by Massachusetts Institute of Technology.

Fausto Intilla

www.oloscience.com