The slow rise of The Andes

The Andes is the world’s longest continental mountain range and the highest outside Asia, with an average elevation of 4000 metres. The question of how quickly the mountains reached such heights has been a contentious one that University of Michigan paleoclimatologist Christopher Poulsen and graduate student Nadja Insel working with Todd Ehlers of the University of Tuebingen in Germany, believe they may now have settled with a new interpretation of isotopic data. Their work suggests that the rise of the Andes was a very gradual process.

Poulsen’s uplifting work suggests that previous interpretations of the evidence have misconstrued changes in oxygen isotope ratios as being due to a rapid rise of the mountain range whereas the more likely explanation is that the changes are due to shifts in ancient climate.

“In the modern climate, there is a well-known inverse relationship between oxygen isotopic values in rain and elevation,” Poulsen explains.
“As a rain cloud ascends a mountain range, it begins to precipitate.
Because atoms of oxygen-18 are more massive than those of oxygen-16, it is preferentially rained out. Thus, as you go up the mountain, the precipitation becomes more and more depleted in oxygen-18, and the ratio of oxygen-18 to oxygen-16 decreases.” Geologists use the ratio of these isotopes, preserved in rock, to infer past elevations and so the rate of rise of a mountain range.

“If the ratio decreases with time, as the samples get younger, the interpretation would typically be that there has been an increase in elevation at that location,” Poulsen adds. He points out that that is the precise conclusion drawn by a series of papers on the uplift history of The Andes published over the past four years. On the basis of oxygen isotope ratios determined by analysis of carbonate rocks, the authors of those papers suggested that the central Andes rose about 2500 to 3500 metres in a mere three million years, Other geologists had assumed that the rise to those heights took place over tens of millions of years.

Unfortunately, elevation is not the only thing to disturb oxygen isotope ratios in precipitation. “It can also be affected by where the vapour came from and how much it rained,” says Poulsen. “More intense rainfall also causes oxygen-18 to be preferentially precipitated.” He and his colleagues were skeptical of the rapid-rise scenario, and so performed climate modelling experiments to investigate whether something other than altitude might have given rise to the shift in ratio observed in carbonate deposits.

Andes: Credit to http://www.flickr.com/photos/atyt/

“The key result in our modelling study is that we identified an elevation threshold for rainfall,” Poulsen says. “Once The Andes reached an elevation greater than 70 percent of the current elevation, the precipitation rate abruptly increased. In our model, the increased precipitation also caused the ratio of oxygen-18 to oxygen-16 to significantly decrease. Our conclusion, then, is that geologists have misinterpreted the isotopic records in the central Andes. The decrease in the ratio is not recording an abrupt increase in elevation; it is recording an abrupt increase in rainfall.”

This conclusion is backed up by geochemical and sedimentological data, Poulsen said. “There is evidence that the central Andes became less arid at the same time that the isotope records show a decrease in the ratio of oxygen-18 to oxygen-16.”

Links

Christopher Poulsen

Science Express, 2010, online

Moon river?

The media was recently drenched with the idea that water had been found on the Moon, offering speculation as to our nearest neighbour offering an oasis-like site for a lunar base from which we could launch missions to Mars and beyond. The truth, if it is ever confirmed, is a little more subtle.

Is moisture on the Moon, simply wishing on a star? (Photo by David Bradley)
Is moisture on the Moon, simply wishing on a star? (Photo by David Bradley)

The Apollo missions of the 1970s had always hinted at the presence of water on the Moon, although its presence in samples brought back to earth was thought to be nothing more than contamination. In 1998, scientists announced that the Lunar Prospector spacecraft had detected 300 million tonnes of water on the moon and hinted that there may be as much as 6 billion tonnes. In July, an analysis of tiny beads of volcanic glass collected by two Apollo missions revealed water trapped inside, suggesting that the Moon’s water had not been entirely vaporized by the violent events that led to its formation. The discovery had implications for the volcanic origin of possible water reservoirs at the Moon’s poles.

However, new evidence released at the end of September based on data from India’s Chandrayaan-1 probe and the Deep Impact and Cassini missions suggests that there may well be some degree of hydration up there. Researchers in India and the US used data from NASA’s Moon Mineralogy Mapper, the M3, aboard the Chandrayyan-1 satellite, which was launched into orbit around the moon in October 2008 to reveal the presence of water on the moon. Chandrayaan’s mission ceased in August 2009.

M3 uses reflectance spectrometry to determine the content of minerals in the thin layer of upper soil on the surface of the moon. The data revealed the presence of chemical bonds between hydrogen and oxygen atoms, like those found between the oxygen atom and its attendant hydrogen atoms in H2O.

However, the next generation of lunar astronauts are not likely to sip from moon springs or splash their silvery boots in lunar puddles because revelations of chemical bonds between hydrogen and oxygen atoms is indicative of water molecules but is even more indicative of hydroxyl ions (OH). It could be that good, old-fashioned H2O forms only when the solar wind doth blow and brings with it hydrogen atoms that can combine with the hydroxyl radicals forming “H+OH” (H2O). It may be that less than a litre of actual water is present per tonne of rock spread across the surface to a depth of a few centimetres and present as water of hydration of the minerals from which the rock is composed.

The rocks and soils that comprise the lunar surface contain about 45 percent oxygen, mostly in the form of silicate minerals. The constant deluge of hydrogen atoms from the solar wind could readily pull oxygen and hydroxyl from the soil and form water molecules on the fly, especially given the hydrogen ions are moving at one third the speed of light when they hit.

Taylor and other M3 team members believe their findings will be of particular significance as mankind continues to plan for a return to the moon. The maps created by M3 could provide mission planners with locations prime for extraction of needed water from the lunar soil.

Following the lunar announcement, Jim Bell, President of The Planetary Society, said: “The possible presence of minor amounts of hydrated material on the Moon is intriguing, though the findings still need to be confirmed by other methods and other investigators. Chandrayaan is another great example of the power and value of international collaboration in space exploration, and The Planetary Society congratulates the entire Chandrayaan, Deep Impact and Cassini teams.”

Researchers still hope to find liquid water at the bottom of the deepest, darkest lunar craters at depths that never see sunlight nor feel the solar wind. Such, hopefully, icy depths are akin to the cold places on the planet Mars where evidence of water ice has been found.

LINKS
Science, 2009, in press
Chandrayaan-1 site

Meteoric rise of life on Earth

Seemingly endless meteor storms that bombarded the Earth four billion years ago helped to create the right growing conditions from which life could first emerge. The same meteoric bombardment may also have had a similar effect on our planetary neighbour Mars.

Richard Court and Mark Sephton of the Department of Earth Science and Engineering, at Imperial College London and their colleagues publish details of a theory that could change our understanding of natural terraforming of the primordial Earth this month in the journal Geochimica et Cosmochima Acta.

Professor Mark A. Sephton

Professor Mark A. Sephton

The researchers have analysed the remaining mineral and organic content of fifteen fragments of ancient meteorites to see how much water vapour and carbon dioxide they would release when subjected to very high temperatures. The experiments aimed to replicate the conditions experienced by the meteoric material as it entered the Earth’s atmosphere billions of years ago.

When a meteor enters a planet’s atmosphere, extreme heat is produced because of the retarding compression of the air due to the meteor travelling at supersonic speeds. This heat causes some of the minerals and organic matter on the meteor’s outer crust to vaporise releasing water and carbon dioxide before it breaks up or hits the ground.

A fragment of the Murchison meteorite was analysed by the IC team

A fragment of the Murchison meteorite was analysed by the IC team

This water source could have added large quantities of moisture to the atmospheres of both Earth and Mars billions of years ago. Moreover, the addition of the greenhouse gas carbon dioxide to the atmosphere would have helped trapped solar energy and so make the primordial planets warm enough for liquid oceans.

Meteoric bombardment of the early Earth may have paved the way for life to emerge

Meteoric bombardment of the early Earth may have paved the way for life to emerge

Court and Sephton used a novel analytic technique known as pyrolysis-FTIR spectroscopy to test the meteorites. The pyrolysis process involved blasting a meteorite fragment with electricity to heat it from room temperature at a rate of 20,000 degrees Celsius per second to 250 Celsius or 1000 Celsius to cause the material to break apart and vaporize. FTIR, or Fourier-transform infra-red spectroscopy, then provides a chemical fingerprint of any small molecules, such as water and carbon dioxide, produced.

For a long time, scientists have been trying to understand why Earth is so water rich compared to other planets in our solar system, Sephton explains. The team found that on average, each meteorite fragment could release 12% of its weight as water vapour and 6% as carbon dioxide gas. These figures, the researchers suggest, are not enough that a few small meteorites could have made a significant contribution to the atmosphere’s water and carbon dioxide levels.

However, the team also analysed data from an ancient meteorite shower called the Late Heavy Bombardment (LHB), which occurred 4 billion years ago, where millions of rocks crashed to Earth and Mars over a period of 20 million years. They calculated that the LHB could have added 10 billion tonnes of carbon dioxide and 10 billion tonnes of water vapour to the planets’ atmospheres every year. This rate of addition is certainly adequate to make both planets warmer and wetter enough to sustain life.

The LHB provides a missing clue. This may have been a pivotal moment in our early history where Earth’s gaseous envelope finally had enough of the right ingredients to nurture life on our planet, adds Sephton.

Because of their chemistry, ancient meteorites have been suggested as a way of furnishing the early Earth with its liquid water, says Court, Now we have data that reveals just how much water and carbon dioxide was directly injected into the atmosphere by meteorites. These gases could have got to work immediately, boosting the water cycle and warming the planet. Of course, the existence of life on Earth is obvious, but habitable conditions on Mars apparently did not last. Unlike Earth, Mars has no magnetic field to shield it from the Sun’s lethal solar wind, its atmosphere was eventually mostly stripped away, and warming volcanic activity subsided. As such any liquid water retreated to the frozen poles leaving behind the barren red planet with which we are almost familiar.

Further reading

Geochim. Cosmochim. Acta, 73 (11), 1 June 2009, 3512-3521

Professor Mark A. Sephton
http://www3.imperial.ac.uk/people/m.a.sephton

Suggested searches

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