Tuesday, April 22, 2014

Earth hydrosphere #2? What really can be said about the ringwoodite discovery




Earth hydrosphere #1 and a big floating hunk of the cryosphere too. (From http://inter3.kuicr.kyoto-u.ac.jp/album/KH94-4/FH000153.jpg)


Earth has a well known hydrosphere--the oceans that make it up cover two-thirds of the planet. But there may be another hydrosphere deep underground. Recently, a mineral that had only been seen in impact - shocked meteorites, ringwoodite, was discovered in a terrestrial diamond1. Pearson and other workers wrote in the journal Nature that this diamond is thought to have originated in the transition zone between the upper and lower mantle--a zone long known by geologists from a change in earthquake seismic wave behavior as they pass through the earth.

Ringwoodite is a high pressure version or polymorph of olivine, which is a common mineral in basalt. It has the same composition as olivine, but the atoms are arranged differently to be more stable at high pressures found in Earth's mid-mantle. Although only one ringwoodite grain has now been found from the Earth, it is commonly found in meteorites. This is because many asteroids experienced collisions in the 4.5 billion years they've orbited the sun; these collisions produce high shock pressure that allows olivine to recrystallize as ringwoodite. Below is an image showing an incidence in a meteorite where recrystallization from olivine to ringwoodite was incomplete. The box on the left image is blown up on the right image (Ol is olivine; Rgt is ringwoodite.):



Ringwoodite had been predicted to be stable deep within the Earth for decades2, and the presence of water has also been hypothesized, but remained controversial. The authors not only have confirmed a terrestrial ringwoodite, but the finding of water (1.4 to 1.5% with uncertainties "as large as 50%") allow one to make predictions of the hydration of Earth's transition zone. The authors claim that the area local to where the ringwoodite formed (and presumably where the diamond grew around it, preserving it) is about 1% hydrated.

The discovery of a ringwoodite mineral on earth is a significant achievement. The discovery that it hosts water is also very exciting. Furthermore, showing that diamonds and the kimberlite source that brought all the material to the surface must have originated in the transition zone is a very significant result. If other grains are found elsewhere, perhaps it can be shown that the transition zone is pervasively hydrated. It could also show that a volatile rich zone deep in the mantle may be linked to diamond formation and explosive kimberlite eruptions (Water is often linked to vulcanism; one reason is that it lowers the melting temperature of rock). It must be stressed, however, that only one 40 micron diameter ringwoodite grain has been discovered.

Unfortunately, the announcement of this discovery has been accompanied by hyperbole. The University of Alberta, where the first author is from, extrapolates the local area surrounding a tiny 40 micron mineral grain to Earth's entire transition zone: "We show that a special region of the Earth, between 400 and 700km, known as the 'transition zone', is an oasis of water in an otherwise very dry deep interior."3 Sci-News.com states "The first land discovery of ringwoodite confirms the presence of huge water reservoirs beneath the surface of the Earth." Other news outlets, from CBC to BBC to Fox News, have announced similar claims.

There are inklings that the transition zone may be hydrated--for example, seismic waves behave peculiarly in the area that could be explained by pervasive hydration. Many workers (some referenced in the Pearson paper) have given evidence that point to a hydrated transition zone and its role in a deep water cycle. The work by Pearson and his co-authors offers one more link, an exciting link, that the mantle's transition zone may be hydrated at least locally. However, extrapolating interpretations made from a single observation to Earth's entire mantle transition zone is not valid. More samples, preferably from wide-ranging locations, are needed to make such a claim. 


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Correction: The amount of water reported was mistakenly first written as 2.5 +/- 50%. The actual number is recorded as "clearly indicative of significant H2O content, and are consistent with a minimum estimate between 1.4 and 1.5 wt% H2O, derived by integrating the spectra in Fig. 2 (see Methods section on FTIR spectroscopy). Although the uncertainty in these estimates may be as large as 50%..." 


1. Pearson et al. Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature. 507, 221-224 (2014)  http://www.nature.com/nature/journal/v507/n7491/full/nature13080.html

2. Ringwood, A. E. & Major, A. The system Mg2SiO4-Fe2SiO4 at high pressures and
temperatures. Phys. Earth Planet. Inter. 3, 89–108 (1970)

3. Hydrous mantle transition zone indicated by ringwoodite included within diamond, University of Alberta Department of Earth and Atmospheric Sciences, http://easweb.eas.ualberta.ca/page/hydrous-mantle-transition 

Thursday, April 17, 2014

Mars isn't static!

One of the old stories has always been that things in space are never changing. The stars were fixed to the celestial sphere and stayed in place like a light fixture with a few thousand bulbs. What made the planets different from the stars was their movement. We now know that stars aren't static either: their light changes, some blow up, some have planets moving around them, some orbit one another, and some whip around our galaxy's center at incredible rates due to the giant black hole there.

Even though we know that the Solar System isn't unchanging, it still is surprising to see a change. Jupiter got walloped by a comet in 1994 and telescopes caught each explosion. The moons Enceladus around Saturn and Io around Jupiter erupt occasionally and the plumes can be seen in space. And now, researchers at NASA have caught a subtle change on Mars. Something common on Earth was captured on mostly dry Mars: a gully changed course.

"A comparison of images taken by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter in November 2010 and May 2013 reveal the formation of a new gully channel on a crater-wall slope in the southern highlands of Mars" http://www.jpl.nasa.gov/news/news.php?release=2014-086

Scientists do not know what time of year that the gully changed.In the area on Mars where this gully lies (Southern Highlands), however, gullies often change in the winter when it is far too cold for liquid water to exist. Martian winters get so cold that carbon dioxide may condense out of the atmosphere and be involved with gully formation. Or perhaps liquid (probably salty, because it didn't freeze in the very cold martian air) water was involved. Either way, this new gully is a good reminder to us that Mars, like Earth, is not static. It's a world where things happen.




Monday, April 14, 2014

Water's connection to us

There's something about water. Sure, we know it is essential to life. It facilitates chemical reactions, either carrying ions or participating itself. It is everywhere on earth--traces of it can be found from the poles to the Atacama Desert, deep in the earth to high in the atmosphere. But there's more to it than its role in biology, meteorology, geology, chemistry. In the desert water brings cool wind and a scent that makes the heart leap--for a desert dweller truly knows the blessing that water is, the Prodigal Son come back, life for the land. In Tucson, the smell and sound of rain brings people to the windows, or sends them outside to hear it, to feel it on their faces. In New Mexico, nineteen Pueblos celebrate the rare rains and artists make a living photographing storms and rainbows in the desert landscape. Elsewhere, even in humid climes, people construct ponds on their land, fountains in their plazas, and build homes in flood zones to be near the whispers and ripples and cascades of water. Vacationers go to the beach and bask by the ocean, the roaring waves themselves soothe bathers by their rhythmic movements.

Our world's connection to water, humanity's connection, is profound. For thousands of years we've revered both the sun and rain for the life they give our fields and us. Our reverence our water and reliance on it is hard-wired in us. A thirst in our genes. It is dear to us.

Though we recognize its value, water is quite common both at home and in space. The Moon has small amounts in its crust. Mars has ancient river valleys where water once flowed, and gullies where it may flow now. Ice lies at the poles and what seem to be ancient shorelines are visible. Further  away, some asteroids show signs of water--from light reflecting off their surface (or emitted really) to jets of vapor coming off like a geyser in Yellowstone. Jupiter's moons are full of it, one with an ice-covered world ocean that reminds us of home. Saturn's beauty is owed to billions of chunks of ice orbiting in an incredibly thin sheet. One of these rings, a faint one, begins in a small icy moon that consistently sends water plumes off its South Pole. Even further away, water makes up much of the moons of Uranus and Neptune, the dwarf planets, asteroids, and comets that lie even further away.  Beyond the furthest icy body, we can see water's fingerprint associated with other stars and gassy nebulae. Water, one of the drivers of life, is everywhere.

That a substance so useful and essential for us is so common, is comforting. The focus of this blog is water in the solar system. This is a broad place, everything in our normal experience except the stars themselves are fair game. Penguins and polar bears roam this realm, as do planets, moons, asteroids, and even dust. Where it is, what does it do, how, and why? It is a shaper of our neighborhood and, we are realizing, necessary for further human exploration beyond the earth.