Tiny Crystal Provides a Big Clue

Lab-grown crystal of blue ringwoodite, about 1.5 times the size of a salt grain (~150 micrometers) Photomicrograph by Jasperox, Wikimedia Commons.

Lab-grown crystal of blue ringwoodite, about 1.5 times the size of a salt grain (~150 micrometers) Photomicrograph by Jasperox, Wikimedia Commons.

Is there an immense ocean, far beneath the surface of the Earth, that replenishes the oceans above? Recent news items describe a deep reservoir containing as much as three-quarters of the Earth’s water supply. Most of these news stories are careful to note that this isn’t some great sloshing underground pool, and you won’t find any fish living there. Rather, the water is “bound up” in mineral deposits and released when these minerals are put under immense pressure. Some news stories compare the minerals to sponges, which is not something you usually associate with rocks. (Here are a couple of examples of the news items: Daily Digest News, The Guardian.)

What’s really going on here? Last March, a research paper in the journal Nature reported the discovery of a tiny crystal of a mineral called ringwoodite, encased in a diamond that was plucked out of shallow river gravel by artisan miners near Juína, Brazil. A research team led by Graham Pearson (University of Alberta, Canada) found the ringwoodite crystal (60 microns, less than the thickness of a sheet of paper) inside a dirty-looking brown diamond that they had bought for about $20 (US), according to an article in Sci-News. (Here’s the article, with a photo of the diamond and ringwoodite crystal.).

This month, a paper in Science used this discovery, along with geological measurements, lab studies, and models, as evidence to support an idea that geologists had been looking at for many years. (Here are a couple of press releases, from the the University of New Mexico and Northwestern University, where the two lead authors are located.)

Scientists suspect that ringwoodite may be common in a “transition zone” that lies 410 to 660 km (250–410 miles) below the Earth’s surface, based on seismic measurements, which track changes in the speed and direction of earthquake vibrations. Ringwoodite has been made in the lab and found in meteorites, but this was the first time that anyone had found a naturally occurring crystal from inside the Earth.

green olivine sand

Green olivine sand on black volcanic rock. Photo by Brocken Inaglory, Wikimedia Commons.

Why do we care about this elusive mineral? Because it’s a high-pressure form of another mineral called olivine. As the name suggests, many forms of olivine are dark green (it can range from yellow to black). The dark-green sands on Papakolea Beach, Hawaii are mostly olivine, and this mineral is common all over the world. At very high pressures, like those you would find more than 660 km beneath the Earth’s surface, olivine transforms into another mineral called perovskite. The presence of one form or another affects the way that earthquake vibrations travel through the Earth. These two minerals have been studied intensively, and geologists use seismic signals as clues to help them map mineral forms and geological activity far beneath the surface, where it’s hard to get the information any other way.

Water and hydroxyl radical

Water molecule (H2O, left) and hydroxyl radical (OH, right). Courtesy of NIST.

Ringwoodite is intermediate between olivine and perovskite. Because of the way its crystals form, they can contain as much as 3% by weight of something called hydroxyl radicals (OH•). (The crystal reported in the Nature paper had 1.5%.) This “radical” has nothing to do with political activism or unorthodox beliefs. Used in a chemistry context, the term refers to a water molecule (HOH, more commonly written H2O) that has one hydrogen atom stripped away, leaving behind a spare electron that it can share with something else, in this case, the elements in ringwoodite’s crystal structure.

Olivine doesn’t contain hydroxyl radicals. When olivine is put under pressure, water from the surrounding environment can be converted into OH•, forced inside, and incorporated into the framework of atoms, forming ringwoodite. The more OH• trapped in the framework, the faster the sound waves travel through it, which is why geologists had suspected that there was a water-containing mineral intermediate between olivine and perovskite.

perovskite

Perovskite collected by A.E. Foote from Magnet Cove, Arkansas. Mineral collection of Bringham Young University Department of Geology, Provo, Utah. Photo by Andrew Silver, USGS.

Put ringwoodite under even more pressure, and it converts into perovskite. The hydroxyl radicals are forced back outside again, where they recombine with hydrogen (which is just about everywhere) to form liquid water. This water causes the perovskite to melt a little, in much the same way that sprinkling salt on ice causes the ice to melt. The scientists who published the paper in Science had actually seen this melting behavior in the lab, when they used a device called a diamond anvil cell to put immense pressure on a ringwoodite crystal and convert it to perovskite.

Geologists had seen seismic waves suddenly slowing down in regions where they had other clues that one rock layer was sinking down below another. A layer of partially melted perovskite that was taking a dive would slow down a seismic wave like this.

Previous examination of ancient minerals called basalts, taken from mid-ocean ridges, suggested that Earth’s upper mantle has a water content of 0.005 to 0.02% water by weight. Lab and modeling studies show that ringwoodite and a related mineral, wadsleyite, can hold between 1 and 3% water by weight. Seismic evidence suggests that the water content below 660 km is much less. Thus, if these studies are right, the transition zone is where most of the water is. If this zone extends all over the world, and its average water content is 1% (a conservative estimate that needs to be verified), this translates into nearly three times as much water as the oceans contain, which is where the news reports got their three-quarters number (three-quarters in the transition zone, one-quarter in the oceans).

Geologists have been working for decades to create a model that “balances the books” on where water comes from and where it goes — a “whole-Earth water cycle”. They had long suspected that there was a subterranean source that acts as a buffer zone to keep the amount of water in the oceans fairly constant. This latest evidence provides more clues to help them fill in the missing pieces.

What Is a Drone?

Second posting in a three-part series (see previous post)

Recent news stories have familiarized us with military drones bearing names like Predator and Reaper. Popular television shows feature tiny spy drones, conjuring images of CIA black ops. You could be forgiven for assuming that drones are a new and pernicious misuse of government power. But what are drones, really, and how are they being used?

The word “drone” is a popular term for any one of several types of unmanned vehicles that fly, swim, or travel over land. Most drones have some type of human guidance, whether it’s a kid at the other end of the kite string or a soldier or sailor sitting at a control panel hundreds of miles away. The variety of functions and capabilities is reflected in a menagerie of abbreviations: UAV (unmanned aerial vehicle), UAS (unmanned aerial system), RPV (remotely piloted vehicle), ROV (remotely operated vehicle), RPA (remotely piloted aircraft), UUV (unmanned underwater vehicle), and the list goes on.

Unmanned ground vehicles range from the Roomba automated vacuum cleaner to DARPA’s Big Dog robotic “pack animal”. REMUS vehicles (Remote Environmental Monitoring UnitS, operate underwater, taking orders from a human at a simple laptop computer or traversing a preprogrammed route. REMUS vehicles have patrolled Puget Sound, monitoring the temperature and salinity of the water. Specially adapted REMUS vehicles have surveyed New York City’s public water mains to check for leaks.

Aerial drone use is certainly not new. You might say that Benjamin Franklin used a drone kite to carry his metal key aloft during his experiments with lightning.

Oil burn experiment

1993 Newfoundland Oil Burn Experiment (Canadian Coast Guard photo)

More recently, miniature helicopters known as ROVs (remotely operated vehicles) flew through a smoke plume and monitored the air during the 1993 Newfoundland Offshore Burn Experiment, a collaboration between the U.S. and Canada in which a contained oil spill was set on fire in order to observe the effects on the surroundings and examine the after-products. (The helicopter in the photo at right is a full-sized, passenger-carrying helicopter carrying support crew for this event.) The Predator drones used in military operations are about the size of a glider plane. Some military surveillance drones are small enough for one person to launch by throwing them into the air (photo below). The U.S. Army is funding development work on hummingbird-sized drones that can fly into small spaces and avoid being noticed.

hand launched drone

Pvt. Patrick Hernandez practices launching a RQ-11B Raven. (USDOD photo by Pamela Redford, Fort Riley Public Affairs)

The Drone Next Door“, a May 7 Future Tense presentation at the New America Foundation (Twitter #FTdrones), focused on aerial drones. These unmanned vehicles operate with various degrees of autonomy. Automated aerial drones can operate without a human steering them, but they follow a specific set of instructions: fly this high, go that fast, travel this far in a specified direction. Autonomous drones can operate independently, executing a mission while making its own decisions under uncertain circumstances: locate and retrieve a specific package, but find your own way past any obstacles and recover from any mishaps you might encounter on the way.

Flying cameras are old technology, said Missy Cummings, associate professor of aeronautics and astronautics at MIT, and one of the U.S. Navy’s first female fighter pilots. What’s new about drones is their ability to make aerial imaging cost effective. The main limitations for drone-mounted sensors are weight and power requirements.

Is there any way of avoiding drone surveillance? Cummings facetiously mentioned anti-UAV hoodies. She noted that for every technology, there is an anti-technology. The Navy is very concerned with GPS denial technology, and is working to develop a drone that does not rely on GPS for navigation. Michael Toscano, president and CEO of the Association for Unmanned Vehicle Systems International, noted that signal interference, intentional or not, could pose a safety issue by disorienting the drone and possibly causing it to crash.

Drones are in widespread use for military operations, but are we in danger of being overrun with drones once their commercial use becomes legal in the U.S.? Konstantin Kakaes, a Schwartz Fellow at the New America Foundation, cited several examples where military technologies failed to make the transition to the civilian world. Nuclear-powered airplanes and nuclear explosions as excavation tools never caught on. President Kennedy pushed for supersonic passenger jets, but the Concorde was a European project, and it was not a commercial success. One success story, GPS navigation, was not predicted to make the transition from military-only use. It succeeded because it provided unique capabilities, and the price came down as it became more widely used.

The KMAX, and unmanned cargo helicopter, proved useful in the remote regions of Afghanistan, but it was not as useful in the U.S. Barriers to technology adoption include production costs and infrastructure requirements such as refueling stations, said Kakaes. A technology that provides a unique capability in a remote, primitive, or hazardous area could lose out to cheaper and better competitors in a modern city.

Drones could, however, prove themselves useful in an urban setting if they could effectively increase capabilities and reduce costs for search and rescue missions (finding survivors of a building collapse, for example), crime scene investigation, traffic accident reporting, and missing person searches, according to Captain Don Roby of the Baltimore County Police Department.

Current FAA rules prohibit commercial use of drones, but under the new rules in 2015, they could reduce costs for traffic reporting and monitor environmental changes, said Matthew Waite, the University of Nebraska-Lincoln professor who founded the Drone Journalism Lab. Waite was not especially concerned about the possibility of airborne paparazzi on every street corner in the near future. “Journalists are horrible pilots,” he said, citing his and his students’ misadventures.

What about scientific research? “Cost is the biggest hurdle for science,” said Robbie Hood, Director of Unmanned Aerial Systems at NOAA. You’re looking at established technology, she said, with the UAV as just another observing system, a “force multiplier for science”. Satellites can provide snapshot views of the ground below, but UASs can stay with a weather system as it develops, providing a more detailed picture. This could enable NOAA to observe a hurricane as it first forms over the open ocean. As climate change opens up shipping lanes in the Arctic, drones will monitor shipping activity, oil spills, and detailed weather reporting that could help prevent ship strandings.

Carter Roberts, president and CEO of the World Wildlife Fund, described how airborne drones are being used to monitor political unrest in areas where sensitive wildlife populations could be harmed. Drones also check for poaching activity, which WWF reports to the governments of the affected areas. Drones provide more immediate feedback than satellite collars, which can cost $10,000 each. Transmitter chips attached to an animal can send text messages to drones overhead much more cheaply. Thermal imaging can be used to reveal the presence of poachers at night, when they are most active. This opens up the possibility of pre-empting the poachers before they make their kill.

Previous post: Don’t Drone Me, Bro 

To come: Hashing it all out: How will we deal with the practical effects of having more unmanned vehicles in our daily lives?