Using satellite data, Brown researchers have for the first time detected widespread water within ancient explosive volcanic deposits on the Moon, suggesting that its interior contains substantial amounts of indigenous water.
A new study of satellite data finds that numerous volcanic deposits distributed across the surface of the Moon contain unusually high amounts of trapped water compared with surrounding terrains. The finding of water in these ancient deposits, which are believed to consist of glass beads formed by the explosive eruption of magma coming from the deep lunar interior, bolsters the idea that the lunar mantle is surprisingly water-rich.
Scientists had assumed for years that the interior of the Moon had been largely depleted of water and other volatile compounds. That began to change in 2008, when a research team including Brown University geologist Alberto Saal detected trace amounts of water in some of the volcanic glass beads brought back to Earth from the Apollo 15 and 17 missions to the Moon. In 2011, further study of tiny crystalline formations within those beads revealed that they actually contain similar amounts of water as some basalts on Earth. That suggests that the Moon’s mantle—parts of it, at least—contain as much water as Earth’s.
“The key question is whether those Apollo samples represent the bulk conditions of the lunar interior or instead represent unusual or perhaps anomalous water-rich regions within an otherwise ‘dry’ mantle,” said Ralph Milliken, lead author of the new research and an associate professor in Brown’s Department of Earth, Environmental and Planetary Sciences. “By looking at the orbital data, we can examine the large pyroclastic deposits on the Moon that were never sampled by the Apollo or Luna missions. The fact that nearly all of them exhibit signatures of water suggests that the Apollo samples are not anomalous, so it may be that the bulk interior of the Moon is wet.”
Detecting the water content of lunar volcanic deposits using orbital instruments is no easy task. Scientists use orbital spectrometers to measure the light that bounces off a planetary surface. By looking at which wavelengths of light are absorbed or reflected by the surface, scientists can get an idea of which minerals and other compounds are present.
The problem is that the lunar surface heats up over the course of a day, especially at the latitudes where these pyroclastic deposits are located. That means that in addition to the light reflected from the surface, the spectrometer also ends up measuring heat.
“That thermally emitted radiation happens at the same wavelengths that we need to use to look for water,” Milliken said. “So in order to say with any confidence that water is present, we first need to account for and remove the thermally emitted component.”
To do that, Li and Milliken used laboratory-based measurements of samples returned from the Apollo missions, combined with a detailed temperature profile of the areas of interest on the Moon’s surface. Using the new thermal correction, the researchers looked at data from the Moon Mineralogy Mapper, an imaging spectrometer that flew aboard India’s Chandrayaan-1 lunar orbiter.
The researchers found evidence of water in nearly all of the large pyroclastic deposits that had been previously mapped across the Moon’s surface, including deposits near the Apollo 15 and 17 landing sites where the water-bearing glass bead samples were collected.
“The distribution of these water-rich deposits is the key thing,” Milliken said. “They’re spread across the surface, which tells us that the water found in the Apollo samples isn’t a one-off. Lunar pyroclastics seem to be universally water-rich, which suggests the same may be true of the mantle.”
The idea that the interior of the Moon is water-rich raises interesting questions about the Moon’s formation. Scientists think the Moon formed from debris left behind after an object about the size of Mars slammed into the Earth very early in solar system history. One of the reasons scientists had assumed the Moon’s interior should be dry is that it seems unlikely that any of the hydrogen needed to form water could have survived the heat of that impact.
“The growing evidence for water inside the Moon suggests that water did somehow survive, or that it was brought in shortly after the impact by asteroids or comets before the Moon had completely solidified,” Li said. “The exact origin of water in the lunar interior is still a big question.”
In addition to shedding light on the water story in the early solar system, the research could also have implications for future lunar exploration. The volcanic beads don’t contain a lot of water—about .05 percent by weight, the researchers say—but the deposits are large, and the water could potentially be extracted.
“Other studies have suggested the presence of water ice in shadowed regions at the lunar poles, but the pyroclastic deposits are at locations that may be easier to access,” Li said. “Anything that helps save future lunar explorers from having to bring lots of water from home is a big step forward, and our results suggest a new alternative.”
Written by Kevin Stacey—https://news.brown.edu/articles/2017/07/moonwater
Holograms Could Detect Signs of Life in Space
In July, the journal Astrobiology published a special issue dedicated to the search for signs of life on Saturn’s icy moon, Enceladus. Included is a paper from Caltech’s Jay Nadeau and colleagues offering evidence that a technique called digital holographic microscopy, which uses lasers to record 3-D images, may be our best bet for spotting extraterrestrial microbes.
No probe since NASA’s Viking program in the late 1970s has explicitly searched for extraterrestrial life—that is, for actual living organisms. Rather, the focus has been on finding water. Enceladus has a lot of water—an ocean’s worth, hidden beneath an icy shell that coats the entire surface. But even if life does exist there in some microbial fashion, the difficulty for scientists on Earth is identifying those microbes from 790 million miles away.
“It’s harder to distinguish between a microbe and a speck of dust than you’d think,” says Nadeau, research professor of medical engineering and aerospace in the Division of Engineering and Applied Science. “You have to differentiate between Brownian motion, which is the random motion of matter, and the intentional, self-directed motion of a living organism.”
Enceladus is the sixth-largest moon of Saturn, and is 100,000 times less massive than Earth. As such, Enceladus has an escape velocity—the minimum speed needed for an object on the moon to escape its surface—of just 239 meters per second. That is a fraction of Earth’s, which is a little over 11,000 meters per second.
Enceladus’s minuscule escape velocity allows for an unusual phenomenon: enormous geysers, venting water vapor through cracks in the moon’s icy shell, regularly jet out into space. When the Saturn probe Cassini flew by Enceladus in 2005, it spotted water vapor plumes in the South Polar Region blasting icy particles at nearly 2,000 kilometers per hour to an altitude of nearly 500 kilometers above the surface. Scientists calculated that as much as 250 kilograms of water vapor were released every second in each plume. Since those first observations, more than a hundred geysers have been spotted. This water is thought to replenish Saturn’s diaphanous E ring, which would otherwise dissipate quickly, and was the subject of a recent announcement by NASA describing Enceladus as an “ocean world” that is the closest NASA has come to finding a place with the necessary ingredients for habitability.
“Water blasting out into space offers a rare opportunity,” says Nadeau. While landing on a foreign body is difficult and costly, a cheaper and easier option might be to send a probe to Enceladus and pass it through the jets, where it would collect water samples that could possibly contain microbes.
Assuming a probe were to do so, it would open up a few questions for engineers like Nadeau, who studies microbes in extreme environments. Could microbes survive a journey in one of those jets? If so, how could a probe collect samples without destroying those microbes? And if samples are collected, how could they be identified as living cells?
The problem with searching for microbes in a sample of water is that they can be difficult to identify. “The hardest thing about bacteria is that they just don’t have a lot of cellular features,” Nadeau says. Bacteria are usually blob-shaped and always tiny—smaller in diameter than a strand of hair. “Sometimes telling the difference between them and sand grains is very difficult,” Nadeau says.
“Some strategies for demonstrating that a microscopic speck is actually a living microbe involve searching for patterns in its structure or studying its specific chemical composition. While these methods are useful, they should be used in conjunction with direct observations of potential microbes,” Nadeau says.
“Looking at patterns and chemistry is useful, but I think we need to take a step back and look for more general characteristics of living things, like the presence of motion. That is, if you see an E. coli, you know that it is alive—and not, say, a grain of sand—because of the way it is moving,” she says. In earlier work, Nadeau suggested that the movement exhibited by many living organisms could potentially be used as a robust, chemistry-independent biosignature for extraterrestrial life. The motion of living organisms can also be triggered or enhanced by “feeding” the microbe electrons and watching them grow more active.
To study the motion of potential microbes from Enceladus’s plumes, Nadeau proposes using an instrument called a digital holographic microscope that has been modified specifically for astrobiology.
In digital holographic microscopy, an object is illuminated with a laser, and the light that bounces off the object and back to a detector is measured. This scattered light contains information about the amplitude (the intensity) of the scattered light and about its phase (a separate property that can be used to tell how far the light traveled after it scattered). With the two types of information, a computer can reconstruct a 3-D image of the object—one that can show motion through all three dimensions.
“Digital holographic microscopy allows you to see and track even the tiniest of motions,” Nadeau says. Furthermore, by tagging potential microbes with fluorescent dyes that bind to broad classes of molecules that are likely to be indicators of life—proteins, sugars, lipids, and nucleic acids. “You can tell what the microbes are made of,” she says.
To study the technology’s potential utility for analyzing extraterrestrial samples, Nadeau and her colleagues obtained samples of frigid water from the Arctic, which is sparsely populated with bacteria; those that are present are rendered sluggish by the cold temperatures.
With holographic microscopy, Nadeau was able to identify organisms with population densities of just 1,000 cells per milliliter of volume, similar to what exists in some of the most extreme environments on Earth, such as subglacial lakes. For comparison, the open ocean contains about 10,000 cells per milliliter and a typical pond might have 1–10 million cells per milliliter. “That low threshold for detection, coupled with the system’s ability to test a lot of samples quickly (at a rate of about one milliliter per hour) and its few moving parts, makes it ideal for astrobiology,” Nadeau says.
Written by Robert Perkins. http://www.caltech.edu/news/holographic-imaging-could-be-used-detect-signs-life-space-78931