A new remote sensing technique for examining olivine, a material that may aid in understanding the early evolution of the Moon, Mars, and other planetary bodies, has been created by planetary scientists at Brown University.
“Olivine is understood to be a major component in the interiors of rocky planets,” said Christopher Kremer, a Ph.D. candidate at Brown University and lead author of a new paper describing the work.
“It’s a primary constituent of Earth’s mantle, and it’s been detected on the surfaces of the Moon and Mars in volcanic deposits or in impact craters that bring up material from the subsurface.”
According to Kremer, current remote sensing methods are effective in detecting olivine from orbit, but scientists would like to do more than just spot it. They would like to have access to information regarding its chemical composition. All olivines contain silicon and oxygen, although some are particularly abundant in iron and others in magnesium.
“The composition tells us something about the environment in which the minerals formed, particularly the temperature,” Kremer said.
“Higher temperatures during formation yield more magnesium, while lower temperatures yield more iron. Being able to tease out those compositions could tell us something about how the interiors of these planetary bodies have evolved since their formation.”
Kremer used mountains of data from the Keck/NASA Reflectance Experiment Laboratory (RELAB), which is housed at Brown, along with academics Carlé Pieters and Jack Mustard to investigate if it could be possible to see that composition via remote sensing.
Spectroscopy is one technique used by scientists to examine the rocks on other planetary bodies. Different wavelengths of light are reflected or absorbed by specific substances or components to varying degrees.
Scientists can determine what substances are present by examining the light spectra that rocks reflect. RELAB performs high-precision spectral measurements on samples whose composition has already been established in the laboratory using conventional methods.
By doing so, the lab offers a reference point for analyzing spectral readings made by spacecraft examining other planetary bodies.
Olivine is understood to be a major component in the interiors of rocky planets. It’s a primary constituent of Earth’s mantle, and it’s been detected on the surfaces of the Moon and Mars in volcanic deposits or in impact craters that bring up material from the subsurface.
Christopher Kremer
Kremer discovered something intriguing in a narrow band of wavelengths that is ignored by the kinds of spectroscopes that are used on orbiting spacecraft while sifting through data from olivine samples examined over the years at RELAB.
“Over the past few decades, there’s been a lot of interest in near-infrared spectroscopy and middle infrared spectroscopy,” Kremer said. “But there’s a small range of wavelengths between those two that’s left out, and those are the wavelengths I was looking at.”
Kremer discovered that a band of wavelengths between 4 and 8 microns could accurately predict the proportion of magnesium or iron present in an olivine sample to within roughly 10%. When certain wavelengths are disregarded, that is a lot better option.
“With the instruments we have now, we could say maybe we have a little bit of this or a little bit of that,” Mustard said. “But with this we’re able to really put a number on it, which is a big step forward.”
The discovery, which was published in Geophysical Research Letters, may spur the development and launch of a spectrometer that can detect these previously unobserved wavelengths. According to Kremer, such a device could yield immediate benefits for comprehending the makeup of the olivine deposits on the Moon’s surface.
“The olivine samples brought back during the Apollo program that we’ve been able to study here on Earth vary widely in magnesium composition,” Kremer said.
“But we don’t know how those differing compositions are distributed on the Moon itself, because we can’t see those compositions spectroscopically. That’s where this new technique comes in. If we could figure out a pattern to how those deposits are distributed, it could tell us something about the early evolution of the Moon.”
There is also a chance for other discoveries. One of the few non-lab instruments that can observe this neglected frequency range is the SOFIA telescope, which is mounted on an airplane. These frequencies were recently used by the equipment to find water molecules on the sunlit lunar surfaces.
“That makes the idea of space-borne spectrometers that can see this range much more attractive, both for water and for rocky material like olivine,” Kremer said.
The research was supported through NASA SSERVI (NNA14AB01A) and a NASA FINESST grant.
