Graphic by Alan Jiang / Sun Contributor

December 9, 2020

Cornell Astronomers Developed Instrument for Discovery of Water on Sunlit Moon

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Cornell astronomers played a key role in developing the instrument responsible for discovering the presence of water on the sunlit surface of the moon this past October. 

NASA’s Stratospheric Observatory for Infrared Astronomy, which housed the device used to make the groundbreaking finding, operates on a special performance Boeing 747 fitted with a 2.5-meter telescope that makes observations high above the Earth’s troposphere. 

A trio of University researchers — astronomy Profs. Terry Herter and Gordon Stacey along with research associate Thomas Nikola — helped devise the Faint Object Infrared Camera, known as FORCAST, for the SOFIA Telescope. FORCAST was the device that made this groundbreaking discovery on the moon. 

Since atmospheric water vapor limits the view of Earth-based telescopes over most infrared wavelengths, SOFIA is optimized to detect radiation that is not otherwise accessible to astronomers on the ground, according to Nikola, who is a research associate at the Cornell Center for Astrophysics and Planetary Science. 

“The water vapor in Earth’s atmosphere prevents us from looking at celestial objects, like objects in the solar system or galaxies, in most of the infrared and submillimeter wavelength range,” Nikola said. “So we have to go above all the water vapor in the Earth’s atmosphere as much as we can so we can better study these objects.”

Nikola explained that an airplane is uniquely suited to house the telescope and observe the cosmos. Unlike high altitude balloons, airplanes are less dependent on weather conditions and the flights are easier to schedule. 

Satellites, another alternative, can be very expensive and their lifetime is usually limited to a few years, Nikola said. Since SOFIA returns to its base airport after each flight, its instrumentation can be exchanged to address different science objectives and sooner be upgraded with “state-of-the-art” cameras and spectrometers. 

“So if new technology comes out, you can put them on SOFIA. It’s not a satellite where you have to [wait for a] 10 year development [period],” Nikola said. “It survives a launch and [has] a much faster turnaround for new technology to put in useful astronomy. So in this sense, it has a lot of potential, and it is used a lot.”

Following NASA’s 1977 purchase of the aircraft, a slew of structural modifications and technological additions transformed the commercial airliner into an airborne observatory. 

According to Nikola, the instrument has both a camera and spectrometer that can detect wavelengths between 5 and 25 micrometers on the infrared region of the electromagnetic spectrum. 

FORCAST’s spectrometer consists of devices that disperse entering radiation, creating a higher spectral resolution spectrum that can distinguish between different emission features, Nikola said. 

The FORCAST device helped make the groundbreaking discovery of discovering the presence of water on the moon. “Big Red” is honored on this instrument, thanks to the Cornell researchers involved in its development.

Often likened to a chemical fingerprint, emission spectra are the distinct set of wavelengths produced by chemical elements and compounds, and are a valuable tool used by astronomers to study materials in space. 

“This tells us basically how the elements are sort of distributed around in the galaxy, the state of the elements and how the abundance of elements evolved over time,” Nikola said. 

Emission spectra of infrared radiation from the sunlit surface of the moon confirmed the presence of water — but it was no easy feat, Nikola said. 

“If we observe something in the infrared, usually what we observe is very, very dim compared to all [the] foreground radiation that we see,” Nikola said. “[It’s] as if you try to observe a candle behind a forest fire.”

In order to detect the dim radiation from Clavius crater — a location on the moon believed to potentially have water — astronomers compared it to Mare Serenitatis, a lunar mare where little to no water is likely present. The emission line produced after subtracting these two points indicated the presence of water, according to Nikola. 

Nikola explained that water has a unique structure, as there are many different ways the bonds between oxygen and hydrogen can bend, rotate and vibrate. In previous observations at other wavelengths, it was difficult to distinguish H2O from other related molecules like hydroxyl, which is just one oxygen atom bonded to one hydrogen atom.

However, this time, the high sensitivity of the FORCAST spectrometer paired with the unique detection capabilities of SOFIA made it possible to observe a spectral feature with a wavelength only produced by water, Nikola said. 

The discovery of water on the moon’s sunlit surface adds another piece to the puzzle of how the solar system might have been created, according to Nikola. 

“By studying the moon, we might get a better estimate of how much water was present during the formation of the earth. So it gives you an additional stepping stone to understand this evolution,” Nikola said. ”How can water be actually conserved over a long time in some of those celestial bodies?”

Although water has previously been found in the cold, shadowed craters of the moon, SOFIA’s discovery of water on the moon’s harsh and hot sunlit surface serves as evidence for broader theories on the origin of lunar water. 

Understanding these origins, Nikola said, could be critical for supporting future NASA exploration missions.

“At some point, humans probably want to have some outpost on the moon,” Nikola said. “So if we know more about the soil on the moon and where we can find various water and other elements, can we survive on the moon? Do we have to have to ship everything out to the moon, or can we maybe extract water from the moon itself?”