Above:
2011 composite satellite image shows the expanse of Arctic sea ice and the Greenland Ice Sheet.
PREFIRE
Why is the North Pole Warm?
Matt Kenyon
As our planet continues to warm, climate scientists need novel ways of exploring the processes that drive change and of measuring the markers that predict such alterations. These scientists recognize that the remote Arctic region of Earth is unique in that it helps regulate the planet’s temperature like a thermostat by radiating back into space much of the excess energy from the Sun that is received in the tropics. However, more than half of these Arctic emissions occur at far-infrared (FIR) wavelengths greater than 15 micrometers and have never been systematically measured from space before.
In 2018, NASA selected the Polar Radiant Energy in the Far Infrared Experiment (PREFIRE) to perform first-of-a-kind infrared and far-infrared measurements of Earth’s atmosphere from space. One key question PREFIRE will attempt to answer is: Why is the Arctic warming faster than the rest of the planet? PREFIRE will fly two CubeSat satellites making radiometric measures of the atmosphere. CubeSats are lower-cost, lightweight, and very small compared to traditional satellites—each is about the size of a loaf of bread.
PREFIRE will use a JPL-designed instrument that uses critical technology from the MDL including a fully custom thermopile detector array. This thermopile array will help probe this little studied portion of the radiant energy emitted by Earth for the first time, seeking clues about Arctic warming, sea ice loss, and ice-sheet melting. Finally, PREFIRE will explore how changes in thermal infrared emissions at the top of Earth’s atmosphere are related to changes in cloud cover and surface conditions below. More than 60 percent of energy radiated from the cold, dry polar regions resides in FIR wavelengths. Energy in these bands is both dynamic and poorly characterized, yet it plays a critical role in defining the rapidly evolving polar climates. The two PREFIRE CubeSats will make radiometric measures of the atmosphere between five to 50 micrometers, completely characterizing the variability in FIR emission on scales of hours to months.
This spectral data provides critical insight into the surface emissivity, its variability, and the atmospheric greenhouse effect, allowing quantitative modeling of the surface/atmosphere feedbacks that are hypothesized to amplify the effects of climate change. The dual spacecraft measurement capability specifically creates sub-diurnal sampling that can test the hypothesis that time-varying errors in current models of FIR surface emissivity cause biases in estimates of the energy exchange between the surface and the atmosphere of the Arctic. Elucidation and mitigation of these biases through PREFIRE measurements and analyses may reduce the large spreads observed in past projected rates of Arctic warming, sea-ice loss, ice sheet melt, and sea level rise.
Performing the required accurate and sensitive radiometric measurements across infrared and far-infrared wavelengths in a miniaturized satellite is challenging, but, fortunately, PREFIRE will have the help of MDL’s focal plane module (FPM). In particular, the FPM will use a thermopile detector array designed and fabricated at MDL with a pixel and format size customized for PREFIRE. The array operates uncooled, so minimal resources are needed to integrate the array into each CubeSat. Each pixel of the thermopile detector array has a broadband optical coating called “gold black” that provides near unity optical efficiency across the entire spectrum that PREFIRE will measure. Finally, the FPM will utilize custom readout integrated circuits built by Black Forest Engineering that show no measurable low frequency noise. Therefore, the entire FPM can observe the Earth over long integration times to enhance the signal-to noise ratio of the measurement.
Principal investigator for PREFIRE is Tristan L’Ecuyer, associate professor of atmospheric and oceanic sciences at the University of Wisconsin–Madison. Principal investigator for the Thermopile Detector is Matt Kenyon at MDL.
Payload is miniaturized and fits within a roughly 1.5U volume CubeSat.
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THERMOPILE Detector Chip: The FPM utilizes a thermopile detector chip customized for PREFIRE in terms of pixel layout and format size. The chip has “gold black” deposited on the pixels to make the pixels sensitive to infrared and far-infrared light.
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