(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)
On a clear night, as you gaze up at the myriad constellations visible in the dark skies of Hawaii, you can often see what looks like a small, bright star traversing the sky. If the object moves steadily across the sky over a period of a few minutes, it is most likely one of thousands of satellites orbiting the Earth.
When satellites have a direct line of sight with the sun, they reflect sunlight, and if they are large enough (typically more than 6 m (20 ft) in length) and low enough (160-640 km or 100-400 mi above the Earth), they can be seen with the naked eye. Since they travel faster than all other celestial objects, they are easy to detect and are frequently observed in the pre-dawn and early evening sky.
One satellite that is particularly notable for its measurement capabilities is NASA’s Terra, which has been orbiting Earth since the end of 1999. The imaging instruments onboard Terra include ASTER (an acronym for the Advanced Spaceborne Thermal Emission and Reflection Radiometer), which measures visible- to long-wave infrared light energy. ASTER provides detailed images of the Earth in 14 different energy ranges (bands) of the spectrum. From its approximately 725-km- (450-mi-) high orbit, ASTER can resolve greyhound-bus-to-football-field-sized features on the Earth’s surface under good conditions. ASTER data are used to create detailed maps of surface temperature and elevation, as well as the amount of light emitted and reflected from the Earth’s surface.
ASTER images, collected over a period of time, can be used to detect changes in temperature and have helped identify volcanoes worldwide that are heating up as they become restless. For example, beginning in June 2013, ASTER detected that temperatures at Pulaweh Volcano, Indonesia, had increased from 6 to 80 degrees above the average background. Pulaweh erupted dramatically on August 10.
ASTER can also detect sulfur dioxide (SO2) emissions, an important measurement for identifying volcanic unrest and eruptive activity. SO2 gas “bubbles out” of magma at very shallow depths, so a change in SO2 release may signal a change in eruptive status for a volcano. SO2 absorbs thermal infrared energy between 8 and 9 microns in wavelength, a light energy region that ASTER routinely measures.
A novel experiment at Kilauea Volcano is helping to improve space-based ASTER measurements of SO2 gas. For several decades, ground-based SO2 emission rates have been regularly measured by the U.S. Geological Survey at Kilauea’s summit, making it one of the best-quantified sources of volcanic SO2 in the world. Currently, there are three measurement systems, each of which provides a unique contribution to our understanding of SO2 emissions. All three ground-based systems exploit sulfur dioxide’s strong absorption of ultraviolet (UV), rather than infrared (IR) radiation.
Our longest-running data set uses a vehicle-mounted, upward-looking UV spectrometer, which we drive beneath and through the eruption plume, measuring the amount of gas above the instrument. Another system uses an upward-looking array of 10 UV spectrometers that, working together, record emission rates continuously during daylight hours (see Volcano Watch, July 12, 2012). Earlier this month, with colleagues from the Cascades Volcano Observatory, we installed a UV camera that images the shape and SO2 content of the plume every 5 seconds. Actively comparing satellite- and ground-based data is helping volcanologists and space scientists improve the quality and usefulness of these several types of measurements.
Measuring SO2 in ground-hugging plumes like Kilauea’s, is challenging. ASTER’s developing capacity to measure SO2 contributes to improvements in eruption monitoring. Detailed imagery from ASTER is also increasing our ability to discriminate the margins of active lava flows, and the size and shape of eruptive fissures and skylights from space. These capabilities are currently being exploited at Kilauea to provide another useful tool for monitoring the hazards of the ongoing eruptions.
The heavens above have provided inspiration, both joyful and melancholy, to artists, musicians, philosophers, poets, and geoscientists. Consider the space-based measurements that are helping to track natural hazards, including volcanic emissions and eruptions, the next time you gaze upon and ponder a starry, starry night.