Satellites could one day be used to warn when a volcano is set to erupt, thanks to a "lucky coincidence" in the skies over Whakaari/White Island.
On December 9, 2019, the popular tourist spot erupted, killing 22 of the 47 people on the island at the time.
Just under an hour later, the European Space Agency's (ESA) Copernicus Sentinel 5P satellite passed directly overhead. Its sensors picked up sulphur dioxide in the plume coming from the volcano.
"By combining the satellite image with a high-resolution wind model, we back-calculated the gas emission rate at Whakaari from 1.5 hours prior to the eruption to 58 minutes after the eruption," said Craig Miller, a senior volcano geophysicist at GNS Science.
"Our results show that a four-fold increase in gas flux occurred approximately 40 minutes prior to the eruption."
GNS had already noticed an increase in sulphur dioxide levels coming from the volcano in the weeks previous, and its alert level was raised from one to two just a few weeks earlier.
But the detection of a significant plume of the sulphur dioxide could be used as an imminent warning.
"It's another tool, really, especially for volcanoes that are remote and hard to get to, where you can't get scientists to the volcano and get machines there measuring gas levels," University of Canterbury volcano expert Ben Kennedy told Newshub, calling it a "lucky coincidence" Sentinel 5P was just passing by at the time.
"This is a promising additional tool. But it's never going to be better than being there in person on the volcano, directly measuring the gas coming out."
Sulphur dioxide is invisible to the naked eye.
"A lot of people would have stood up on the hills above Whakatāne and looked out towards Whakaari and seen there's frequently a plume of gas coming off Whakaari," said Dr Kennedy.
"The plume that we tend to see with our eyes is mostly water vapour. Whether or not we see a plume of water vapour is quite dependent on what the atmospheric humidity is... which is why scientists need machines to pick out the volcanic gases.
"The volcanic gases will all come out together and what you'll see is mostly water vapour, the steam - but there will be sulphur dioxide, carbon dioxide and other gases in there as well."
Adrian Pittari of the University of Waikato's School of Science said the detection of sulphur dioxide doesn't necessarily mean an eruption is imminent, but it "does help to determine if the volcano is undergoing a heightened state of unrest, thus increasing the risk of an eruption".
"In hindsight, the satellite measured the plume after the 2019 eruption so it would not have made a difference to any pre-eruption warnings at that time. However, we now know that continued surveillance of sulphur dioxide plumes by this satellite adds a new tool to monitoring the 'vital signs' of Whakaari/White Island and our other active volcanoes in New Zealand."
"There will be more satellites in the future that are capable of measuring gases," said Dr Kennedy. "I suspect there will be satellites in the future that do target volcano monitoring... I hope in the future it will lead to better monitoring of volcanoes."
The Copernicus Sentinel 5P satellite's primary mission is to "perform atmospheric measurements with high spatio-temporal resolution, to be used for air quality, ozone and UV radiation, and climate monitoring and forecasting", according to the ESA's website.
It was launched in 2017. In 2020 it detected drops in pollution in China as the country went into lockdown in response to the COVID-19 outbreak.
The research was published on Saturday in the journal Science Advances.
