Astrobiologists from the United States and Germany have recorded the highest level of UV radiation from the Sun yet known at the Earth’s surface.

You might expect the highest radiation levels of this type on Earth to be somewhere in Antarctica – underneath the hole in Earth’s ozone layer. This layer of Earth’s stratosphere contains higher concentrations of ozone gas (O 3 ) than the rest of the atmosphere, and it protects life on Earth by absorbing harmful UV radiation from the Sun. In the southern hemisphere above Antarctica, when the ozone develops, the ozone layer is at its thinnest and more UV radiation gets through to pummel to the surface.

However, the record-setting UV measurements in the study were not gathered in Antarctica. Instead, the team found a spot in the tropics where UV radiation reaches levels that would be harmful for unprotected life. Measurements were collected in and area of the Andes where there is an abundance of life, including human settlements.

Life at 5917 Meters

The measurements were taken by astrobiologists studying high-altitude lakes near the Licancabur volcano (altitude: 5,917 meters). The data was collected as part of the High Lakes Project, which was supported by the NASA Astrobiology Institute under the SETI Institute team (2003-2008) and led by Nathalie A. Cabrol of the SETI Institute and NASA Ames Research Center.

“We wanted to deploy dosimeters and study UV variations as a function of elevation,” Cabrol told Astrobiology Magazine. “The two dosimeters were within 10 km of each other (one at the foot of Licancabur, the other at the summit) – which turned out to be a blessing when it came time to verify our data considering the levels we were observing.”

It’s not uncommon to have high UV levels at altitude. At midday on the top of a mountain – where you’re also higher up in the atmosphere – radiation levels are naturally greater than at the base of the mountain. However, the levels Cabrol and her team measured were not expected.

“We came to the Andes because we knew we would find high levels of UV due to the latitude, elevation, and atmospheric conditions, but we were certainly not expecting these kinds of levels. Not at all,” said Cabrol.

In a press release from SETI, Cabrol explained:

“A UV index of 11 is considered extreme, and has reached up to 26 in nearby locations in recent years. But on December 29, 2003, we measured an index of 43. If you’re at a beach in the U.S., you might experience an index of 8 or 9 during the summer, intense enough to warrant protection. You simply do not want to be outside when the index reaches 30 or 40.”

The team believes that many factors came together as a ‘perfect storm’ at the research site.

“The ozone layer is naturally thin at those elevations and latitude, and UV radiation is maximum in summer,” said Cabrol. “Those natural factors were compounded by a great atmospheric instability due to seasonal storms. The occurrence of a very large solar storm at the same time as these peaks may have played a role, which is not yet explained but the temporal coincidence remains intriguing.”

Astrobiology at Altitude

Nathalie Cabrol has spent a great deal of time exploring high altitude lakes in South America while working on numerous projects with the Astrobiology Program. Her interest in the region began with studies concerning the habitability of early Mars. Conditions in the Andes are different from Mars, but specific aspects of the environment, including the UV flux, can provide clues about how life might have survived in a young martian landscape. When Cabrol began work on the High Lakes Project, she was also studying Gusev crater on Mars, which had just been selected as the landing site for NASA’s Mars Exploration Rover, Spirit.

“I was working on identifying ancient lakes at the surface of Mars,” said Cabrol. “I wanted to go to this region of the Andes because the geology, morphology, mineralogy, composition, and overall physical environment (e.g., temperature, UV, very low relative humidity) provide the best terrestrial analogy to early Mars.”

“The analogy we stumbled into in terms of UV was certainly a lot more than we had bargained for!”

The work led to further studies on the lakes, and soon biologists became interested in the environment as a place to study life’s limits on Earth.

“We started the study of the Andean lakes’ physical habitats, but then microbiologists became really interested by samples from these lakes,” said Cabrol. “What we found there was certainly an abundance of life, but a lack of biodiversity, which makes sense. Only the species most resilient to high UV adapt there. For instance, in the Licancabur summit lake, 35% of the lake’s microbial population was composed of only 4 related species.”

“While there are some similarities between lakes, there are also major differences within a few kilometers,” Cabrol continued. “In an extremely arid climate, local factors strongly dictate what you find in those lakes, and who will survive. That might be something to keep in mind for Mars.”

Cabrol’s work also opened up high altitude lakes as sites to test equipment and techniques that could one day be used on a mission to Saturn’s moon Titan. With support from the Astrobiology Science and Technology for Exploring Planets (ASTEP) element of the NASA Astrobiology program, Cabrol and her team are currently testing a floating, robotic platform that collects and analyzes samples while plying the waters of these remote lakes as part of a separate project. The idea is that similar technology could one-day provide access to Titan’s hydrocarbon seas.

Future Habitability

The data that Cabrol and her team collect has benefits in our understanding of Earth’s environment and the future of life on our planet. The UV study could hint at how life on Earth could be at risk of high radiation exposure if ozone levels thin or become more unstable globally. Cabrol points out that the factors that led to the observed spikes in UV are not rare.

“There is so much to be learned out there. We need abundant data to clearly understand the mechanisms behind these spikes, and for that we need more instruments and more logging time,” said Cabrol. “We need a great depth of statistical data to understand the precursor signs (and we think there are precursor signs, such as the increase in the UV ratio in the preceding hours or days), and the types of environmental factors that will generate such spikes. Although the Altiplano and Andes are scarcely populated, there are a number of small towns and villages for which a monitoring and warning system would provide beneficial protection.”

In the SETI release, David Black, president and CEO of the SETI Institute, commented, “this is an excellent example of how astrobiology — which includes understanding the atmospheres of other planets — is germane to contemporary concerns here on Earth.”

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Nathalie Cabrol talks about how lakes on Earth are ‘sentinels’ of climate change – and how they could have also played the same role on early Mars. Credit: SETI Institute (YouTube)

Additional support for this study include the NASA Astrobiology Institute’s Lewis and Clark Fund, the NASA Ames Director’s Discretionary Fund, and National Geographic.