ESA's Sentinel-3 radar altimeter has captured reflections from the surface of the Salar de Uyuni – a salt flat at 3600 m of altitude in Bolivia – since the spacecraft began orbiting in 2016. During the rainy season, water fills the basin with a centimetre-thick layer and it appears to a radar altimeter as a flat ocean, revealing a number of intriguing puzzles.
Backscatter measurements of the return echoes were used to quantify the smoothness of the water surface. We have often observed across the entire basin constant values that correspond to the theoretical prediction of a water surface with ripples less than 0.5 mm high, which is less than a thirty-second of the radar altimeter wavelength. However, wind at the Salar de Uyuni should roughen the water surface and spread reflections. How is it possible that there is hardly a ripple in the water with winds freely blowing over the 10,000 square kilometres?
With the mystery unravelled, an expedition was launched to take measurements of wind, water salinity, depth, etc., coinciding with the Sentinel 3 radar altimeter passing directly overhead at 10:17 UTC on 20 February 2024. The logistics for getting to this location were complicated. Based on our current knowledge a field survey in the interior of the Salar de Uyuni, in presence of water, was never conducted before. The hypersaline water posed a serious risk to equipments and the local support was crucial in securing a high-clearance truck to reach the centre of the Salar de Uyuni in time for the satellite overpass. Figure 1 shows the expedition truck at the location. On the truck bed there is a wind sock and the wind speed gauge. Water measurements and drone flight operations were conducted simultaneously.
Overall, the observations pin down some possibly relevant conditions (as shown in Figure 2). For example, the water depth: various references mention depths as high as 30 cm. We measured 1.8 cm. Possibly, such shallow depth would inhibit wave formation. Surprisingly, we also observed a water current. On the one hand, a gravity current should be constant. Instead, there was clear evidence that the water current varied, probably responding to the wind direction change. Some salt flakes floating on the surface were also identified. The big flakes (~cm size) are few in number, but potentially there might be many more microscopic flakes. Maybe salt flakes grow from micro to large in the natural evolution. Where do they come from? Might they be a plausible surfactant argument contributing in damping out waves?
We used various video modalities to sense surface roughness, e.g., tracking the motion of small beads floating on the surface or viewing the sun reflection from the top using a drone. Drone imaging showed that the reflection appeared as a compact, circular shape on the water surface, indicating the unique specular properties of the water. While the ground-based observations are fixed in space and vary over time, the drone would have the potential of observing a slanted path in space-time, providing a three-dimensional perspective of the Salar de Uyuni.
We experienced the spectacular mirror effect for which the Salar de Uyuni is famous for. It is astonishing that this is possible in the natural world. We questioned if this happens on a year-to-year basis during the raining season. Our scientific study uses the radar altimeter on board the Sentinel-3 satellite constellation that has been collecting around four hundreds thousand echo bursts along six ground tracks within the Salar de Uyuni boundary.
Despite what is believed, the smoothness of the surface evolves spatially and temporally for the radar altimeter. The surface starts becoming radar smooth at the beginning of the wet season in December. The peak period is from late January to early March. In late February, visitors have the highest chance of seeing the mirror effect, as approximately 50% of the bursts are radar smooth. From late April to November, the surface becomes radar smooth only on very rare occasions. A connection with rainfall patterns is found in response to the onset and variability of climate fluctuations. This scientific study shows how satellite observations can provide geoscientists with new tools for studying Earth’s surface unusual phenomena.
When the area fills with water and the surface is smooth as glass, the mirror effect is astonishing, and has become an increasingly popular attraction for visitors to the Bolivian Altiplano. Therefore, its possible prediction would aid visitors to better plan. Radar altimeters promise new monitoring capabilities for detecting when and where the mirror effect might occur.
The multimedia team working for Nature produced a video that you can here https://youtu.be/O3qXO9ofgDU