This intriguing astronaut photo shows a bright-white, salty surprise lurking at the summit of a giant volcano in the Sahara. The ancient, lava-covered peak also holds hidden, shadowy waterways, despite its extremely arid environment.
Emi Koussi (also known as Emi Koussou) is a pyroclastic shield volcano in northern Chad. It's located within the Tibesti Massif, a volcanic mountain range that straddles the border between Chad and Libya. The volcano's dome-shaped cone is up to 43 miles (69 kilometers) across and reaches a maximum elevation of 11,204 feet (3,415 meters) above sea level, making it the tallest peak in the Sahara, according to NASA's Earth Observatory.
Emi Koussi is classified as extinct, and there are no historical records of it erupting, according to the Smithsonian Institution's Global Volcanism Program. However, scientists know that it was once highly active because its gradual slopes are covered with ancient lava. Some experts think this once-molten rock is around 2 million years old and likely flowed with low viscosity, "more like motor oil than toothpaste," Earth Observatory representatives previously wrote.
At the volcano's summit lies a complex caldera system that spans up to 9 miles (15 km) across and is covered with various volcanic vents and cones (see photos below). A second, smaller crater, dubbed Era Kohor, lies at the caldera's southern end.
Nestled within Era Kohor, a thick crust of brilliant-white salt covers the lowest part of the caldera's floor. This crystal layer, which reaches up to 3,300 feet (1,000 m) across, reflects light in such a way that astronauts commonly confuse it for snow, which rarely falls in the area.
The salty slab within the secondary Era Kohor crater is up to 3,300 feet (1,000 m) across and several feet thick. It was left behind by an ancient lake that once filled the caldera. (Image credit: Stefan Thüngen/wikimedia)The salty compound, known as natron, is a mix of sodium carbonate decahydrate, sodium bicarbonate, sodium chloride and sodium sulfate. It was left behind by an ancient salty lake that once filled Era Kohor but has long since evaporated.
This is not the only notable natron deposit in the Tibesti Massif. Around 150 miles (240 km) northwest of Emi Koussi (also in northern Chad) lies a volcanic caldera known as Trou au Natron (or Doon Orei), which contains a layer of the salty substance that is arranged with a pair of cones in a way that makes it look like a giant skull when viewed from above. This unusual deposit lies next to another hefty volcano, dubbed Toussidé, which is also covered with tendrils of ancient lava.
While Emi Koussi's ancient activity was mostly centered at its summit, a pair of volcanic cones can be seen on the mountain's northern flank. The lava that quickly flowed from these openings likely helped to create a series of ravines between the mountain and a volcanic plateau, dubbed Tarso Ahon, which is partly visible in the top left of the photo.
These ravines are very narrow and deep, meaning they are near-permanently cast in shadow and much cooler than their surroundings. As a result, they can often hold liquid water, despite the Tibesti Massif receiving as little as 0.8 inches (20 millimeters) of rain a year. Two of the largest canyons — which are 2,000 feet (600 m) and 700 feet (250 m) deep, respectively — contain permanent waterways that flow east and west of the volcano, according to the Earth Observatory.
False-color satellite images, captured by ESA's Copernicus satellite (main) and NASA's Terra satellite (inset), show off the convoluted topography of Emi Koussi and its summit caldera. (Image credit: Main: ESA/Copernicus Sentinel data (2017); second: NASA/ASTER/GLOVIS)Emi Koussi's slopes are covered with dry "stream channels," which were carved out by millennia of rain flowing down its flanks. Current rainfall patterns would struggle to create such structures. However, thousands of years ago, during the mid-Holocene, the average rainfall was at least 10 times greater, according to a 2025 study that compared the ancient lakes of Era Kohor and Trou au Natron. (The Holocene is the current geological epoch, which began about 11,700 years ago, at the end of the last ice age.)
This is not the first time that astronauts have gazed down upon Emi Koussi. In fact, the volcano was one of the first geological formations photographed from space, when the crew of NASA's Apollo 7 mission snapped the imposing peak in October 1968.
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'> A surprising slab of salt shines at the summit of the Sahara's tallest peak — Earth from space
statement. "These tokens contain complete image information."
Collapsing the visual perception process
In conventional visual perception, light signals have to go through multiple stages. A sensor captures the signals, and an analog-to-digital converter turns the light into pixels. That data is then stored temporarily before it's shuttled to a separate chip, where the image is cut into square "compartments" — much like dividing a photo into a grid of tiles. Each compartment is then converted into a token for an AI model to read.
One study widely cited by other academics found that an analog-to-digital converter is responsible for 66% of an image sensor's energy consumption, on average. Moving visual data processing off the chip and into the cloud can increase overall energy consumption further.
LightTok's solution to the energy problem is to collapse the five stages into one by building the sensing, memory and computation processes into the same pixel. The researchers achieved this through an array based on a technology called a single-layer molybdenum disulfide floating-gate phototransistor that can sense light, remember what it sensed, and then factor that into a calculation.
The chip physically eliminates data movement, which is the main source of energy waste.
Liang Shi-Jun, physics professor at Nanjing University
Molybdenum disulfide is a 2D material that reacts well to light and can be grown in sheets one atom thick. The phototransistor converts incoming photons — particles of light — into an electrical current, while the floating gate is an isolated component inside the phototransistor that can trap and hold an electrical charge, rather than disappearing after the light goes away.
Liang Shi-Jun, a physics professor at Nanjing University, summarized the chip to Chinese state-run news agency Xinhua. "The chip physically eliminates data movement, which is the main source of energy waste," Liang said. "Light comes in, tokens come out" — hence the name "LightTok."
LightTok achieved 87.3% accuracy in image recognition during tests — compared to the conventional, multi-step process described above —- while being 10 times more energy efficient at converting light into tokens, the researchers reported in the study.
A more efficient physical world
At present, LightTok's maximum resolution is just 32 by 32 photosensitive pixels — the light-sensitive compartments on an image sensor that capture the visual data. This is far inferior to the quality of current smartphone cameras, let alone drones and autonomous hardware.
Nevertheless, Miao said in the statement that there's an opportunity for the technology to be scaled up using the complementary metal-oxide-semiconductor manufacturing process — the same method used to fabricate chips found in smartphones and laptops as well as sensors in drones.
The researchers believe that if LightTok can eventually scale successfully, it could transform the operation of remote sensing technology. For example, a drone scanning a disaster zone or remote area could potentially fly longer because less energy is required for visual processing.
Kumar Sokka, CEO of Acre Security, a company that provides real-world sensing for critical infrastructure, described the work as a "small-scale demonstration." However, "the direction [of the research] matters to anyone working in the physical world," added Sokka, who was not involved in the new research.
Sokka, who previously spent 15 years at industrial automation company Rockwell Automation, told Live Science in an email that too much of the conversation around physical AI has centered around AI models. In truth, the bigger challenge has been the energy cost of "getting what a sensor sees into a form a model can actually use, right where the sensing happens," he said.
The massive amount of energy typically required to turn raw data, such as light, into tokens "is wasteful when you're running perception on a robot or an edge device with a tight power budget," Sokka noted. However, processing at the point of detection may be "an enabler for pervasive physical AI, and a clever one, but it's not a cure for the whole problem," he added.
Can you match these ancient devices to their pictures? Find out with our computing quiz!
'> New LightTok chip converts light directly into 'tokens' for AI — slashing energy use in drones and other autonomous machines
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