28 April 2026

Today’s Visualized watches a robot untie itself. But first, catch up on the latest science news, including how HIV drove human evolution in a little over a decade.

Atmospheric Science | Science

The stratosphere is filled with particles so small even scientists overlooked them

NASA’s WB-57 high-altitude research aircraft flies over the Arctic National Wildlife Refuge during a 2023 SABRE mission to study particles in the stratosphere. NASA

Some of the particles with the biggest impact on stratospheric chemistry are so small they have remained mostly invisible to scientists—until now. New research in Science suggests the lower stratosphere contains tons more ultrafine aerosol particles than previously thought; they are carried upward by rising air currents and atmospheric mixing from the troposphere, the lowest layer of Earth’s atmosphere. Despite their tiny size, the specks add up to large amounts of surface area where atmospheric reactions can occur, some of which can break down ozone and scatter sunlight.

To study these hard-to-detect particles, researchers used instruments aboard NASA’s WB-57 high-altitude research aircraft during the Stratospheric Aerosol Processes, Budget, and Radiative Effects (SABRE) mission in 2023, and measured aerosols up to 19 kilometers above Earth. Particles smaller than 150 nanometers across made up most of the total aerosol surface area, accounting for as much as 90% of the surface area in some regions.

These sub-150-nanometer particles also acted like sponges for condensable vapors: Gases that might otherwise form new aerosols or bulk up larger ones instead stuck to the tiny particles. As they aged, the specks collided, clumped together, and gradually grew larger and fewer.

“Understanding these small particles is critical for predicting how the stratosphere would respond to any sort of perturbation, whether natural, like a volcano, or human-caused,” said lead author Ming Lyu in a statement. The overlooked particles could also complicate one proposed geoengineering strategy that aims to cool the planet by injecting aerosol precursors into the stratosphere to reflect sunlight, as the added material may stick to them first.

Read the paper

Genetics | News from Science

How an HIV/AIDS tragedy spurred human evolution

Before the arrival of powerful anti-HIV drugs, AIDS took such a heavy toll in one region of South Africa that it left a mark on the human genome. In just over a decade, the virus drove changes to the frequency of immune-system genes, a new study shows. As access to antiretroviral drugs increased two decades ago, those evolutionary forces eased, and the genetic changes slowed.

The researchers, who reported their findings yesterday in Proceedings of the National Academy of Sciences, assessed genetic changes in the population of KwaZulu-Natal, the hardest hit province in South Africa—a country that today is home to 20% of the estimated 40.8 million people in the world living with HIV. The study relied on stored blood samples collected between 1998 and 2025 from nearly 1600 mothers in the province, with and without HIV infections, and more than 400 of their babies.

The study focused on variations in human leucocyte antigen (HLA) genes that control a key immune response that clears infected cells. One group of these HLA genes hampered this clearance (“susceptible”) and the other (“protective”) bolstered it. Prior to anti-HIV drugs becoming widely available, the frequency of the susceptible genes decreased and the protective ones went up—a reflection of the huge impact the virus had on mortality in the region.

Other studies have shown how infectious diseases, including malaria and tuberculosis, have altered the human genome. But those changes took thousands of years. “That’s what is quite exciting about this is: You can see how rapidly evolution actually can occur,” said immunologist Philip Goulder, who led the study.

Read the full story

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Visualized

This is knot your average robot. Hong et al./Science (2026)

Got your knickers in a knot? They might have engineering potential!

A team of researchers has created a miniature soft robot out of a simple knot design. The base is a millimeter-thick fiber with a stiff core and a flexible outer coating. When twisted into a pretzel, the fiber builds up tension like a spring. Then, when the temperature rises to between 60°C and 90°C, the flexible coating loosens, causing the knot to pop untied and jump almost 2 meters high.

The mechanism is fairly simple: a conversion of stored elastic energy into kinetic energy. But to tune the bot’s behavior, the team adjusted the fiber’s materials and how the two layers twist. And depending on the kind of knot, the robot could flip, spin, or make a series of complex movements in the air. The group even added a thin, leaflike attachment to mimic how tree seedlings fall; different positionings caused the robot to fly far away, boomerang, or spin the whole way down.

The jumping knots won’t just be a party trick. The researchers envision using the knots to plant seeds for reforestation or agricultural needs, given how forcefully they land on the ground and could penetrate into soils. Early experiments showed that pine and arugula seeds delivered with the bots successfully germinated. The robots even worked on wet, sandy, and snowy surfaces.

“We often start by exploring interesting phenomena,” lead author Shu Yang said in a statement. “Then we ask how far we can push them and whether they can solve real problems.”

Read the paper

Et Cetera

Percolation perfection

Earth scientists recently tested the equation to describe how hot water percolates through coffee grounds, thereby determining how to, mathematically, pull the perfect shot. They were inspired to perform the study as a teaching tool for budding volcanologists. The work is “genuinely exciting and shows how methods developed in one field can open new perspectives in another,” said one coffee science expert.

Royal Society Open Science Paper | Read more at Science News

Out of thin air

A new metal-organic material made from cadmium and carbon can pull water from the air when it is exposed to UV light; it could lead to passive, sustainable, sun-powered methods for dehumidifying air. Although the material will need to be tweaked to make it less toxic and more easily manufactured at scale, the work is “a genuinely fresh contribution to supramolecular chemistry,” said one photochemist.

Journal of the ACS Paper | Read more at C&EN

Ready to split

The Turkana Rift may break Africa into two continents sooner than scientists thought. Seismological surveying has found the crust at the center of the rift is a mere 13 kilometers deep—meaning it’s probably only a few million years away from splitting the continent apart. “We found that rifting in this zone is more advanced, and the crust is thinner, than anyone had recognized,” said one of the researchers behind the work.

Nature Communications Paper | Read more at Scientific American

[AI] really is forcing us to rethink fundamental questions—what is a mathematical proof? What is a paper? What is the purpose of our profession?
—Terence Tao, University of California, Los Angeles

Nature | 27 April 2026 | Davide Castelvecchi

Last but not least

I was already impressed with leaf cutter ants before I learned their nests can have 7000 chambers—and somehow, they still know their way around.

Christie Wilcox, Editor, ScienceAdviser

With contributions from Ana Georgescu, Jon Cohen, and Hannah Richter

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