In the sea, soft-bodied sea slugs crawl around without shells to protect them. Instead, they sport dazzling colors and flashy patterns to warn would-be predators that they’re toxic. Now, researchers have learned that many of these vibrant hues come from structures under the slugs’ skin.

These slugs, called nudibranchs (NU-dih-branks), are sometimes dubbed the “butterflies of the sea.” Their colors are “exceptionally bright,” says materials scientist and physicist Samuel Humphrey. He works at the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany. Nudibranchs’ range of colors and patterns exceeds those of other ocean dwellers and is only like that of butterflies and birds, he says.

Not much was known about nudibranch colors, Humphrey says. But in nature, some of these colors — such as white, blue and purple — are hard to get from pigments. (Those are molecules that provide color.) That made Humphrey’s team suspect the slugs’ colors instead arise from light waves bouncing off structures on their bodies. Such structural color can create hues as intense as those seen on the sea slugs.

Humphrey collected one species of nudibranchs off the coast of France. Others, he and his colleagues bought from aquarium shops. The researchers looked at the creatures under the microscope and saw tiny dots of colors that combined to create a shade. For instance, a nudibranch might look purple, but zoom in, and you may find specks of red and blue. “It’s a bit like mixing paints until you get the color you want,” Humphrey says.

Actually, he adds, “it’s better than that.” Sea slugs’ hues arise from specific wavelengths, or colors, of light bouncing off them and reaching our eyes. And the team found that, for a given color, nudibranchs reflect up to 80 percent of light that hits them. That makes their hues more vivid than those produced by pigments, which create colors by absorbing light. Losing light means losing brightness, Humphrey explains.

The researchers shared their results in the March issue of the Proceedings of the National Academy of Sciences.​

With a powerful microscope, the team saw stacks of tiny plates under nudibranchs’ skin. The plates were crystals made of guanine. The thickness of the plates and the distance between them in a stack determined the colors that they reflected. That allows sea slugs to wear a rainbow of colors made of just one material.

The many tiny dots of color on sea slugs’ skin creates an effect akin to how TVs make images with many spots of only a few colors. This is called pixelation. “It feels like a thing that humans have invented,” Humphrey says. “But actually, evolution had invented it millions of years ago.”

Data Dive:

1. How many types of colored light make up the violet hue of Hypselodoris tryoni (Figure A)?
2. How many types of colored light make up the beige or white region of the same slug (Figure B)? Which color of light has the highest reflectance? What is that color’s peak wavelength?
3. Which sea slug here has the color with the highest reflectance? (Hint: Not all the y-axes are exactly the same.) What is that color and what is the peak wavelength?
4. How many different colors of light are in the region of Chromodoris annae (Figures E and M)? Based on those colors, how do you think this area of the slug looks to the eye?

For the first time, scientists have measured metal pollutants leaching from a piece of space junk in the atmosphere. They caught a burst of lithium as a SpaceX rocket burned up high above Earth’s surface. Such pollutants can damage the ozone layer and may impact global climate.

Humans have been launching metal things into orbit around Earth for nearly 70 years. But the pace of launches has skyrocketed in the past decade. And private companies plan to boost the number of launches in the near future. Consider SpaceX’s Starlink system. This system provides internet to far-flung places. It will eventually consist of more than 40,000 satellites. Already, nearly 10,000 Starlink satellites orbit Earth.

Each piece of Starlink equipment has a planned life of about five years. After that, the satellite burns up in the upper atmosphere. This releases metals. They include lithium, aluminum and copper. Those metals may speed up chemical reactions that destroy ozone. That’s a problem, because the ozone layer of Earth’s atmosphere protects life on Earth from getting too much ultraviolet (UV) light from the sun.

Metal pollution from space junk may also affect Earth’s climate. Such metals could change the balance of light that reaches the ground or gets bounced back to space.

The stratosphere is the layer of the atmosphere that extends from 14 to 64 kilometers (9 to 31 miles) above Earth. A 2023 study found that about 10 percent of particles there contain pollutants from burnt-up satellites and rocket parts.

Scientists wanted to see if they could directly trace such particles to a piece of space debris. They got their chance on February 19, 2025. That day, the researchers observed a cloud of lithium about 100 kilometers (60 miles) above Germany. It was blowing away from the upper stage of a Falcon 9 rocket. This rocket part contains fuel and an engine. The rocket part made a fireball as it broke apart over Ireland and the United Kingdom.

“A few hours after the reentry of this rocket, we could see 10 times more lithium than we would have observed otherwise,” says Claudia Stolle. She’s a meteorologist at the Leibniz Institute of Atmospheric Physics. That’s in Germany.

The measurements relied on lidar. Lidar sends out laser pulses and detects the light that bounces back. The laser light is tuned to specific wavelengths that reflect off particular materials, such as lithium.

The team also ran computer models of the atmosphere. These showed that the wind had carried the lithium from the spot where the rocket came down to the area over Kühlungsborn, Germany. That’s where the lidar was located.

The researchers shared their findings February 19. The work appeared in Communications Earth and Environment.

The technique could help monitor potential harms from space debris. Meteorites — space rocks that fall to Earth — bring metals into the atmosphere. But falling space junk may one day boost metal pollution by around 40 percent, Stolle’s team estimated. Tracking such contaminants and their effects will become increasingly important.

More companies and countries are planning to launch satellites into orbit, Stolle notes. “All of them will burn up sooner or later.”

Data Dive:

1. Look at Figure A. When did the density of lithium increase?
2. What was the altitude of the lithium plume?
3. What was the lithium density at the center of the plume?
4. Look at Figure B. How tall was the plume, top to bottom, at 1:30 a.m. on February 20?
5. If the data continued recording after 2:00 a.m. on Feb 20, what would it likely look like?