Space weathering

Space weathering is the kind of wear and tear that happens to anything in the rough conditions of outer space. It affects objects like the Moon, Mercury, asteroids, comets, and many moons of other planets, which do not have atmospheres to protect them.

The main causes of space weathering include crashes from galactic cosmic rays and solar cosmic rays, plus effects from the solar wind like irradiation and sputtering. Tiny space rocks and micrometeorites also hit these surfaces repeatedly.

Understanding space weathering is very important. It changes how we see the surfaces of planets and moons from far away. Knowing about these changes helps scientists understand what they are really looking at when they study space using special tools.

History

We learn about space weathering mostly from rocks and soil brought back by the Apollo program. Space is very harsh, and tiny pieces of space dust, called micrometeorites, and high-energy particles constantly hit the Moon's surface. This can melt, break apart, or turn parts of the Moon’s soil into gas.

One of the first things scientists found was called “agglutinates.” These are tiny pieces formed when micrometeorites melt a bit of the Moon’s soil, mixing it with nearby dust and glass. Agglutinates are very common and can make up most of the Moon’s soil. They look black because they contain tiny bits of iron.

Space weathering also changes the surface of individual soil grains. It can add thin layers, trap gases like hydrogen and helium, and leave marks from solar flares. These changes affect how the Moon’s soil looks when we study it with special tools, especially in light of certain colors like ultraviolet, visible, and near infrared wavelengths. Tiny bits of metallic iron play a big role in these color changes.

Effects on spectral properties

On the Moon, space weathering changes how the surface looks. Over time, the Moon's surface gets darker, looks redder, and its special color patterns become less clear. These changes happen because tiny bits of iron form on the surface. We can see this in the Moon's craters. New craters have bright lines around them because they show fresh material, but these lines fade as the surface darkens from space weathering.

Space weathering on asteroids

Space weathering can also happen on asteroids, even though their environment is different from the Moon. Impacts in the asteroid belt are slower, so they create less melting and vapor. Fewer solar wind particles reach the asteroid belt, and smaller asteroids have more impacts and lower gravity, meaning their surfaces change more quickly. Because of these differences, space weathering happens more slowly on asteroids.

Scientists noticed something strange: the colors seen in the spectra of asteroids often don't match the colors of meteorites we find on Earth. For example, S-type asteroids look different from the most common meteorites, called ordinary chondrites. But some asteroids near Earth show colors that change over time, suggesting that their surfaces are being altered. Spacecraft like the Galileo Spacecraft and NEAR Shoemaker found differences in color at fresh craters on asteroids Gaspra and Ida, and Eros. The Hayabusa mission to the asteroid Itokawa also showed signs of space weathering in the soil it brought back. Even though Itokawa is very small, its surface shows changes from space weathering. Most of the color changes from weathering happen quickly, in the first hundred thousand years, which makes it harder to use color to figure out how old asteroids are.

Space weathering on Mercury

The environment on Mercury is very different from that of the Moon. During the day, Mercury gets much hotter, and at night it gets much colder. These temperature changes can change how space weathering affects the surface. Because of where Mercury is in our solar system, it gets hit by more tiny space rocks, and they hit faster than on the Moon. This makes Mercury better at creating melted material and vapor compared to the Moon. Scientists think that for every area, Mercury creates about 13.5 times more melted material and 19.5 times more vapor than the Moon does.

When we look at Mercury from Earth using telescopes, its light appears to slope upward toward red colors. We don’t see certain colors that would show the presence of iron in minerals like pyroxene. This suggests that either Mercury’s surface has no iron, or the iron has changed because of space weathering, making the surface look redder.