Our eyes are remarkable organs. Millions of receptors called rods and cones line our retinas, converting light into signals that our brains use to see the world around us. The colors we perceive vary among individual humans, including people who have color vision deficiency. Quite literally, we don’t always see eye-to-eye with one another.
Just as color is subjective among humans, so too are the colors in the stunning images our robotic spacecraft send back to Earth. Cameras on our space probes act as proxies for our own eyes, but what they see isn’t necessarily what our eyes would see.
“Most of the cameras that we have on our spacecraft and space telescopes don’t take color pictures,” said Heidi Hammel, vice president for science at the Association of Universities for Research in Astronomy, and vice president of The Planetary Society’s board of directors. “They take black and white pictures that are tuned to specific wavelengths of light.”
Tuning space cameras to specific wavelengths of light helps scientists answer important questions about the objects they study, such as what minerals are on the surface of an asteroid, or what elements are in a nebula. Because each space mission has its own unique science questions to answer, different spacecraft are equipped with cameras tuned to different wavelengths.
Human eyes can generally see light with wavelengths between 380 and 700 nanometers. This covers the entirety of our visual rainbow, from purple at 380 nanometers to red 700 nanometers.
Many space cameras use filters to capture images in narrow slices of our visual range. The Cassini spacecraft’s Wide Angle Camera, for instance, had filters that centered on 460 nanometers (blue light), 567 nanometers (green light), and 648 nanometers (red light). By combining images from all three filters, image processors could create approximate true-color images of Saturn and its moons that were close, but not exactly identical to how our eyes would see them.
“It’s a combination of a little bit of art and science, but you can’t just trust a picture that you see and think that’s how it would look to your eyes,” Hammel said.
We can’t say for certain what the worlds of our Solar System look like to us until we see them with our own eyes from an orbiting spaceship, but we can dispel some standard myths. Here’s a tour of the planets that examines what we know about each world’s true colors.
What color is Mercury?
Mercury has a gray, slightly brownish appearance, somewhat similar to Earth’s Moon. Our best global pictures of the planet were made using images partially comprised of light from outside our visual range.
This image of Mercury comes from data captured by NASA’s MESSENGER spacecraft in 2008: