As December begins, simply pointing binoculars at bright, ocher Mars places dim, lime-tinged Uranus in the same field of view. That red-and-green traffic light conjunction, low in the southwest after sunset, provides an easy way for novices to find the rarely seen Uranus. The conjunction also highlights the two most deeply tinted planets, raising an age-old question: What gives planets their distinctive colors?
Throughout most of history, celestial tints were no more than Rorschach tests of human cultures. Ancients looked at the somewhat reddish hue of Mars and saw fire or blood, leaving us with warlike Mars associations that are still evident in terms such as martial arts and court martial. By the 18th century, scientists recognized that planets shine because they reflect sunlight. As Isaac Newton had demonstrated, sunlight contains all the colors of the rainbow, which when blended together appear white to the human eye. A surface retains this neutral whiteness if all colors bounce back equally. Conversely, any distinct coloration shows that some portion of the rainbow has been absorbed, and that provides intriguing information about what exists on the planet's surface.
For example, Earth looks blue not because of its oceans but because of the atmosphere. Astronauts looking down at our planet see the same azure we do when gazing up at the sky. The cause is atmospheric molecules that are just the right size to scatter the sun's short, blue wavelengths. Air scatters blue light 10 times more readily than it scatters red.
Martian air, by contrast, is too thin to scatter much light. This is evident from telescopic images of the planet, although hints of blue peek out around the edges of the disk, where we can peer through the thickest slice of its atmosphere. The dominant hue of Mars, yellow-orange, comes from the iron oxide—ordinary rust—in the soil, which gets kicked up by 100-mile-per-hour winds that episodically change the sky from blue to a bizarre pink. Every decade or two, global storms blow the rusty dust around the entire planet; one such storm started last July and may still be raging. These dust bowls give Mars a more golden tint.
Color-enhanced images clarify the tonal variety of the solar system: Mars' rusty, cratered surface (A), Uranus' blue-green, methane-tinged atmosphere (B), and Saturn's buttery, bland cloud tops (C).Photographs courtesy of NASA/JPL
Distant, frigid Uranus derives its vivid coloration from methane, the same gas that sometimes hovers in an invisible cloud over marshes on Earth. The compound is composed of one carbon atom bound to four hydrogen atoms. Methane absorbs red light, so copious quantities of the gas in Uranus' atmosphere boost its blue-scattering effect, painting the world a dramatic aqua. When viewed through binoculars, the planet's color appears further amplified because at low light levels our retinas respond to blue-green far more readily than they do to any other color.
Pan to pale Jupiter—an easy task, as it reaches its closest approach of the year on December 31, when it rises in the east at sunset and rides high and dazzling at midnight. Even small telescopes reveal its famous Great Red Spot, more like a weak pink spot, and the faint bands crossing the off-white disk. On a gas-planet such as Jupiter, these are the colors of turmoil. Warm colors denote upheaval and chemical activity. Observations from NASA's Galileo space probe suggest Jupiter's atmosphere is driven by fast-rising, rapidly cooling gases and by charged particles raining down from the planet's powerful magnetic field. Scientists still haven't identified the exact chemistry behind the colors, but it likely includes compounds of sulfur and phosphorus, the stuff of kitchen matches.
To the right of Jupiter, Saturn shines brighter than at any time in the past quarter century, yet it displays almost no color. That blandness betrays another set of conditions: lots of stability, little methane. So if Santa doesn't bring you a chemistry set, no matter. This month the sky provides its own.
Learn more about aqua-tinted Uranus at pds.jpl.nasa.gov/planets/ welcome/uranus.htm. And for more from NASA about Mars, see mars.jpl.nasa.gov/mgs.