| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Skywatcher Helium Rain Likely
This is today’s forecast for planet Venus. Weather is a common trait among most of the planets in our solar system, but it rarely bears much resemblance to what we are used to on Earth. Global dust clouds envelop Mars. Giant gas planets in the outer solar system whip up storms the size of smaller planets. With salmon colored storms and butterscotch skies, alien weather is a rich subject for speculation and research. The only planet where we can be fairly sure weather does not occur is on Mercury. The basic ingredients needed for weather are an atmosphere and heat. And while Mercury is hot, the planet has no atmosphere to speak of. Its weak gravity allows almost all gases to float up and escape into space, much the way helium does on Earth. With no atmosphere to hold in the heat, the temperature rises to 425° Celsius in the sunshine, and drops as low as -180° Celsius at night—the most extreme temperature swing in the solar system. In complete contrast, Venus experiences almost no surface temperature variations at all. Not long ago, astronomers speculated that the second planet from the Sun would be much like Earth. They knew that its mass was similar to ours, and that it had an atmosphere. Then in the 1960s, spacecraft visits revealed something quite different. Venus’ blistering 480° Celsius (900 ° Fahrenheit) is hot enough to melt lead, and though farther from the Sun, the planet is even hotter than Mercury. The planet’s carbon dioxide-rich atmosphere is to blame, creating a “greenhouse effect” gone wild. We know carbon dioxide as a colorless gas because light visible to the human eye passes through it unaffected. Invisible infrared wavelengths, on the other hand, do not. When sunlight hitting Venus’ surface heats the ground, some of it radiates back up as infrared light. But atmospheric carbon dioxide bounces it right back to warm the surface further. Furthermore, the thick atmosphere distributes heat evenly around the planet, keeping Venus hot even at night. Forecasting on Venus is easy—every day is just like the next. The only pictures we have of the Venusian surface were sent back in the 1970s. Soviet Venera 7 and Venera 9 spacecraft survived the planet’s punishing temperature and crushing pressure for about an hour apiece. Their images show a very dry world under a yellow-orange sky colored by 50-kilometer-high clouds. This is where toasty temperatures come in handy; pure sulfuric acid rain evaporates before it reaches the ground. A light surface wind blows with surprising force thanks to the dense atmosphere, but disturbs little at ground level. Most of the action takes place in the higher, cooler regions of the atmosphere. Computer models of Venusian weather suggest that slow convection—a gentle flow of hot gas into cold—lower in the atmosphere is somehow being whipped into a fierce global wind system that whirls around the entire planet in about four days. Finding the exact mechanism for this “super-rotation” is difficult because dense sulfuric acid clouds block our view. At night a mysterious light show brightens the skies of Venus. For years, observers have reported a faint, sporadic glow on Venus’s unlit side. Known as “ashen light,” this phenomenon is notoriously difficult to spot. Recently, astronomers using the University of California’s Keck telescope on Mauna Kea in Hawaii imaged the faint glow in detail. The glow they found is the same shade of green as that emitted when one oxygen molecule collides with another, suggesting that this may be the light’s origin. Yet loose oxygen molecules are scarce on Venus. What little is present probably comes from the breakdown of carbon monoxide molecules by sunlight. In this acidic environment, however, oxygen does not last long before it is pulled into bonds with other compounds. Scientists suspect the planet’s rapidly whirling atmosphere speeds oxygen to the dark hemisphere in time for the molecules to collide and glow. Astronomers had hints of Martian weather before the space age allowed them to see it up close. Occasionally, details on Mars would blur away as though someone had picked up the planet and shaken it like an Etch-a-Sketch to erase its features. It happened again in 1971. When the Mariner 9 spacecraft prepared to photograph Mars from orbit, observers watched in frustration as surface detail smeared from view. It turns out a windstorm had scattered fine particles across the entire world. Storms like this happen regularly, most recently in 2001. In that year, The Hubble Space Telescope and Mars Global Surveyor witnessed a planet-swallowing dust storm envelop Mars’s atmosphere. The dust absorbed sunlight and heated the air, which caused more convection and more winds, kicked up more dust, and raised the air temperature nearly 50°C. The dust blocked sunlight from reaching the planet's surface, so temperatures on the ground dipped slightly during the day, but actually increased at night. The distance between Mars and the Sun varies by 26 million miles over the course of its year. Such an elliptical orbit makes Martian seasons more extreme than ours, with temperatures ranging from -130 to 27°C. Summer in Mars's southern hemisphere coincides with the planet's closest approach to the sun. At this time, the south ice cap nearly disappears and the Red Planet enters its dusty season. Large dust storms come and go as warming air rushes toward cooler areas.
No matter the season, the fine dust perpetually suspended in the atmosphere tints Martian skies a pinkish-brown color which some describe as butterscotch. These dust particles can reflect and scatter sunlight as well as absorb it. When the Sun is low in the sky, the dust gives it a halo of blue light. This blue light mixes with the pink sky to create purple sunsets and sunrises. Aside from the dust, Mars’s atmosphere also contains a small amount of water vapor. At night, the water crystallizes to form wispy ice-clouds that sublimate away in the heat of the morning Sun. The ice particles look blue from a distance because they are small enough to scatter blue light. On Earth, rosy-fingered dawn turns clouds pink against a blue sky. A Martian dawn looks exactly opposite; its blue clouds against a pink sky are another reminder that Mars is a very alien world. Even from Earth, weather on planet Jupiter is obvious. As a gas giant, Jupiter is essentially a huge ball of atmosphere. Because Jupiter is ten times larger than Earth, tremendous gravitational pressures are constantly squeezing its component materials. This process, along with another that probably includes an internal rain of liquid helium, generates more energy than the planet receives from the Sun. Given these ingredients, it is not at all surprising that the weather on this world is spectacular. Bands of light and dark clouds on the planet’s disk are clearly visible even through a pair of binoculars. Spacecraft have revealed intricate vortices between these bands which indicate the presence of weather fronts and convection currents. These white cloud zones and dark belts are Jupiter’s main weather features. The dominant theory used to be that the white zones are hot and rising up from below, and that the dark belts are cold and sinking back down. Information from the Cassini spacecraft, which flew past Jupiter in early 2001, suggests just the reverse. The white clouds seem to be sinking, while the dark belts are welling up. This discovery may solve the mystery of why Jupiter’s rainbow bands are so colorful. The chemicals that make up Jupiter’s atmosphere—largely hydrogen and some helium, with traces of water, ammonia, methane, sulfur, and other chemicals—turn plenty of different colors when warm. Yet at –240°C, the temperature of the top of Jupiter's immense atmosphere, these gases should be colorless. Astronomers suspect that warmer compounds rising up from below help dye Jupiter’s skies.
By far the most famous atmospheric feature on this planet is the Great Red Spot. About three Earth diameters across, and a deep salmon-pink, the Great Red Spot does not rise from anywhere, but instead spins high in Jupiter’s clouds, looking tantalizingly like a hurricane. Earth’s hurricanes and cyclones are huge, swirling masses of extreme low pressure fed by hot, humid air. But the Great Red Spot is actually a high pressure system protruding like a mountain into the upper reaches of Jupiter’s atmosphere. It rotates at speeds of about 400 km/h, about twice those of Earth’s worst storms. This high pressure system has been around for at least as long as humans have known about it—nearly 400 years. After all of this time, we still do not know what caused it, what keeps it going, or even what makes the Great Red Spot red. Saturn’s weather is very similar to Jupiter’s. Vast cyclonic storms that may even contain lightning churn its atmosphere. Both planets spin very fast, stretching their thick clouds into discrete bands that stream along at nearly 500 meters per second near the equator. The highest of Saturn’s three cloud decks is composed of crystallized ammonia at a bone-chilling –130°C. These high clouds appear white and obscure what is happening beneath. If not for them, Saturn might be much more colorful. A layer of ammonia comes next, giving Saturn its yellowish hue. Below this are water clouds that likely hover over a vast ocean of liquid hydrogen. At nearly three billion kilometers from the Sun, and with no internal source of heat, Uranus lacks the prerequisites for weather. But the pearly blue hue of Uranus is a sign that plenty of methane gas floats around in its atmosphere. When sunlight hits methane, red light is absorbed and blue light bounces back. Neptune’s deeper blue means chemicals other than methane might be at work. Yet its azure atmosphere is far from inert. The fastest winds in the solar system howl here at a blistering 1,600 kilometers per hour. These are fueled by an internal combustion system that emits three times more heat than gets delivered to these distant reaches by the sun. Neptune’s internal fires keep it at the same temperature as Uranus, a relatively balmy –214°C, although it is one and a half times farther from the Sun. Nevertheless, new research suggests the planet may even have seasons. Far from the Sun’s warmth, planet Pluto travels a wacky elliptical orbit that occasionally pulls it even closer to the Sun than Neptune. Though Pluto’s tiny size means its gravity is weak, the planet does have an atmosphere—sometimes. When nearest those warm rays, as happened in 1989, Pluto thaws to –212°C—warm enough to evaporate gases frozen solid for more than a century. These gases will reform into ice when the planet sails far away. But even at the height of a Plutonian summer, places where the ice is thick may be as cold as -228°C. The temperature differential powers tremendous winds in Pluto's upper atmosphere. Stormy skies don’t stop at the edge of our solar system. Just this year, astronomers spotted signs of truly extreme weather on a planet orbiting the distant star HD 209458, located 150 light years away in the direction of the constellation Pegasus. This large, Jupiter-sized planet sometimes passes right in front of the star, briefly dimming its light. Astronomers have found that a type of ultraviolet light usually absorbed by hydrogen gas is dimmed more noticeably and over a much larger area than the rest of the light from the star. Astronomers speculate that heat from the star is evaporating the planet’s atmosphere, creating a trailing cloud of hydrogen gas three times the size of the planet itself. Now that’s an atmosphere! Kirsten Vanstone is an astronomer and writer based in Toronto. |
