weather on worlds beyond

WEATHERWISE ■ JANUARY / FEBRUARY 201212

by Michael Carroll

Neptune’s remarkably clear skies may support beachball-sized raindrops, as seen in this image of a future atmospheric probe.

WEATHER ON WORLDSBEYOND

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Somewhere above the orange fog, clouds boil into the upper troposphere. As they condense, rain falls to blanket the rolling hills below. Flash floods pour through dry riverbeds, rolling stones toward the lowlands. But this

noonday shower falls in cryogenic temperatures and dim twilight. The rain is not water, but rather liquid methane. The monsoon season has returned to Saturn’s moon Titan.

The Weather at Home and NearbyIf the physical universe has a goal, it is to keep

things in balance. Dry regions on Earth pull humidity from moist areas in an effort to evenly distribute the water vapor; winds carry warm air to cooler regions. Our weather is a mechanism in constant search of equilibrium.

Hot air forms over the region of a planet pointing most directly at the sun. On Earth, this point is near the equator. Here, hot air rises, moves away from the equator and drifts toward the poles. It cools at high altitude and sinks back down, migrating back toward the equator. This circulating carousel of atmosphere is called, generically, a Hadley cell. Hadley cells are arranged like donuts encircling the planet parallel to the equator.

A simple model of these cells is found on the planet next door to ours, Venus. There, air rises above the equator, flows toward the poles where it cools, then returns again to the equator, completing an airy conveyor belt from the planet’s waist to its scalp. On Earth, our 24-hour daily spin twists cloud systems into swirls and vortices, but Venus’ lazy 243-day rotation leaves air currents undisturbed. Instead, Venusian clouds are driven by solar heating.

Venus is closer to the sun than Earth, while Mars is a bit farther out. With rocky, solid surfaces, they are the most Earth-like of any planet in our solar system. Venus is nearly Earth’s twin in size. Mars is roughly half as far across. The weather on both is a simplified version of terrestrial patterns.

At 25°, Mars has a nearly identical axial tilt to Earth’s, creating similar seasons. Its day—the time it takes to spin around its axis once—is only a few minutes longer than ours. But the Martian atmosphere is one-thousandth the density of Earth’s at sea level, so liquid water cannot exist on the surface. Mars is so cold that some of its carbon dioxide atmosphere is locked into its polar caps as dry ice. Still, Martian meteorology echoes that of Earth; we’ve witnessed great dust storms, whirling dust devils, and even snowfall near the poles. Martian dust tints the sky orange during the day, and dims to an eerie blue at sunset.

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The Gas Giants: Stormy WeatherWhile the weather on Mars and Venus par-

allels that of Earth, meteorology becomes sur-real beyond the orbit of Mars. Among the outer planets, we find the coldest temperatures, fiercest winds, and most alien skies. Four gigantic plan-ets rule this frigid realm: Jupiter, Saturn, Uranus, and Neptune. In this region of bitter darkness and numbing cold, the sun glows like a distant ember. Storms large enough to swallow the entire Earth rage for decades. Earthly gases freeze into liquid or ice. Snows of ammonia and methane ice crystals drift from electrified clouds.

All four outer worlds have belts of clouds encircling them parallel to the equator. The cloud bands remain stable within their latitudinal paths. On Earth, bands of clouds are broken up by storms at temperate latitudes. Continents block airflow, triggering pressure waves that also mix things up. Storms come and go on the order of days or, in the case of cyclones and monsoons, weeks. But the massive storms of the gas giants may last for decades. Astronomers have observed Jupiter’s Great Red Spot for nearly 400 years.

Unlike the terrestrial planets, the outer worlds have no well-defined delineation between solid interior and atmosphere. With no solid surface to prop up the air, their atmospheres are self-supporting. Atmospheric pressure acts as a foundation against the force of gravity, keeping gases in balance with the pull of the planet. This balance is called hydrostatic equilibrium. Because there is no solid surface from which to

Atmospheric circulation on Venus (left) is a simplified version of that on Earth.

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Two views of Martian dust devils. NASA’s Mars Reconnaissance Orbiter captured the view from orbit (left) while the Spirit Rover saw the phenomenon from Columbia Hills.

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Jupiter and SaturnJupiter is the largest planet in our solar system.

This behemoth could swallow a thousand Earths with room to spare. More massive than all the other planets and their moons put together, the planet is, in effect, a giant ball of weather. Like Saturn, Jupiter’s highest cloud deck consists of ammonia ice crystals. At 60 km beneath the scattered ammonia, the rich, rust-brown cloud deck swirls, and this gives Jupiter its dark banding. This layer of ammonium hydrosulfide cloud may be laced with rich organic compounds generated from energy farther below. Here and there, pale blue-gray water clouds break through the ruddy plain, boiling up from beneath. The water cloud deck is the lowest, floating nearly 100 miles below the highest ammonia cirrus. Here, temperatures rise above the melting point of water, and rain falls into an eternal night of crushing pressures and searing temperatures, as hydrogen turns into a liquid, and then into a fluid metal.

Jupiter’s most famous storm is known as the Great Red Spot (GRS). The GRS is as far across as two Earths, and has raged for at least three centuries. During that time, it has changed color, shape, and latitude. It rotates anticyclonically, that is, counterclockwise. The storm is actually a vast dome. Its form has the appearance of a

measure altitude, scientists instead use pressure as a reference point, with one bar (equivalent to Earth’s atmosphere at sea level) as the starting point.

The atmospheres of this planetary quartet are dominated by hydrogen and helium—the ancient building blocks of the solar system. The gas giants, Jupiter and Saturn, orbit closer in than their siblings, and have warmer atmospheres seasoned by ammonia. Their cores suffer from such great pressures that hydrogen actually transforms into a liquid metal. Farther out in the solar system, the smaller planets Uranus and Neptune have cores of ice, and are thus called the ice giants. Their hydrogen/helium skies include methane.

The atmospheres of the giants are a rich brew of complex chemistry that manufactures organic compounds. This transformation takes place at the hand of forces that include lightning, rising currents from interior heat, cloud condensation, and photodissociation. This titanic chemistry experiment dyes the clouds of Jupiter and Saturn in tans, browns, and blues. Farther out, methane tints Uranus and Neptune. Neptune’s clear air reveals a rich teal cloud deck, while hydrocarbon hazes stain Uranus to a pale green.

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Jupiter and Saturn each have a triad of cloud decks, from chilled ammonia ice to water clouds below.

The colorful clouds of Jupiter are laced with organic materials, sulfur and ammonia, and complex organic molecules.

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Half a billion kilometers beyond Jupiter, we reach the lord of the rings, Saturn. The golden giant receives one fourth of the solar energy that Jupiter does (one hundredth that of Earth). Predictions held that its meteorology would be less vibrant than Jupiter’s because of its distance from the sun. With less heat to drive air movements, researchers expected Saturn’s winds to be more subdued than Jupiter’s, and that Uranus and Neptune would continue the trend toward quieter weather. But distance does not bring calm, as Caltech’s planetary scientist Andrew Ingersoll cautions. “Actually, the winds do not decrease as you move out in the solar system. That’s quite dramatic for Neptune, because it gets only 5 percent of the amount of energy that Jupiter does on a per-area basis. All the giant planets have stronger wind fields than Earth.”

Saturn’s winds may be the strongest of all. Air masses race in west-blowing jets, moving opposite—or retrograde—to the direction of the planet’s spin. These currents generate bizarre storm systems. A line of features called the “string of pearls” spreads a 60,000 km-long parade of clearings in Saturn’s upper cloud deck. Strange, donut-shaped clouds occupy another retrograde jet. The clouds resemble smoke rings, with clear air in the center.

Those pearls and donuts are just the beginning of the meteorological madness Saturn has to offer. In the south, smack in the middle of one of those retrograde wind streams, lightning-laced Thunderstorm Alley simmers. Limited to a line along the southern latitude of 30°, its tempests last for months.

Lightning can only be generated in water clouds. Clouds of ammonia or methane do

hurricane, but unlike hurricanes, the center of the GRS is a high-pressure region. Clouds flow out from the center in an S-shaped pattern. The cyclone forces streams of cloud to flow around it. Observers have clocked winds within the spot at up to 600 kph.

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This true-color view of Saturn’s northern hemisphere shows ring shadows draped across bluish clouds. The moon Mimas is at bottom.

Saturn’s “string of pearls” seen in infrared. These clearings form a string of hot spots where interior heat pours into the upper atmosphere.

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Uranus and NeptuneUranus and Neptune are twins in size, each

encompassing as much volume as 60 Earths. These frigid worlds arrange their cloud decks similarly to Jupiter and Saturn, but their atmospheres lack ammonia clouds. Instead, brilliant white cirrus clouds of methane ice float above a blanket of lower clouds, which may also be methane. Deeper water clouds may contain ammonia, but if so, they are hidden beneath the middle deck.

The remarkable colors of Uranus and Neptune result from methane. Methane absorbs red light, leaving the bluer parts of the spectrum to reflect back at the observer. A ruddy haze shifts the color of Uranus toward green. Uranus retains a subtle version of the same belts and zones exhibited by its gas giant siblings, but its weather is more subdued than that of Jupiter or Saturn. Why? The answer lies within the depths of these outer worlds. Jupiter, Saturn, and Neptune all put out more energy than they receive from solar heating. This internal heat drives the weather from within. But Uranus is far colder compared to its surroundings. Temperatures on Uranus and Neptune are nearly equal at –323°F, even though Neptune receives only four-ninths the solar energy that Uranus does. Uranus’ temperature appears to be in equilibrium with the incoming solar energy, leading to an atmosphere that is less mixed from interior to surface.

Uranian winds are difficult to judge. In Voyager and Hubble Space Telescope images, few distinct clouds appear. Voyager was only able to image the southern hemisphere, as the north was in darkness. (Unlike other planets, Uranus spins on its side.) The few discrete clouds that have been followed yield equatorial wind velocities of up to 430 kph.

As Uranus has moved along its 84-year orbit, day has finally dawned in the north, and Uranus has shown some dramatic changes. MIT’s Heidi Hammel studies both Uranus and Neptune using

not build the kind of electrical charges that lead to lightning. In Thunderstorm Alley, water clouds erupt from the lower regions of Saturn’s troposphere. The location and limit of thunderstorm activity present a mystery that is only deepening as summer comes to the northern hemisphere. New clouds are exploding in the north, exactly at the same distance from the equator.

Saturn’s polar regions also challenge traditional models. One of its most striking enigmas blankets the northern pole. When NASA’s Cassini Saturn Orbiter arrived in 2004, the shadows of Saturn’s rings had darkened the northern surface for years; clouds in the north were distinctly bluer than the rest of the planet. Carolyn Porco, imaging team leader of Cassini, now believes she knows the cause. “We think it has to do with the fact that it’s the winter hemisphere; we got there at the depths of northern winter. The rings cast a shadow that makes the atmosphere cold. The air gets so cold that the level where clouds can form gets lower in the atmosphere. Above the clouds, it gets clearer and clearer, and you’re getting a lot of Rayleigh [scattering of blue light].” Since Saturn passed equinox and its ring shadows have migrated to the opposite hemisphere, the northern pole has warmed, and the blue tint is fading. The blue tinting is now appearing in the south as the ring shadows shift to that hemisphere.

The poles of Saturn display other wondrous features. Locked directly over the south pole, a whirlpool gazes from concentric cloud bands like a vast eye. The storm’s cliff-like rim rises 25 to 40 miles high, reminiscent of a terrestrial hurricane’s eye. Up in the north, a colossal hexagon drapes over territory the diameter of two Earths. The baffling stream of air is stable and long-lived, as seen in Voyager spacecraft images in the 1980s. The hexagon’s cause is not well understood, making it one of the great planetary mysteries of today.

A titanic methane anvil boils into the troposphere of Uranus. A jet stream spreads these storms into a thin line hundreds of miles long, forming a shape researchers call a “tadpole”.

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could tell that one dark spot was a comparatively long-lived storm. She charted its movements for many months. The storm resembled a similar dark spot on Neptune.

Both planets have distinct clouds, but not as many as models predict. In any atmosphere, a battle rages between upwelling air currents that keep particles afloat and gravity that pulls particles down. For clouds to form, particles must stay airborne long enough to assemble into a group large enough to become visible. Larry Sromovsky, a researcher at the University of Wisconsin–Madison’s Space Science and Engineering Center, has been studying this battle as it relates to the atmospheres of the ice giants. “If there is not much vertical mixing, sedimentation will win, even if there is not a rapid formation of big particles.” Another factor, says Sromovsky, is the abundance of what are called condensation nuclei—small particles that act as cloud seeds, attracting liquid to form droplets. If the nuclei are few and far between, larger droplets will condense, falling more rapidly and forming fewer clouds.

Kevin Baines of NASA’s Jet Propulsion Laboratory suggests another concept for the missing clouds: “When methane condenses, it may condense so rapidly, and there’s so much of it, that over a few seconds to a minute you go from a little droplet that grows to the size of a beach ball, and the beach ball falls.” Baines imagines that giant methane raindrops fall from the air too quickly to form clouds.

Uranus seems bland compared to the stunning disk of Neptune. Neptune’s clouds are intrinsically blue, says Heidi Hammel. “There is some kind of coloring agent in the atmosphere that gives it the more bluish color compared to Uranus.” What that agent is remains a mystery.

White clouds of methane crystals glide across subtle belts and zones, sometimes spiraling into cyclonic features. Unlike the clouds on Jupiter and Saturn, clouds on Neptune evolve rapidly, masking the dynamics of the blue world’s atmosphere. Outer planets researchers have difficulty measuring Neptune’s wind speeds because they are unsure if a given cloud feature is the same cloud that they studied even an hour earlier.

In Neptune’s clear upper atmosphere, at a pressure of about 100 millibars (1/10th that of Earth’s surface pressure), temperatures hover at about –360°F. Temperatures rise with depth. At a pressure comparable to Earth at sea level, temperatures reach –334°F. Beneath this level, the air is warm enough for methane to exist as a liquid or vapor. Methane cools and condenses

two powerful instruments: the Hubble Space Telescope and the Keck Ten Meter Telescope atop Hawaii’s Mauna Kea. Hammel has found features on Uranus that resemble the diversity of Neptune’s meteorology. “We saw lots of clouds popping up all over Uranus. The Hubble and Keck Telescope images showed all sorts of transient rapid cloud activity.”

It appears that the weather on the green giant is becoming more similar to that on Neptune. While dark storms—similar to Jupiter’s Great Red Spot—last for roughly five years on Neptune, the duration of those on Uranus is unknown because of constraints on Earthly schedules. Hubble Space Telescope and the Keck Ten Meter are the only two facilities in the world with enough spatial resolution to detect these features, and observing time on them is limited. Hammel’s team is able to use the Hubble for a scant six hours a year, and time on the Keck is scarcely twice that. Despite these limitations, Hammel

The bizarre landscape of Neptune’s moon Triton. Through a rarefied atmosphere, we see ice giant Neptune with its great blue storms, looming behind Triton’s alien geysers of nitrogen.

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Neptune presents yet another puzzle: As it moves in its elongated orbit and its distance from the sun increases, Neptune’s atmosphere has been steadily warming. Hammel is intrigued. “I don’t think we understand our atmospheres in the solar system as well as people think we do. There may be mechanisms that we are not fully accounting for as we model planetary atmospheres. That includes Earth.”

MoonsNo planetary tour would be complete without

a stop at two moons—one at Saturn (Titan) and one at Neptune (Triton).

TitanSaturn’s planet-sized moon, Titan, has an

atmosphere denser than Earth’s. Its alien weather showcases the closest thing to Earth’s hydrological cycle, and scientists want to study its complex chemistry as a possible model of Earth’s early pre-biology.

Titan’s murky nitrogen atmosphere is rich in methane. Like water on Earth, liquefied methane rains from the sky, pooling in lakes, and then evaporates to condense as clouds once again. NASA’s Cassini Orbiter uses radar to peer through Titan’s dense orange fog. The school-

into clouds, upwelling high into the stratosphere. Some storms traverse 50 to 100 km of altitude.

Despite the transient nature of its clouds, Neptune skies are visited by a few long-lived structures. One of the first features Voyager resolved in its 1989 flyby was a large, blue storm reminiscent of Jupiter’s Great Red Spot. The storm was approximately the same size relative to the planet—a deep pool of sapphire floating in a cobalt sea of clouds. The Great Dark Spot spanned the distance of Earth’s diameter. This oval-shaped vortex rotated anticyclonically within the streams of Neptune’s surrounding atmosphere. As air deflected up over the disturbance, methane clouds condensed at its edge. These types of clouds are known as orographic clouds, and are often seen on the leeward side of mountains on Earth.

By the time the Hubble Space Telescope began operations in 1993, the Great Dark Spot of 1989 had vanished without a trace. But Heidi Hammel has spotted similar storms since then. “They seem to last about 5 years or so,” she explains. “In 1994 we got our first Hubble images, and we saw that big dark spot in the north.” Hammel’s team found several others over the course of nearly a decade, but none recently. “We’ve had sort of a dry spell when it comes to dark spots on Neptune.”

Saturn’s planet-sized moon Titan hosts a methane cycle similar to the water cycle on Earth. This artist’s concept shows a location called Hotei Arcus, a region that may have super-chilled volcanic activity.

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bus-sized spacecraft has spotted river valleys and eroded slopes marking outflow regions, along with methane-filled lakes as large as the Black Sea.

Methane rain may fall in seasonal downpours. Researcher Elizabeth Turtle of Johns Hopkins’ Applied Physics Laboratory suggests that Titan goes through a shift of weather patterns to the spring/summer hemisphere during the equinox season. “Similar behavior is observed on Earth,” she explains, “but it’s restricted to the tropics … where surface winds from the northern and southern hemispheres converge, causing upwelling.” Rainfall patterns shift from one side of Earth’s equator to the other each spring/fall, Tuttle says. Her observations indicate that a similar shift occurs on Titan.

Cassini charted Titan’s sluggish winds on a global scale by using a natural weather vane: sand dunes. Radar images revealed vast tracts of dunes that rival any desert on Earth. The European Space Agency’s Huygens probe charted currents on its way to a landing there. The coffee-table–sized craft sampled gases and sensed moisture, even in the surface. Huygens recorded hurricane-force winds 120 km above the ground, but below 8 km, the air calmed. Huygens’ weather report from the surface: a temperature of –179°C and a pressure of 1.47 times the density of Earth’s at sea level.

TritonAs Voyager 2’s images of Neptune’s moon Triton

appeared on screens at NASA’s Jet Propulsion Laboratory in 1989, a hush settled across the press

room. Gasps were heard amidst mumbled words, such as “weird” and “creepy.” Triton’s alien, weathered surface spread before the scientists as a tortured battlefield of pockmarks and fissures. Its atmosphere turned out to be 10,000 times less dense than Earth’s (15 microbars). Triton’s feeble atmosphere consists mostly of nitrogen. Because of Triton’s low gravity, the outer fringes of its atmosphere waft to an altitude of 800 km above the frigid surface. Hazes drift among eight-kilometer-tall plumes from nitrogen geysers.

Winds, clouds, hazes, and even air pressure are driven by the freezing and thawing of Triton’s polar caps. As winter arrives at one of the poles, Triton’s nitrogen migrates there, freezing to the surface. The entire atmosphere collapses twice a year—when it is winter on one pole or the other. The strange moon only has “weather” during the spring and fall, because it has an active atmosphere only during those seasons.

For now, our cosmic wanderlust must be fed by telescopic observations and our robotic emissaries. But one day, perhaps soon, humans may get a first-hand view of the blue sunsets of Mars or the methane monsoons of Titan.

For more on the subject, see Michael Carroll’s newest book, Drifting on Alien Winds: Exploring the Skies and Weather of Other Worlds (Springer, 2011). W

MICHAEL CARROLL is a science writer and astronomical artist. He has a painting on Mars—in digital form—on the Phoenix Lander. He lives in Colorado where there is nice weather and oxygen to breathe.

Winds of the giant planets vary in surprising ways.

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