Cassini captured this 2011 image of the latest Saturnian superstorm about three months after it began. The planet's rings can be seen cutting horizontally across the bottom of the image. Image credit: NASA/JPL-Caltech/Space Science Institute

Jul 30, 2015 Day after day, Cassini keeps watch for Saturn storms

Sen—There is nothing in science quite like the robotic exploration of space. It’s grand on the most daunting of scales—Cassini’s silent, effortless glide through the void has taken it billions of kilometers over the course of nearly two decades. A machine wrought here on Earth has not just endured the austerity that separates our world from others, it has thrived in it. But that grandeur belies an intricacy that astonishes and a relentlessness that nearly terrifies.

Each week, the mission publishes a document it calls “Significant Events,” which details in broad strokes the activities undertaken by the spacecraft. The schedule varies week to week as the science objectives change, but one thing is always clear: The spacecraft never has a moment at rest. Take this excerpt from May 5:

“UVIS made radial profiles of Saturn's atmosphere in an observation lasting 1.25 hours. Next, UVIS, with ISS and VIMS riding along, proceeded to image Saturn's thermosphere for almost five hours. … Next, ISS, CIRS and VIMS executed another 90-minute Titan monitoring observation. … Finally today, VIMS took the helm to start an 18-hour observation of Saturn's faint G ring and its broad E ring. Two other telescopic remote-sensing instruments -- CIRS and UVIS -- participated.”

Day after day the spacecraft twists and turns, craning to catch the best glimpse possible of all the Saturn system has to offer. But after reading months of these reports, a regularity began to jump out at me. Nearly every day, in the midst of this unending sprint, Cassini takes time to perform the same observation. May 3 highlights this:

“ISS, CIRS and VIMS made a 90-minute observation in the Titan monitoring campaign. The huge hazy moon was 2.3 million kilometers from Cassini's telescopes. Next, ISS turned to Saturn for a two-minute storm-watch observation. ISS then spent 45 minutes looking near the planet for small objects, part of the satellite orbit campaign...”

By the next day, the author of “Significant Events” that week was already tired of writing it. May 4:

“...ISS carried out another one-hour satellite orbit observation. Two-minute ISS storm-watch observations occurred four more times this week; two were today, and two more were on Tuesday. VIMS participated in one of them each day.”

What are these storm-watch observations? And why does Cassini seem take more of them than anything else?

Take a passing glance at Saturn’s cloud tops and it would be easy to dismiss its atmosphere as among the least interesting in the Solar System. Of all the worlds with atmospheres (Venus, Earth, Mars, Jupiter, Saturn, Titan, Uranus, Neptune), perhaps only that of Uranus appears more bland upon a first viewing.

Surely the most intriguing atmosphere is that of Jupiter. Its alternating red and white stripes caught the attention of even the earliest telescopic astronomers; among the first to make detailed drawings was Giovanni Cassini, future namesake of our favorite spacecraft. Although they would take more than three hundred years to be discovered, it turns out Saturn, too, has such stripes. The Voyager flybys would provide the first depictions of their faint pattern in the early 1980s.

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This 2004 image captured by the Hubble Space Telescope reveals the broad bands of clouds which make up Saturn's upper atmosphere. Image credit: NASA,ESAand E. Karkoschka (University of Arizona)

Before we dive in too deeply though, let’s answer a couple of common questions related to atmospheres. Perhaps most obviously: What is an atmosphere, actually? At its most basic level, an atmosphere is the collection of gases surrounding a celestial object. It turns out, though, that every object has at least a tiny trace of gas around it, rendering such a broad definition pretty useless!

Instead, astronomers typically use a different metric for defining what has an atmosphere and what doesn’t. Gases are made up of tiny particles, all constantly bouncing around at high velocity. If an atom or molecule is more likely to collide with another atom or molecule than it is to escape to space, it belongs to an object’s atmosphere (next time you cough, there’s an itsy-bitsy, teeny-tiny probability that a molecule of your breath hits nothing and escapes directly to space; it’s probably never happened in the history of the Earth, though). Particles which are more likely to escape than to collide belong to a body’s exosphere.

All worlds have an exosphere—Earth’s starts about 500 km up—but not every object also has an atmosphere. Confusingly, astronomers will lazily use the term atmosphere to refer to the exosphere for objects at which that’s all that is present. This is why you’ll occasionally hear about the atmosphere of Mercury or the Moon. The exospheres of those objects are constantly escaping, only to be replenished from the surface.

Now that we have a way to determine which worlds have an atmosphere, how far do they extend? This is where things get kind of weird. For a rocky planet like Earth, the answer is clear-cut: our atmosphere starts at the ground and extends outwards to the bottom of the exosphere (called the exobase, for those playing jargon bingo at home!). But what about the gas giants? After all, they’re made of, well, gas—does this mean that their atmospheres extend all the way down?

Here’s the truth: aside from the very basics, we know surprisingly little about the interiors of these giant planets. They are so massive that the pressures found within are greater than we can simulate in labs here on Earth. Most likely, if one were to travel deeper and deeper into a gas giant, the gas comprising it would get denser and denser until it smoothly transitioned to a liquid. Ultimately, near the core one might even find these elements in solid form. This means there’s no solid (pardon the pun!) answer to the bottom edge of a gas giant’s atmosphere.

Of course, we still need to measure stuff from somewhere, so planetary scientists generally do a very Earth-centric thing: we define the reference point for a gas planet’s atmosphere to be the level at which the atmospheric pressure is equal to 1 bar, the (approximate) pressure at sea level on the Earth! If you flew your spaceship to this location and hopped out onto the wing, you wouldn’t need a spacesuit. A lot of clothes and an oxygen mask, to be sure, but you could ditch that bulky pressure suit in your closet.

Just like on Earth, the atmospheres of these giant planets can have clouds which float at different levels. It’s variations in these cloud bands which give the four outer worlds their distinctive look. Jupiter has more helium and ammonia, while Saturn is host to a larger ratio of hydrogen and methane. Also just like Earth, atmospheres can brew storms and this brings us back to those mysterious Cassini observations.

If you’ve ever been caught in an afternoon thunderstorm in the summer when the sky was clear not an hour earlier, then you know how a storm can quickly blow up and just as quickly fade away. When Cassini arrived at Saturn in 2004, in was unprepared for these rapid changes. In an email to Sen, Prof Carl Murray at Queen Mary University of London, a Cassini imaging team member, explains:

“Although numerous, carefully planned Cassini images of the planet Saturn have been taken throughout the mission, the sudden appearance of storms and various phenomena in the planet’s atmosphere made us aware that we lacked a plan to make regular observations of the planet. Such observations would help in quickly discovering and then tracking unusual features such as storms.”

To be more responsive to these short-scale changes in Saturn’s atmosphere, the imaging team modified existing small satellite astrometry observations to be bookended by brief—often just two minutes long—exposures. The Storm Watch observations use Cassini’s wide-angle camera to capture the planet through several filters. If bad weather whips up, mission scientists are less likely to miss it.

But not every storm on Saturn is short lived. Its most fearsome storms are also among its most enigmatic. Since the latter part of the 19th century, astronomers have charted six great storms that left their mark on the entire planet, each occurring around a quarter-century after the last. The latest flared up in December 2010 and didn’t dissipate until late the following year.

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This Cassini Storm Watch observation captured the birth of the 2010-2011 superstorm on December 5th, 2010. It can be seen as the small, white blemish along the right edge of the visible part of the planet. Image credit: NASA/JPL-Caltech/Space Science Institute

The thunderhead of these mega storms can prove truly terrifying—the densest part can be larger than the Earth! For more than a century, scientists have puzzled over the surprising regularity with which they appear. Over the last five years, astronomers have been studying the data collected by Cassini about this latest incarnation and finally we might have the answer.

To understand what might be going on, we need to know one more thing about how atmospheres work: convection. If you heat a pocket of gas up, it tends to rise and, as it travels upwards, it can transfer that heat and cool down. As it cools, the gas becomes denser and begins to sink. It’s an effective method for moving around heat and the same process is used in many modern ovens to ensure even cooking.

Since the interior of Saturn is warmer than space, convection is always occurring. The upper layers of the atmosphere, however, also contain something else familiar to us: water vapor. Under the right conditions, these clouds can rain down into the planet below. And just like getting caught in a summer rainstorm, when this happens things below get cooled off.

Now that the lower layers of gas are cooler, convection in this region can be suppressed. But without fresh deliveries of heat, the top of the atmosphere then begins to cool to an even lower temperature. Eventually the temperature gradient becomes too great and the cycle gets kickstarted once more, launching up a huge pocket of warm gas, enabling a tremendous thunderhead to form.

So why does this happen every 20-30 years? It turns out that it’s actually rather difficult to cool the thin gas near the outer edge of Saturn’s atmosphere and that such a process itself might take decades.

These storms are more than just beautiful. They can also help us understand a tremendous amount about the composition and working of the planet’s atmosphere. For example, if the hypothesis we just considered is correct, it implies that the atmospheric composition of water must be at least one per cent.

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This Cassini image from January 2011 shows the superstorm's tail in false color. The colors represent the different depths of clouds containing methane, allowing scientists to map its distribution about the planet. The rings, which are free of methane, appear as a horizontal blue line. Image credit: NASA/JPL-Caltech/Space Science Institute

The intense air currents within the storm—Saturn has among the highest wind speeds in the Solar System—can also dredge up material from deeper below. By one estimate, material from 160 km down can be dragged to the surface. This enables Cassini to study the portion of the planet’s atmosphere otherwise out of reach of its sensors.

Of course, as large as the superstorms of Saturn can grow, they’re no match for the largest and most famous tempest out there: Jupiter’s Great Red Spot. Unlike the comparatively short-lived storms on Saturn, the Great Red Spot has been raging for at least as long as we’ve had the capability of detecting it—more than 350 years! Why has this Earth-swallowing cyclone persisted for so long? We don’t yet have a good answer, but it could depend of the different atmospheric compositions of the two planets.

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The Voyager 1 spacecraft took this dramatic image of Jupiter's Great Red Spot during its flyby in 1979. Image credit: NASA/JPL

To really understand what’s going on, we’ve got to get up close and personal with this wild weather. On Dec. 7, 1995, the Galileo spacecraft dropped a 339-kg probe into the atmosphere of Jupiter. Despite unluckily descending into highly-unusual cloud formation, it managed to return valuable data from more than 150 km down into the atmosphere. A similar mission to Saturn in the form of a New Frontiers-class spacecraft is on the long-term roadmap for planetary science, but given NASA’s current funding levels and science priorities, it’s likely to be decades before such a machine reaches the ringed planet.

We will get one close look in late 2017, when Cassini concludes its mission by plunging into Saturn, but the spacecraft isn’t designed to withstand the heat and pressure of a high-speed atmospheric entry. Even so, it will represent our best look to date at this seemingly-quiet world that manages to launch some of the largest storms in the Solar System.

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