Simulating Titan’s Atmosphere in the Lab

Image credit: ESA
It takes at least three elements to harbor life as we know it: water, energy and an atmosphere. Among Mars and the moons around both Jupiter and Saturn, there is evidence of one or two of these three elements, but less is known if a complete set is available. Only Saturn’s moon, Titan, has an atmosphere comparable to Earth’s in pressure, and is much thicker than the martian one (1% of Earth’s sea level pressure).

The most interesting point about simulations of Titan’s hydrocarbon haze is that this smoggy component contains molecules called tholins (from the Greek word, muddy) that can form the foundations of the building blocks of life. For example, amino acids, one of the building blocks of terrestrial life, form when these red-brown smog-like particles are placed in water. As Carl Sagan pointed out, Titan may be regarded as a broad parallel to the early terrestrial atmosphere with respect to its chemistry and in this way, it is certainly relevant to the origins of life.

This summer, NASA’s Cassini spacecraft, launched in 1997, is scheduled to go into orbit around Saturn and its moons for four years. In early 2005, the piggybacking Huygens probe is scheduled to plunge into the hazy Titan atmosphere and land on the moon’s surface. There are 12 instruments onboard the Cassini Spacecraft orbiter, and 6 instruments onboard the Huygens Probe. The Huygens probe is geared primarily towards sampling the atmosphere. The probe is equipped to take measurements and record images for up to a half an hour on the surface. But the probe has no legs, so when it sets down on Titan’s surface its orientation will be random. And its landing may not be by a site bearing organics. Images of where Cassini is in its current orbit are continuously updated and available for view as the mission progresses.

Astrobiology Magazine had an opportunity to talk with research scientist, Jean-Michel Bernard of the University of Paris, about how to simulate Titan’s complex chemistry in a terrestrial test tube. His simulations of Titan’s environment build on the classic prebiotic soup, first pioneered fifty years ago by University of Chicago researchers, Harold Urey and Stanley Miller.

Astrobiology Magazine (AM): What first stimulated your interest in the atmospheric chemistry of Titan?

Jean-Michel Bernard (JB): How do two simple molecules (nitrogen and methane) create a very complex chemistry? Does chemistry become biochemistry? The recent discoveries of life in extreme conditions on Earth (bacteria in the South Pole at -40?C and archaea at more than +110?C in the vicinity of hydrothermal sources) allow to suppose that life could be present on other worlds and other conditions.

Titan has astrobiological interest because it is the only satellite in the solar system with a dense atmosphere. Titan’s atmosphere is made of nitrogen and methane. The energetic particles coming from the Sun and Saturn’s environment allow complex chemistry, such as formation of hydrocarbons and nitriles. The particles also generate a permanent haze around the satellite, rains of methane, winds, seasons Recently, lakes of hydrocarbons seem to have been detected on Titan’s surface. I think that this discovery, if it is confirmed by the Cassini-Huygens mission, will be of great interest.

It would make Titan an analog to the Earth, since it would have an atmosphere (gas), lakes (liquid), haze and soil (solid), the three necessary environments for the appearance of life.

The composition of Titan’s haze is unknown. Only optical data are available and they are difficult to analyze due to the complexity of this carbonaceous material. Many experiments have been carried out in order to mimic the chemistry of Titan’s atmosphere, most notably the aerosols analogs named “tholins” by Carl Sagan’s group. It seems that tholins could be involved in the origin of life. Indeed, hydrolysis of these Titan aerosol analogs gives rise to the formation of amino acids, the precursors of life.

AM: Can you describe your experimental simulation for extending the Miller-Urey experiments in a way that is customized for Titan’s low temperatures and unique chemistry?

JB: Since the Miller-Urey experiments, many experimental simulations of supposed prebiotic system have been carried out. But after the retrieval of Voyager’s data, it appeared necessary to come back to this approach to simulate Titan’s atmosphere. Then several scientists carried out such simulation experiments by introducing a nitrogen-methane mixture in a system like Miller’s apparatus. But a problem became obvious due to the difference between the experimental conditions and Titan’s conditions. The pressure and temperature were not representative of Titan’s environment. Then we decided to carry out experiments which reproduce the pressure and the temperature of Titan’s stratosphere: a gas mixture of 2% of methane in nitrogen, a low pressure (about 1 mbar) and a cryogenic system in order to have a low temperature. Furthermore, our system is placed in a glove box containing pure nitrogen in order to avoid contamination by ambient air of the solid products.

AM: What do you consider the best energy source for triggering Titan’s synthetic chemistry: the magnetosphere of Saturnian particles, solar radiation, or something else?

JB: Scientists debate about what energy source would best simulate the energy sources in Titan’s atmosphere. Ultraviolet (UV) radiation? Cosmic rays? Electrons and other energetic particles coming from Saturn’s magnetosphere? All these sources are involved, but their occurence depends of the altitude: extreme ultraviolet radiation and electrons in the ionosphere, UV light in the stratosphere, while cosmic rays occur in the troposphere.

I think the appropriate question should be: What is the experimental goal? If it is to understand the hydrogen cyanide (HCN) chemistry in Titan’s stratosphere, a simulation with UV radiation of HCN is appropriate. If the goal is to determine the effects of electric fields generated by galactic cosmic rays in the troposphere, a corona discharge of a simulated Titan-atmosphere is preferable.

In studying Titan’s stratospheric conditions, we chose to use an electric discharge in our simulation. This choice is contested by a minority of scientists because the main energy source in Titan’s stratosphere is UV radiation. But our results validated our experiment. We detected all the organic species observed on Titan. We predicted the presence of CH3CN (acetonitrile) before its observation. We detected for the first time dicyanoacetylene, C4N2, an unstable molecule at room temperature that has also been detected in Titan’s atmosphere. The middle infrared signature of the solid products created in our experiment was in line with Titan observations.

AM: How are your results part of the planned atmospheric testing for the Cassini-Huygens probe?

JB: After collaborating with a team from the Observatoire Astronomique de Bordeaux in France, we determined the dielectric constants of aerosol analogs. This will allow us to estimate how Titan’s atmosphere and surface properties could affect the performance of the Cassini-Huygens radar experiments. The altimeter onboard the Huygens probe could be affected by the aerosol properties, but complementary experiments must be carried out to confirm this result.

Two years ago, we introduced a gas mixture, N2/CH4/CO (98/1.99/0.01). The goal was to determine the impact of carbon monoxide, the most abundant oxygenated compound on Titan. Surprisingly, we detected oxirane in the gaseous phase as the major oxygenated product. This unstable molecule was discovered in the interstellar medium but theoretical models do not predict it for Titan’s chemistry. Yet maybe this molecule is present on Titan.

Currently, we are analyzing the first molecules, radicals, atoms and ions (or ‘species’) created inside our experimental reactor. We are using infrared spectrometry and UV-visible emission to study excited species like CN, CH, NH, C2, HCN, C2H2. Next, we will observe the correlation between the abundance of these species and the structures of the solid products. . Coupling these experimental results with a theoretical model developed in collaboration with the University of Porto in Portugal, we will have a better understanding about the chemistry occurring into the experimental reactor. This will allow us to analyze the Cassini-Huygens data and Titan’s haze formation.

Our team is involved at the mission science level as well, as one of the scientists of the mission is also in our group at the Laboratoire Inter-Universitaire des Syst?mes Atmosph?riques, LISA). Our laboratory tholins will be used as guides to calibrate several of the instruments on the Huygens probe and the Cassini orbiter.

There are 18 instruments on board the probe and orbiter. Calibration tests are needed for gas chromatography and mass spectroscopy [GC-MS]. The GC-MS will identify and measure chemicals in Titan’s atmosphere.

Calibration tests are also needed for the Aerosol Collector and Pyrolyser (ACP). This experiment will draw in aerosol particles from the atmosphere through filters, then heat the trapped samples in ovens to vaporize volatiles and decompose the complex organic materials.

The Composite Infrared Spectrometer (CIRS), a thermal measuring instrument on the orbiter, also needs to be calibrated. Compared to previous deep space missions, the spectrometer onboard Cassini-Huygens is a significant improvement, with a spectral resolution ten times higher than the Voyager spacecraft’s spectrometer.

AM: Do you have future plans for this research?

JB: Our next step is an experiment developed by Marie-Claire Gazeau, called “SETUP”. The experiment has two parts: a cold plasma in order to dissociate nitrogen, and a photochemical reactor in order to photodissociate methane. This will give us a better global simulation of Titan’s condition.

Original Source: NASA Astrobiology Magazine

Titan Could Help the Study of Oceanography

Image credit: Mark Robertson-Tessi
After a 7-year interplanetary voyage, NASA?s Cassini spacecraft will reach Saturn this July and begin what promises to be one of the most exciting missions in planetary exploration history.

After years of work, scientists have just completed plans for Cassini?s observations of Saturn?s largest moon, Titan.

“Of course, no battle plan survives contact with the enemy,” said Ralph Lorenz, an assistant research scientist at the University of Arizona?s Lunar and Planetary Laboratory in Tucson.

The spacecraft will deploy the European Space Agency?s Huygens probe to Titan for a January 2005 landing. Nearly half the size of Earth, frigid Titan is the only moon in the solar system with a thick atmosphere. Smog has prevented scientists from getting more than a tantalizing hint of what may be on the moon?s amazing surface.

“Titan is a completely new world to us, and what we learn early on will likely make us want to adjust our plans. But we have 44 flybys of Titan in only four years, so we have to have a basic plan to work to.”

Scientists have long thought that, given the abundant methane in Titan’s atmosphere, there might be liquid hydrocarbons on Titan. Infrared maps taken by the Hubble Space Telescope and ground-based telescopes show bright and dark regions on Titan’s surface. The maps indicate the dark regions are literally pitch-black, suggesting liquid ethane and methane.

Last year, data from the Arecibo telescope showed there are many regions on Titan that are both fairly radar-dark and very smooth. One explanation is that these areas are seas of methane and ethane. These two compounds, present in natural gas on Earth, are liquid at Titan’s frigid surface temperature, 94 degrees Kelvin (minus 179 degrees Celsius).

Titan will be an outstanding laboratory for oceanography and meteorology, Lorenz predicts.

“Many important oceanographical processes, like the transport of heat from low to high latitudes by ocean currents, or the generation of waves by wind, are known only empirically on Earth,” Lorenz said. “If you want to know how big waves get for a given windspeed, you just go out and measure both of them, get a lot of datapoints, and fit a line through them.

“But that’s not the same as understanding the underlying physics and being able to predict how things will be different if circumstances change. By giving us a whole new set of parameters, Titan will really open our understanding of how oceans and climates work.”

Cassini/Huygens will answer many questions, among them:

Are the winds strong enough to whip up waves that will cut cliffs in the lakesides? Will they form steep beaches, or will the strong tides caused by Saturn’s gravity be a bigger effect, forming wide, shallow tidal flats?

How deep are Titan’s seas? This question bears on the history of Titan’s atmosphere, which is the only other significant nitrogen atmosphere in the solar system, apart from the one you’re breathing now.

And do the oceans have the same composition everywhere? Just as there are salty seas and freshwater lakes on Earth, some seas on Titan may be more ethane-rich than others.

Lorenz began working on the Huygens project as an engineer for the European Space Agency in 1990, then earned his doctorate from the University of Kent at Canterbury, England, while building one of the probe’s experiments. He joined the University of Arizona in 1994 where he started work on Cassini’s Radar investigation. He is a co-author of the book, “Lifting Titan’s Veil” published in 2002 by Cambridge University Press.

Original Source: UA News Release

Getting Closer to Saturn

Image credit: ESA

NASA’s Cassini spacecraft is on track to reach Saturn in summer 2004. Before it reaches Saturn, however, the spacecraft will release a tiny probe called Huygens that will parachute into Titan, Saturn’s largest moon, to give scientists an idea of what’s underneath those thick clouds. Astronomers think that the environment on Titan is very similar to primordial conditions here on planet Earth billions of years ago. Huygens will take more than a thousand pictures and countless samples as it travels down to the surface in January 2005.

This time next year, ESA?s Huygens spaceprobe will be descending through the atmosphere of Saturn?s largest moon, becoming the first spacecraft to land on a body in the outer Solar System.

Earlier this month, the giant ringed planet Saturn was closer to Earth than it will be for the next thirty years. All the planets orbit the Sun as if on a giant racetrack, travelling in the same direction but in different lanes.

Those in the outer lanes have further to travel than those on the inside lanes. So, Earth regularly ?laps? the further planets. On New Year?s Eve 2003, Earth overtook Saturn, drawing closer than at any time in the next three decades.

Through a small telescope, Saturn is normally visible as a creamy yellow ?star?. You may be able to see the ring system that the planet is famous for, and its largest moon Titan will show up as a tiny dot of light.

That tiny dot is the destination for ESA?s Huygens probe and may hold vital clues about how life began on Earth. Titan is the only moon with a thick atmosphere in the Solar System.

Astronomers think this atmosphere might closely match the one Earth possessed millions of years ago, before life began. Certainly Titan?s atmosphere is rich in carbon, the chemical necessary for life on Earth. What is more, this is all stored in ?deep freeze?, ten times further from the Sun than the Earth.

The big mystery is Titan?s surface, which is hidden by a cloud layer. This is why ESA built Huygens, to probe through this layer which is impenetrable by Earth-based observations.

In January 2005, Huygens will parachute below the clouds to see what is really going on. Its battery of instruments will return over 1000 images as it floats down and samples the chemistry of this exotic place.

The Titan probe was named Huygens in honour of the Dutch astronomer who discovered Titan in 1655. Launched in October 1997, Huygens is currently in space, hitching a ride on NASA?s Cassini spacecraft.

So look forward to seeing more of Saturn and a tiny European spacecraft called Huygens, that in one year?s time will make an historic landing in the quest to uncover the origins of life.

Original Source: ESA News Release

235 Days to Saturn

Image credit: NASA/JPL

NASA’s Cassini spacecraft is on final approach to Saturn, and so far, the view is just getting better and better. The Saturn-bound spacecraft captured this photograph of the Ringed Planet on November 9th at a distance of 114 million km. The smallest features visible are 668 kilometres across, so the resolution is going to get much better as it gets closer. Five of the planet’s many moons can also be seen in this photograph (they were digitally enhanced to be easier to see). Cassini will finally arrive at Saturn on July 1, 2004.

A cold, dusky Saturn looms in the distance in this striking, natural color view of the ringed planet and five of its icy satellites. This image was composed of exposures taken by Cassini’s narrow angle camera on November 9, 2003 at 08:54 UTC (spacecraft event time) from a distance of 111.4 million km (69.2 million mi) — about three-fourths the distance of the Earth from the Sun — and 235 days from insertion into Saturn orbit. The smallest features visible here are about 668 km (415 mi) across, which is a marked improvement over the last Cassini Saturn image released on November 1, 2002. New features such as intricate cloud patterns and small moons near the rings should become visible over the next several months as the spacecraft speeds toward its destination.

Some details within Saturn’s massive ring system are already visible. Structure is evident in the B ring, the middle and brightest of Saturn’s three main rings. The 4800 km (2980 mi)-wide Cassini Division is the distinctive dark, central band that separates the outermost A ring from the brighter B ring. Interestingly, the outer edge of the B ring is maintained by a strong gravitational resonance with the moon Mimas, also visible in this image (see below). The 325 km (200 mi)-wide Encke gap in the A ring, near the outer edge of the ring system, is also visible, as is the fainter C ring, interior to the B ring.

With a thickness of only a few tens of meters or less, the main rings span 274,000 km (171,000 mi) from one end to the other? about three-quarters of the distance between the Earth and the Moon.

Saturn’s multi-banded, multi-hued atmosphere is also apparent at this distance. In this composite made of images taken through broadband blue, green, and red spectral filters, the color is very close to what the human eye would see. The different hues of yellow, brown and red seen in the illuminated southern hemisphere are more delicate and subtle than the colors on Jupiter. Coloration on both Jupiter and Saturn is caused by small colored particles mixed with the white ammonia clouds. The ammonia clouds on Saturn are deeper and thicker than those on Jupiter because ammonia gas condenses at a deeper level in Saturn’s colder atmosphere. The composition of the colored particles is not known but is thought to include sulfur and nitrogen as key constituents at middle and low latitudes.

In the southern polar region, a dusky haze is visible, more gray than the light-brown at middle latitudes. This polar haze may be produced by energetic electrons and protons in the aurorae which destroy methane gas, leading to the formation of a haze of complex hydrocarbons.

Most of Saturn’s northern hemisphere is in shadow of the rings, with the exception of a small sliver visible on the limb. (Light passing through the Cassini Division illuminates the higher altitudes in the atmosphere.) This sliver appears bluer than the visible southern hemisphere, probably due to molecular scattering by hydrogen at these altitudes above the haze and clouds. As the Cassini tour unfolds over the next five years and beyond, we will have an opportunity to see how the colors change with time, whether due to changing seasonal heating or to some other mechanism.

Five Saturnian satellites can also be seen in this image. The brightnesses of these bodies have been increased three- to ten-fold to enhance visibility. The satellites are, on the left, from brightest to faintest, Rhea (1530 km, 951 mi across), Dione (1120 km, 696 mi), and Enceladus (520 km, 323 mi); and on the right, from brightest to faintest, Tethys (1060 km, 659 mi) and Mimas (392 km, 244 mi).

From the Voyager encounters in 1980 and 1981, we know that each of Saturn’s icy moons possesses intriguing features. Enceladus is the most reflective body in the solar system; both Mimas and Tethys exhibit large craters on their surfaces; Dione and Rhea have curious streaks of bright, wispy material. Cassini will make very close approaches to Rhea, Dione and Enceladus, returning images in which features as small as 50 meters or less will be detectable. Images with details finer than those seen by Voyager (~ 2 km, 1.3 mi) will be returned from all five moons.

Cassini will enter Saturn orbit on July 1, 2004.

The Cassini-Huygens mission is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Office of Space Science, Washington, D.C.

Original Source: NASA/JPL News Release

Astronomers Peer Through Titan’s Clouds

Image credit: NASA

Astronomers from Cornell university have used the Arecibo radio telescope to peer through the thick clouds on Titan, Saturn’s largest moon. The radar signatures on the surface of Titan seem to indicate a liquid surface; although, the researchers say the signals could also mean smooth solid surfaces too. More answers will come next year when the Huygens probe carried by the Cassini spacecraft will drop through the clouds and send back information about the surface of Titan.

The smog-shrouded atmosphere of Titan, Saturn’s largest moon, has been parted by Earth-based radar to reveal the first evidence of liquid hydrocarbon lakes on its surface. The observations are reported by a Cornell University-led astronomy team working with the world’s largest radio/radar telescope at the National Science Foundation’s (NSF) Arecibo Observatory.

The radar observations, reported in the journal Science on its Science Express Web site (Oct. 2, 2003), detected specular — or mirrorlike — glints from Titan with properties that are consistent with liquid hydrocarbon surfaces. Cornell astronomer Donald Campbell, who led the observation team, does not rule out that the reflections could be from very smooth solid surfaces. “The surface of Titan is one of the last unstudied parcels of real estate in the solar system, and we really know very little about it,” he says.

The observations were made possible by the 1997 upgrade of the telescope’s 305-meter (1,000 feet) diameter dish, which has greatly increased the sensitivity of what was already the world’s most powerful radar system. The observatory is managed by the National Astronomy and Ionosphere Center (NAIC), based at Cornell in Ithaca, N.Y., which has been operating the huge telescope for the NSF since 1971.

Campbell, who is associate director of NAIC as well as a Cornell professor of astronomy, notes that for more than two decades astronomers have speculated that the interaction of the sun’s ultraviolet radiation with methane in Titan’s upper atmosphere — photochemical reactions similar to those that cause urban smog — could have resulted in large amounts of liquid and solid hydrocarbons raining onto Titan’s frigid surface (minus 290 degrees Fahrenheit, or minus 179 degrees Celsius). Campbell explains that radar signals would specularly reflect — or glint — from liquid surfaces on Titan, similar to sunlight glinting off the ocean. Although Titan’s underlying surface is thought to be water ice, the complex chemistry in the upper atmosphere might have resulted in the icy surface being at least partly covered in liquid ethane and methane and solid hydrocarbons, says Campbell. One class of the solid hydrocarbons, often referred to as Titan tholins, was artificially created in a campus laboratory by a team led by the late Cornell astronomer Carl Sagan.

Titan, which is about 50 percent larger than the Earth’s moon, is the only satellite in the solar system with a dense atmosphere. This atmosphere is transparent to radio/radar waves and partially transparent at short infrared wavelengths but is opaque at visible wavelengths.

The observations were made in November and December of both 2001 and 2002. The radar signal takes 2.25 hours to travel to Titan and back. The Arecibo radar operates at a 13-centimeter wavelength (2,380 megahertz), and the transmitted power is close to one megawatt (the equivalent of about 1,000 microwave ovens). Both the Arecibo telescope and the NSF’s new 100-meter Robert C. Byrd Green Bank Telescope were used to receive the extremely weak radar echoes.

Next summer, NASA’s Cassini spacecraft, launched in 1997, is scheduled to go into orbit around Saturn and its moons for four years. The piggybacking Huygens probe is scheduled to plunge into the hazy Titan atmosphere and land on the moon’s surface.

On Campbell’s team for the Arecibo radar observations of Titan were Gregory Black, the University of Virginia; Lynn Carter, Cornell graduate student; and Steven Ostro, Jet Propulsion Laboratory.

The Arecibo Observatory part of NAIC which is operated by Cornell University under a cooperative agreement with the NSF. NASA provides partial support for Arecibo’s planetary radar program. The Robert C. Byrd Green Bank Telescope is part of the National Radio Astronomy Observatory, an NSF supported institution operated under cooperative agreement by Associated Universities Inc.

Original Source: Cornell News Release

Three Views of Saturn

Image credit: Hubble

The planet Saturn reached its maximum tilt towards the Earth last Spring, and astronomers took advantage of the situation to image the ringed planet in three wavelengths of light using the Hubble Space Telescope: ultraviolet, visible, and infrared. Saturn tilts at an angle of 26-degrees and experiences seasons in its hemispheres like the Earth as it travels around the Sun; its orbit takes nearly 30 years. Particles in Saturn’s atmosphere reflect different wavelights of light differently, so the different images can help fill in pieces of missing information.

This is a series of images of Saturn, as seen at many different wavelengths, when the planet’s rings were at a maximum tilt of 26 degrees toward Earth. Saturn experiences seasonal tilts away from and toward the Sun, much the same way Earth does. This happens over the course of its 29.5-year orbit. This means that approximately every 30 years, Earth observers can catch their best glimpse of Saturn’s South Pole and the southern side of the planet’s rings. Between March and April 2003, researchers took full advantage to study the gas giant at maximum tilt. They used NASA’s Hubble Space Telescope to capture detailed images of Saturn’s Southern Hemisphere and the southern face of its rings.

The telescope’s Wide Field Planetary Camera 2 used 30 filters to snap these images on March 7, 2003. The filters span a range of wavelengths. “The set of 30 selected filters may be the best spectral coverage of Saturn observations ever obtained,” says planetary researcher Erich Karkoschka of the University of Arizona. Various wavelengths of light allow researchers to see important characteristics of Saturn’s atmosphere. Particles in Saturn’s atmosphere reflect different wavelengths of light in discrete ways, causing some bands of gas in the atmosphere to stand out vividly in an image, while other areas will be very dark or dull. One image cannot stand by itself because one feature may have several interpretations. In fact, only by combining and comparing these different images, in a set such as this one, can researchers interpret the data and better understand the planet.

By examining the hazes and clouds present in these images, researchers can learn about the dynamics of Saturn’s atmosphere. Scientists gain insight into the structure and gaseous composition of Saturn’s clouds via inspection of images such as these taken by the Hubble telescope. Over several wavelength bands, from infrared to ultraviolet, these images reveal the properties and sizes of aerosols in Saturn’s gaseous makeup. For example, smaller aerosols are visible only in the ultraviolet image, because they do not scatter or absorb visible or infrared light, which have longer wavelengths. By determining the characteristics of the atmosphere’s constituents, researchers can describe the dynamics of cloud formation. At certain visible and infrared wavelengths, light absorption by methane gas blocks all but the uppermost layers of Saturn’s atmosphere, which helps researchers discern clouds at different altitudes. In addition, when compared with images of Saturn from seasons past (1991 and 1995), this view of the planet also offers scientists a better comprehension of Saturn’s seasonal changes.

Original Source: Hubble News Release

Saturn’s Winds are Slowing Down

Image credit: NASA

When the Voyager spacecraft zipped past Saturn in 1980/81, they clocked the ringed planets equatorial winds at 1700 km/h. But a team of Spanish and American astronomers recently measured the motions of clouds and storms on Saturn using the Hubble Space Telescope and found they were only going 990 km/h. Although the equatorial winds have slowed down, other jets further away from the equator are still moving the same speed. This has led the astronomers to believe that the slow-down has something to do with the change of seasons on Saturn.

Saturn, one of the windiest planets, has recently had an unexpected and dramatic change in weather: its equatorial winds have subsided from a rapid 1700 km/hr during the Voyager spacecraft flybys in 1980-81 to a modest 990 km/hr from 1996 to 2002. This slow-down in the winds has been detected by a Spanish-American team of scientists, including Richard French of Wellesley College in Massachusetts, who report their findings in the June 5 issue of the journal, Nature. (5 June 2003, Vol. 423, pp. 623-625)

Using Hubble Space Telescope (HST) images of the ringed giant planet, the scientists (A. Sanchez-Lavega, S. Perez-Hoyos, J. F. Rojas, and R. Hueso from Universidad Pais Vasco in Bilbao, Spain, and French from Wellesley College), measured the motions of cloud features and storm systems on the ringed giant planet.

“One of the major mysteries in atmospheric sciences is why the giant planets Jupiter and Saturn — huge spheres composed mainly of hydrogen and helium — have an alternating pattern of east-west winds, which vary in direction with latitude,” explains French. “Unlike winds on terrestrial planets like Earth, which are powered primarily by sunlight, winds on the giant planets have an additional energy source in the heat that escapes from their deep interiors. Even though the strength of this interior heat is a mere fraction of the sunlight on Earth, the giant planets’ winds are ten times more intense than terrestrial winds.”

The role of these interior energy sources in sustaining these strong winds in giant planets and understanding why the maximum speed is reached at the equator constitute major challenges to theories of atmospheric motion in planets and stars.

There currently are two quite different explanations for the system of jets on giant planets. At one extreme, the winds are thought to extend very deep into the interior of the planet, tapping the heat released from the planet to drive their motions. At the other extreme, the atmospheric circulation is modelled as on the terrestrial planets, driven by the solar heat deposited in a shallow upper atmospheric layer. Both explanations have important drawbacks, and neither can account for the strong equatorial winds.

One way to test these models is to analyse the long-term behaviour of the winds by measuring their sensitivity to changes in the amount of sunlight due to seasonal effects or to other influences. Previous studies showed that Jupiter?s winds are quite stable, and not sensitive to seasonal changes, but little was known about Saturn, whose muted cloud features are much harder to measure.

Using the high-resolution capability of the Wide Field Planetary Camera onboard the HST, the Spanish-American team has been able to track enough cloud elements in Saturn to measure the wind velocity over a broad range of latitudes. The equatorial winds measured in 1996-2001 are only half as strong as was found in 1980-81, when the Voyager spacecraft visited the planet. In contrast, the windy jets far from the equator have remained stable and show a strong hemispheric symmetry not found in Jupiter.

The different behaviour of Saturn?s winds could have a simple explanation, note the scientists. The long seasonal cycle in Saturn?s atmosphere (one Saturn year is about thirty terrestrial years) and the equatorial shadowing by the planet?s giant rings could account for the sudden slowdown in the equatorial winds. Rather than being tied to the deep interior of Saturn, driven primarily by internal heat, the equatorial winds could be in part a shallow surface phenomenon, affected as well by seasonal variations in sunlight. In fact, Saturn?s equatorial region has been the location of giant storm systems, such as those seen in 1990 and 1994. These storms may have induced strong dynamical changes, perhaps resulting in the observed weakening of the equatorial winds.

Another possibility is that the winds measured by the team are at higher altitudes where the winds are likely to decrease in speed. In the Nature article, the team notes that Saturn?s non-equatorial winds have remained unchanged during this period, resembling Jupiter in this respect, which hints that these winds could be more deeply rooted.

New HST observations by the Spanish-American team are planned for the end of this year. The new data and the high-resolution imaging to be obtained by the NASA-ESA Cassini orbital mission expected to arrive at Saturn in mid-2004 will enable them and other scientists to learn whether the current wind pattern will persist or will change over the course of Saturn?s seasonal cycle. In either case, notes French, “these results will be important tests of our theoretical understanding of winds on the giant planets.”

Original Source: Wellesley College News Release

Saturn Obscured by the Moon on Wednesday

Astronomers will have a treat on Wednesday when the planet Saturn sneaks behind a quarter moon and then return approximately an hour or so later. This planetary eclipse is called an occultation, and it will be best viewed from North-eastern part of North America. The exact time of the occultation depends on your location, so follow the links to find the various times in different cities.