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.