Category: 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

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

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.