Sky and Telescope - May 2012 - (Page 24)

Stormy Weather
0.01 0.06 0.1 0 66 90

Stratosphere Tropopause
Great White Spot

Stratospheric haze

Atmospheric pressure (bars)

Tropospheric haze
Upward velocity 150 meters/ second (335 mph)

Troposphere

Ammonia (NH3) clouds Ammonia hydrosulﬁde (NH4SH) clouds Water (H2O) clouds
S&T: LEAH TISCIONE / SOURCE: AGUSTÍN SÁNCHEZ-LAVEGA

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370

CLOUD STRUCTURE According to the author’s model, the 2010 Great White Spot originated as a thunderstorm in a water cloud deep below the visible layers of Saturn’s atmosphere. The storm ascended rapidly into the troposphere, where it could be easily spotted by amateur and professional astronomers.

changes in the stratospheric winds and in the abundances of some minor gases that were detected with infrared instruments on ground-based telescopes and Cassini.

A Giant Thunderstorm
All of the evidence suggests that the initial GWS was a giant thunderstorm about 100 times the size of typical Earth storms. For example, intense lightning activity within the storm produced the radio outbursts in the head detected by Cassini on December 5, 2010. My colleagues and I have developed computer models of the GWS phenomenon that try to reproduce the observations and also probe the dynamical structure of Saturn’s atmosphere below the upper ammonia cloud

S AT U R N ’ S S E A S O N S Saturn seasons are due to the 26.7° tilt of the planet’s rotational axis with respect to the orbital plane. But at tropical and equatorial latitudes, the ring shadowing of the upper atmosphere varies during Saturn's orbit, which ampliﬁes seasonal eﬀects. Measurements of Saturn’s eastwest zonal winds during the last Saturn year show that they remained stable despite the planet’s strong seasonal cycle.

Ultraviolet

Methane

PROBING THE DEPTHS Don Parker took these images on January 2, 2011. The GWS appeared bright through an ultraviolet ﬁlter but was barely visible in an infrared (methane) ﬁlter. Images such as these show conditions at diﬀerent altitudes, and support the idea that the original disturbance consisted of warm gas and ice crystals that ascended from deeper in the atmosphere.

layer. According to our models, Saturn’s 2010 GWS grew rapidly in brightness and area due to the formation and expansion of dense cumulus clouds of ammonia crystals — behavior somewhat like terrestrial thunderstorms. This is why we usually say that the GWS “erupts” in the planet. Accordingly, the storm clouds in the head resulted from hot, moist gas rising rapidly from Saturn’s deeper atmosphere. On Earth, the air is a mixture of nitrogen and oxygen, with water providing moisture. In contrast, Saturn’s drier atmosphere consists mainly of hydrogen and helium, with ammonia and water providing most of the moisture. Based on models of moist convection in Saturn’s atmosphere, we think that within the storm head, water-moist gas ascended from a depth of about 250 km below Saturn’s upper clouds at speeds of 150 meters per second (335 mph), about three times faster than the velocities typical of severe thunderstorms on Earth. High-resolution Cassini images of the GWS head showed abundant clusters of cumulonimbus (tall, vertical) clouds that appeared as a single compact bright spot in unresolved ground-based images. This observation supports our thunderstorm model. Another important conclusion from our models is that Saturn’s water abundance must have been high where the GWS erupted, about ﬁve times or more than the amount we expected from the Sun’s abundance of oxygen, which is taken as a reference for the giant planets because they

24 May 2012 sky & telescope

Depth (kilometers)

Table of Contents for the Digital Edition of Sky and Telescope - May 2012

Sky and Telescope - May 2012
Table of Contents
Spectrum
Letters
75, 50 & 25 Years Ago
News Notes
Saturn’s Raging Superstorm
Stars Above, Earth Below: Astronomy in National Parks
The Remarkable Case of Comet Lovejoy
In This Section
May’s Sky at a Glance
Binocular Highlight
Planetary Almanac
Northern Hemisphere’s Sky
Sun, Moon, and Planets
Celestial Calendar
Exploring the Solar System
Deep-Sky Wonders
Going Deep
S&T Test Report
New Product Showcase
Telescope Workshop
Planetary Imaging with Your DSLR Camera
Gallery
Focal Point

Sky and Telescope - May 2012