Lightning Mixes a Dark and Stormy Brew at Saturn

The story of lightning in Saturn’s atmosphere has been building slowly. It first showed up years ago as electrostatic discharges lasting fractions of a second, heard as static and interference in the natural radio signals emitted by Saturn. The visible flash of lightning was harder to capture. Recently, Cassini scientists were excited to obtain long-sought photographic evidence of lightning in the atmosphere. Now, accumulating visible-light and infrared observations are providing fresh evidence that the dark clouds associated with Saturn’s thunderstorms contain dark substances such as soot and other carbon products that are forged when lightning strikes methane.

A presentation today by Kevin Baines, of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., and Mona Delitsky, of California Specialty Engineering, La Canada Flintridge, Calif., discussed the new results. They presented the findings at the American Astronomical Association’s Division of Planetary Sciences meeting in Pasadena.

The authors, who also contributed to a paper about the subject published at the end of last year, say that the effects of lightning on methane, which produces a rich collection of carbon substances, may explain the unusually dark spectral characteristics of some of Saturn’s clouds -- ones that have been observed in the vicinity of thunderstorms. Saturn’s lightning strikes are thought to be comparable in strength or slightly weaker than Earth lightning. They are sufficiently strong to cook up some interesting chemistry in Saturn’s clouds.

Scientists have seen that the clouds that form in the lightning region on Saturn appear to darken with time, creating circular dark spots in the atmosphere. The effect of sunlight on concentrations of atmospheric constituents might explain the darkening, but new analysis of visible-light and infrared observations indicates that these dark clouds represent a distinctly different type of material. Lightning’s effect on methane appears to create products such as graphite and carbon soot. Boosted to higher altitudes by the energy of thunderstorms, the dark material rises to higher levels in the atmosphere where it can be seen.

Lightning is thought to occur predominantly in regions of Saturn’s atmosphere where there is a strong up and down motion caused by moist cloud convection. Electrical charge may build up from friction between frozen and liquid particles that rise and fall in the convecting clouds.

The methane is cooked into carbon by lightning about 100 kilometers (62 miles) below the visible atmosphere. Other lightning-generated products become mixed with upper level condensates such as water and ammonium hydrosulfide. The resulting carbon-contaminated particles are icy, solid and unusually dark. The convection causes ammonia gas in the upper atmosphere above the lightning-generation level to rise and bright ammonia clouds become visible. This is followed by the appearance of the dark clouds that bring their cargo of carbon soot to the light of day to be viewed by Cassini.

To explain the dark clouds observed throughout the spectrum from the visible to near infrared observed by the visual and infrared mapping spectrometer (VIMS) onboard Cassini, VIMS team scientist Baines and chemist Delitsky analyzed results from a variety of laboratory spark experiments that tested how methane-laden chemistry like Saturn’s responds to the effect of lightning. They found that lightning discharges can reduce hydrocarbons such as methane to elemental carbon. Repeated electrostatic discharges into methane gas yield solid carbon, acetylene and other hydrocarbons. Temperatures inside a lightning strike on Saturn can reach over 30,000 Kelvin (53,540 degrees Fahrenheit), far hotter than needed to produce solid carbon from methane in Saturn’s atmosphere. That conversion can occur at temperatures as low as 2,000 Kelvin (3,140 degrees Fahrenheit).

The authors say that carbon fired deep inside Saturn may be the dominant darkener of the clouds found in and near Saturn’s thunderstorms. In addition, “lightning on Saturn will input large amounts of energy to a narrow column of atmosphere and generate products at high energies such as radicals and ions. After the column cools down, the new chemical species recombine and are frozen into a new chemical equilibrium which includes carbon soot but also incorporates a veritable soup of organic compounds and exotic species that include mixtures of sulfur, nitrogen, and phosphorus,” said Baines. Lightning, therefore, may kick-start a process that leads to increasingly complex chemistry in Saturn’s atmosphere.

The work directly addresses a Cassini mission objective to investigate the sources and nature of Saturn’s lightning and another to investigate the planet’s chemistry and dynamics, and is helping to characterize the dynamics that connect Saturn’s visible upper atmosphere with its deep interior. Ultimately, said Baines, “understanding more about lightning’s effects sheds light on the chemical processes at work in atmospheres that on some planetary bodies could lead to the origin of life.”

This Cassini Science League entry is an overview of a science paper authored, or co-authored, by at least one Cassini scientist. The information above was derived from or informed by the following publications:

1) “Storm clouds on Saturn: Lightning-induced chemistry and associated materials consistent with Cassini/VIMS spectra,” Kevin H. Baines (JPL); Mona L. Delitsky (California Specialty Engineering, La Canada Flintridge, Calif.); Thomas W. Momary, Robert H. Brown (University of Arizona, Tuscon); Bonnie J. Buratti (JPL); Roger N. Clark (U.S. Geological Survey, Denver, Colorado.); Philip D. Nicholson (Cornell University, Ithaca, N.Y.), Planetary and Space Science, Volume 57, Issues 14-15, December 2009, Pages 1650-1658

2) “Lightning-produced Carbon Species in the Atmosphere of Saturn,” abstract, Mona Delitsky (California Specialty Engineering, Flintridge, California) and K. H. Baines, JPL, presented at American Astronomical Society, Division of Planetary Sciences, 2010 Annual Meeting, October 2010, Pasadena, California

-- Mary Beth Murrill, Cassini science communication coordinator