Venus Green, But Not with Envy

Physicsweb.org; Jan. 18, 2001; News: "Night-time on Venus" 

SRI International Jan. 18, 2001; Press Release: "SRI International Makes First Observation of Atomic Oxygen Emission in the Night Airglow of Venus"

Hubble Space Telescope Image

Oxygen atoms in Earth's atmosphere absorb solar radiation during the day and emit this energy at night at the green and red spectral emission lines of atomic oxygen. So it would not be surprising to think that Venus -- showered with almost twice as much solar energy due to its proximity to the Sun -- should also demonstrate this "nightglow" effect. Yet, the 1975 visits to Venus by Russian Venera 9 and 10 orbiters did not detect nightglow from atomic oxygen. The lack of oxygen emission spectra was attributed to the lack of free oxygen in a Venusian atmosphere that is 96% carbon dioxide.

A quarter-century down-the-road, Venus' enigmatic reputation has just been enhanced with recent Earth-based observation of the emissions not seen by the Russian vehicles. A team led by Tom Slanger, of the SRI International, examined the night-side of the planet with one of the largest telescope in the world: the 10-m Keck I telescope in Hawaii. The revolutionary Keck has instrumentation 100 times more sensitive than that used on the Venera probes. It was able to detect spectral lines of oxygen in the atmosphere of Venus that was 25 times greater than the upper limit deduced from the Russian spacecraft data.

Two spectral lines were studied: green light emitted when atomic oxygen drops from a high to a lower state, followed by red light emitted as the electrons go to a ground state. Though every green emission should be followed by a red emission, the green line was eight times stronger than the red and similar to Earth's auroral glow in intensity. The scientists believe the oxygen-poor atmosphere of Venus produces these emissions by the following process: energetic ultraviolet light on the dayside of Venus splits carbon dioxide into carbon monoxide and monatomic oxygen (O). Powerful winds sweep the oxygen atoms to the night side where they combine to form molecular oxygen (O₂) and emit the green spectral line.

The disparity between green and red emissions have been attributed to different conditions at various altitudes in the Venusian atmosphere, different processes at work that produce the emissions, or the need to re-evaluate how oxygen can be excited to emit radiation.

Galileo probe image

The Keck results lead to a reinterpretation of what the detection of oxygen emission lines in an atmosphere means. Since they can indicate the presence of atomic oxygen interactions, these emissions will no longer exclusively be associated with atmospheric molecular oxygen. This research may find application in the spectral examination of extra-solar planet atmospheres. In the future it may be possible to examine and interpret these emissions in light of the known interactions found in our solar system.

The team is trying to reconcile the Keck derived findings with the Venera orbiter observations. Speculation is that the difference may be attributed to the solar cycle, which was near peak when the Keck observations were made in November 1999. "We do not understand how the variability can be this large", said Slanger. [revised 12/20/2005: changed “Southwest Research Institute” to “SRI International”]

No Need for Lightning Insurance on Venus

Jet Propulsion Laboratory; Jan. 19, 2001; Press Release: "Cassini Scientists See No Sign of Lightning on Venus "

Cassini probe

The Saturn-bound Cassini probe made gravity assist flybys of Venus in 1998 and 1999. During this energy-boosting maneuver, the craft performed some radio-science experiments in the vicinity of Venus. The research, just published in the journal Nature, is important for what the radio experiments didn't detect: lightning.

University of Iowa physicist Donald Gurnett and seven co-authors write that Cassini's antennas did not detect any impulsive high-frequency (0.125 to 16 MHz) radio signals. Gurnett's group was looking for "sperics," signals that cause static on AM radio (0.54 to 1.70 MHz for U.S. commercial stations) during thunderstorms on Earth. As validation of Cassini's lightning detection capabilities, it detected continuous spherics at up to 70 impulses per second on its flyby of the Earth in August 1999.

Reasons for not detecting lightning on Venus range from atmospheric differences, to the Cassini instrumentation not being adequate for detecting the type of lightning that predominates on that planet.  For instance lightning on Earth is divided into two main types: intense cloud-to-ground and weaker cloud-to-cloud discharges. "Because clouds over Venus are at very high altitudes of 40 kilometers (25 miles) or more, it is likely that lightning at Venus, if it exists, is primarily cloud-to-cloud," explains Gurnett. Another type of electrical discharge on Earth is called a "sprite." This slow discharge emits low-frequency signals as it travels from clouds to the ionosphere above. If Venus has this type of electrical discharge it would be difficult to detect. Also radio signals below about 1 MHz cannot penetrate the Venusian ionosphere -- lightning below that frequency may not be detected.

Venera on Venus

"If lightning exists in the Venusian atmosphere, it is either extremely rare, or very different from terrestrial lightning," Gurnett says. Gurnett believes that Earth-like lightning "would have been easily detectable" by Cassini.

Controversial "discoveries" of lightning in the Venusian atmosphere date to 1978 when a Russian Venera lander detected low-frequency signals there. In 1979 a researcher thought NASA's Pioneer Venus spacecraft had returned data indicating lightning. As recently as 1990 the Galileo Jupiter-mission, in its gravity assist maneuver around Venus, detected some small signals that were interpreted as lightning. Even though the detection equipment was similar to Cassini's, Galileo was 60 times further from Venus -- so those results are considered less conclusive.

Venus' Ashen Face?

Skypub.com; Jan. 19, 2001; News: "Case for 'Ashen Light' Weakens" 

Adv. Space Res.; Vol. 10, No. 5, (1990); C. T. Russell and J. L. Phillips: "The Ashen Light"  

Galileo probe image

Recent findings concerning emission processes in the atmosphere of Venus may help resolve the question of a Venusian phenomena observed by Earth-based telescopes for over 300-years. This visual phenomenon is known as the "Ashen Light" -- a rare, faint glow on the night side of Venus. But there has always been a debate as to the validity of these observations and no agreement on what process can caused intense nightglow on Venus to be visible on Earth.

The case for the existence of the Ashen Light has been made many times. For instance a 1990 article by C. T. Russell and J. L. Phillips in Advances in Space Research points out that this Venusian glow has been spotted "on many hundreds of occasions" since Giovanni Riccioli first observed it on January 9, 1643. They site a 1969 study that tallies "129 sightings made between 1954 and 1962." At least once these lights were "observed simultaneously and independently by 2 professional astronomers" and on many occasions by amateur observers. To quote Russell and Phillips: "The fact that many totally independent observers have observed the Ashen Light simultaneously, that some of these simultaneous observations were made by professional astronomers and that the observations persist today with improved instrumentation suggest the phenomenon is real."

But the Sky and Telescope website reports that recent science casts doubt on any mechanism that could generate night side light on Venus visible to an observer on Earth. This is based on the study of faint emission lines from oxygen in the Venusian atmosphere by the Tom G. Slanger team and the search for lighting discharges by the Donald Gurnett team (above two NewsNotes).

Slanger's Keck telescope study indicates that the green airglow emissions caused by atomic oxygen chemistry on the dark side of the planet might be visible to an orbiting astronaut, but is too faint to be visually detected in the types of telescopes employed by Ashen Light observers.

The Gurnett team's conclusion that intense lightning strikes were not detected by two visits of the Cassini spacecraft seems to rule-out the theory that rapid-fire lighting strikes were the source of nightglow on Venus.

It may not be "case closed," but for now the case for the Ashen Lights looks very dim.  

Venus All Wet in the Past?

Washington University in St. Louis; Jan. 24, 2001; Press Release: "Study suggests Venus could have been wet planet"

The dry and hot-enough-to-melt-lead surface of Venus may have been wet with water in the past. Indirect evidence of this is the high ratio of heavy hydrogen (deuterium) to the more common lighter form of hydrogen in the Venusian atmosphere. This has been interpreted as the result of the lighter form of hydrogen escaping the planet and leaving high accumulations of deuterium. Hydrogen, of course, is one of the main ingredients of water. The other, oxygen, is locked-up in abundance by the planet's dense carbon dioxide atmosphere. But that evidence is not conclusive because we don't know the ratio of deuterium/hydrogen that the planet started out with.

Surface of Venus from Magellan probe radar mosaic

Experiments at Washington University, in St. Louis, on a mineral known as "tremolite" may lead to more definitive conclusions regarding Venus' water history. Tremolite is formed by lava and magma in the presence of water. Detecting tremolite on Venus would indicate a water presence in the planet's past. But according to accepted theory the mineral should decompose quickly at high temperatures -- erasing clues to the existence of water. Logically the super-high temperatures at the surface of Venus (the dense atmosphere won't allow for a cool down even at night) should have destroyed any tremolite long ago.

But researchers at the Planetary Chemistry Laboratory in Earth and Planetary Sciences, at Washington University, decided to determine the decay rate for tremolite, at various temperatures, to understand if it might be worth looking for on the planet. Graduate student Natasha M. Johnson and Professor Bruce Fegley, Jr., Ph.D., conducted over 200 experiments -- heating samples to temperatures considered hot by Venus standards, for periods as long as 20 months. By periodically weighing the samples they document the decay rate for the mineral. By conducting the high-temperature experiments, says Johnson: "Ours is the first study that investigates hydrous mineral decomposition rates with applications to Venus."

Pioneer-Venus orbited the planet

The experiments showed (surprise!) that a tremolite sample would take about 4-billion years to decay by half at the surface of Venus. This is much more stable than previously thought and now offers a "litmus test" for determining if water existed at all on the surface. Probes using infrared reflectance spectroscopy could be built-today to look for tremolite and related minerals on the Venusian surface.