Is there lightning in Titan's atmosphere?
Despite the lack of evidence for lightning on Titan, many scientists consider it to be a strong possibility. Therefore, the European Space Agency's Huygens probe has been designed (and tested) to withstand lightning strikes as it descends through the Titan atmosphere.
"Lightning on Titan may be rare because of low solar input, low temperature, low gravity, etc., but nevertheless it may be possible," says Tetsuya Tokano, a scientist at the University of Cologne's Institute for Geophysics and Meteorology.
Developing models and improving prediction capabilities for lightning phenomena on Titan are important in light of the impact of lightning strikes on the Huygens probe. The scientists gathered at Nice also discussed the work in progress in this area of research.
There are basically two possible charging mechanisms on Titan that could lead to lightning strikes: charging by free electrons and ions, or charging by collision.
"Methane clouds are necessary for charging, and there is evidence for occasional clouds in the troposphere," Tokano said. "Once it is formed, a cloud rapidly attracts a large number of free electrons, which are abundant in Titan's troposphere. As a consequence, the negative space charge in the cloud may cause a cloud-to-ground lightning strike in Titan's lower troposphere."
"The collisional charging mechanism, on the other hand, appears to be less efficient since the charge transfer itself may be limited at Titan's cold temperatures and no substantial charge redistribution takes place in the cloud due to the weak updraft and gravitation."
Are aerosols on Titan sticky?
As part of the Cassini-Huygens mission to Saturn and Titan, the Huygens probe is set to sample aerosols as it descends through Titan's atmosphere. However, these aerosol particles may cover the surfaces of the detectors during the descent, preventing Huygens from achieving its purpose.
Whether this happens depends primarily on how sticky the aerosol particles are, which will in turn be a function of their age.
Dr Vladimir Dimitrov of the University of Tel Aviv presented a very interesting study in which he described how time affects hydrocarbon aerosols in Titan's atmosphere and what this implies for the Huygens mission.
"Aging and charging of aerosols are very favorable phenomena with respect to the functioning of the Huygens probe," said Dimitrov, "because they essentially weaken the possibility of damaging the detectors onboard the probe."
As a result of external factors, or spontaneously, an aerosol particle changes its properties over time, becoming much more inert, dense and hard -- and much less sticky.
Moreover, external radiation charges the aerosol particles, giving them the ability to capture external electrons.
"Altogether," concluded Dimitrov, "the combined effect of aging and charging at the altitude range where the Huygens probe will operate decreases the interference with the measurements by a factor of 50 to 100, and continuous trouble-free operation of Huygens is ensured."
We must still wait for the best close-up view
Meanwhile, with three years to go before Cassini-Huygens reaches Titan, future observations with increased spectral resolution and adaptive optics systems are important in order to prepare for the Cassini-Huygens observations.
"But we have to be patient," says Jean-Pierre Lebreton, ESA's Huygens project scientist. "We have to wait for Cassini-Huygens to be able to reveal the secrets of Titan hidden behind the thick orange haze curtain that shrouds the atmosphere."
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