Regarding the latest findings about another Titanian puzzle, its extremely low frequency (ELF) radio wave, captured by the Permittivity, Waves and Altimetry (PWA) sensor on the Huygens Atmosphere Structure Instrument (HASI) during its descent into the moon's ground, spacEurope counted with the precious help from Fernando Simões, CETP/IPSL-CNRS, France, and a member of the PWA team.
Let's read what he has to tell us.
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"The Permittivity, Waves and Altimetry (PWA) analyzer, a subunit of the Huygens Atmospheric Structure Instrument (HASI) detected an Extremely Low Frequency (ELF) electromagnetic signal during the descent of the Huygens probe through the atmosphere of Titan.
The electromagnetic signal has a narrow band at about 36 Hz.
The interpretation of the signal is not straightforward because several models have failed so far to explain the nature of this signal. There are three major scenarios for explaining the origin of this signal: electronics problem - malfunction of PWA or interference with other instruments; mechanical vibrations - interference with the booms that hold the electrodes of the dipole antenna, the parachutes or the Huygens craft; natural signal – resonance within the cavity of Titan.
The first scenario has been ruled out because the signal was observed neither during pre-flight testing nor during flight checkouts. Additional modes of PWA have functioned nominally and there is no obvious correlation with other instruments. Furthermore, the signal has never been observed on Earth during balloon campaigns aimed to simulate the descent on Titan.
The second scenario involves a “microphonic” effect induced by the vibrations of the antenna, parachutes, or the Huygens vessel. In fact, mechanical vibrations with a frequency close to the measured signal could induce such effect. For example, if the antenna has a natural resonance close to 36 Hz the effect could be explained. So far, this branch is still under active investigation and more reliable experiments at low temperature are foreseen. However, there is no clear evidence of such effect.
The last scenario involves an explanation related with the electromagnetic environment of Titan. On Earth, similar signals are related with the Schumann resonance, which is a natural phenomenon associated with the Earth cavity. When an electromagnetic wave propagates in the surface-ionosphere cavity of the Earth it is possible to reach a resonant state if the wavelength is an integer of cavity perimeter. Although it has a different nature, the effect resembles a sound box or a guitar. When the player forces the strings they vibrate at a natural frequency related with their physical properties, namely the length. Using this analogy we can say that Titan cavity is the guitar. The electromagnetic waves are the strings and the player, obviously, is the excitation source. On Earth, the source is lightning but the signal seems too strong to be explained by thus phenomenon. In fact, there is no definite evidence about lightning on Titan, and the signal is even stronger than on Earth. Therefore, we should look for another source and this is the major concern - that’s why scientists think it might be an artifact.
Now let’s assume the resonance is natural, even if we do not understand exactly what might produce such signal. It could be generated by an interaction between the ionosphere of Titan and Saturn magnetosphere and, actually, some people are studying this possibility. However, assuming the signal is natural and that it is a resonance in the cavity of Titan it is possible to study wave propagation in the cavity. How? On Earth, the surface is a good mirror and the waves are trapped between the surface and the ionosphere that acts as the other mirror, hence waves circle the planet. The situation is different on Titan because the surface is not a good mirror, which means waves can penetrate the soil and eventually are reflected below the surface. Theoretical models predict a subsurface water-ammonia ocean, which would work as a good reflector for these waves. Using the guitar analogy, the musician could play the instrument and, by sound listening, conclude whether there is an object inside. In fact, typical sound produced by strings would change if guitar cavity, the ‘sound box’, has an object inside. A professional player would easily identify the problem. That’s what I and my and colleagues are trying to do.
Of course, the situation it is much more difficult because PWA was not specially designed for such studies. Furthermore, they do not know exactly the properties of the guitar they are playing with… This idea provides an interesting way to investigate the subsurface of Titan but before they have to be sure that the signal is not an artifact. Then, using the same analogy, they have to find out whether they are dealing with a bad quality guitar or if really there is an object inside.
The presence of a subsurface ocean will also be assessed by the Cassini orbiter using a different, and more established, technique. Let’s kept tuned and see how Titan continues to surprise us."
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‘A new numerical model for the simulation of ELF wave propagation and the computation of eigenmodes in the atmosphere of Titan: did Huygens observe any Schumann Resonance?’, a paper by Fernando Simões et al., published in a special issue of the Planetary and Space Science magazine dedicated to Huygens results provides more information about the theme.
Let's read what he has to tell us.
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
"The Permittivity, Waves and Altimetry (PWA) analyzer, a subunit of the Huygens Atmospheric Structure Instrument (HASI) detected an Extremely Low Frequency (ELF) electromagnetic signal during the descent of the Huygens probe through the atmosphere of Titan.
The electromagnetic signal has a narrow band at about 36 Hz.
The interpretation of the signal is not straightforward because several models have failed so far to explain the nature of this signal. There are three major scenarios for explaining the origin of this signal: electronics problem - malfunction of PWA or interference with other instruments; mechanical vibrations - interference with the booms that hold the electrodes of the dipole antenna, the parachutes or the Huygens craft; natural signal – resonance within the cavity of Titan.
The first scenario has been ruled out because the signal was observed neither during pre-flight testing nor during flight checkouts. Additional modes of PWA have functioned nominally and there is no obvious correlation with other instruments. Furthermore, the signal has never been observed on Earth during balloon campaigns aimed to simulate the descent on Titan.
The second scenario involves a “microphonic” effect induced by the vibrations of the antenna, parachutes, or the Huygens vessel. In fact, mechanical vibrations with a frequency close to the measured signal could induce such effect. For example, if the antenna has a natural resonance close to 36 Hz the effect could be explained. So far, this branch is still under active investigation and more reliable experiments at low temperature are foreseen. However, there is no clear evidence of such effect.
The last scenario involves an explanation related with the electromagnetic environment of Titan. On Earth, similar signals are related with the Schumann resonance, which is a natural phenomenon associated with the Earth cavity. When an electromagnetic wave propagates in the surface-ionosphere cavity of the Earth it is possible to reach a resonant state if the wavelength is an integer of cavity perimeter. Although it has a different nature, the effect resembles a sound box or a guitar. When the player forces the strings they vibrate at a natural frequency related with their physical properties, namely the length. Using this analogy we can say that Titan cavity is the guitar. The electromagnetic waves are the strings and the player, obviously, is the excitation source. On Earth, the source is lightning but the signal seems too strong to be explained by thus phenomenon. In fact, there is no definite evidence about lightning on Titan, and the signal is even stronger than on Earth. Therefore, we should look for another source and this is the major concern - that’s why scientists think it might be an artifact.
Now let’s assume the resonance is natural, even if we do not understand exactly what might produce such signal. It could be generated by an interaction between the ionosphere of Titan and Saturn magnetosphere and, actually, some people are studying this possibility. However, assuming the signal is natural and that it is a resonance in the cavity of Titan it is possible to study wave propagation in the cavity. How? On Earth, the surface is a good mirror and the waves are trapped between the surface and the ionosphere that acts as the other mirror, hence waves circle the planet. The situation is different on Titan because the surface is not a good mirror, which means waves can penetrate the soil and eventually are reflected below the surface. Theoretical models predict a subsurface water-ammonia ocean, which would work as a good reflector for these waves. Using the guitar analogy, the musician could play the instrument and, by sound listening, conclude whether there is an object inside. In fact, typical sound produced by strings would change if guitar cavity, the ‘sound box’, has an object inside. A professional player would easily identify the problem. That’s what I and my and colleagues are trying to do.
Of course, the situation it is much more difficult because PWA was not specially designed for such studies. Furthermore, they do not know exactly the properties of the guitar they are playing with… This idea provides an interesting way to investigate the subsurface of Titan but before they have to be sure that the signal is not an artifact. Then, using the same analogy, they have to find out whether they are dealing with a bad quality guitar or if really there is an object inside.
The presence of a subsurface ocean will also be assessed by the Cassini orbiter using a different, and more established, technique. Let’s kept tuned and see how Titan continues to surprise us."
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
‘A new numerical model for the simulation of ELF wave propagation and the computation of eigenmodes in the atmosphere of Titan: did Huygens observe any Schumann Resonance?’, a paper by Fernando Simões et al., published in a special issue of the Planetary and Space Science magazine dedicated to Huygens results provides more information about the theme.
