The 10 most important scientific discoveries done by the Huygens probe on Titan

On 14 January 2005, 12 years ago, ESA’s Huygens probe landing rewrote the history of space exploration on moon of Saturn. In the incredible descent 2 hours and 27 minutes, and in the 72 minutes that followed before the end of the batteries, the probe picked up a treasure trove of data. Here are his most important achievements.

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Study of the Titan atmosphere.
Before the arrival of Huygens on Titan, scientists knew that the satellite’s atmosphere is made up largely of nitrogen, with methane percentages. But not so much was known about the conditions of pressure and temperature at different altitudes. During the descent, the on-board instrument HASI (Huygens Atmospheric Structure Instrument) has determined the conditions of temperature, atmospheric density and pressure from 1400 km altitude, until the surface of Titan. In the picture below Titan’s atmosphere at varies heights from 5 different made by Huygens in the direction of the 4 cardinal points.

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<2/10> The super winds of Titan.
During the descent, Huygens probe has been moved by winds of 165.8 km from the surface of Titan. Because of turbulent atmosphere, with gales which confirmed a thesis of scientists, called “super rotation” of Titan, with the atmosphere of the moon moves faster than its surface. But there’s more: at high altitude winds moving in the same direction as the rotation of the moon and tend to decrease towards the surface. In the upper atmosphere, at 120 km altitude, blowing at 120 m / s (430 km / h). During the Huygens descent have been recorded winds of 108 km / h at 55 km altitude, down to 36 km / h at 30 km of altitude and 14 km / h at 20 km altitude. At certain point, the winds ceased and then have reversed their motion at 7 km altitude, giving rise to a movement in the opposite direction in the last 15 minutes of descent.

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<3/10> The mystery of the origin of methane.
Two of the major open questions on Titan concern the origin of methane present in its atmosphere, and the persistence of it (since the light from the Sun periodically destroys methane emissions). In his descent to Titan, Huygens has observed a steady increase of the gas, increased by 40% after landing. Further analysis clarified that methane does not come from superficial microorganisms but, perhaps, by liquid reserves located beneath the icy surface of the satellite. In the picture below are depicted as they might be underground hydrocarbon reserves.

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<4/10> The origin of the atmosphere of nitrogen.
Earth and Titan are the only two objects in our Solar System to have a nitrogen atmosphere. The discovery is not new: even the Voyager spacecraft had recorded the presence of nitrogen. But thanks to instruments inside Hugens was possible to have direct confirmation. But the data collected by the European probe have been really important to understand what may be the source of Titan’s nitrogen atmosphere: it may have been produced by violent meteorite impacts that have dissolved the ammonia ice on the surface, maybe about four billion years ago. In the image, how Titan changes with the seasons.

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<5/10> Radioactive decay. The mass spectrometer of Huygens has discovered traces of radiogenic argon below 18 km of altitude. This gas originates solely from decay of potassium-40, a radioactive isotope that is contained in rocks. The only possible presence of the rocks on Titan could be underground the ocean of hydrocarbons and the crust of ice. Because the emission duration of potassium-40 is 1.3 billion years old, much less the age of Titan, the analysis of this isotope in the atmosphere provides important data on the history of volcanism of this satellite.
In the photo is depicted the region called Sotra Facula reconstructed on basis of data collected by the Cassini spacecraft during its mission on Titan.

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<6/10> The analysis of the composition of Titan’s fog. Until the descent of Huygens, no one could know for sure if the fog that shrouds Titan arrived or not up to its surface (then confirmed by the lander). The Huygens Spectral Radiometer (DISR) has also identified differences in the composition of the mist at different levels, showing that it assumes a lighter color and a coarser composition closer to the surface, and that is dissipated – not entirely – only at 30 km height. In the photo, 6 stereographic images taken by Huygens during its descent show the fog on Titan at different heights.

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<7/10> The measurement of the chemical composition of the aerosols. The analysis of two samples of atmospheric particles collected on Titan at 130-135 and 20-25 km altitude has shown that the major constituents of its aerosol are nitrogen and carbon for both altitudes. These substances have probably originated in the upper atmosphere of the satellite, where sunlight alters photochemically the gases like methane. In the picture below, how the aerosols form the fog of Titan.


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<8/10> The presence of rivers and lakes dried up. Hidden by a thick blanket of fog, Titan’s surface has remained mysterious until the instruments of Huygens (Descent Imager/Spectral Radiometer), did not record his first spectacular images during the descent. The images have revealed a plateau furrowed by a large number of channels, similar to terrestrial rivers, with steep valleys and deep canyons up to 100 m. The rivers seem to merge into a vast dark expanse, periodically flooded with methane and ethane. Nevertheless, at the moment of landing, the dark region appeared as the empty bed of a huge lake. In the photo, the most beautiful image captured by Huygens on the ground.

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<9/10> The hypothesis of the presence of an underground ocean.
One of instruments of Huygens deputy to the detection of extremely low frequency radio signals (Schumann resonances) linkable to atmospheric lightning, has identified an interesting fact. Although not observed on Titan, lightning similar to those on Earth, the altimeter has detected abnormal signals at a height between 140 and 40 km from the surface of the satellite. The scientists have hypothesized,that the satellite’s atmosphere acts like a giant electric circuit, in which the two poles are, on the one hand, the ionospheric current of Titan excited by Saturn’s magnetosphere; on the other hand, a large submerged ocean made up of water and ammonia at 55-80 km from a crust of ice.

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<10/10> The discovery of dunes that “disappear” from the radar. With big surprise, the engineers at NASA’s mission met many difficulties in locating the Huygens landing site on radar maps provided by Cassini. This is because the landing site of the lander turned out to be an expanse of ice covered by deposits of organic material (ie carbon-based) invisible to radar. Locate Huygens was made possible thanks to the presence of two large dark dunes about 30 km from the site, visible to all instruments. The dunes are formed perhaps nitrile particles and similar hydrocarbon sand grains, the size of 100-300 microns. Titan: a frozen world, but very dynamic. In the picture, the landing site of probe, reconstructed with a mosaic of several images.



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