Titan, ten months after the successful landing of the Huygens probe
1er novembre 2005
On 30 November 2005, the journal Nature publishes on line articles presenting the first scientific results from the European Huygens probe, which landed on Titan on 14 January 2005. Huygens is part of the ESA/NASA Cassini-Huygens mission. The analyses of the data collected provide a wealth of unique information on the surface and atmosphere of Titan, revealing a complex and fascinating world. The Observatoire de Paris is deeply involved in this mission, with many scientists collaborating to four of the six instruments aboard and one "Interdisciplinary Scientist".
Figure 1 : Panoramic mosaic composed of images recorded by Huygens/DISR between altitudes of 17 and 8 km. Narrow channels cut a brighter terrain and flow in a lower-lying dark plain, possibly consisting of dry lakebeds. This is an Earth-like topography with evidence of prior fluid flow. The landing site is close to the center of the picture. Click on the image to enlarge it
Figure 2 : Surface reflectivity measured on the landing site with the DISR lamp turned on (red line). The visible portion of the spectrum is consistent with laboratory-produced tholins, thought to be analogs of Titan’s photochemical aerosols (black curves). Water ice is likely responsible for the absorption seen at 1500-1600 nm. The decrease of the reflectivity with wavelength beyond 830 nm is due to an unidentified material. Click on the image to enlarge it titan-fig2.gif
Figure 3 : Spectrogram recorded by the GCMS on the surface of Titan. The signal is plotted versus the ratio of mass m to charge z of the component considered. The GCMS ionizes the constituent, once, twice, etc ; (or possibly splits it). For example, N2, ionized once, is at 28. Ionized twice, it is at 14. Click on the image to enlarge it
A remarkable measurement is that of the variation with altitude, below 140 km height, of the abundance ratio between methane and nitrogen. Constant in the stratosphere of Titan, this ratio starts to grow in the troposphere below 32 km altitude up to 8 km, where it becomes constant until the surface. This behavior suggests that methane is saturated at 8 km, altitude where it could condense and form fog. A remarkable phenomenon was observed on the surface. Two minutes after the impact, the abundance ratio of methane increased abruptly by 40% (Figure 4). This is correlated with the increase in the inlet temperature of the GCMS (marked "inlet") whose radiation heats the surface (initially at -179°C) which thus degasses. The temperature of the inlet climbs up until 85°C. Other species degassed (Figure 4) : ethane, carbon dioxide, and most probably other hydrocarbons including benzene. It could be the index of the presence on the surface of much more complex organic compounds, responsible for the color of the dark material observed by DISR.
Figure 4 : Top : Emission on the surface of N2 (higher curve) and of CH4 (lower curve), versus time, in seconds. The moment of the impact is indicated by the vertical line. Bottom : Inlet temperature (inlet) of the GCMS versus time Click on the image to enlarge it titan-fig4.gif
Figure 5 : The profiles of temperature, pressure and density from the altitude of 1500 km down to the surface of the satellite have been obtained. In the high atmosphere, density and temperature are higher than expected. Several layers of temperature inversion testify both a strong stratification and a remarkable temporal variability of the atmosphere. In the low stratosphere and the troposphere the measures confirm the behaviour described by the existing models based on the measures done more than twenty years ago by Voyager 1. Click on the image to enlarge it
Figure 6 : During the descent (starting from an altitude of 150 km) positive and negative electrical charges have been detected : these measurements have been used to derive the electrical conductivity profile and to probe for the first time the lower ionospheric layer induced by cosmic rays. A conductivity peak has been found at about 60 km, even if the values are much lower than those of the Earth’s atmosphere conductivity. Click on the image to enlarge it titan-fig6.jpg
Figure 7 : The on board accelerometers recorded the Huygens probe impact with the Titan surface, giving some indication on the soil nature : the probe touched down on a solid surface, which has properties similar to wet sand. The temperature and pressure sensors continued to monitor the meteorological conditions for almost half an hour after impact, indicating a constant temperature of -180°C and a stable pressure of 1.47 atm. Click on the image to enlarge it
The data collected "in situ" by HASI are essential to the calibration of the measures carried out from the other instruments of the Huygens probe and constitute the "ground truth" for the observations carried out from the Cassini instruments, thus contributing in meaningful way to the Titan global knowledge.
References Tomasko et al. 2005 : Rain, winds and haze during the Huygens probe’s descent to Titan’s surface. Niemann et al. 2005 : The abundances of constituents of Titan’s atmosphere from the GCMS instrument on the Huygens probe. Fulchignoni et al. 2005 : In situ measurements of the physical characteristics of Titan’s environment. Nature (publications on line on 30 novembre, on paper on 8 decembre)
Contact DISR : Bruno Bézard (Observatoire de Paris, LESIA) HASI & SSP Marcello Fulchignoni (Observatoire de Paris, LESIA) GCMS Daniel Gautier (Observatoire de Paris, LESIA)
Dernière modification le 4 mars 2013