Titan, ten months after the successful landing of the Huygens probe |
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Titan, ten months after the successful landing of the Huygens probe |
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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".
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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 |
DISR (Descent Imager / Spectral Radiometer) is the spectro-imaging instrument of Huygens (PI: Marty Tomasko, Univ. Arizona). The LESIA, a department of the Observatoire de Paris, delivered the detectors, electronics, and mechanical shutter of the infrared spectrometers. Throughout the descent and after landing, DISR recorded spectra and images of the atmosphere and surface (Tomasko et al. 2005). Spectral measurements from the visible to the infrared indicate that atmospheric aerosols extend continuously down do the surface, with particle number densities of a few tens per cm3. Produced by methane photochemistry, these particles are irregular and composed of several hundreds of 0.05-micron monomers. Assembling mosaics from images of the surface (visible below about 55 km) lead to the construction of a descent trajectory, allowing extraction of the wind profile. The winds are prograde (eastward) above 10 km, which confirms the super-rotation of the atmosphere predicted by general circulation models. Around 7 km, the wind reversed back to the west as the probe probably entered the more turbulent boundary layer. Although DISR did not directly image liquid bodies, there is compelling evidence for fluid flow (Figure 1). A bright terrain is cut by narrow channels that flow into a darker, lower-lying plain. Dentritic, deeply incised (50-100 m) channels likely imply methane rain. A network with short, stubby, and rectilinear channels may imply spring-fed flows. The landing site is reminiscent of a dry lakebed with rounded cobbles 10-15 cm in diameter, probably made of water ice, lying above a finer-grained substrate that looks like gravel. At an altitude of 700 m, DISR turned on a lamp to cast off the strong absorption of sunlight by atmospheric methane. This allowed us to measure the abundance of this gas in the lower atmosphere (5%) and spectrally analyze the surface (Figure 2). It is dark and reflects at most 15-20% of incident light at wavelengths around 830 nm. The visible spectrum is similar to those of tholins which are organic compounds synthesized in the laboratory. Further in the infrared, the reflectivity decreases with wavelength, unlike any organics measured in the laboratory. The absorption feature centered at 1540 nm can be attributed to water ice. The surface would then incorporate "dirty" ice, coated with photochemical particles, and mixed with an unidentified dark material.
| 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 |
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The Gas Chromatograph Mass Spectrometer (GCMS) is an instrument mainly built in the United States, with the participation of France, Germany and Austria. The Principal Investigator is Hasso Niemann (Goddard Space Flight Center, Greenbelt, Maryland) who had already built the spectrometer installed on board the Galileo probe on Jupiter. An example of mass spectrum measured by the GCMS (in this case on the surface of Titan) is shown in Figure 3.
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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 first spectacular result is the determination of the isotopic ratio 14N/15N in the molecular nitrogen N2, which is the principal constituent of the atmosphere of Titan. The value found, 0.67 times the terrestrial ratio, is interpreted as resulting from the preferential exhaust of 14N compared to 15N. On this basis, the models suggest that 2 to 5 times the initial nitrogen mass disappeared from the atmosphere of Titan since its formation, 4.5 billion years ago. The GCMS also measured the isotopic ratio 12C/13C and found it equal to 82.3 +-1, that is a little less than the terrestrial value of 90. This difference is not interpreted yet. In any case, it is certainly not due to a biological activity (as found on the Earth in the organics related to life) since then 12/C13C would have been higher than 90. Finally, the GCMS measured the isotopes 40Ar and 36Ar of argon. The detection of 40Ar, which comes from the radioactive decay of the potassium (40K) contained in silicates, implies a communication, at least episodical, between the interior of Titan and the atmosphere. 36Ar, though in very small quantity, was probably trapped in the ices contained in the planetesimals which formed Titan. 36Ar is primordial, since it was formed in the Sun.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 |
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The ensemble of available information at the present time on Titan (camera ISS, infra-red Spectrometre VIMS, Radar on board Cassini) suggests that the methane, which is destroyed by the solar radiation in a few tens of million years, is renewed continuously or episodically from the interior of Titan, where it is trapped with high pressure in a crystalline structure named chlarate hydrate, intensively studied in laboratory of geophysics, in particular by oil tankers. It is plausible that this methane was trapped initially in the primitive solar nebula in the ices which formed Titan.
The Huygens Atmospheric Structure Instrument (HASI) is a multi sensor package (accelerometers, thermometers, barometers and passive and active electrodes) which has been designed and built in Italy, UK, Finland, France, Spain and Austria. The goal of the HASI experiment was to measure the physical quantities characterizing Titan's atmosphere during the Huygens entry and descent phases and at the surface. Prof. Marcello Fulchignoni (LESIA, Observatoire de Paris/Université Denis Diderot-Paris 7) is the Principal Investigator of the experiment.
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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 |
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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.
Huygens observations revealed a satellite governed by Earth-like geophysical processes albeit with quite different chemistry. They provide some clues on the origin and evolution of Titan but several important questions remain unanswered. The Cassini spacecraft, currently orbiting Saturn, carries on the exploration of Titan to complement on a larger scale the data gathered by the Huygens mission.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)