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Titan, ten months after the successful landing of the Huygens probe(Forwarded)



 
 
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Old December 2nd 05, 09:04 PM posted to sci.space.news
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Default Titan, ten months after the successful landing of the Huygens probe(Forwarded)

Observatoire de Paris
Paris, France

Contact:

DISR:
Bruno Bézard, Observatoire de Paris, LESIA
Tél: 33 1 45 07 77 17 Fax: 33 1 45 34 76 83

HASI & SSP:
Marcello Fulchignoni, Observatoire de Paris, LESIA
Tél: 33 1 45 07 75 39 Fax: 33 1 45 07 71 10

GCMS Daniel Gautier, Observatoire de Paris, LESIA
Tél: 33 1 45 07 77 07 Fax: 33 1 45 34 76 83

30 November 2005

Titan, ten months after the successful landing of the Huygens probe

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".

DISR

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.

GCMS

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.

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.

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.

HASI

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.

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.

Perspectives

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 November, on paper on 8 December)

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/...titan-fig1.jpg (299KB)]
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.

[Figure 2:
http://www.obspm.fr/actual/nouvelle/...titan-fig2.gif (26KB)]
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.

[Figure 3:
http://www.obspm.fr/actual/nouvelle/...titan-fig3.gif (48KB)]
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.

[Figure 4:
http://www.obspm.fr/actual/nouvelle/...titan-fig4.gif (34KB)]
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.

[Figure 5:
http://www.obspm.fr/actual/nouvelle/...titan-fig5.jpg (17KB)]
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.

[Figure 6:
http://www.obspm.fr/actual/nouvelle/...titan-fig6.jpg (32KB)]
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.

[Figure 7:
http://www.obspm.fr/actual/nouvelle/...titan-fig7.jpg (50KB)]
The on board accelerometers recorded the Huygens probe impact with the
Titan surface, giving some indication on the soil natu 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.


 




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