Andrew Yee
December 1st 05, 05:28 AM
ESA News
http://www.esa.int
30 November 2005
Rain, winds and haze during the descent to Titan
The high-resolution images taken in Titan's atmosphere by the Descent
Imager/Spectral Radiometer (DISR) were spectacular, but not the only
surprises obtained during descent. Both DISR and the Doppler Wind
Experiment data have given Huygens scientists much to think about.
The irreversible conversion of methane into other hydrocarbons in Titan's
stratosphere implies a surface or subsurface 'reservoir' of methane.
Although the NASA/ESA/ASI Cassini orbiter has not seen a global surface
reservoir, and DISR images do not show liquid hydrocarbon pools on the
surface either, this instrument's images do reveal the traces of flowing
liquid.
The DISR imagers provided views of Titan's previously unseen surface, thus
allowing a deeper understanding of the moon's geology. Surprisingly like
Earth, the brighter highland regions show complex systems draining into
flat, dark lowlands, possibly dry lake or river beds.
Images taken after landing in one of these lowland areas show more than 50
stones which vary between 3 mm and 15 cm in diameter. No rocks larger than
15 cm are seen. This size distribution suggests that rocks larger than 15
cm cannot be transported to the lakebed, while small pebbles (less than 5
cm) are quickly removed from the surface.
From these features, along with apparent 'ponds' and elongated 'islands'
oriented parallel to the 'coastline', the scientists can propose
explanations for the nature of the brightness variations spread throughout
the images.
They appear to be controlled by a flow of 'runny' liquids (consistent with
methane, ethane or both) down slopes, whether caused by precipitation or
springs.
The light*dark brightness difference can be explained by the 'irrigation'
of the bright terrain, with darker material being removed and carried into
the channels, which discharge into the region 'offshore', thereby
darkening it.
'Aeolian' (wind) processes, such as gusts, and Titan's low gravity may aid
this migration.
The surface science lamp worked exactly as planned, permitting surface
reflection measurements even in strong methane absorption bands.
Operations after landing included the collection of successive images as
well as spectral reflectance measurements of the surface illuminated by
the lamp from an assumed height of roughly 30 cm.
The infrared reflectance spectrum -- the rise and fall of brightness at
different wavelengths of light -- measured for the surface is unlike any
other in the Solar System. There are signs of organic materials such as
'tholins', and dips in the brightness consistent with water ice are also
seen. However, the most intriguing feature in the surface spectrum is an
infrared signature of a material not matched by any combination of spectra
of ices and complex organics found on Earth.
These spectra also show a methane abundance near the surface of 5 +/- 1%,
which is in precise agreement with the 4.9% in situ measurements made by
the probe's Gas Chromatograph Mass Spectrometer. The corresponding
relative humidity of methane is about 50%.
Therefore, the surface is not 'bone dry', but this does rule out extensive
ground fogs in the vicinity of the landing site caused by methane alone.
Taken together, these new observations make clearer the role of methane in
shaping the surface of Titan and how it is recycled into the atmosphere.
The substantial relative humidity of methane and the obvious evidence of
fluid flow on the surface provide evidence for methane 'rain' and
subsequent evaporation. Some hints of 'cryovolcanic' flows may also be
present in the images.
By assembling the panoramic mosaics, the Huygens scientists could
determine the descent trajectory as part of an iterative process of image
reconstruction. The trajectory could be used to derive the probe ground
track and see how wind speeds changed with altitude.
They found that the probe drifted steadily east-northeast due to Titan's
'prograde' (in the direction of rotation of the moon) winds. It slowed
from near 30 to 10 m/s between altitudes of 50 and 30 km and then slowed
more rapidly (from 10 to 4 m/s) between altitudes of 30 and 20 km.
The winds dropped to zero and reversed at around 7 km, near the expected
top of the planetary boundary layer, producing a west-northwestwardly
motion for about 1 km during the last 15 minutes of the descent.
The Doppler Wind Experiment (DWE) data which were obtained from two
Earth-based telescopes have confirmed the findings of the DISR and
provided a high-resolution vertical profile of Titan's winds.
The DWE not only confirmed the considerable turbulence above 120 km and
the eastward drift in prograde winds, but also the weak retrograde
(westward) winds near the surface.
Significantly, this experiment provided the first in situ confirmation of
Titan's 'superrotation' (the atmosphere is moving faster than the
surface). Unexpectedly, it also found a layer of very low wind velocity
between 60 and 100 km altitude, which is presently unexplained.
Notes to editors:
This summary is based on papers which appear on line in Nature, on 30
November 2005.
For more information:
Marti Tomasko, PI Descent Imager/Spectral Radiometer,
Lunar and Planetary Laboratory, Univ. of Arizona, USA
E-mail: mtomasko @ lpl.arizona.edu
Bruno Bezard, Observatoire de Paris-Meudon, France
E-mail: bruno.bezard @ obspm.fr
Michael Bird, PI for the Doppler Wind Experiment
Radioastronomisches Institut, Universitaet Bonn, Germany
E-mail: mbird @ astro.uni-bonn.de
Robindro Dutta-Roy
Radioastronomisches Institut, Universitaet Bonn, Germany
E-mail: duttaroy @ astro.uni-bonn.de
Jean-Pierre Lebreton, ESA Huygens Mission Manager
E-mail: jplebret @ rssd.esa.int
Related articles
* At Saturn and Titan
http://www.esa.int/SPECIALS/Cassini-Huygens/index.html
* Looking at Mars
http://www.esa.int/SPECIALS/Mars_Express/index.html
* Highlights of ESA's Huygens mission
http://www.esa.int/SPECIALS/Results_from_Mars_Express_and_Huygens/SEMTFSULWFE_0.html
* Titan's turbulence surprises scientists
http://www.esa.int/SPECIALS/Results_from_Mars_Express_and_Huygens/SEMLKRULWFE_0.html
* Tide out on Titan? A soft solid surface for Huygens
http://www.esa.int/SPECIALS/Results_from_Mars_Express_and_Huygens/SEM80TULWFE_0.html
* First 'in situ' composition measurements made in Titan's atmosphere
http://www.esa.int/SPECIALS/Results_from_Mars_Express_and_Huygens/SEMK1TULWFE_0.html
* Huygens 3D animation of Titan's surface
http://www.esa.int/SPECIALS/Cassini-Huygens/SEMO8G808BE_0.html
* Huygens landing site animation
http://www.lpl.arizona.edu/DISR/
Related links
* Mars Express instruments
http://www.esa.int/SPECIALS/Mars_Express/SEMUC75V9ED_0.html
* Huygens instruments
http://www.esa.int/SPECIALS/Cassini-Huygens/SEM9W82VQUD_0.html
* Cassini instruments
http://www.esa.int/SPECIALS/Cassini-Huygens/SEMY182VQUD_0.html
[NOTE: Images supporting this release are available at
http://www.esa.int/SPECIALS/Results_from_Mars_Express_and_Huygens/SEM23TULWFE_1.html
]
http://www.esa.int
30 November 2005
Rain, winds and haze during the descent to Titan
The high-resolution images taken in Titan's atmosphere by the Descent
Imager/Spectral Radiometer (DISR) were spectacular, but not the only
surprises obtained during descent. Both DISR and the Doppler Wind
Experiment data have given Huygens scientists much to think about.
The irreversible conversion of methane into other hydrocarbons in Titan's
stratosphere implies a surface or subsurface 'reservoir' of methane.
Although the NASA/ESA/ASI Cassini orbiter has not seen a global surface
reservoir, and DISR images do not show liquid hydrocarbon pools on the
surface either, this instrument's images do reveal the traces of flowing
liquid.
The DISR imagers provided views of Titan's previously unseen surface, thus
allowing a deeper understanding of the moon's geology. Surprisingly like
Earth, the brighter highland regions show complex systems draining into
flat, dark lowlands, possibly dry lake or river beds.
Images taken after landing in one of these lowland areas show more than 50
stones which vary between 3 mm and 15 cm in diameter. No rocks larger than
15 cm are seen. This size distribution suggests that rocks larger than 15
cm cannot be transported to the lakebed, while small pebbles (less than 5
cm) are quickly removed from the surface.
From these features, along with apparent 'ponds' and elongated 'islands'
oriented parallel to the 'coastline', the scientists can propose
explanations for the nature of the brightness variations spread throughout
the images.
They appear to be controlled by a flow of 'runny' liquids (consistent with
methane, ethane or both) down slopes, whether caused by precipitation or
springs.
The light*dark brightness difference can be explained by the 'irrigation'
of the bright terrain, with darker material being removed and carried into
the channels, which discharge into the region 'offshore', thereby
darkening it.
'Aeolian' (wind) processes, such as gusts, and Titan's low gravity may aid
this migration.
The surface science lamp worked exactly as planned, permitting surface
reflection measurements even in strong methane absorption bands.
Operations after landing included the collection of successive images as
well as spectral reflectance measurements of the surface illuminated by
the lamp from an assumed height of roughly 30 cm.
The infrared reflectance spectrum -- the rise and fall of brightness at
different wavelengths of light -- measured for the surface is unlike any
other in the Solar System. There are signs of organic materials such as
'tholins', and dips in the brightness consistent with water ice are also
seen. However, the most intriguing feature in the surface spectrum is an
infrared signature of a material not matched by any combination of spectra
of ices and complex organics found on Earth.
These spectra also show a methane abundance near the surface of 5 +/- 1%,
which is in precise agreement with the 4.9% in situ measurements made by
the probe's Gas Chromatograph Mass Spectrometer. The corresponding
relative humidity of methane is about 50%.
Therefore, the surface is not 'bone dry', but this does rule out extensive
ground fogs in the vicinity of the landing site caused by methane alone.
Taken together, these new observations make clearer the role of methane in
shaping the surface of Titan and how it is recycled into the atmosphere.
The substantial relative humidity of methane and the obvious evidence of
fluid flow on the surface provide evidence for methane 'rain' and
subsequent evaporation. Some hints of 'cryovolcanic' flows may also be
present in the images.
By assembling the panoramic mosaics, the Huygens scientists could
determine the descent trajectory as part of an iterative process of image
reconstruction. The trajectory could be used to derive the probe ground
track and see how wind speeds changed with altitude.
They found that the probe drifted steadily east-northeast due to Titan's
'prograde' (in the direction of rotation of the moon) winds. It slowed
from near 30 to 10 m/s between altitudes of 50 and 30 km and then slowed
more rapidly (from 10 to 4 m/s) between altitudes of 30 and 20 km.
The winds dropped to zero and reversed at around 7 km, near the expected
top of the planetary boundary layer, producing a west-northwestwardly
motion for about 1 km during the last 15 minutes of the descent.
The Doppler Wind Experiment (DWE) data which were obtained from two
Earth-based telescopes have confirmed the findings of the DISR and
provided a high-resolution vertical profile of Titan's winds.
The DWE not only confirmed the considerable turbulence above 120 km and
the eastward drift in prograde winds, but also the weak retrograde
(westward) winds near the surface.
Significantly, this experiment provided the first in situ confirmation of
Titan's 'superrotation' (the atmosphere is moving faster than the
surface). Unexpectedly, it also found a layer of very low wind velocity
between 60 and 100 km altitude, which is presently unexplained.
Notes to editors:
This summary is based on papers which appear on line in Nature, on 30
November 2005.
For more information:
Marti Tomasko, PI Descent Imager/Spectral Radiometer,
Lunar and Planetary Laboratory, Univ. of Arizona, USA
E-mail: mtomasko @ lpl.arizona.edu
Bruno Bezard, Observatoire de Paris-Meudon, France
E-mail: bruno.bezard @ obspm.fr
Michael Bird, PI for the Doppler Wind Experiment
Radioastronomisches Institut, Universitaet Bonn, Germany
E-mail: mbird @ astro.uni-bonn.de
Robindro Dutta-Roy
Radioastronomisches Institut, Universitaet Bonn, Germany
E-mail: duttaroy @ astro.uni-bonn.de
Jean-Pierre Lebreton, ESA Huygens Mission Manager
E-mail: jplebret @ rssd.esa.int
Related articles
* At Saturn and Titan
http://www.esa.int/SPECIALS/Cassini-Huygens/index.html
* Looking at Mars
http://www.esa.int/SPECIALS/Mars_Express/index.html
* Highlights of ESA's Huygens mission
http://www.esa.int/SPECIALS/Results_from_Mars_Express_and_Huygens/SEMTFSULWFE_0.html
* Titan's turbulence surprises scientists
http://www.esa.int/SPECIALS/Results_from_Mars_Express_and_Huygens/SEMLKRULWFE_0.html
* Tide out on Titan? A soft solid surface for Huygens
http://www.esa.int/SPECIALS/Results_from_Mars_Express_and_Huygens/SEM80TULWFE_0.html
* First 'in situ' composition measurements made in Titan's atmosphere
http://www.esa.int/SPECIALS/Results_from_Mars_Express_and_Huygens/SEMK1TULWFE_0.html
* Huygens 3D animation of Titan's surface
http://www.esa.int/SPECIALS/Cassini-Huygens/SEMO8G808BE_0.html
* Huygens landing site animation
http://www.lpl.arizona.edu/DISR/
Related links
* Mars Express instruments
http://www.esa.int/SPECIALS/Mars_Express/SEMUC75V9ED_0.html
* Huygens instruments
http://www.esa.int/SPECIALS/Cassini-Huygens/SEM9W82VQUD_0.html
* Cassini instruments
http://www.esa.int/SPECIALS/Cassini-Huygens/SEMY182VQUD_0.html
[NOTE: Images supporting this release are available at
http://www.esa.int/SPECIALS/Results_from_Mars_Express_and_Huygens/SEM23TULWFE_1.html
]