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Highlights of ESA’s Huygens mission

Published by Sigurd De Keyser on Wed Nov 30, 2005 10:06 pm
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ESA; board the NASA/ESA/ASI Cassini spacecraft, ESA’s Huygens probe was released on 25 December 2004. It reached the upper layer of Titan’s atmosphere on 14 January 2005 and landed on the surface after a parachute descent of 2 hours and 28 minutes.

Clear images of the surface were obtained below 40 km altitude – revealing an extraordinary world, resembling Earth in many respects, especially in meteorology, geomorphology and fluvial activity. The images show strong evidence for erosion due to liquid flows, possibly methane, on Titan.
The probe descended over the boundary between a bright icy terrain eroded by fluvial activity, and a darker area that looked like a dry river- or lakebed. Huygens landed in the dark area. Water-ice pebbles up to a few centimetres in diameter were scattered near the landing site, and the surface here was found to have the consistency of loose wet sand.

Winds were found to blow predominantly in the direction of Titan’s rotation, west to east winds, with speeds up to 450 km/h above an altitude of 120 km. The winds decreased with decreasing altitude and then and changed direction close to the surface. An unexpected layer of high wind-shear was encountered between altitudes of 100 and 60 km.

Huygens also surprised the scientists by finding a second lower ionospheric layer, between 140 km and 40 km, with electrical conductivity peaking near 60 km, and its instruments may also have recorded the signature of lightning.

‘Haze’ was detected all the way down to the surface, contrary to the predictions of pre-Huygens models. It was predicted that the atmosphere would be clear of ‘haze’ in the lower stratosphere, below around 60 km. Fortunately, the haze was transparent enough for good images of the surface to be obtained below 40 km.

Huygens enabled studies of the atmosphere and surface, including the first in-situ sampling of the organic chemistry and the aerosols below 150 km. These confirmed the presence of a complex organic chemistry in both the gas and the solid phase, which reinforces the idea that Titan is a promising place to observe chemical pathways involving molecules that may have been the building blocks of life on Earth.

Argon 40 was also detected at the surface and its presence indicates that Titan has experienced in the past, and is most likely still experiencing today, internal geological activity.

Titan’s turbulence surprises scientists

Strong turbulence in the upper atmosphere, a second ionospheric layer and possible lightning were among the surprises found by the Huygens Atmospheric Structure Instrument (HASI) during the descent to Titan’s surface.

HASI provided measurements from an altitude of 1400 km down to the surface of the physical characteristics of the atmosphere and surface, such as temperature and density profiles, electrical conductivity, and surface structure. The Huygens SSP made measurements just above and on the surface of Titan.
High-altitude atmospheric structure had been inferred from earlier solar occultation measurements by Voyager, but the middle atmosphere (200–600 km) was not well determined, although telescopic observations indicated a complex vertical structure.

Very little was known about the surface of Titan because it is hidden by a thick ‘haze’ – initial speculation was that the surface was covered by a deep hydrocarbon ocean, but infrared and radar measurements showed definite albedo contrasts —possibly consistent with lakes, but not with a global ocean.

Earlier observations showed that the surface pressure on Titan was comparable to that on Earth, and that methane formed a plausible counterpart to terrestrial water for cloud and rain formation. There was also speculation on the possibility of lightning occurring in Titan’s atmosphere that could affect the chemical composition of the atmosphere.

HASI found that in the upper part of the atmosphere, the temperature and density were both higher than expected. The temperature structure shows strong wave-like variations of 10-20 K about a mean of about 170 K. This, together with other evidence, indicates that Titan’s atmosphere has many different layers.

Models of Titan’s ionosphere predicted that galactic cosmic rays would produce an ionospheric layer with a maximum concentration of electrons between 70 and 90 km altitude. HASI also surprised the Huygens team by finding a second lower ionospheric layer, between 140 km and 40 km, with electrical conductivity peaking near 60 km.

HASI may also have seen the signature of lightning. Several electrical field impulse events were observed during the descent, caused by possible lightning activity in the spherical waveguide formed by the surface of Titan and the inner boundary of its ionosphere.

The vertical resolution of the temperature measurement was sufficient to resolve the structure of the planetary boundary layer. This boundary layer had a thickness of about 300 m at the place and time of landing. The surface temperature was accurately measured at 93.65±0.25 K and the pressure 1467±1 hPa (very close to measurements made earlier by Voyager, about 95K and 1400 hPa).

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.

Tide out on Titan? A soft solid surface for Huygens

The Surface Science Package (SSP) revealed that Huygens could have hit and cracked an ice ‘pebble’ on landing, and then it slumped into a sandy surface possibly dampened by liquid methane. Had the tide on Titan just gone out?

The SSP comprised nine independent sensors, chosen to cover the wide range of properties that be encountered, from liquids or very soft material to solid, hard ice. Some were designed primarily for landing on a solid surface and others for a liquid landing, with eight also operating during the descent.
Extreme and unexpected motion of Huygens at high altitudes was recorded by the SSP’s two-axis tilt sensor tilt sensor, suggesting strong turbulence whose meteorological origin remains unknown.

Penetrometry and accelerometry measurements on impact revealed that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol). Huygens landed on a relatively soft surface resembling wet clay, lightly packed snow and either wet or dry sand.

The probe had penetrated about 10 cm into surface, and settling gradually by a few millimetres after landing and tilting by a fraction of a degree. An initial high penetration force is best explained by the probe striking one of the many pebbles seen in the DISR images after landing.

Acoustic sounding with SSP over the last 90 m above the surface revealed a relatively smooth, but not completely flat, surface surrounding the landing site. The probe’s vertical velocity just before landing was determined with high precision as 4.6 m/s and the touchdown location had an undulating topography of around 1 metre over an area of 1000 sq. metres.

Those sensors intended to measure liquid properties (refractometer, permittivity and density sensors) would have performed correctly had the probe landed in liquid. The results from these sensors are still being analysed for indications of trace liquids, since the Huygens GCMS detected evaporating methane after touchdown.

Together with optical, radar and infrared spectrometer images from Cassini and images from the DISR instrument on Huygens, these results indicate a variety of possible processes modifying Titan’s surface.

Fluvial and marine processes appear most prominent at the Huygens landing site, although aeolian (wind-borne) activity cannot be ruled out. The SSP and HASI impact data are consistent with two plausible interpretations for the soft material: solid, granular material having a very small or zero cohesion, or a surface containing liquid.

In the latter case, the surface might be analogous to a wet sand or a textured tar/wet clay. The ‘sand’ could be made of ice grains from impact or fluvial erosion, wetted by liquid methane. Alternatively it might be a collection of photochemical products and fine-grained ice, making a somewhat sticky ‘tar’.

The uncertainties reflect the exotic nature of the materials comprising the solid surface and possible liquids in this extremely cold (–180 °C) environment.

First ‘in situ’ composition measurements made in Titan’s atmosphere

Unique results from the Aerosol Collector and Pyrolyser (ACP) and the Gas Chromatograph Mass Spectrometer (GCMS) have given scientists their first in situ chemical data on Titan’s atmosphere, including aerosols, chemical composition and isotopes.

Two of Titan’s key unknowns are the origin of the molecular nitrogen and methane in the atmosphere, and the mechanisms by which methane is maintained in the face of rapid destruction by photochemistry (chemical processes that are accompanied by or catalysed by the emission or absorption of visible or ultraviolet light).
The GCMS measured chemical composition and isotope abundances from 140 km altitude to the surface and confirmed the primary constituents were nitrogen and methane, and that the haze in the atmosphere is primarily methane.

From isotopic ratio measurements, the Huygens scientists obtained two key findings. The carbon isotope ratio (12C/13C) measured in methane suggests a continuous or periodic replenishment of methane in the atmosphere, but no evidence was found of active biological systems.

The nitrogen isotope ratio (14N/15N) suggests to the scientists that the early atmosphere of Titan was five times denser than it is now, and hence lost nitrogen to space.

Argon 36 was detected for the first time, but not xenon or krypton. However, the argon was found in low abundance, which is especially interesting because of the huge, nitrogen-dominated atmosphere and because about 50% of the mass of Titan is water ice, known to be a potentially efficient carrier of noble gases.

This low abundance implies the atmosphere was condensed or captured as ammonia, instead of nitrogen. The non-detection of the other noble gases, a surprising finding, will also fuel theories of the origin and evolution of Titan’s atmosphere.

The composition of surface vapours obtained by GCMS after landing shows that Huygens landed on a surface wet with methane, which evaporated as the cold soil was heated by the warmer probe. The surface was also rich in organic compounds not seen in the atmosphere, for example cyanogen and ethane, indicating a complex chemistry on Titan’s surface as well as in the atmosphere.

Argon 40 was also detected at the surface and its presence indicates that Titan has experienced in the past, and is most likely still experiencing today, internal geological activity.

Titan’s aerosols play an important role in determining atmospheric thermal structure, affecting the processes of radiative heating and cooling. They can help to create warm and cold layers that in turn contribute to circulation patterns and determine the strengths of winds.

The ACP obtained direct measurements of the chemical make-up of these aerosol particles. From an analysis of the products obtained by pyrolysis (chemical decomposition of organic materials by heating) of aerosols at 600°C, ammonia and hydrogen cyanide were the first molecules identified.

This is of prime importance because ammonia is not present as a gas in the atmosphere, hence the aerosols must include the results of chemical reactions that may have produced complex organic molecules. They are not simply condensates.

Aerosol particles may also act as condensation nuclei for cloud formation, and are the end-products of a complex organic chemistry which is important in astrobiology. Indeed, Titan offers the possibility to observe chemical pathways involving molecules that may have been the building blocks of life on Earth.

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