Tide out on Titan? A soft solid surface for Huygens (Forwarded)

ESA News
http://www.esa.int

30 November 2005

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.

Notes to editors:

This summary is based on a paper which appears on line in Nature, on 30
November 2005.

For more information:

John Zarnecki, PI Surface Science Package
Open University, Milton Keynes, UK
E-mail: J.C.Zarnecki @ open.ac.uk

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_...FSULWFE_0.html
* Titan's turbulence surprises scientists

http://www.esa.int/SPECIALS/Results_...KRULWFE_0.html
* Rain, winds and haze during the descent to Titan

http://www.esa.int/SPECIALS/Results_...3TULWFE_0.html
* First 'in situ' composition measurements made in Titan's atmosphere

http://www.esa.int/SPECIALS/Results_...1TULWFE_0.html
* Huygens 3D animation of Titan's surface
http://www.esa.int/SPECIALS/Cassini-...8G808BE_0.html
* Huygens landing site animation
http://www.lpl.arizona.edu/DISR/

Related links

* Mars Express instruments
http://www.esa.int/SPECIALS/Mars_Exp...C75V9ED_0.html
* Huygens instruments
http://www.esa.int/SPECIALS/Cassini-...W82VQUD_0.html
* Cassini instruments
http://www.esa.int/SPECIALS/Cassini-...182VQUD_0.html

[NOTE: An image supporting this release is available at
http://www.esa.int/SPECIALS/Results_...0TULWFE_1.html
]