# Presentation: 794724

## Dunes on Titan

- Cassini CHARM Talk			27th February 2007
- Ralph D Lorenz
- Johns Hopkins University
- Applied Physics Laboratory
- Laurel, MD
- http://www.lpl.arizona.edu/~rlorenz

## out now!                                                                coming soon – end March 2007 

## HST 940nm Map (Smith et al., 1996) projected onto a Globe, viewed from 4 longitudes  (0,90, 180, 270)

- What makes dark areas dark ?

## Re-analysis of Voyager orange filter images shows weak (~5%) but consistent contrasts

- (Richardson et al., 2004)

- Resultant map compares well with HST and other near-IR maps. Human with red (ideally polarizing) glasses could see Titan’s surface

## What could be happening ?

## Dunes form by process called ‘saltation’ - particles jump/bounce along the ground driven by the wind. 

- Dunes form by process called ‘saltation’ - particles jump/bounce along the ground driven by the wind.
- Wind is compressed by topography - as dune grows, aerodynamic stress on crest of dune removes material faster than it is supplied : reach an equilibrium state with dune slowly moving forwards

## Calculations showed only weak winds (U* ~ UCd0.5 ~ 0.03 m/s) needed to lift particles in Titan’s thick atmosphere, low gravity environment. 

- Calculations showed only weak winds (U* ~ UCd0.5 ~ 0.03 m/s) needed to lift particles in Titan’s thick atmosphere, low gravity environment.
- But also showed (Lorenz et al., JGR, 1995) that thermally-driven winds are low anyway, so saltation seemed marginal. (Only the fastest winds drive saltation.Weibull distribution)

## T3 ‘cat scratches’

- Dark subparallel (E-W) streaks observed, apparent indicators of flow of something. No topographic signature.

## Aeolian features, like this terrestrial analog ?  But could cat-scratches just be some sort of liquid seeps?

## Correlation of ‘lobster’ cat-scratch region and optically-dark region (individual scratches not resolved in ISS) was noted after T3. This suggested that the optically-dark area Belet would be observed with T8 SAR

## Reprojected DISR mosaic by E. Karkoschka, LPL, UA

## Slide 12

## Slide 13

## Slide 14

## T8 SAR : not just dark streaks, but bright uprange edges indicating slopes

- 150x100km

##  Linear dunes in the Namib Desert 

- Shuttle handheld digital camera STS107

## Now, Namibia, desert streaming into ocean,    waves of bright sand diving into dunes of dark water    -visible rhythms of blue and brown,    sea and sand dance upon my strings.				Astronaut Story Musgrave 				9 Jun 1999

- Now, Namibia, desert streaming into ocean,    waves of bright sand diving into dunes of dark water    -visible rhythms of blue and brown,    sea and sand dance upon my strings.				Astronaut Story Musgrave 				9 Jun 1999

## Dune morphology is a function of variability in wind direction, and availability of sand.   Linear dunes require modest variability (often bidirectional) 

- Dune morphology is a function of variability in wind direction, and availability of sand.   Linear dunes require modest variability (often bidirectional)

- http://www.geog.ucl.ac.uk/~hmomiji/

## Linear dunes not common in the Americas, but dominant type globally (esp. Simpson, Arabian, Sahara, Namib..)

- Moenkopi Plateau, N. Arizona

## Earth Observation image (c) Terraserver, Inc of linear dunes in the Arabian desert (Yemen)

## Dunes ‘flow’ around hills (no ‘wake’) - depostional, longitudinal (linear) dunes 

- 200x70km

## T17 

## Gravitational Tide in Titan’s atmosphere

- see  Lorenz,  Symposium on Titan, Toulouse 1991
- and Tokano and Neubauer, Icarus, 2002

- What drives the wind ? What causes it to change ?  Seasons or tides?

## instantaneous wind and pressure field from Tokano & Neubauer

- Fluctuating windfield with meridional component, ~0.5m/s, as required for dunes
- orbit-averaged windfield gives net equatorward transport from 45 latitude....
- dunes at the poles ?

## Dune morphology is a function of variability in wind direction, and availability of sand.   Linear dunes require modest variability (often bidirectional) 

- Dune morphology is a function of variability in wind direction, and availability of sand.   Linear dunes require modest variability (often bidirectional)

- http://www.geog.ucl.ac.uk/~hmomiji/

## Dunes so far appear longitudinal everywhere, except in one spot (S of Adiri) where flow around apparent topographic rise may ‘straighten’ fluctuating flow - forming transverse dunes 

## Brightness varies with incidence angle

## Radarclinometry 

## Sand seas are remarkably flat (on scales >> those of the dunes..)

- http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1329.pdf

##  30x20km segment of SIR-C L-band  image of Namib desert acquired in 1994. Data Take 58.40 Processing ID 44419 

-  *30x20km segment of SIR-C L-band  image of Namib desert acquired in 1994. Data Take 58.40 Processing ID 44419*

-  *75x50 km segment of Cassini Ku-band SAR acquired October 2005 during T8 flyby over Belet*

- zoom to 30x20km

## SIR-C / Cassini comparison.    T3 morphology consistent with shallow (thinning) dunes - lower sand supply ?

- http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1249.pdf

## Tapering linear dunes form cluster of star dunes at apices.

## C

**B**

**A**

**C**

**C**

**B**

**A**

## Dunes cover about 40% of the tropics observed so far – extrapolation suggests around 20% of total surface is covered.  But why only in tropics?

- http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1412.pdf

## Sahara

- Arabian

## Belet : Malay afterworld reached by a flower-lined bridge 
Shangri-La : Tibetan mythical land of eternal youth. 
Senkyo : Japanese ideal realm of aloofness and serenity, freedom from wordly cares and death.
Fensal : In Norse mythology, magnificent mansion of Frigga, to which she invited all married couples who had led virtuous lives on Earth to enjoy each other's company forever. 

- Belet : Malay afterworld reached by a flower-lined bridge
- Shangri-La : Tibetan mythical land of eternal youth.
- Senkyo : Japanese ideal realm of aloofness and serenity, freedom from wordly cares and death.
- Fensal : In Norse mythology, magnificent mansion of Frigga, to which she invited all married couples who had led virtuous lives on Earth to enjoy each other's company forever.

## NB – for dunes, low latitudes must be usually (or at least sometimes) dry.   But we see channels not far from Huygens landing site, so the area is not always dry......   

- The global cycle then drives methane from equatorial and tropical to polar regions. _Methane is probably not stable in tropical regions on geological time scales,_

- NB – for dunes, low latitudes must be usually (or at least sometimes) dry.   But we see channels not far from Huygens landing site, so the area is not always dry......

## Sand Composition

- Dunes material is radar-dark. Emissive. Consistent with low dielectric constant. Probably not silicate rock.  Ice, organics?
- Material is optically dark - favors organics? VIMS sees specific signature
-  Material either has to be erodable into sand-sized grains, or can stick into sand-sized aggregates that are not readily destroyed.

## Implications - Sand Budget

- Belet dunefield (at least 200x1500km) with dune height of ~100m demands an origin for ~104-105 km3 of material
- Exceeds likely impact production (main source on Venus)
- Fluvial erosion, or photochemical production, seem most likely candidates
- (Formation time ~104 yr. Global transport time ~107 yr)

## Guabanito

- Xanadu

- Kerguelen Facula

- T13 RADAR in Shangri-La  - pronounced southward deviation of dunes

## Low-latitude part of T21 shows dunes with striking style of interaction with topography : apparent ‘blocking’. (transport direction appears generally to the east everywhere)

## Dune Orientations reveal global wind pattern 

- Jani Radebaugh at BYU has measured nearly 10,000 individual dunes!

- http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1412.pdf

## Hot-air balloon (‘Montgolfière’) can make many times round-the-world flights.

- Exploit (predictable?) tidal winds to direct drift...

## Because the tidal wind introduces a significant periodic meridional component to the wind, balloons do not drift just E-W, but sail across a band of latitudes. 

- Because the tidal wind introduces a significant periodic meridional component to the wind, balloons do not drift just E-W, but sail across a band of latitudes.
- Tokano and Lorenz, GCM Simulation of Balloon Trajectories on Titan, *Planetary and Space Science*, 2006

## Outlook

- This discovery opens a whole new world for aeolian studies
- Implications for sand supply, cohesion and windfield, supporting tidal wind model?
- (so far) high latitude dark regions have no dunes. Weaker tidal winds?  No sand left (but still dark)?.  Too damp for sand to move ?
- Dunes were instrumental in helping co-locate DISR and RADAR imagery

## Outlook (cont’d)

- Large areas of deep material with consistent fine-grained material makes useful ‘calibrator’ target area for remote sensing
- Dune patterns give us a way to map winds for future missions like a balloon
- Dunes the easiest place for a Titan lander ?