# Presentation: 065481

## The Secrets of Iapetus Research(Or: The Desperate Hunt for a Solution of One Mystery,With the Outcome of Ending up with Two)

- The Secrets of Iapetus Research(Or: The Desperate Hunt for a Solution of *One* Mystery,With the Outcome of Ending up with *Two*)

- Tilmann Denk
- (Freie Universität Berlin, Germany)
- in behalf of the Cassini Imaging Team
- CHARM telecon, JPL, Pasadena (CA)
- Start UTC: 2005-025T19:00 UTC (Jan 25)

## Content

- Introduction
- The hemispheric brightness dichotomy mystery and its various solution attempts in earlier centuries
- The Cassini ISS (imaging) planning effort
- Cassini arrives at Saturn
- Cassini senses Iapetus at close quarters
- The second mystery

## Introduction – The Speaker

- Name: Tilmann Denk
- Born: 1965  age = 39
- Education: Aerospace technology
- Employer: Freie Universität Berlin
- Cassini: Imaging team associate
- Profession: Icy satellite photographer

## Introduction – Who is Iapetus?

- A Saturnian satellite (3rd largest)
- Discovery: Jean-Dominique Cassini 1671/72
- Physics: *D* ~ 1440 km (hard to say exactly);                *ρ* = 1.1 g/cm3; *M* = 1.8*1021 kg
- Orbit: *a* = 3,560,800 km; *e* = 0.0284;     *i* = 7.57° (*tilt* = 14.84°; *P**node* = 3153.7 yrs);     *P* = 79.33 d (sync. rotation)
- Solar system size ranking (w/o TNOs): 20
- => General assessment: Very unusual!

## Iapetus‘ Hemispheric Brightness Dichotomy: Basics

- Discovered by J.-D. Cassini in 1671
- Dark leading side as dark as a fresh street
- Bright trailing side (almost) as bright as fresh snow
- Unexplained since >333 years
- Serious explanation attempts since early 1970‘s
- One of the oldest riddles of planetology(or even *the* oldest?)
- First imaged by Voyager 1 & 2 (1980/81)
- Dark hemisphere named „Cassini Regio“

## Slide 6

## Brightness Dichotomy:The Astronaut‘s Helmet

- Brightness Dichotomy:The Astronaut‘s Helmet

- helmet‘s hinge
- (also known as „the moat“)

## Idea # 1 (Soter 1974): Cassini Regio = Phoebe dust

- Iapetus‘ Hemispheric Brightness Dichotomy: Why is it?

## Idea # 1 (Soter 1974): Cassini Regio = Phoebe dust
Idea # 2 (Voyager 1980/81):Endogenic origin of dark material

- Idea # 2 (Voyager 1980/81):Endogenic origin of dark material

- Iapetus‘ Hemispheric Brightness Dichotomy: Why is it?

## Idea # 1 (Soter 1974): Cassini Regio = Phoebe dust
Idea # 2 (Voyager 1980/81):Endogenic origin of dark material
Idea # 3 (various authors): Phoebe or interplanetary dust just alters the leading side

- Idea # 2 (Voyager 1980/81):Endogenic origin of dark material
- Idea # 3 (various authors): Phoebe or interplanetary dust just *alters* the leading side

- Iapetus‘ Hemispheric Brightness Dichotomy: Why is it?

## Idea # 1 (Soter 1974): Cassini Regio = Phoebe dust
Idea # 2 (Voyager 1980/81):Endogenic origin of dark material
Idea # 3 (various authors): Phoebe or interplanetary dust just alters the leading side
Idea # 4 (various authors):Catastrophic collisions (IA, HY, outer sat., TI)

- Idea # 2 (Voyager 1980/81):Endogenic origin of dark material
- Idea # 3 (various authors): Phoebe or interplanetary dust just *alters* the leading side
- Idea # 4 (various authors):Catastrophic collisions (IA, HY, outer sat., TI)

- Iapetus‘ Hemispheric Brightness Dichotomy: Why is it?

## Albedo-dichotomy origin summary

- Possible classification of Iapetus global albedo dichotomy origin hypotheses:
- 1. Exogenic origin/ dust is coming in over long time
- (a) Dark grayish dust from Phoebe hits Iapetus's leading side and gets chemically altered/ reddened       (Soter 1974, Burns *et al.* 1979, 1996; Hamilton 1997)
- (b) Dark reddish dust from smaller retrograde outer saturnian satellites covers Iapetus's leading side (Buratti *et al.* 2002)
- 2. Asymmetric exogenic influence removes thin ice veneer from leading side,     but not from trailing side and poles
-   ***(a) Viewpoint of orbit mechanics:***
- (a1) Circumsaturnian dust is the cause (from Phoebe or other outer Saturnian satellites)        (Cruikshank *et al.* 1983; Bell *et al.* 1985; Buratti and Mosher 1995)
- (a2) Interplanetary micrometeoroid flux is the cause        (Cook and Franklin 1970; Squyres and Sagan 1983; Wilson and Sagan 1996)
-   ***(b) Viewpoint of physical processes on the surface:***
- (b1) Exposing of the dark subsurface layer by bright material erosion (Cook and Franklin 1970; Wilson and Sagan 1996)
- (b2) enrichment of formerly intimately mixed dark material due to sublimation of the bright ice component        (Cruikshank *et al.* 1983; Bell *et al.* 1985; Buratti and Mosher 1995)
- (b3) synthesis of the dark material by chemical/irradiation processes made possible by bright icy material erosion        (Squyres and Sagan 1983)
- 3. Exogenic origin/ dust and debris originates from a major single event
- (a) from collision with reddish Hyperion (Matthews 1992, Marchi *et al.* 2002)
- (b) from collision with Iapetus itself (Tabak and Young 1989)
- (c) from collision of an outer saturnian satellite with a heliocentric object with the result of a retrograde debris cloud      crossing Iapetus's orbit (Denk and Neukum 2000)
- (d) material comes from Titan (Owen *et al.* 2001)
- 4. Endogenic origin: Dark material from interior (Smith *et al.* 1981, 1982)

## May Cassini help?

- We are optimistic! 

## July/ August 2004: Leading side, southern hemisphere (up to 15 km/pxl)

- The Cassini ISS Planning Effort(for 2004)

## Cassini ISS Plans, Jul/Aug 2004

## Three giant impact basins (Ø 390 to 550 km)
Craters on the southern hemisphere
Craters within Cassini Regio

- Craters on the southern hemisphere
- Craters within Cassini Regio

- Jul/Aug 2004: Images & discoveries

## Jul/Aug 2004: Big impact structures

- Cassini Regio: ~390 km basin

- Trailing side: ~500 km basin

- Jul/Aug 2004: Big impact structures

## July/ August 2004: Leading side, southern hemisphere (up to 15 km/pxl)
October 2004: Leading side, anti-Saturn hemisphere (up to 6.7 km/pxl)And: Sub-Saturn bright terrain in ‚Saturn shine‘

- October 2004: Leading side, anti-Saturn hemisphere (up to 6.7 km/pxl)And: Sub-Saturn bright terrain in ‚Saturn shine‘

- The Cassini ISS Planning Effort(for 2004)

## Cassini ISS Plans, Oct 2004

- „Sun shine“ observations

- „Saturn shine“ observations

## Oct 2004: Images & discoveries

- Peculiar equatorial wispy streak in Cassini Regio
- 180°W meridian is a color boundary (confirmed)
- High „Voyager mountains“ confirmed
- ‚Saturn shine‘ images of „moat“

## Mountains

- Voyager 2

- Voyager 2

- Cassini

- Cassini

- Cassini

- Cassini

## Mountains

- Cassini

- Voyager 2

## Saturn-facing Hemisphere

- Cassini in October 2004
- in “Saturn shine”
- 9.3 km/pxl; 44° “phase”;
- sub-S/C lat/lon = 0°/316°

- Voyager 1 data (1980)

- Cassini observations (July 2004)

## July/ August 2004: Leading side, southern hemisphere (up to 15 km/pxl)
October 2004: Leading side, anti-Saturn hemisphere (up to 6.7 km/pxl)And: Sub-Saturn bright terrain in ‚Saturn shine‘
Christmas holidays 2004: OPNAVs of leading side (up to 4.3 km/pxl, low phase angle)

- October 2004: Leading side, anti-Saturn hemisphere (up to 6.7 km/pxl)And: Sub-Saturn bright terrain in ‚Saturn shine‘
- Christmas holidays 2004: OPNAVs of leading side (up to 4.3 km/pxl, low phase angle)

- The Cassini ISS Planning Effort(for 2004)

## Cassini ISS Plans, Dec 2004

## Dec 2004: Images & discoveries

- Ø ~ 400 km

- Ø ~ 550 km

- A forth large basin (Ø ~ 400 km)
- „Moat“ is a 250 km sized crater

## Stunning landform: The „bellyband“
~2000 km long ridge or chain of mountains
Exactly aligned along the equator
Up to 20 km high (depends on selection of reference radius)
Narrow peak, residing on a broader bulge

- ~2000 km long ridge or chain of mountains
- Exactly aligned along the equator
- Up to 20 km high (depends on selection of reference radius)
- Narrow peak, residing on a broader bulge

- Dec 2004: Images & discoveries

## Voyager images might show it as well...

## New-Year‘s Eve 2004/05: 2nd closest flyby of nominal mission: 123,400 km altitude = up to 740 m/pxl
A gift from the Tour Gods
8 ISS mosaics: from moderate 50° to high 140° phase angles; from 1 km/pxl to 740 m/pxl to 1.1 km/pxl

- A gift from the Tour Gods
- 8 ISS mosaics: from moderate 50° to high 140° phase angles; from 1 km/pxl to 740 m/pxl to 1.1 km/pxl

- The Cassini ISS Planning Effort(for 2004 + 2 days)

## First mosaic: ISS_00BIA_GLOBCOLA001_PRIME(2004-366T01:53 UTC; 00:42 hrs)

- First mosaic: ISS_00BIA_GLOBCOLA001_PRIME(2004-366T01:53 UTC; 00:42 hrs)

- Trigger #                               2157
- Telemetry Rate                          S&ER5a (40)
- number footprints                       6
- number of images                        **61**
- total Data Volume (predict)             282 Mb
- 2x3 clr (1x2 color) mosaic
- 170000 km alt.; **1.0 km/pxl** (NAC 1sum);
**d_IA=1410 pxl; 50 deg phase**;
- sub-S/C lat/lon = +29/70;
- sun illuminates surface from left
- Clear filters: 2 NAC shutters per footprint:
- 180 ms (bright) and 1.2 sec (dark terrain)
- 1x2 multi-color mosaic with substantial overlap: *Southern footprint (ftprt 1):* 12 col, 3 pol NACs; shutter times "optimized" for dark terrain.
*Northern footprint (ftprt 2):* 20 col, 6 pol NACs; shutter times "optimized" for bright terrain;
- 4 WAC IR polarizers.
- WAC 4-color context (Vio, Grn, Ir1, Ir3)

- north
- pole

## 4th mosaic: ISS_00CIA_LIMBTOPOD001_PRIME(2004-366T18:30 UTC; 00:30 hrs)  

- 4th mosaic: ISS_00CIA_LIMBTOPOD001_PRIME(2004-366T18:30 UTC; 00:30 hrs)

- Trigger #                               2160
- Telemetry Rate                          S&ER3 (24)
- number footprints                       7
- number of images                        **26**
- total Data Volume (predict)             92 Mb
- 2x3 NAC clr mosaic
- 123400 km alt.; **740 m/pxl** (NAC 1sum);
**d_IA=2000 pxl; 93 deg phase**;
- sub-S/C lat/lon = +63/27;
- sun illuminates surface from left
- 2 NAC shutters per footprint:
- 260 ms (bright) and 1.5 sec (dark terrain)
- Ftprt 1: 4-color NAC 2sum (getting mini-piece of trailing side)
- Last ftprt: graylight imaging
- (6 long exp NAC; 1sum and 2sum)
- WAC 4-color context (Vio, Grn, Ir1, Ir3)

## 7th mosaic: ISS_00CIA_GRAYLITEH001_PRIME(2005-001T04:00 UTC; 01:00 hrs) 

- 7th mosaic: ISS_00CIA_GRAYLITEH001_PRIME(2005-001T04:00 UTC; 01:00 hrs)

- Trigger #                               2165
- Telemetry Rate                          S&ER3 (24)
- number footprints                       13
- number of images                        **34**
- total Data Volume (predict)             123 Mb
- 3+3+2 NAC clr mosaic
- 136000 km alt.; **820 m/pxl** (NAC 1sum);
**d_IA=1760 pxl; 123 deg phase**;
- sub-S/C lat/lon = +60/319;
- sun illuminates surface from left (Saturn also)
- 2 NAC shutters per footprint:
- 320 ms (bright) and 2.0 sec (dark terrain)
- 1x4 NAC mosaic for graylight imaging
- (4 shutters/ftprt; 1sum and 2sum)
- WAC 4-color context (Vio, Grn, Ir1, Ir3)

## Huygens targeting: 16/17 Dec 2004

- Huygens release: 25 Dec 2004
- Cassini deflection: 28 Dec 2004
- MIA uplink: 29 Dec 2004
- Iapetus observations: 31 Dec 04 – 02 Jan 05

## ISS_00BIA_GLOBCOLA001 (1 km/pxl)

- ISS_00BIA_GLOBCOLA001 (1 km/pxl)

## The „bellyband“

## Craters at center of dark Cassini Regio

## „Landslide crater“ (Ø ~ 120 km)

- „Landslide crater“ (Ø ~ 120 km)

## Dark material streaksin transition zone

- Dark material streaksin transition zone

## Craters within dark terrain with north-facing bright walls

## „Saturn shine“ image (1.1 km/pxl) of trailing side

- „Saturn shine“ image (1.1 km/pxl) of trailing side

## Content

- Introduction
- The hemispheric brightness dichotomy mystery and its various solution attempts in earlier centuries
- The Cassini ISS (imaging) planning effort
- Cassini arrives at Saturn
- Cassini senses Iapetus at close quarters
- The second mystery

## New-Year‘s Eve flyby hope: Solve the brightness dichotomy origin riddle the brilliant idea is still missing

New-Year‘s Eve flyby discovery: The „bellyband“  is as unique and exciting and unexplained as the dichotomy the brilliant idea is also missing 

- New-Year‘s Eve flyby hope: Solve the brightness dichotomy origin riddle *the* brilliant idea is still missing
- New-Year‘s Eve flyby discovery: The „bellyband“  is as unique and exciting and unexplained as the dichotomy *the* brilliant idea is also missing 

- The Second Mystery

## The Final „Hope“: Sep 2007

**Planned Cassini flybys at Iapetus (selection)***
- Date                      min. altitude     best ISS pixel scale         visible hemisphere**
- -----------------------------------------------------------------------------------------------------------------------------
- Jul 2004                  2,500,000 km     15 km/pxl       CR, southern bright terrain, SP
- Oct 2004                 1,110,000 km       6.7 km/pxl    anti-Saturn side (western CR), SP
- 31Dec04/ 01Jan05	  123,400 km      740 m/pxl      northern CR, high northern latitudes
- -----------------------------------------------------------------------------------------------------------------------------
- Nov 2005	                 416,000 km       2.5 km/pxl     eastern and central CR, north. bright terrain
- Jan 2006                   879,000 km       5.3 km/pxl     eastern CR and trans. zone, "moat" partially
- Apr 2006                   603,000 km       3.6 km/pxl     leading side, SP
- Sep 2007                    ~1,270 km#      ~12 m/pxl     CR (high phase), trailing side (low phase)
- -----------------------------------------------------------------------------------------------------------------------------
- *...Included are all flybys in 2004 and all flybys below 106 km altitude until June 2008.
- **...CR = Cassini Regio (dark material hemisphere); SP = south pole.
- #...10 Sep 2007: targeted flyby altitude not fixed yet; might be between ~1000 and ~1400 km.

## ISS observations 10 Sep 2007

- (Current status of imaging plan at IA+00:30 to IA+03:00)

## Images from Cassini
(ISS Team home page):

http://www.ciclops.org
-------------
LPSC conference abstracts about Iapetus (early February):
http://www.lpi.usra.edu/meetings/lpsc2005/

- (ISS Team home page):
- _[http://www.ciclops.org](http://www.ciclops.org/)_
- -------------
- LPSC conference abstracts about Iapetus (early February):
- _[http://www.lpi.usra.edu/meetings/lpsc2005/](http://www.lpi.usra.edu/meetings/lpsc2005/)_