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Tidal-Tectonic Processes and Their Implications for the Character of Europa’s Icy Crust. Greenberg, Geissler, Hoppa, and Tufts 2002. Life on Europa. Evolution and State of Europa Two Linked Concepts:. Tidal Heating. global scale. Cycloidal Ridges. Tidal Stresses. Chaotic Terrain. - PowerPoint PPT Presentation
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Tidal-Tectonic Processesand Their Implications for
the Character ofEuropa’s Icy Crust
Greenberg, Geissler, Hoppa, and Tufts
2002
Life on
Europa
Tidal Heating
Tidal Stresses
global scale
Chaotic Terrain
Cycloidal Ridges
Evolution and State of EuropaTwo Linked Concepts:
Europa, what a place … Life, huh?What do we have to think about to test this idea?
Why are the cracks dirty?
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Core
Silicate Mantle
Ocean
Crust
qc
qm
qo
qcr
Tc
Ts
€
+ ˙ E grad + ˙ E g
tidal
€
+ ˙ E gtidal?
qs ≈ 100 mW / m2
• SS heat transfer and crust thickness.
• Tidal heating ~ qcr
• Conduction v. convection in ice;
•thin ice (~10 km); Tidal stresses can break it.•thick ice (~25 km; Nimmo and Manga, 2002); Tidal stresses cannot break it.
A Heat Balance Favoring Life?
An ice thickness near Cilix that does not favor Life?
€
L flex ~D
Δρg
⎛
⎝ ⎜
⎞
⎠ ⎟
1/4
∝ dice
3/4 ≈ 25 km
Nimmo et al., 2003
To explore effects of tidally-driven (or any) dynamics, we need a geologic time scale…
Subjove hemisphere in natural color
• Stratigraphy gives relative ages; consistent with intermittent and periodic changes.
• Crater counts (and a cratering model) give an age (in principal!); There are not enough of them.
• Relaxation of topography around craters (with an ice model).
From: Prockter et al. (2002)
Stratigraphy:Crosscutting Relationships
PSRD Discoveries (http://www.psrd.hawaii.edu/)
Stratigraphy:Crosscutting “Lenticulae”
Manannán Crater very thin ice
Pwyll Craterthin ice
Cilix Craterthick ice
Craters. Not enough for statistics, but v. interesting!
We don’t know time very well but the geology permits us to certainly entertain the idea of periodic tidal
forcing acting over many length (and time?) scales.
Back to tides….
Tidal streses and energetics basics:what we have to think about
• The Tidal potential on Europa: a 4+ body problem; nonsynchronous orbit
• Tidal heating on Europa from dissipation (can only occur if e>0):
• Energy is extracted from the orbit(s), causing them to evolve:
• Ganymede, Europa and Io are in a 1:2:4 “Laplace resonance”: How is this maintained? What keeps Europa in a nonsynchronous orbit?
€
Ftide θ ,ϕ ,t( ) = ∇Vtidal = ∇Vbulge +∇Vradial +∇Vlibrational +∇Vnon-synch
€
˙ E diss =nEst
tot
Qs
; n = 2π /Ts
€
Est /cycle
librational , Est /cycle
radial ,...∝e 2m p
2
˜ μ sa6
€
˙ e s ≈ −˙ E
2es
nowE tot
orbit ; 0 < es <1
€
τ e,damp ∝Qs ˜ μ sa5 n
€
h = na2 1− e2
-h is fixed: If e goes down (circularize orbit) a must go up (satellite moves out)
Perijove
Apojove
P AEuropa’s Tides over 1 orbit: Fourier Components
Total Tide Total Tide
Total Tide Total Tide
PP
P P
CC C
Orbital Evolution: Io-Europa-Ganymede-Jupiter system
One Picture for the origin of Laplace Resonance (shown in the next movie): It’s because of Io (I don’t understand this).
1. Io moves outward and becomes tidally-locked with Jupiter. (Dissipation in Io results in a declining e)
2. Europa moves out, in turn.
3. Europa and Io become locked in a resonance and then the pair become locked with Ganymede
Question: Io is very active volcanically. This means Qmantle is changing on time scales of 106-108 years. If Q goes down e goes up and a must go down. How stable is this resonance?
Origin of Resonance
Cycloidal Cracking
Cycloidal Cracking
“Dawn” - crack opens perpendicular to tidal force, travels northeast
“Noon” - force rotates, crack travels west
“Dusk” - force rotations, crack travels southeast
“Night” – not enough stress to shear, crack stops
Next day: Repeat!
Formation
Global Lineaments
Conamararegion
Cadmus and Minos
1. Astypalaea Linea2. Thynia Linea3. Libya Linea4. Agenor Linea5. Cadmus Linea6. Minos Linea
Global Lineament Orientation
T TCC
Thin shell, Constant D
Strike Slip Faulting
Tidal Walking
SplittingRight Lateral Shear
CompressionLeft Lateral Shear
(Time-Dependent?) Ridge formation
Ridge Formation
Habital N
iches
Tides, water and life?