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?