Balloon-BorneElectromagnetic Sounding ofElectromagnetic Sounding of the Lithospheric Thickness of

VenusRobert E GrimmRobert E. Grimm

Southwest Research Institute

Comparative Tectonics and pGeodynamics of Venus, Earth,

and Rocky Exoplanets

1

y pMay, 2015

Electromagnetic Sounding of Venus

• Goal: Understand global geodynamics of Venus.

• Objective: Determine thickness of the thermal lithosphere and its geographic variations.– Complementary to / surrogate for heat flow.

• Investigation: Determine electrical conductivityInvestigation: Determine electrical conductivity structure of the interior.

• Measurements: Frequency dependent impedance• Measurements: Frequency-dependent impedance by the Magnetotelluric (MT) method.

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• Auxiliary results: Electromagnetic environment, crustal magnetism.

EM SoundingGrant and West 1968

• Determines electrical structure from i d i

Source

Grant and West, 1968

inductive response.– Is distinct from

tipropagativemethods (radar).Natural or artificial– Natural or artificial sources.Many techniques– Many techniques.

– Skin Depth (km) = 0.5 �U/f = 0.5 �T/Vf = frequency Hz; T = period sec

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f = frequency, Hz; T = period, secU = resistivity, :-m; V = conductivity, S/m

Low-Frequency EM Spectrum

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The Magnetotelluric Method

• Horizontal magnetic fields H are a measure of the total current Ifl i i h dflowing in the ground.

• Electric fields E are sensitive to conductivity and are measured asconductivity and are measured as a voltage drop V.

• Impedance of the ground Z = V/I

Cond ctor

= E/H– Measure orthogonal horizontal

components at surface, Ex/Hy and Conductory

Ey/Hx

– Convert impedance to apparent resistivity Ua.

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– Inversion Ua(f) �o U(z)

Sample Terrestrial MTInversions are not inherently nonunique, unlike potential fields.y q , p

However, depth to conductors are better resolved than depth to resistors (ambiguity in conductivity-thickness product)

km

oThickness of lithosphere is a well-posed problem

1600 kmMT profile across northwestern Canada (Jones et al., 2005).

Log resistivity scale: Red = 10 :-m (conductive), Blue = 104 :-m (resistive)

Major conductor at 50-200 km depth (outlined in black) tracks top of

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Major conductor at 50 200 km depth (outlined in black) tracks top of asthenosphere but at shallower depth (graphite?)

Subducted slab (suture zone) is imaged between double black lines.

More Terrestrial MTMT mapping of lithospheric thickness in Europe ( Korja, 2007).

Cross-Sections: Red = conductive; Blue resistiveBlue = resistive.

Map: Magenta = thick lithosphere, cyan = thin

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EM Exploration Depths are Large On Venus

Conductivity-temperature relations for olivine as f i f H O

0

Smoothed Earth Model

Layered Earth Model Venus L =100 km

Wet Dry

Venus function of H2O content (Poe et al., 2010)

“Wet” = 200 ppm H2O

50

100

150

200h, k

m

Venus L = 200 kmWetDry pp 2200

250

300

350

Dep

th

aVenus

L = 400 km

2.5

3

epth

, km b

1 2 3 4 5 6400

Log10

U, : -mDry

Wet

1.5

2g 10

E

xplo

r. D

Exploration depth 100 km achieved at ~10 Hz instead of

-6 -5 -4 -3 -2 -1 0 1 21

Log10

Freq., Hz

Log0.01 Hz

Lightning on Venus !

Fi ld li d i l l l i d di d b VEX (R ll l 2007)• Field-aligned, circularly polarized energy discovered by VEX (Russell et al., 2007) • Diagnostic signature of a whistler wave that is vertically refracted through

ionosphere as it traverses from below. • Whistler dispersion arises from impulsive source = lightning• Whistler dispersion arises from impulsive source = lightning.

– Extrapolated flash rate ~18/sec (20% Earth)• Implies presence of global Schumann resonances 10-30 Hz.

– Transverse electromagnetic (TEM) waves confined to atmospheric waveguide g ( ) p gby conducting boundaries (ionosphere & ground)

Detectable anywhere on the planet

Properties of the Waveguide

• TEM: half-wavelength > waveguide height

TM

waveguide height• Px = Ez x By

• Finite boundary d i i i

TEM

conductivities cause leaky waveguide: small Ex appears.

Ionobase

Apparent Resistivity-�Ui

�U

• Can show that TEM impedance at any altitude is a linear function of the �Um

Flight Altitude

signed impedances of the boundaries (or use square roots of

10Surface

0 +�Ug

( qapparent resistivity).

Aerial EM Simulation for Venus

Ra = 9580

1. Use mantle-convection model to

010002000

generate representative 2D temperature variations (CITCOM:0 2000 4000 6000 8000 10000

800 1000 1200 1400 1600

variations (CITCOM: Newtonian temper-ature dependent i it )A B viscosity)

2. Assign conductivity throughout the model domain using laboratory relations for dry and “wet” olivine

Amy Barr

relations for dry and wet olivine.3. Assign ionosphere a smoothly varying conductivity.4. Numerically compute EM fields in iono-atm-ground that result from ay p g

10-Hz wave applied at LH boundary.5. Assess recovery of ground conductivity (apparent resistivity).

Aerial EM Simulation for Venus

InversionDry = Solid; Wet = DashedL = 250 km

L = 360 km

L = 250 kmdT/dz = 4.1 K/km0

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Convection Models w/ Iono

L = 760 km

L 360 kmdT/dz = 2.6K/km

25

50

dT/dz = 1.2 K/km75

100

epth

, km

L, km True dT/dz Recovered

125

150

De

Std Dev. dT/dzStd. Dev

250 4.1 r 1.0 3.9 r 0.8

175

360 2.6 r 0.4 2.8 r 0.3760 1.2 r 0.1 1.4 r 0.1

3 4 5 6 7 8200

Log10 Resistivity, :-mFails for “Wet”

Lithosphere

Implementation• Nominal Measurement: Horiz E and Horiz

B (Magnetotelluric method).• Better Measurement: Horiz and Vertical E

Kerry Neil

Keith Harrison

Better Measurement: Horiz and Vertical E (Wave-Tilt Method)

z Electrode(difference with x-average)

+x Electrode–x Electrode

• Best Measurement: Attach large-area electrodes to inside

magnetometer

gsurface of balloon.

Dan Durda

Summaryi ffi i b h• EM is an efficient way to probe the

interior of Venus from tens to hundreds of kilometershundreds of kilometers.– Single platform, ground contact not

required, no transmitter, deepest q , , ppenetration of any geophysical method except earthquake seismology.S iti t lith h i thi k– Sensitive to lithospheric thickness

• Requires programmatic intestinal fortitudefortitude– Terrestrial EM testing straightforward.– VEGA balloons successful 1985

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– Longstanding JPL test program; ongoing engagement in Europe