Balloon-Borne Electromagnetic Sounding of the Lithospheric Thickness of Venus Robert E. Grimm Southwest Research Institute

Comparative Tectonics and Geodynamics of Venus, Earth, and Rocky Exoplanets May, 2015 1 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:Determineelectricalconductivity: Determine electrical conductivity structure of the interior.

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

• Auxiliary results: Electromagnetic environment, crustal

magnetism. 2 EM Sounding

Grant and West , 1968 •Determines electrical structure from Source idinducti ve response. – Is distinct from propagative methods (radar). – Natural or artificial sources. – Many techniques. – Skin Depth (km) = 0.5 /f = 0.5 T/ f=frequencyHz;T=periodsecf = frequency, Hz; T = period, sec

= resistivity, -m; = conductivity, S/m 3 Low-Frequency EM Spectrum

4 The Magnetotelluric Method

•Horizontal magnetic fields H are a measure of the total current I flowi ng i n th e ground . •Electric fields E are sensitive to conductivity and are measured as a voltage drop V. • Impedance of the ground Z = V/I = E/H –Measure orthogonal horizontal

components at surface, Ex/Hy and Condu ctor Ey/Hx –Convert impedance to apparent

resistivity a.

–Inversion a(f) (z)

5 Sample Terrestrial MT Inversions are not inherently nonunique,,p unlike potential fields. However, depth to conductors are better resolved than depth to resistors (ambiguity in conductivity-thickness product) Thickness of lithosphere is a well-posed problem km

1600 km MT 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 asthenosphere but at shallower depth (graphite?)

Subducted slab (suture zone) is imaged between double black lines. 6 More Terrestrial MT

MT mapping of lithospheric thickness in Europe ( Korja, 2007). Cross-Sections: Red = conductive; Blue = resistive . Map: Magenta = thick lithosphere, cyan = thin

7 EM Exploration Depths are Large On Venus

Smoothed Earth Layered Earth Model Venus L =100 km Model Conductivity-temperature Wet Dry relations for olivine as 0 Venus fifunction ofHf H2O content 50 L = 200 km 100 (Poe et al., 2010) Wet 150 Dry , km, “Wet” = 200 ppmH2O

h 200

250 Dept 300 Venus a 350 L = 400 km 400 1 2 3 4 5 6 Dry 3 Log , -m 10 Wet b 2.5 epth, kmepth, Exploration depth 100 km 2

achieved at ~10 Hz instead of Explor. D 1.5 10 g

0.01 Hz Lo 1 -6 -5 -4 -3 -2 -1 0 1 2 Log Freq., Hz 10 Lightning on Venus !

• Field -aligned , ci rcul arl y pol ari zed energy di scovered b y VEX (R ussell 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. –Extrapolated flash rate ~18/sec (20% Earth) •Implies presence of global Schumann resonances 10-30 Hz. –Transverse electromagg()netic (TEM) waves confined to atmosp heric waveg uide by conducting boundaries (ionosphere & ground) Detectable anywhere on the planet Properties of the Waveguide TM •TEM: half-wavelength > waveguide height • P = E x B TEM x z y •Finite boundary condiiiductivities cause leaky waveguide: small Ex appears. Apparent •Can show that TEM -i Resistivity Ionobase impedance at any altitude is a linear function of the m signed impedances of the Flight Altitude boundaries (or use square roots of apparent resistivity). Surface 0 +g 10 Aerial EM Simulation for Venus

1. Use mantle- Ra = 958 convection model to 0 1000 generate representative 2000 2D temperature variations (CITCOM: 0200040006000800010000 Newtonian temper- 800 1000 1200 1400 1600 ature dependent ABAmy Barr viit)iscosity) 2. Assign conductivity throughout the model domain using laboratory relations for dry and “wet” olivine. 3. Assign ionosphere a smoothly varying conductivity. 4. Numericallyyp compute EM fields in iono-atm-ground that result from a 10-Hz wave applied at LH boundary. 5. Assess recovery of ground conductivity (apparent resistivity). Aerial EM Simulation for Venus Dry = Solid; Wet = Dashed Inversion L=250kmL = 250 km Convection Models w/ Iono 0 dT/dz = 4.1 K/km

25 L = 360 km dT/dz = 2.6K/km 50 L = 760 km 75 dT/dz = 1.2 K/km

100

pth, km L, km True dT/dz Recovered e D Std Dev. dT/dz 125 Std. Dev

150 250 4.1 1.0 3.9 0.8 360 2.6 0.4 2.8 0.3 175 760 1.2 0.1 1.4 0.1

200 3 4 5 6 7 8 Fails for “Wet” Log Resistivity, -m 10 Lithosphere Implementation

Kerry Neil •Nominal Measurement: Horiz E and Horiz Keith Harrison B (Magnetotelluric method). • 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 surface of balloon.

Dan Durda magnetometer Summary •EM iffiiis an efficient way to prob bhe the interior of Venus from tens to hundreds of kilometers. –Single platform, ground contact not reqq,uired, no transmitter, ,p deepest penetration of any geophysical method except earthquake seismology. – SititSensitive to lithosph eri c thick ness •Requires programmatic intestinal fortitude – Terrestrial EM testing straightforward. –VEGA balloons successful 1985 – Longstanding JPL test program; ongoing engagement in Europe 15