Balloon-Borne ofElectromagnetic Sounding of of Venus · 2015. 5. 20. · Balloon-Borne ofElectromagnetic Sounding of of Venus GrimmRobert E . Grimm Institute Comp arative Tectonics
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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.
2
• 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
3
f = frequency, Hz; T = period, secU = resistivity, :-m; V = conductivity, S/m
Low-Frequency EM Spectrum
4
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
25
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
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