balloon-borne ofelectromagnetic sounding of of venus · 2015. 5. 20. · balloon-borne...
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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
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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.
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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
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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)
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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.
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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
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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
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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).
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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).
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Aerial EM Simulation for Venus
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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
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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
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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