Inferring internal structures of solar system bodies from electromagnetic induction
Krishan Khurana
Institute of Geophysics and Planetary Physics and D t f E th d S
Dept. of Earth and Space Sciences, S i
UCLA, Los Angeles, CA, USA. kkhurana@igpp.ucla.edu,
Christopher T. Russell:
A C l b ti
A Celebration of a Life in Science
f Lif i S i
Honored and privileged to work with Chris Russell
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Have been a coauthor on 71 papers with Chris.
Venus
2
Earth
3
Jupiter 13
Io and torus 12
Europa
6
Ganymede 4
Callisto
3
Saturn
17
Enceladus
7
Tethys
1
Rhea
2
Titan
2
Instruments 3
Lunar Interior Magnetic Sounding
1
Hood et al. 1999, GRL
Hood et al. 1999
The principle behind electromagnetic induction
Eddy currents
BInduced(t)
BPrimary(t)
The primary and secondary fields shown separately
shown separately
The total field
–Eddy currents generate a secondary or induced field which reduces the primary field inside the conductor. –The induced field can be detected with a sensor.
5
For Galilean moons, Jupiter provides the primary field
• The Galilean satellites are located in the inner and middle magnetosphere iddl
t h
of Jupiter.
• Because the dipole and Because the dipole and
rotation axes of Jupiter are not aligned, the
are not aligned, the moons experience a varying field in their frame.
Galilean moons in Jupiter’s magnetosphere
Io Europa 1800‐2000
400‐500
2000
50
57 104 150‐340 145‐700
27‐53
76‐330 0.2‐0.4
0.1‐0.6 1.0‐2.1
0.2‐1.1 0.2‐0.4
0.1‐0.5
Ganymede Callisto 70‐140
5‐50 4
0.2
177 323 130‐1700 30‐6500 190‐1400 230‐4400 0.1‐1.1 0.02‐8.5
0.1‐0.8 0.03‐1.2
0.04‐0.7 0.02‐1.2
No shock forms upstream of the moons
B (nT)
‐3
p.s
p s (cm )
V (km/s)
VA (km/s)
CS (km/s) MA
Ms
Mms
7
The Galilean satellites viewed at different scales
Record of impact craters reveals the surface ages of solar system bodies
8
Sources of magnetic field near Io
•
Jupiter + its current sheet
– Obtained from Khurana (1997) magnetospheric model.
•
Plasma interaction currents
– Calculated from 3
Calculated from 3‐D
D MHD simulations
MHD simulations
•
Electromagnetic induction from a subsurface conductor.
conductor
– Obtained from 3 layer spherical shell models. •
Permanent internal field
– Obtained from modeling of residual field
9
Galileo Observations at Europa
•
Galileo Flew by Europa 12 times, out of which data for 3 passes were lost because of spacecraft malfunction. •
E4 and E14 passes showed signatures consistent with induced dipolar fields from currents
dipolar fields from currents flowing near the surface. The direction of the dipole moment was directed towards Europa in both cases (as expected)
both cases (as expected). •
E14
A subsequent pass (E26) confirmed that the dipole moment flipped in response to the different orientation of Jupiter’s field as expected from theory.
Khurana et al. 1998, Nature
Khurana et al. (1998, Nature)
Confirmation of Inductive response from Europa
100% response
Induced moment in a perfectly conducting sphere
Kivelson et al. 2000, Science
11
Galileo Observations at Callisto
During the C3 flyby, the magnetic field of Jupiter was di t d di ll
directed radially outward.
t
d
During the C9 flyby, the magnetic field of Jupiter was di t d di ll i
directed radially inwards.
d
The observed induction signature also showed opposite polarities.
l iti
This confirms that electromagnetic induction and not a permanent dipole is the t
t di l i th
source of the observed signature.
Khurana et al. (1998, Nature)
12
Ganymede: A Moon with Magnetism
(with thanks to Torrence Johnson)
The inductive response from Ganymede
Myo = 49 nT
82% response
100% response
Induced moment in a Induced
moment in a
metallic sphere
Kivelson et al. Icarus, 2002
Magnetospheric currents
Flow
x Flow
• Magnetopause current
• Tail current
Tail current
• Field‐aligned current
Jia et al. 2008
Current uncertainties and assumptions in our knowledge for Europa
• The work on inferring an ocean is solid.
• But:
– The thickness, depth and composition of the ocean cannot be constrained very well with the available data.
data
– ocean flow currents from electrodynamic induction??? y
y
Driven by tides? By thermal convection?
• Assumptions:
– the inducing signal has a single harmonic in it (at the rotation frequency of Jupiter).
– The ocean can be represented as a spherical shell.
16
Determining ocean properties from induction Hand and Chyba, Icarus, 2007
Hand and Chyba 2007
The induction response factor as a function of conductivity, ocean thickness and ice shell thickness
for the three layer model. Marked on the figure is the range of response factor deduced by Schilling
et al. (2004)
(
) (horizontal
(
dotted lines).
) The upper limit imposed on the conductivity of the solution
from saturation effects are marked by the two vertical lines. Figure reproduced from Hand and
Chyba (2007)
18
What would a Europa Orbiter measure?
•
•
•
•
By using the nominal orbit of Europa orbiter (polar orbit at an altitude of ~ 200 km), we h
have calculated the expected l l t d th
t d
field at the orbiter for three different scenarios. No ocean curves are black, a 3 km thick ocean ( = 2.75 S/m) km thick ocean (
= 2 75 S/m)
would produce the red signal and a 100 km thick ocean ( = 2.75 S/m) would produce the cyan signal. One can easily distinguish between the three cases.
Three periodicities are visible –
the 2 hour orbit period of the spacecraft, the 11.1 hr rotation period of Jupiter and the 85 hour orbital period of Europa.
19
Sounding at two multiple frequencies
Longer period waves penetrate deeper and are especially useful in studying the mantle and the core.
Khurana et al 2009
20
Electromagnetic induction from Io
Different sources of perturbations
•
Jupiter + its current sheet
– Obtained from Khurana (1997) magnetospheric model.
•
Plasma interaction currents
– Calculated from 3
Calculated from 3‐D
D MHD simulations
MHD simulations
•
Electromagnetic induction from a subsurface conductor.
conductor
– Obtained from 3 layer spherical shell models. •
Permanent internal field
– Obtained from modeling of residual field
21
I24 Data, MHD, solid mantle, Magma ocean
I24 magnetic
ti field
fi ld observed
b
d andd modeled
d l d
500
Bxx (nT)
400
300
200
100
0
-100
800
By (nT)
700
600
500
400
300
200
-1700
1700
Bz (nT)
-1800
-1900
-2000
-2100
2200
-2200
-2300
DOY: 284 04:20
1999-Oct-11
04:25
04:30
04:35
04:40
04:45
Melt fraction for three magma ocean models is 5%, 20% and 100%
Thickness = 50 km.
22
Khurana et al. 2011, Science
I27 Data, MHD, solid mantle, Magma ocean
400
Bx (nT)
200
0
-200
-400
-600
-800
1200
By (nT)
1000
800
Kh
Khurana et al. 2011, Science
t l 2011 S i
600
400
200
0
-1200
Bz (nT)
-1400
-1600
-1800
-2000
-2200
-2400
DOY: 53
2000-Feb-22
2000
Feb 22
13:40
13:45
13:50
13:55
Melt fraction for three magma ocean models is 5%, 20% and 100%
Khurana et al. 2011, Science
14:00
23
Model predicted dipole moment vs. observed
for all four flybys
Dipole moments Mx and My
Mx, My (modell)
M
200
0
-200
-600.00
-400.00
-200.00
0.00
200.00
Mx, My (observed)
400.00
600.00
Khurana et al. 2011, Science
24
Giant planets and their major icy satellites
25
What will keep Chris busy into his nineties?
Happy 70th Dear Chris.