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Comparative Planetology
  Overview
 The
Earth as a Template
  Understanding
  The
Outer Planets
  Jupiter,
  The
the governing parameters
Saturn, Neptune & Uranus
Terrestrial Planets
  Mercury,
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Venus, Mars
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What is Comparative Planetology?
to
Understanding the Governing Parameters:
The known unknowns:
Atmosphere, Ionosphere, Magnetosphere,
Composition, Solar Radiation Flux, Solar Wind
Are there unknown unknowns?
…
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Comparative Planetology
The Earth as a Template:
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Comparative Planetology: Earth Template
Parameter Space:
Size
Magnetic Field (dipolar)
Solar Wind (variable)
Ionosphere (conductivity)
~ 6378 km
~ 31,000 nT
~ 450 km/s , 8 cm-3, 6 nT
Variable: latitude,
direction, and
day/night
Dynamics:
Sub-storms and Geomagnetic Storms -Generally governed by the Solar Wind and
perturbations in upstream conditions (consider
dynamic pressure changes and magnetic field
orientation/strength, Mach number of the flow)
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Comparative Planetology
Magnetospheres of the Outer Planets (MOP):
MOP: - can learn about them by comparing with Earth (which we
know a lot better)
- can learn even more about the universe in general because
there are very important differences between Earth and MOP.
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Comparative Planetology
MOP -- Overview
Intrinsic Magnetic Field Strengths Vary
Planet
Earth
Jupiter
Saturn
Uranus
Neptune
Distance (AU)
1.0
5.2
9.5
19.2
30.1
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Mag Mom (ME)
1
20,000
580
49
27
Radius / RE
1.0
11.0
9.5
4.2
3.9
surface B (T)
3.1E-05
0.00043
2.1E-05
2.1E-05
1.4E-05
magnetopause L
11
45
21
27
26
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Jupiter
Knowns:
R ~ 71,500 km
B ~ 430,000 nT
SW ~ 1 nT,
~ 0.2 cm-3
~ 400 km/s
Unknowns:
Plasma sources
Moons
Rotation Rate?
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Jupiter
Jupiter -- Momentum Coupling
 
 
 
 
As plasma from Io flows
outwards its rotation
decreases (conservation
of angular momentum)
Sub-corotating plasma
pulls back the magnetic
field
Curl B -> radial current
J x B force enforces
rotation
Khurana 2001
Field-aligned currents
couple magnetosphere
to Jupiter’s rotation
Cowley & Bunce 2001
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Jupiter Aurora
Oval Rotates with Jupiter, and there’s more…
Jupiter’s
Aurora - The
Movie
Fixed magnetic
co-ordinates
rotating with
Jupiter
Clarke et al.
Grodent et al.
HST
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Jupiter Aurora
The main auroral oval is the
signature of Jupiter’s attempt
to spin up its magnetosphere
-- magnetically maps to the
region where corotation
breaks down.
Clarke et al.
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Jupiter Aurora Footprints
The aurora ‘footprints’
magnetically map to the
orbital location of three
Galilean moons: Io,
Europa, and Ganymede
-- they track around
around the oval with the
orbital speed of the
moons relative to
rotation.
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Jupiter Aurora Footprints
This was quite useful
for constraining
magnetic mapping and
the radial location of
corotation breakdown.
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Jupiter Moons - Internal Plasma Source
Io
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Jupiter Moons - Internal Plasma Source
Pilan Plume
Io’s Active
Volcanoes
Prometheus
Pilan 5 months apart
Pele
InfraRed
Tvashtar erupts during New Horizons Flyby
Jupiter Moons - Internal Plasma Source
After Spencer & Schneider 1996
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Io Plasma Torus
Warm Torus:
90% of plasma
Ne~2000 cm-3 O+ S++
Ti~100eV Te~5eV
UV power ~ 2 x 1012 W
Cold Torus:
Ne~1000 cm-3 S+
Ti~Te~1 eV
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Source:
Extended clouds
O, S, SO, SO2, S2..?
~1 ton/s ~3 x 1028 ions/
s
Dn/n~2% per rotation
Local Io Source? ~20%?
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Io Plasma Torus
Cassini UVIS - PI Larry Esposito, University of Colorado
•  Movie - 45 days as Cassini approached Jupiter
•  Integration over multiple lines in the EUV
E
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= direction of dipole tilt
W
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Io Plasma Torus
Io spews out volcanic material (1000 kg/
s), which ionizes (S+, S++, O+, SO2+)
Observing the Io
plasma torus from
Earth using highly
visible spectral lines
like sodium.
Sodium
Sodium
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Io Plasma Torus
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Io Plasma Torus
Radial
Distance
Results in Interchange Instability - Spreading of the Torus
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Connerney et al.
The Io Aurora
Clarke et al.
Infrared
Io
Ultraviolet
- energetic particles bombard atmosphere
- ‘wake’ emission extends halfway around Jupiter
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The Io Aurora
Downward
Current
Upward
Current
Stag
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nan
t flow
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Ganymede: A Magnetosphere within a
Magnetosphere
Torrence Johnson
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Ganymede’s Magnetosphere
Radius = 2634 km
B = 720 nT
Jovian ‘Wind’
~ 174 nT
~ 184 km/s
~ 3-6 cm-3
Heavy mass ions
(O+)
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Ganymede’s Magnetosphere & Aurora
HST
observations
of oxygen
emissions
- McGrath
Aurora on
Ganymede
Trailing Side =
Upstream
North Polar Cap
Leading Side =
Downstream
South Polar Cap
Khurana & Pappalardo
Jupiter -- Magnetospheric Dynamics
Equatorial View
Polar View
Dawn
Dawn
Dusk
Dusk
Side View
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Jupiter -- Magnetospheric Dynamics
Equatorial View
Dawn
Polar View
Dawn
Dusk
Hill Region
Subcorotating
plasmasheet
Dusk
Main Auroral Oval
Side View
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Jupiter -- Magnetospheric Dynamics
Equatorial View
Dawn
Dusk
Polar View
Dawn
Inward motion
-> less load on
Dusk
ionosphere
Outward motion
-> more load on
ionosphere
Outer Magnetosphere
“Cushion Region”
Side View
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Jupiter -- Magnetospheric Dynamics
? ? ?
Equatorial View
Dusk
Dawn
Polar View
Dawn
How
important is
the Dungey
Cycle?
Dusk
? ? ?
How Much of
Polar Flux is
Open?
Side View
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Relating
Auroral
Regions to
Dynamics
Dusk
Dawn
Tail Reconnection?
Less load =
weaker currents
Cusp?
More load =
stronger
currents,
filaments
Dayside Reconnection?
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Jovian Dynamics -- Some Outstanding
Questions
•  How is angular momentum transferred from Jupiter
to the magnetosphere? Slippage in ionosphere vs.
potential regions?
•  What happens in the magnetotail?
•  Are plasma losses via ~steady, small-scale crossfield “drizzle” or occasional, large, plasmoid events?
•  What triggers disruptions?
•  What are the roles of Io’s volcanism vs. solar wind
in magnetospheric variability? Other moons?
•  Can we decipher the clues from the aurora?
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Upcoming Mission to Jupiter: Juno
Juno flies through the
polar regions
•  traverses field lines that
couple to all regions of the
magnetosphere
•  acceleration region of
auroral particles
•  source regions of radio
emissions
•  crosses Io wake (and
fluxtube, with luck)
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Saturn
Knowns:
Unknowns:
R ~ 60,300 km
Neutral / Plasma sources
B ~ 21,000 nT
Moons
SW ~ 1 nT, < 0.1 cm-3, ~ 400 km/s
Rings
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Saturn - Role of Enceladus
The Geysers of Enceladus
In 2006, it was discovered that Enceladus
had geysers of water spewing from its
surface. This is the probable mass source
for the (mostly water ion) E-Ring plasma
torus.
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Saturn - Role of Enceladus
Enceladus’ plume is the probable mass source for the
(mostly water ion) plasma torus, as well as the neutrals and
ice/dust that make up the E-ring, extending from 3-9 Saturn
radii.
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Saturn - Role of Enceladus
Electron flux (upper panel) and ion flux (lower) from Enceladus flyby
electron
E3: March 12, 2006
Wake e- depleted
Stagnant ions
ion
Dust impacts
Plume
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Saturn - Role of the Rings
Saturn’s Rings
Saturn’s rings are
labeled by letters A, B,
C, etc.
Energetic (plasma)
particles are absorbed
by the rings, not in the
gap.
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Energy in eV
Cassini Division
Cassini
Division
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Saturn - Plasma Radial Transport
Saturn’s Plasmasphere
The rotationally dominated Saturnian magnetosphere has a much
larger “plasmasphere” than Earth. It is the E-Ring cold torus, mostly
water group ions, which extends out to 9 Saturn radii. -- Outside the
E-ring torus, Saturn’s magnetosphere contains warmer, less dense
plasma, more like Earth’s plasma sheet.
Saturn Injections
Warmer plasma gets injected
into the cold plasma torus by an
unknown mechanism.
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Perhaps an interchange
instability?
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Saturn Aurora
Saturn Aurora are
still not completely
understood.
The auroral forms
are often“spiral”
shaped, and the
brightening seems
to happen on
the dayside…
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6 Average Zones
X
Simultaneous
Cassini SW
Observations
Jan 2004
Low field rarefaction region
Minor compression event
Intermediate field strength
rarefaction region
Major compression region
Saturn Aurora - Substorm-like or Something Else?
Saturn Aurora - Substorm-like or Something Else?
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Earth - Jupiter - Saturn
Earth’s magnetospheric dynamics are driven by variations in the
solar wind, which generate substorms and geomagnetic storms
reflected in perturbations to the magnetic field morphology as
well as in the aurora.
Jupiter’s magnetosphere appears to be rotationally driven, due
to both the rapid rotation of the planet and the internal plasma
loading provided by Io. The aurora is only somewhat impacted
by solar wind changes. Magnetospheric interactions with the
Galilean moons is also reflected by the auroral footprints.
Saturn’s magnetosphere rotates rapidly, and contains both
sources and sinks of plasma from Enceladus and the rings. The
aurora appears to be dependent on solar wind variations, but
responds quite differently than what we observe at the Earth.
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Giant Planet Magnetospheres
Uranus & Neptune
- 
- 
- 
- 
Highly asymmetric,
Highly non-dipolar
Complex transport (SW
+ rotation)
Multiple plasma sources
(ionosphere + solar
wind + satellites)
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Non-magnetized Objects
How do Venus, Mars, and Titan interact with a
magnetized plasma?
No intrinsic magnetic field
Atmosphere/Ionosphere (Venus > Titan > Mars)
Similar magnetized ‘wind’ conditions
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Venus
Radius ~ 6051km
Gravity ~ 8.9 m/s2
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Without a global
magnetic field, the
solar wind interacts
directly with Venus’
upper atmosphere,
specifically the
ionosphere.
Ions outside the
ionopause become
trapped on the solar
wind magnetic field
and carried away,
‘pickup ions.’
0.72 AU from Sun
Atm -> 92 bars, <T> ~ 737 K (464 C)
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Venus
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Mars
While Mars does not
have a global
magnetosphere,
localized crustal
magnetism can
create ‘mini
magnetospheres’
Ions outside the
ionopause or minimagnetopause
become pickup ions.
Radius ~ 3396 km
Gravity ~ 3.7 m/s2
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1.5 AU from Sun
Atm -> 6.36 mb, <T> ~ 213 K (-60 C)
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Mars
D. Brain
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Due to the magnetic
anomalies, Mars has a partial
magnetopause. The edges
appear to be correlated with
observations of solar wind
particles precipitation, or
aurora! Much too diffuse to
observe with HST.
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Titan
Mostly interacts with the
magnetized plasma of
Saturn’s magnetosphere,
very close to Saturn’s
dayside magnetouause.
Occasionally lies in the
magnetosheath or even in the
solar wind!
Radius ~ 2575 km
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Where is Titan?
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Titan’s Near-Space Environment
Snowden et al. 2007
Draping of Saturn’s magnetic field due to
interaction with Titan’s ionosphere/atmosphere.
Rotational flow is Subsonic but Super Alfvénic.
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Titan - Saturn - Solar Wind Coupling
Winglee et al., 2009
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Mercury -- The Closest Magnetosphere to
the Sun
A relatively small
magnetosphere, close to the
Sun.
Very susceptible to solar wind
conditions and changes.
Very diffuse and unique
atmosphere… much to learn
from Messenger!
Radius ~ 2440 km
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Magnetic Field ~ 340 nT
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Mercury’s Magnetosphere
No atmosphere
thus no ionosphere
but exosphere
No plasmasphere
Weak magnetic field
Multi-ion plasma
Small magnetosphere
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Messenger: Mission to Mercury
Why go to Mercury?
Science Questions Driving the Mission and Instrumentation -Q.1. Why is Mercury so dense?
Q.2. What is the geologic history of Mercury?
Q.3. What is the nature of Mercury's magnetic field?
Q.4. What is the structure of Mercury's core?
Q.5. What are the unusual materials at Mercury's poles?
Q.6. What volatiles are important at Mercury?
‘Mercury: The Key to Terrestrial Planet Evolution’
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Messenger: Mission to Mercury
http://messenger.jhuapl.edu/instruments/index.html
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Mercury Messenger
Radius ~ 2575 km
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Evidence for Subsurface
Oceans
Image credits: Planetary Photo Journal
Jovian
System
Io (5.9 RJ)
Europa (9.5 RJ)
Ganymede (15.1 RJ)
Callisto (26.6 RJ)
Clarke, 2002
Image Credit: John Spencer
The wobble of Jupiter’s magnetic equator over the orbital
path of the Galilean moons causes a periodic dB/dt local to
the moons.
Induced vs.
Intrinsic
Magnetic Fields
Galileo: E4
Galileo: E14
Kivelson et. al., 2000
Galileo: E26
Astrobiology
Implications