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EART160 Planetary Sciences
Francis Nimmo
Course Overview
• Foundation class for Planetary Sciences pathway
• Introduction to formation and evolution of planetary
bodies in this Solar System
• Focus on surfaces, interiors and atmospheres of
planetary bodies, especially solid ones
Course Outline
• Week 1 – Introduction, highlights, missions, solar
system formation, cosmochemistry
• Week 2 – Terrestrial planet surfaces (1)
• Week 3 – Terrestrial planet surfaces (2)
• Week 4 – Terrestrial planet interiors
• Week 5 – Midterm; Planetary atmospheres
• Week 6 – Orbital dynamics
• Week 7 –Giant planets & extra-solar planets
• Week 8 - Satellites
• Week 9 – Asteroids, Meteorites and Comets
• Week 10 – Recap. and putting it all together; Final
Logistics
• Website: http://www.es.ucsc.edu/~fnimmo/eart160
• Optional text – Hartmann, Moons & Planets, 5th ed.
• Prerequisites –
– One of: Math 11B or 19B; and
– One of: Phys 6A or Phys 5A.
• WARNING: I am going to assume a good working
knowledge of single-variable calculus and freshman physics.
You will need to be able to set up and solve “word
problems”. Don’t be under any illusions – this is a
quantitative course.
• Grading – based on weekly homeworks (40%), midterm (20%),
final (40%).
• Homeworks due on Fridays (not this week)
• Plagiarism – see Syllabus for policy (posted on web)
• Office hours – MWF 12:10-1:10 (A219 E&MS) or by
appointment (email: [email protected])
• Questions? - Yes please!
Expectations
• Homework typically consists of 3 questions
• If it’s taking you more than 1 hour per question
on average, you’ve got a problem – come and
see me
• Late homework penalized by 10% per day
• Midterm/finals consist of short (compulsory)
and long (pick from a list) questions
• Showing up and asking questions are usually
routes to a good grade
Summer Research Opportunities
• There are many programs, usually paid, for summer
undergraduate research positions in planetary science
• Most of the deadlines are within the next month!
• There is a list of some of these programs on the class
website
http://es.ucsc.edu/~fnimmo/eart160
There are also going to be many planetary sciences talks
this term – searching for a new professor!
This Week
• Introductory stuff
• Highlights
• Formation of the solar system and planets:
•
•
•
•
What is the Solar System made of?
How and how fast did the planets form?
How have they evolved subsequently?
[How typical is our Solar System?]
Don’t hesitate to ask questions – it’s what I’m here for
Highlights (1)
1. The surface of Titan
2. New craters on Mars
What is the
fluid?
Why do we care?
Highlights (2)
3. Subsurface oceans
4. An unexpected particle
How do we know?
How did it form?
Highlights (3)
5. Enceladus geysers
6. Direct imaging of exoplanets
250 km
diameter
What is the energy source?
Any Earths out there?
HR8799
Mission Highlights
Phoenix (Mars)
Lunar frenzy
Kaguya (Japan)
Mercury, the last
unknown
(MESSENGER)
Chandrayaan-1
(India)
Chang’e (China)
Selected Missions
Mission
Target
Dates
Agency
Notes
Cassini/Huygens
Saturn
1997-
NASA/ESA Doppler shift . . .
M.E.R.
Mars
2003-
NASA
Still going . . .
Mars Express
Mars
2003-
ESA
First Mars radar
MESSENGER
Mercury
2004-
NASA
Third flyby
Rosetta
Comet
2004-
ESA
Deep Impact
Comet
2005
NASA
What next?
M.R.O.
Mars
2005-
NASA
30cm resolution!
New Horizons
Pluto
2006-
NASA
Jupiter flyby
Dawn
Vesta/Ceres
2007-
NASA
Vesta 2011
L.R.O./GRAIL
Moon
2009/2011
NASA
Lunar orbiters
Kepler
Exoplanets
2009
NASA
M.S.L.
Mars
2013(?)
NASA
BepiColombo
Mercury
2013
ESA
$300m overrun
What does the Solar System consist of?
•
•
•
•
The Sun
99.85% of the mass (78% H, 20% He)
Nine Eight Planets
Satellites
A bunch of other junk (comets, asteroids, Kuiper
Belt Objects etc.)
Where is everything?
Note logarithmic scales!
V E
Me
Ma
Terrestrial planets
J
S
U
N P
KB
Gas giants Ice giants
1 AU is the mean Sun-Earth distance = 150 million km
Nearest star (Proxima Centauri) is 4.2 LY=265,000 AU
Me
V E Ma
Inner solar system
1.5 AU
Note
log scales!
Outer solar system
Basic data
Distance
(AU)
Porbital
(yrs)
Protation
(days)
Sun
24.7
Mercury
0.38 0.24 58.6
Venus
0.72 0.62 243.0R
Earth
1.00 1.00 1.00
Mars
1.52 1.88 1.03
Jupiter
5.20 11.86 0.41
Saturn
9.57 29.60 0.44
Uranus
19.19 84.06 0.72R
Neptune
30.07 165.9 0.67
Pluto
39.54 248.6 6.39R
Mass
(1024kg)
2x106
0.33
4.87
5.97
0.64
1899
568
86.6
102.4
0.013
Radius
(km)
r
695950
1.41
5.43
5.24
5.52
3.93
1.33
0.68
1.32
1.64
2.05
2437
6052
6371
3390
71492
60268
24973
24764
1152
See e.g. Lodders and Fegley, Planetary Scientist’s Companion
(g cm-3)
Solar System Formation
• The basic characteristics of this Solar System –
composition, mass distribution, angular
momentum distribution – are mainly
determined by the manner in which the solar
system originally formed
• So to understand the subsequent evolution of
the planets (and other objects), we need to
understand how they formed
In the beginning . . .
Elemental abundance
(log scale)
• Elements are generated by nucleosynthesis within stars
• Heavier elements (up to Fe) are formed by fusion of
lighter elements: H -> He -> C -> O
• Elements beyond Fe are produced by nuclei absorbing
neutrons
• Elements are scattered
during stellar explosions
(supernovae) and form
clouds of material
(nebulae) ready to form
the next generation of
stars and planets
From Albarede, Geochemistry: An introduction
Solar System Formation - Overview
• Some event (e.g. nearby supernova) triggers
gravitational collapse of a cloud (nebula) of dust and gas
• As the nebula collapses, it forms a spinning disk (due to
conservation of angular momentum)
• The collapse releases gravitational energy, which heats
the centre; this central hot portion forms a star
• The outer, cooler particles suffer repeated collisions,
building planet-sized bodies from dust grains (accretion)
• Young stellar activity (T-Tauri phase) blows off any
remaining gas and leaves an embryonic solar system
• These argument suggest that the planets and the Sun
should all have (more or less) the same composition
• Comets and meteorites are important because they are
relatively pristine remnants of the original nebula
Jeans Collapse
• A perturbation will cause the density to increase locally
• Increased density -> increased gravity -> more material
gets sucked in -> runaway process (Jeans collapse)
Collapsing cloud
M,r
R
GM 2
Gravitational potential energy ~
R
M
Thermal energy ~ kTN ~ kT

M=mass r=density
k=Boltzmann’s constant
=atomic weight
N=no. of atoms
T=temperature (K)
Equating these two and using M~rR3 we get:
a
r crit
kT
~
GR 2
Does this
make sense?
Example: R=60 light years T=50 K gives rcrit~10-20 kg m-3
This is 6 atoms per c.c. (a few times the typical interstellar value)
Sequence of events
• 1. Nebular disk
formation
• 2. Initial coagulation
(~10km, ~104 yrs)
• 3. Runaway growth (to
Moon size, ~105 yrs)
• 4. Orderly growth (to
Mars size, ~106 yrs),
gas loss (?)
• 5. Late-stage collisions
(~107-8 yrs)
Accretion timescales (1)
• Consider a protoplanet moving through a planetesimal
swarm. We have dM / dt ~ r s vR2 f where v is the relative
velocity and f is a factor which arises because the
gravitational cross-sectional area exceeds the real c.s.a.
Planet
density r
fR
vorb
f is the Safronov number:
2
Where does
f  (1  (ve / v) )
R
Planetesimal
Swarm, density rs
this come from?
 (1  (8GrR / v ))
where ve is the escape velocity, G
is the gravitational constant, r is
the planet density. So:
2
2
dM / dt ~ r s vR2 (1  (8GrR 2 / v 2 ))
Accretion timescales (2)
• Two end-members:
– 8GrR2 << v2 so dM/dt ~ R2 which means all bodies increase in
radius at same rate – orderly growth
– 8GrR2 >> v2 so dM/dt ~ R4 which means largest bodies grow
fastest – runaway growth
– So beyond some critical size (~Moon-size), the largest bodies
will grow fastest and accrete the bulk of the mass
•Growth timescale increases with increasing distance (why?):
a, AU
ss,g cm-2 n, s-1
t, Myr
1
10
2x10-7
5
5
1
2x10-8
500
25
0.1
2x10-9
50,000
Approximate timescales t to form an
Earth-like planet. Here we are using
f=10, r=5.5 g/cc. In practice, f will
increase as R increases. Here s is
the nebular density per unit area and
n is 2p /orbital period.
Note that forming Neptune
is problematic!
Late-Stage Accretion
• Once each planet has swept up debris out of the area where its
gravity dominates that of the Sun (its feeding zone, or Hill
sphere), accretion slows down drastically
• Size of planets at this point is determined by the radius of the
Hill sphere and local nebular density, ~ Mars-size at 1 AU
• Collisions now only occur because of mutual perturbations
between planets, timescale ~107-8 yrs
Agnor et al. Icarus 1999
Complications
• 1) Timing of gas loss
– Presence of gas tends to cause planets to spiral inwards, hence
timing of gas loss is important
– Since outer planets can accrete gas if large enough, the relative
timescales of planetary growth and gas loss are important
• 2) “Snow line”
– More solid material is available beyond the snow line, which
allows planets to grow more rapidly
• 3) Jupiter formation
– Jupiter is so massive that it significantly perturbs the nearby area
e.g. it scattered so much material from the asteroid belt that a
planet never formed there
– It must have formed early, while the nebular gas was still
present. How?
Dust grains
Timescale Summary
Runaway growth
~Moon-size
(planetesimal)
~0.1 Myr
Orderly growth
~Mars-size
(embryo)
~1 Myr
Late-stage accretion
(Giant impacts. Gas loss?)
~Earth-size
(planet)
~10-100 Myr
Observations (1)
• Early stages of solar system formation can be imaged directly – dust
disks have large surface area, radiate effectively in the infra-red
• Unfortunately, once planets form, the IR signal disappears, so until
very recently we couldn’t detect planets (now we know of ~150)
• Timescale of clearing of nebula (~1-10 Myr) is known because young
stellar ages are easy to determine from mass/luminosity relationship.
Thick disk
This is a Hubble image of a young solar
system. You can see the vertical green
plasma jet which is guided by the star’s
magnetic field. The white zones are gas
and dust, being illuminated from inside by
the young star. The dark central zone is
where the dust is so optically thick that the
light is not being transmitted.
Observations (2)
• We can use the presentday observed planetary
masses and
compositions to
reconstruct how much
mass was there initially
– the minimum mass
solar nebula
• This gives us a constraint on the initial nebula conditions e.g.
how rapidly did its density fall off with distance?
• The picture gets more complicated if the planets have moved . . .
• The observed change in planetary compositions with distance
gives us another clue – silicates and iron close to the Sun,
volatile elements more common further out
An Artist’s Impression
The young Sun
solid planetesimals
gas/dust
nebula
Cartoon of Nebular Processes
Disk cools by radiation
Polar jets
Hot,
high r
Dust grains
Infalling
material
Nebula disk
(dust/gas)
Cold,
low r
Stellar magnetic field
(sweeps innermost disk clear,
reduces stellar spin rate)
• Scale height increases radially (why?)
• Temperatures decrease radially – consequence of lower
irradiation, and lower surface density and optical depth
leading to more efficient cooling
What is the nebular composition?
• Why do we care? It will control what the planets are
made of!
• How do we know?
– Composition of the Sun (photosphere)
– Primitive meteorites (see below)
– (Remote sensing of other solar systems - not yet very
useful)
• An important result is that the solar photosphere
and the primitive meteorites give very similar
answers: this gives us confidence that our
estimates of nebular composition are correct
1.4 million km
Solar photosphere
Note sunspots
(roughly Earth-size)
• Visible surface of the Sun
• Assumed to represent the
bulk solar composition (is
this a good assumption?)
• Compositions are obtained
by spectroscopy
• Only source of information
on the most volatile
elements (which are
depleted in meteorites):
H,C,N,O
Primitive Meteorites
• Meteorites fall to Earth and can be analyzed
• Radiometric dating techniques suggest that they formed
during solar system formation (4.56 Gyr B.P.)
• Carbonaceous (CI) chondrites contain chondrules and
do not appear to have been significantly altered
• They are also rich in volatile
elements
• Compositions are very
similar to Comet Halley,
also assumed to be ancient,
unaltered and volatile-rich
1cm
chondrules
Meteorites vs. Photosphere
• This plot shows the
striking similarity between
meteoritic and
photospheric compositions
• Note that volatiles (N,C,O)
are enriched in
photosphere relative to
meteorites
• We can use this
information to obtain a
best-guess nebular
composition
Basaltic Volcanism Terrestrial Planets, 1981
Nebular Composition
• Based on solar photosphere and chondrite compositions,
we can come up with a best-guess at the nebular
composition (here relative to 106 Si atoms):
Element
H
He C
N
O
Ne
Mg Si
Log10 (No. 10.44 9.44 7.00
Atoms)
6.42 7.32 6.52 6.0
Condens.
Temp (K)
120
180
--
78
--
--
6.0
S
Ar
5.65
5.05 5.95
1340 1529 674
40
• Blue are volatile, red are refractory
• Most important refractory elements are Mg, Si, Fe, S
Data from Lodders and Fegley, Planetary Scientist’s Companion, CUP, 1998
This is for all elements with relative abundances > 105 atoms.
Fe
1337
Planetary Compositions
• Which elements actually condense will depend on the
local nebular conditions (temperature)
• E.g. volatile species will only be stable beyond a “snow
line”. This is why the inner planets are rock-rich and the
outer planets gas- and ice-rich
• The compounds formed from the elements will be
determined by temperature (see next slide)
• The rates at which reactions occur are also governed by
temperature. In the outer solar system, reaction rates
may be so slow that the equilibrium condensation
compounds are not produced
Three kinds of planets . . .
• Nebular material can be divided into “gas” (mainly
H/He), “ice” (CH4,H2O,NH3 etc.) and “rock”
(including metals)
• Planets tend to be dominated by one of these three
end-members
• Proportions of gas/ice/rock are roughly 100/1/0.1
• The compounds which
actually condense will depend on
the local nebular conditions
(temperature)
• E.g. volatile species will only
be stable beyond a “snow line”.
This is why the inner planets are
rock-rich and the outer planets
gas- and ice-rich
Gas-rich
Rock-rich
Ice-rich
Temperature and Condensation
Nebular conditions can be used to predict what components of
the solar nebula will be present as gases or solids:
Mid-plane
Photosphere
Earth Saturn
Temperature profiles in a young (T
Tauri) stellar nebula, D’Alessio et al.,
A.J. 1998
Condensation behaviour of most abundant elements
of solar nebula e.g. C is stable as CO above 1000K,
CH4 above 60K, and then condenses to CH4.6H2O.
From Lissauer and DePater, Planetary Sciences
Terrestrial (silicate) planets
Venus
Earth
Mars
Mercury
Moon
Io
Ganymede
• Consist mainly of silicates ((Fe,Mg)SiO4) and iron (plus FeS)
• Mercury is iron-rich, perhaps because it lost its mantle during a
giant impact (more on this later)
• Volatile elements (H2O,CO2 etc.) uncommon in the inner solar
system because of the initially hot nebular conditions
• Some volatiles may have been supplied later by comets
• Satellites like Ganymede have similar structures but have an ice
layer on top (volatiles are more common in the outer nebula)
Gas and Ice Giants
90% H/He
75% H/He
10% H/He
10% H/He
• Jupiter and Saturn consist
mainly of He/H with a rockice core of ~10 Earth masses
• Their cores grew fast enough
that they captured the nebular
gas before it was blown off
• Uranus and Neptune are
primarily ices (CH4,H2O,NH3
etc.) covered with a thick
He/H atmosphere
• Their cores grew more
slowly and captured less gas
Figure from Guillot, Physics Today, (2004). Sizes are to scale. Yellow is molecular
hydrogen, red is metallic hydrogen, ices are blue, rock is grey. Note that ices are not just
water ice, but also frozen methane, ammonia etc.
How old is the solar system?
• We date the solar system using the decay of long-lived radioactive
nuclides e.g. 238U-206Pb (4.47 Gyr), 235U-207Pb (0.70 Gyr)
• These nuclides were formed during the supernova which supplied
the elements making up the original nebula
• The oldest objects are certain meteorites, which have an age of
4550 Myr B.P. (see figure)
• Some meteorites once
contained live 26Al, which has a
half-life of only 0.7 Myr. So
these meteorites must have
formed within a few Myr of 26Al
production (in the supernova).
• So the solar system itself is
also 4550 Myr old
Meteorite isochron (from Albarede,
Geochemistry: An Introduction)
Summary
• Solar system formation involved collapse of a large gas
cloud, triggered by a supernova (which also generated
many of the elements)
• Solar system originally consisted of gas:ice:rock in ratio
100:1:0.1 (solar photosphere; primitive meteorites)
• Initial nebula was dense and hot near the sun, thinner,
colder further out
• Inner planets are mainly rock; outer planets (beyond the
snow line) also include ice and (if massive enough) gas
• Planets grow by collisions; Mars-sized bodies formed
within ~1 Myr of solar system formation
• Late-stage accretion is slow and involved large impacts
Important Concepts
•
•
•
•
•
•
•
•
•
•
•
•
Minimum mass solar nebula
Stellar nucleosynthesis
Solar photosphere
Jeans collapse
T-Tauri phase & gas loss
CI chondrite
Accretion
Escape velocity
Snow line
Planetesimals
Runaway growth
Astronomical unit (AU)
End of Lecture
Forming Jupiters
• Individual gas giants probably form by gas accreting
onto a pre-existing large solid planet
• How big does the initial solid planet have to be?
Gravitational P.E. per unit ~ GM
R
mass of gas
R
Solid core
M,r
Gas
M=mass r=density
k=Boltzmann’s constant
N=no. of atoms per kg
T=temperature (K)
Thermal energy per unit
mass of gas
~ kTN
Equating these two and using M~rR3 we get:
M crit
 NkT 
~

 G 
3/ 2
r 1/ 2 Does this
make sense?
Example: r=5000 kg m-3 T=1000 K gives Mcrit~ 6x1023 kg (=Earth)
This is actually a bit low – real value is more like 8-10 MEarth