Solar System Physics Flashcards
What does the solar system consist of?
the Sun orbited by the 8 planets (4 terrestrial, 4 jovian), minor bodies
including dwarf planets (e.g. Pluto), asteroids, comets, and other debris left over from the formation of
the solar system.
planet
in orbit around a star, and is massive enough to be spherical
(or nearly spherical!) and to have cleared its own orbit of other objects.
2 groups of planets in the solar system
terrestrial and jovian
each has very distinct properties
what is gravity responsible for?
orbits of planets around the Sun, and moons/satellites around planets.
Gravitational potential energy released by matter falling towards a gravitating object…
is converted to kinetic energy, which can in turn be converted to other forms (e.g. heat).
surface gravity
Surface gravity is the gravitational acceleration (force per unit mass) at a planetary surface.
tidal force
The tidal force is the difference in gravitational force experienced by 2 parts of an object.
If a planetary atmosphere is hotter than the escape temperature for a given atom or molecule…
then that atom or molecule is not present in the atmosphere.
When is the escape temperature higher?
for more massive planets (stronger gravitational acceleration)
and for heavier atoms/molecules (need more energy to go fast enough to escape).
Scale height
Atmospheric pressure decreases exponentially with increasing height. The rate at which it
decreases is given by the scale height.
If the atmospheric temperature increases…
the scale height increases, and the atmosphere extends further out from the planet or star.
Jovian planets are primarily composed of…
Hydrogen and Helium
outer layers of jovian planets
Are gaseous, therefore rotate differentially (not all latitudes have the same angular speed) and are slightly oblate due to the centrifugal force being stronger at their equators.
outer layers of Jupiter and Saturn
The gaseous outer layers of Jupiter and Saturn display complex flows (zones and belts, storms,
vortex structure).
Jovian planets core
Jovian planets have cores composed of dense ‘soups’ of rock and ices, or rock alone
strong magnetic field in Jupiter and Saturn
Liquid metallic hydrogen in Jupiter’s and Saturn’s interiors produces a strong magnetic field.
Weak magnetic fields in Uranus and Neptune
Ionic ‘oceans’ in Uranus and Neptune produce weaker magnetic fields.
how are the interiors of jovian planets heated
heated by the gravitational potential energy released as heavier elements sink slowly towards the core.
Why are rings thin?
Rings are thin because inelastic collisions in the vertical direction remove energy and linear
momentum from the colliding particles, while angular momentum is conserved.
Roche stability limit
Within a critical radius, known as the Roche stability limit, an object will be pulled apart by tidal
forces. Saturn’s rings lie mostly inside this radius.
What do tidal forces acting on moons cause?
frictional heating of the moons
what can tidal friction on a planet do?
slow down its rotation
Because angular momentum of planet and moon must be conserved,
this means the planet speeds up, so its orbital radius increases.
The inner 2 Galilean moons
rocky
show more evidence of volcanic or tectonic activity, and less evidence
of impact cratering
The outer 2 Galilean moons
rock/ice
layers of the sun
The Sun can be divided into 3 interior layers (core, radiative zone, convective zone) and 3 outer
atmospheric layers (photosphere, chromosphere, corona).
what happens in the suns core?
In the core, nuclear fusion of H to He is taking place, providing the Sun’s energy.
Radiative zone in the sun
In the radiative zone energy is transported by photons which scatter very frequently and distribute
energy through the interior.
Convection zone of the sun
In the convective zone energy is transported by rising and falling flows of gas.
The visible solar surface (photosphere)
approximately a blackbody radiator, obeying the StefanBoltzmann Law and Wien’s Displacement Law.
Temperature in sun
The temperature decreases from the core to the photosphere, then increases from photosphere
into chromosphere and corona. It is not known how this increase happens.
The solar atmosphere
permeated by a strong magnetic field which is generated in its interior by a dynamo, and emerges at sunspots.
Solar magnetic field
imposes complicated structures on the solar atmosphere and is responsible for solar activity, which varies on an approximately 11-year cycle.
Solar wind
The solar wind is a constant stream of hot gas from the Sun’s surface.
structure of terrestrial planets
The terrestrial planets have a metallic core (formed by gravitational differentiation), a rocky mantle
and a thin crust.
Earth’s magnetic field
Earth’s magnetic field implies a liquid core and a dynamo.
Mercury’s magnetic field
weak
Mars and Venus magnetic field
neither Mars nor Venus shows evidence of a current planetary dynamo.
Atmospheres of Venus and Mars
predominantly CO2 and N2
Venus atmosphere
very high pressure
Mars’ atmosphere
very low pressure
what makes Earth’s atmosphere special?
The Earth is the only planet with substantial O2, which is generated by plant life.
Surfaces of terrestrial planets
The surfaces of the terrestrial planets have been shaped by impact cratering, volcanism, tectonic
activity and erosion.
surface of Mercury and the moon
substantial evidence of impact cratering but no recent volcanic or tectonic activity, or erosion.
surface of Venus
Venus is thought to still be very volcanically active.
surface of Mars
Mars shows evidence of weathering by wind and flowing water in the past.
How to find radius and mass of the Earth
The radius and mass of the Earth can be obtained from astronomical measurements
interior structure of the Earth
deduced by seismology using pressure (P) and shear (S) waves.
what does the seismic shadow for shear waves provide evidence for?
a liquid core
How can the age of the Earth be obtained?
The age of the Earth (and other solar system objects) can be obtained by radioactive dating using
long-lived radioisotopes. It is ~4.5 billion years old.
3 ways a planet can be heated
- accretionary heating
- radiogenic heating
- solar heating
accretionary heating: what is the energy loss?
as mass falls towards protoplanet, loses potential energy.
Ulost=Ur-UR
if from great distance r»R so Ulost=GMm/R
initial temperature of Earth: considering mass of dust falling towards early Earth
loses PE as it falls, gains KE.
Suppose contains N atoms, average mass per atom m.
N=M/m
KE converted to thermal energy E=3.2kbT
so Etot=3/2NkbT=3/2kbTM/m
how hot was newly formed Earth?
from energy conservation:
GMm/R=3/2kbTM/m
T=2/3 GmM/kbR
if assume accreting material is silicon, T=8*10^4K
cooling of Earth
temp changes, luminosity (energy loss rate) changes
differential equation gives cooling time of 8*10^4 years to cool to 300K
what does radioactive decay of elements inside Earth release?
heat energy
how to estimate how much power is produced by radiogenic heating?
- consider reaction and associated half life
- work out fraction of total mass converted to energy
- energy released per kg by knowing how much of an atom in 1kg of Earth.
- power per kg=energy per kg/half life
- x by M earth
what does radiogenic heating cause?
it may drive convection currents in the mantle, carrying energy from the deep interior to the Earth’s surface.
Albedo
much of the flux reflected back into space and not contributing to heating (albedo).
A=sunlight reflected/sunlight received
solar power receveid by planet
cross-sectional area times the flux
power reflected= AP
power absorbed=(1-A)P
power radiated by a planet: what assumption is made?
planet radiates like a blackbody
power radiated by planet: over long timescale what will happen?
planet will come into equilibrium such that power in=power out
temperature of planet due to solar heating derivation
equate pin and pout for expression for Tp
substitute for F
combine together
assumptions made for temperature of a planet due to solar radiation:
- temperature and the albedo are the same everywhere on the planet
- the planet behaves like a perfect blackbody
- all latitudes receive an equal amount of incoming solar radiation
why when comparing theoretical and observed temperatures of planets, do venus and earth have far higher observed temps?
greenhouse effect
(jupiter to neptune slightly higher but due to internal heating)
greenhouse effect on Earth
atmosphere transparent to visible light.
solar radiation heats surface, re-radiates at infrared wavelengths
atmosphere not transparent at infrared wavelengths so IR absorbed
this heats up atmosphere and it re-radiates IR, heating up surface.
why is the greenhouse effect on Venus runaway?
venus closer to sun, water exists as water vapour, trapping IR.
Scale height for H20 increases, UV and X-rays dissociate water molecules.
Without water, atmosphere is mainly CO2
definition of planet
orbits sun
massive enough to be spherical
has cleared its orbit of debris
definition of dwarf planet
orbiting sun
massive enough to be spherical
has not cleared its orbit of planetesimals
definition of minor planet (includes asteroids)
orbiting sun but not massive enough to be spherical and orbit not cleared
definition of trans-neptunian object (Kuiper Belt objects)
dwarf/minor planet orbiting sun at a greater average distance than Neptune
largest and most massive asteroids in the asteroid belt?
Ceres, Vesta and Pallas
what is the structure in the asteroid belt (kirkwood gaps) due to?
orbital resonances with Jupiter
asteroids line up with jupiter and receive gravitational tugs that deflect them into new orbit (empty locations)
two theories for the origin of the asteroid belt
- debris left over from the break-up of a planet
- primordial rocks that never managed to accrete to form a planet
which theory favoured for origin of asteroid belt?
primordial rocks
not enough mass to make up a moon
differences in chemical composition of asteroids shows they don’t have a common origin
gravity of jupiter would prevent asteroids accreting into a more massive body
how to we learn about asteroids?
some visited by spacecraft but also learn a lot from meteorites (asteroid fragments that survive impact with the Earth
what does the chemical composition of an asteroid tell us?
their age
chemical composition can be determined from spectra, observations from Earth or by spacecraft
3 main asteroid groups
Carbonaceous (C-type)
Silicate (S-type)
Metal (M-type)
carbonaceous asteroids
about 75% of population
primordial (unchanged)
darkest, least reflective, contain primarily carbon
dominate outer parts of asteroid belt
silicate asteroids
about 17% of population
undergone significant melting and reformation
younger than C-type
brighter (stony, mineral composition)
dominate inner parts of belt
metal asteroids
around 8% of population
cores of progenitor bodies, disrupted through collisions
consist primarily of iron and nickel.
what are the trojans?
two groups of asteroids orbiting about 60 degrees ahead and behind jupiter
The trojans are grouped around lagrange points L4 and L5. What are these?
where the gravitational forces from jupiter and the sun balance the centripetal force due to orbital motion
what are comets?
“dirty snowballs”
rocky/icy objects orbiting sun on highly eccentric elliptical paths
why do comets contain large amounts of ice?
spend most of their lives very far from the Sun (Kepler 2) so contain large amounts of ice
as one nears the sun, it produces a coma and tail (outgassing)
structure of a comet
coma - atmosphere of gas and dust, surrounds the nucleus.
ion tail - ionised gas from coma pushed away from sun by solar wind
dust tail - follows curved path due to outward force exerted by Sun’s radiation pressure on dust particles
comet tails around the sun
tails get longer the closer a comet gets to the sun
as approaches sun, tails stream behind it but after passing perihelion the comet tails will stream in front of the coma and nucleus.
short period comets
period less than 200 years
aphelion outside the orbit of neptune, originating in the Kuiper belt
eg: Halley’s comet, T=76y
long period comets
10^4-10^7 years
originate in the oort cloud
oort cloud
a spherical cloud of debris from the formation of the solar system extending to around light year
contains 10^12-10^13 comets with a total mass of a few hundred Earth masses
meteoroid
the object itself, originally part of a comet or asteroid
meteor
visible streak of light in the sky produced when a meteor enters the Earth’s atmosphere
meteorite
remains of the meteoroid if it reaches the Earth’s surface intact
meteor shower
rate of around 1000 meteors an hour, tend to come from a particular direction - radiant point.
occurs when the Earth [asses through a trail of debris behind a comet
(usual rate around 20 an hour)
interaction of meteoroid with the atmosphere
KE and momentum of a meteoroid reduced by air friction as it enters atmosphere
heating and mass loss leaves a glowing tail
meteorite surfaces are blackened and fused due to high temperatures created as they pass through atmosphere.
meteorite classification: 4 possibilities
chondrite (86%)
achondrite (8%)
iron (5%)
mixture (1%)
chondrite meteorite
primordial
from outer belt
achondrite meteorites
from molten rock in crusts or other asteroids, moon or mars
ejected into space due to impacts with parent body
iron meteorites
iron-nickel composition implies origin in the cores of asteroids
mixture meteorites
stony-iron composition
outer cores of asteroids?
kinetic energy of a meteoroid
considering cubical meteoroid of side d, density p and speed v.
Ek=1/2pd^3v^2
stopping height of a meteoroid
air resistance slows meteoroid down as encounters Earth’s atmosphere
assume will stop when encounters mair = its own mass
mair=volume of air encounteredxdensity
set m meteroid = m air
luminosity of a meteoroid
estimate from energy loss during deceleration
power dissipated=energy/time
sub in stopping time=H/v
most observations come from height<30k so can use to estimate radiation flux at Earth assuming 10% of power dissipated emerges as light.
comparison of the rotational angular momentum of the sun with the orbital angular momentum of Jupiter
L=RMV=RMwR where w=2pi/T
for sun gives: 2.8x10^42 kgm^2s^-1
for jupiter: 1.6x10^43 kgm^2s^-1
Jupiter more than 6 times larger than sun. Did sun transfer angular momentum to planets?
The Nebular Hypothesis
everything is solar system formed from a large cloud of interstellar gas and dust which collapsed under its own gravity
evidence of such disks in other systems that are still forming
Timeline - first 100,000 years
nebula becomes gravitationally unstable and starts to collapse
central region heats up forming protostar
rotation speeds up as material falls inwards and cloud flattens to disk
Timeline - 100,000 to 10 million years
nebula exists with protostar surrounded by proto-planetary disk
protostar becomes a T Tauri star, heated by gravitational contraction
further out material accretes to form planetesimals
beyonf 4 or 5 au, icy planetesimals can form (jovian planets)
why would the jovian planets need to form within 10 million years?
still enough gas to build them up
how do terrestrial planet form?
through accretion of multiple planetesimals into a more massive body.
(takes 10-100 million years for Earth sized object to form within approx 3 au)
Timeline - 10 million to 0.1 billion years
solar system consists of protoplanets and planetesimals orbiting proto-sun, nebular gas has mostly dissipated
large impacts during this time (explains Venus, Earth-Moon system etc.)
Timeline - >0.1 billion years
temp and pressure in core of proto-sun high enough for nuclear fusion
asteroid belt and Kuiper belt form as other planetesimals cleared
period of late heavy bombardment
Jovian planets migrated to current orbits?
Timeline - 1 billion years
end of heavy bombardment and migration
first simple life on Earth (Australia fossils)
Problems with Solar System formation timeline
- not enough gas in protoplanetary disk to let Jupiter and Saturn reach their current masses if they formed in current orbits
- cannot fully explain all impact craters on terrestrial objects created during heavy bombardment
- absence of super-Earths in the inner Solar system compared to exoplanet observations
what would resolve problems with Solar System formation timeline?
if Jupiter and Saturn formed closer to Sun, first migrated inwards, then outwards
(gravity would prevent massive rocky planets forming)
three main methods to detect extrasolar planets
- radial velocity
- transit method
- direct detection.
- and 2. use the effect of the planet on its star
radial velocity method
for massive stars, wobble (due to orbit of centre of mass) can be detected by doppler shift of spectral lines emitted by star
Transit method
stars light intensity decreases when the planet passes between the star and our line of sight
Direct imaging method
light from star removed by components in telescope that destructively interfere with starlight
light from planet unaffected and computer processing used to detect planet
works best infrared and for planets far from their star
