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Astronomy 1 > Solar System Physics > Flashcards

Solar System Physics Flashcards

1
Q

What does the solar system consist of?

A

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.

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2
Q

planet

A

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.

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3
Q

2 groups of planets in the solar system

A

terrestrial and jovian
each has very distinct properties

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4
Q

what is gravity responsible for?

A

orbits of planets around the Sun, and moons/satellites around planets.

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5
Q

Gravitational potential energy released by matter falling towards a gravitating object…

A

is converted to kinetic energy, which can in turn be converted to other forms (e.g. heat).

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6
Q

surface gravity

A

Surface gravity is the gravitational acceleration (force per unit mass) at a planetary surface.

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7
Q

tidal force

A

The tidal force is the difference in gravitational force experienced by 2 parts of an object.

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8
Q

If a planetary atmosphere is hotter than the escape temperature for a given atom or molecule…

A

then that atom or molecule is not present in the atmosphere.

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9
Q

When is the escape temperature higher?

A

for more massive planets (stronger gravitational acceleration)
and for heavier atoms/molecules (need more energy to go fast enough to escape).

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10
Q

Scale height

A

Atmospheric pressure decreases exponentially with increasing height. The rate at which it
decreases is given by the scale height.

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11
Q

If the atmospheric temperature increases…

A

the scale height increases, and the atmosphere extends further out from the planet or star.

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12
Q

Jovian planets are primarily composed of…

A

Hydrogen and Helium

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13
Q

outer layers of jovian planets

A

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.

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14
Q

outer layers of Jupiter and Saturn

A

The gaseous outer layers of Jupiter and Saturn display complex flows (zones and belts, storms,
vortex structure).

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15
Q

Jovian planets core

A

Jovian planets have cores composed of dense ‘soups’ of rock and ices, or rock alone

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16
Q

strong magnetic field in Jupiter and Saturn

A

Liquid metallic hydrogen in Jupiter’s and Saturn’s interiors produces a strong magnetic field.

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17
Q

Weak magnetic fields in Uranus and Neptune

A

Ionic ‘oceans’ in Uranus and Neptune produce weaker magnetic fields.

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18
Q

how are the interiors of jovian planets heated

A

heated by the gravitational potential energy released as heavier elements sink slowly towards the core.

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19
Q

Why are rings thin?

A

Rings are thin because inelastic collisions in the vertical direction remove energy and linear
momentum from the colliding particles, while angular momentum is conserved.

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20
Q

Roche stability limit

A

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.

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21
Q

What do tidal forces acting on moons cause?

A

frictional heating of the moons

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22
Q

what can tidal friction on a planet do?

A

slow down its rotation

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23
Q

Because angular momentum of planet and moon must be conserved,

A

this means the planet speeds up, so its orbital radius increases.

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24
Q

The inner 2 Galilean moons

A

rocky
show more evidence of volcanic or tectonic activity, and less evidence
of impact cratering

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25
Q

The outer 2 Galilean moons

A

rock/ice

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26
Q

layers of the sun

A

The Sun can be divided into 3 interior layers (core, radiative zone, convective zone) and 3 outer
atmospheric layers (photosphere, chromosphere, corona).

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27
Q

what happens in the suns core?

A

In the core, nuclear fusion of H to He is taking place, providing the Sun’s energy.

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28
Q

Radiative zone in the sun

A

In the radiative zone energy is transported by photons which scatter very frequently and distribute
energy through the interior.

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29
Q

Convection zone of the sun

A

In the convective zone energy is transported by rising and falling flows of gas.

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30
Q

The visible solar surface (photosphere)

A

approximately a blackbody radiator, obeying the StefanBoltzmann Law and Wien’s Displacement Law.

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31
Q

Temperature in sun

A

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.

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32
Q

The solar atmosphere

A

permeated by a strong magnetic field which is generated in its interior by a dynamo, and emerges at sunspots.

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33
Q

Solar magnetic field

A

imposes complicated structures on the solar atmosphere and is responsible for solar activity, which varies on an approximately 11-year cycle.

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34
Q

Solar wind

A

The solar wind is a constant stream of hot gas from the Sun’s surface.

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35
Q

structure of terrestrial planets

A

The terrestrial planets have a metallic core (formed by gravitational differentiation), a rocky mantle
and a thin crust.

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36
Q

Earth’s magnetic field

A

Earth’s magnetic field implies a liquid core and a dynamo.

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37
Q

Mercury’s magnetic field

A

weak

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38
Q

Mars and Venus magnetic field

A

neither Mars nor Venus shows evidence of a current planetary dynamo.

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39
Q

Atmospheres of Venus and Mars

A

predominantly CO2 and N2

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40
Q

Venus atmosphere

A

very high pressure

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41
Q

Mars’ atmosphere

A

very low pressure

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42
Q

what makes Earth’s atmosphere special?

A

The Earth is the only planet with substantial O2, which is generated by plant life.

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43
Q

Surfaces of terrestrial planets

A

The surfaces of the terrestrial planets have been shaped by impact cratering, volcanism, tectonic
activity and erosion.

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44
Q

surface of Mercury and the moon

A

substantial evidence of impact cratering but no recent volcanic or tectonic activity, or erosion.

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45
Q

surface of Venus

A

Venus is thought to still be very volcanically active.

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46
Q

surface of Mars

A

Mars shows evidence of weathering by wind and flowing water in the past.

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47
Q

How to find radius and mass of the Earth

A

The radius and mass of the Earth can be obtained from astronomical measurements

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48
Q

interior structure of the Earth

A

deduced by seismology using pressure (P) and shear (S) waves.

49
Q

what does the seismic shadow for shear waves provide evidence for?

A

a liquid core

50
Q

How can the age of the Earth be obtained?

A

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.

51
Q

3 ways a planet can be heated

A
  1. accretionary heating
  2. radiogenic heating
  3. solar heating
52
Q

accretionary heating: what is the energy loss?

A

as mass falls towards protoplanet, loses potential energy.
Ulost=Ur-UR

if from great distance r»R so Ulost=GMm/R

53
Q

initial temperature of Earth: considering mass of dust falling towards early Earth

A

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

54
Q

how hot was newly formed Earth?

A

from energy conservation:
GMm/R=3/2kbTM/m
T=2/3 GmM/kbR

if assume accreting material is silicon, T=8*10^4K

55
Q

cooling of Earth

A

temp changes, luminosity (energy loss rate) changes

differential equation gives cooling time of 8*10^4 years to cool to 300K

56
Q

what does radioactive decay of elements inside Earth release?

A

heat energy

57
Q

how to estimate how much power is produced by radiogenic heating?

A
  1. consider reaction and associated half life
  2. work out fraction of total mass converted to energy
  3. energy released per kg by knowing how much of an atom in 1kg of Earth.
  4. power per kg=energy per kg/half life
  5. x by M earth
58
Q

what does radiogenic heating cause?

A

it may drive convection currents in the mantle, carrying energy from the deep interior to the Earth’s surface.

59
Q

Albedo

A

much of the flux reflected back into space and not contributing to heating (albedo).

A=sunlight reflected/sunlight received

60
Q

solar power receveid by planet

A

cross-sectional area times the flux

power reflected= AP
power absorbed=(1-A)P

61
Q

power radiated by a planet: what assumption is made?

A

planet radiates like a blackbody

62
Q

power radiated by planet: over long timescale what will happen?

A

planet will come into equilibrium such that power in=power out

63
Q

temperature of planet due to solar heating derivation

A

equate pin and pout for expression for Tp
substitute for F
combine together

64
Q

assumptions made for temperature of a planet due to solar radiation:

A
  1. temperature and the albedo are the same everywhere on the planet
  2. the planet behaves like a perfect blackbody
  3. all latitudes receive an equal amount of incoming solar radiation
65
Q

why when comparing theoretical and observed temperatures of planets, do venus and earth have far higher observed temps?

A

greenhouse effect

(jupiter to neptune slightly higher but due to internal heating)

66
Q

greenhouse effect on Earth

A

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.

67
Q

why is the greenhouse effect on Venus runaway?

A

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

68
Q

definition of planet

A

orbits sun
massive enough to be spherical
has cleared its orbit of debris

69
Q

definition of dwarf planet

A

orbiting sun
massive enough to be spherical
has not cleared its orbit of planetesimals

70
Q

definition of minor planet (includes asteroids)

A

orbiting sun but not massive enough to be spherical and orbit not cleared

71
Q

definition of trans-neptunian object (Kuiper Belt objects)

A

dwarf/minor planet orbiting sun at a greater average distance than Neptune

72
Q

largest and most massive asteroids in the asteroid belt?

A

Ceres, Vesta and Pallas

73
Q

what is the structure in the asteroid belt (kirkwood gaps) due to?

A

orbital resonances with Jupiter

asteroids line up with jupiter and receive gravitational tugs that deflect them into new orbit (empty locations)

74
Q

two theories for the origin of the asteroid belt

A
  1. debris left over from the break-up of a planet
  2. primordial rocks that never managed to accrete to form a planet
75
Q

which theory favoured for origin of asteroid belt?

A

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

76
Q

how to we learn about asteroids?

A

some visited by spacecraft but also learn a lot from meteorites (asteroid fragments that survive impact with the Earth

77
Q

what does the chemical composition of an asteroid tell us?

A

their age

chemical composition can be determined from spectra, observations from Earth or by spacecraft

78
Q

3 main asteroid groups

A

Carbonaceous (C-type)
Silicate (S-type)
Metal (M-type)

79
Q

carbonaceous asteroids

A

about 75% of population

primordial (unchanged)
darkest, least reflective, contain primarily carbon

dominate outer parts of asteroid belt

80
Q

silicate asteroids

A

about 17% of population

undergone significant melting and reformation

younger than C-type

brighter (stony, mineral composition)

dominate inner parts of belt

81
Q

metal asteroids

A

around 8% of population

cores of progenitor bodies, disrupted through collisions

consist primarily of iron and nickel.

82
Q

what are the trojans?

A

two groups of asteroids orbiting about 60 degrees ahead and behind jupiter

83
Q

The trojans are grouped around lagrange points L4 and L5. What are these?

A

where the gravitational forces from jupiter and the sun balance the centripetal force due to orbital motion

84
Q

what are comets?

A

“dirty snowballs”
rocky/icy objects orbiting sun on highly eccentric elliptical paths

85
Q

why do comets contain large amounts of ice?

A

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)

86
Q

structure of a comet

A

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

87
Q

comet tails around the sun

A

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.

88
Q

short period comets

A

period less than 200 years
aphelion outside the orbit of neptune, originating in the Kuiper belt

eg: Halley’s comet, T=76y

89
Q

long period comets

A

10^4-10^7 years

originate in the oort cloud

90
Q

oort cloud

A

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

91
Q

meteoroid

A

the object itself, originally part of a comet or asteroid

92
Q

meteor

A

visible streak of light in the sky produced when a meteor enters the Earth’s atmosphere

93
Q

meteorite

A

remains of the meteoroid if it reaches the Earth’s surface intact

94
Q

meteor shower

A

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)

95
Q

interaction of meteoroid with the atmosphere

A

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.

96
Q

meteorite classification: 4 possibilities

A

chondrite (86%)
achondrite (8%)
iron (5%)
mixture (1%)

97
Q

chondrite meteorite

A

primordial

from outer belt

98
Q

achondrite meteorites

A

from molten rock in crusts or other asteroids, moon or mars

ejected into space due to impacts with parent body

99
Q

iron meteorites

A

iron-nickel composition implies origin in the cores of asteroids

100
Q

mixture meteorites

A

stony-iron composition
outer cores of asteroids?

101
Q

kinetic energy of a meteoroid

A

considering cubical meteoroid of side d, density p and speed v.

Ek=1/2pd^3v^2

102
Q

stopping height of a meteoroid

A

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

103
Q

luminosity of a meteoroid

A

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.

104
Q

comparison of the rotational angular momentum of the sun with the orbital angular momentum of Jupiter

A

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?

105
Q

The Nebular Hypothesis

A

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

106
Q

Timeline - first 100,000 years

A

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

107
Q

Timeline - 100,000 to 10 million years

A

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)

108
Q

why would the jovian planets need to form within 10 million years?

A

still enough gas to build them up

109
Q

how do terrestrial planet form?

A

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)

110
Q

Timeline - 10 million to 0.1 billion years

A

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.)

111
Q

Timeline - >0.1 billion years

A

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?

112
Q

Timeline - 1 billion years

A

end of heavy bombardment and migration

first simple life on Earth (Australia fossils)

113
Q

Problems with Solar System formation timeline

A
  1. not enough gas in protoplanetary disk to let Jupiter and Saturn reach their current masses if they formed in current orbits
  2. cannot fully explain all impact craters on terrestrial objects created during heavy bombardment
  3. absence of super-Earths in the inner Solar system compared to exoplanet observations
114
Q

what would resolve problems with Solar System formation timeline?

A

if Jupiter and Saturn formed closer to Sun, first migrated inwards, then outwards

(gravity would prevent massive rocky planets forming)

115
Q

three main methods to detect extrasolar planets

A
  1. radial velocity
  2. transit method
  3. direct detection.
  4. and 2. use the effect of the planet on its star
116
Q

radial velocity method

A

for massive stars, wobble (due to orbit of centre of mass) can be detected by doppler shift of spectral lines emitted by star

117
Q

Transit method

A

stars light intensity decreases when the planet passes between the star and our line of sight

118
Q

Direct imaging method

A

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

Astronomy 1 (6 decks)

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