1
Q
What is the best idea of how the solar system formed?
A
The nebular model
2
Q
What uncertainties come with the nebular model?
A
e.g. angular momentum and the formation of the planets
3
Q
What evidence supports the nebular model?
A
observational evidence from the Hubble telescope
4
Q
Explain how to nebular model works.
A
- Collapse of a (normally stable) molecular cloud into the solar nebula with a central proto-Sun
- Flattening of the nebula into a circustellar protoplanetary disc (to conserve angular momentum)
- Planet formation sweeps up gas and dust (high melting point mateial forms near sun)
- Elimination of remaining gas and dust via accretion and high velocity T Tauri solar winds
- Sun enters half way through the Main Sequence and begins fusing hydrogen
5
Q
What is a molecular cloud?
A
A molecular cloud is a large volume of cold, dense gas in interstellar space/a galaxy, consisting of mostly hydrogen H2 (98%) and helium and some metals, that capable of collapsing to form new stars
6
Q
Outline the evidence that the collapse of a molecular cloud and the formation of the solar system were driven by a supernova
A
Meteorites contain decay products of short-lived isotopes such as 26Al and 60Fe; These isotopes must have formed by nucleosynthesis in a supernova shortly before the formation of the solar system.
7
Q
What is the weight, size and density of a molecular cloud?
A
Typically a million solar masses
50 ly across
desnity of 109 particles per cubic metre
8
Q
How many molecular clouds are there in the Milky Way and what form do they take?
A
2000 known
typically form a ring halfway between the Sun and the galactic centre
9
Q
What determines if and how fast a molecular cloud will collapse?
A
- Gravity is the driving force, but many clouds are sable - held open by gas pressure
- The relationship between T, P and collapse rate is described by the Jeans intsability
- The Jeans mass is the mass at which the gravity of the dense cloud overwhelms its internal pressure (therefore allowing for collapse to take place)
10
Q
What triggers the collapse of a molecular cloud?
A
Increasing the mass via addition of new gas
- collsion of clouds
- passage through a galactic spiral arm (denser part of galaxy)
Changing the distrobution of mass within a cloud
- supernova shock slams into cloud
11
Q
What does 26Al and 60Fe decay to and what are the half-lives of these decay products?
A
26Al decays to 26Mg with a half-life of 700kyr
60Fe decays to 60Ni with a half-life of 2.6Myr
(Excess 26Mg is found in CAIs in meteorites dated to 4568Myr)
12
Q
What happens to a molecular cloud as it collapses?
A
- A collapsing cloud tends to fragment as the Jeans criterion is reached in various regions
- These fragments are typically a few ly across
- Contraction results in heating
- Angular momentum forces a spherical nebula to flatten into a 200-AU circumstellar disc with a central proto-star over about 100kyr (preserving angular momentum)
13
Q
What is a protoplanetary disc?
A
- A circumstellar disk with planets forming
- Observed around a very high fraction of stars in young clusters
- Lasts about 10Myr before being cleared by T Tauri stellar winds and accretion
- Angluar momentum is shed via viscous drag (collsiions dissipate angular momentum); T Tauri winds and possible magnetic braking.
- Initially hot; later cooling allows formation of solids in the inner disk
- Outer disk retains (methane and ammonia) ices
In the image “empty” rings represent locations of new born planets
14
Q
What do the “empty” rings represent in this protoplanetary ring?
A
The locations of new born planets
15
Q
Sketch a cross-section through a protoplanetary disc, labelling significant features
A
- Proto-star at the centre;
- very hot gaseous inner disk;
- beyond this is a colder outer dust disc;
- the disc thickens further away from the proto-star;
- frost line marked at a sensible point in the dusty outer disc.
16
Q
What happens in the hot, gaseous inner disc of a protoplanetary disc?
A
- Material blown away from the proto-star by radiation pressure
- cooling allows solids to condense out of the vapour phase
17
Q
What makes up the colder, outer disc of a protoplanetary disc?
A
- A mixture of gases and solid matter (dust);
- Ices, organic matter and hydrogen
(Organic matter comes from the molecular cloud)
18
Q
Outline the significance of the frost line to planetary formation
A
A planet forming at the frost line would have benefited from a large amount of mass in the form of water ice. H2O is stable and doesn’t evaporate.
Therefore, the concentration of ices beyond the “frost line” aids planetary formation
19
Q
Outline the evolution of a protoplanetary disc
A
- Angular momentum is shed via viscous drag
- Cooling of inner disc allows condensation of micrometre-scale metal and silicate dust/smoke
- Condensation sequence of (i) metal and silicates, (ii) water, and (iii) ices
- Disc lasts around 10Myr before destruction via solar wind and accretion (so planetary formation must occur within 10Myr)
- Proto-Sun enters Main Sequence and begins stable fusing of hydrogen after 50-100Myr
20
Q
Explain the concept of the condensation sequence with regard to the protoplanetary disc
A
The condensation sequence describes the sequence in which solid materials formed via condensation of the hot gas of the protoplanetary disc
21
Q
What are the strengths of the nebular model?
A
- Well supported by observations
- Capable of explaining the formation of dust that can ultimately form planets
- Capable of producing planetary systems that share a common direction of orbit
- Broadly supported by the variations in planetary cheistry
- Refractory silicate planets in the inner solar system
- Gas giants and ice giants further out
22
Q
What are the outstanding questions that negate the nebular model?
A
- How exactly does the inner disc shed its angular momentum?
- Viscous drag? T Tauri wind? Magnetic braking?
- Why are other star systems so different?
- Hot Jupiters and Hot Neptunes
23
Q
What is a Hot Jupiter?
A
A hot Jupiter is a gas giant orbiting very close to its star, heated by its star to over 1000 K
24
Q
Why is Jupiter not a Hot Jupiter?
A
This is the Grand Tack hypothesis:
Inwards migration of Jupiter and Saturn began via interactions with the disc, then Jupiter and Saturn entered a 1:2 orbital resonance, forcing inwards migration to halt and reverse.
25
What is a meteoroid?
A small piece of rock in space \<10m or so
26
What is a meteorite?
A meteoroid after it has struck the ground
27
What is a meteor?
A meteoroid produces a meteor, fireball or bolide upon passage through the atmosphere, via **ram pressure**
Meteors are also produced by micrometeoroids via direct energy transfer from gas molecule collisions
28
What is an asteroid?
An asteroid is bigger than 10m yet not big enough to have attained hydrostatic equilibrium (become spherical)
29
What is a bolide?
A large fireball meteor that explodes in the atmosphere.
30
What are the 3 main types of meteoroid/asteroid?
Irons
Stones
Stony irons
31
Stony meteorids/asteroids are divided into..?
Chondrites and achondrites
32
What are the types of chondrite?
**Ordinary chondrites**
**Carbonaceous chondrites** (look like coal and smell organic)
**Enstatite chondrites** (Fe silicates)
33
What are iron meteorites?
90% iron, 10% nickel
Fragments of the cores of differentiated planetesimals
34
What are stony irons?
Mixtures of iron and silicates
Mesosilicates - irregularly textured breccias
Pallasites - peridot olivine in iron-nickel
35
What are the characteristics of achondrites?
* Lack of chondrules
* Melted and/or differentiated
* Includes the martian and lunar meteorites
36
What are the characteristics of chondrites?
* Contain chondrules
37
What is the most common type of meteorite found on Earth?
Ordinary chondrite
And they are typically thermaly metamorphosed
38
What are the characteristics of an enstatite chondrite?
* Rich in enstatite and chemically reduced
* Oxygen isotopic composition similar to that of terrestrial and lunar rocks
39
What are the characteristics of a carbonaceous chondrite?
* Contains ~2.7 wt. % C, including amino-acids and nucleobases *(useful for origins of life?)*
* Composed of chondrules, matrix and CAls
* Subjected to aqueous alteration and/or thermal metamorphism
* A chemically "primitive" composition
* similar to the composition of the Sun, excluding the most volatile elements e.g. H, He, Li
* CAls - the oldest solar syetem materials (Ca Al inclusions)
40
Where do irons, stony-irons and achondrites come from? And are they pristine examples of the disc?
Irons, stony-irons and achondrites come from differentiated asteroids
* Later liberated by impacts
* Indicates that core formation is relatively easy
* Plenty of information about alteration processes, rather less about conditions in the nebula
41
Are chondrites pristine samples of the disc?
–Unfortunately not
–Thermal metamorphism and/or aqueous alteration
–But some are better than others!
42
What are calcium-aluminium inclusions, and how do they relate to the condensation sequence?
CAIs are refractory inclusions in carbonaceous chondrites that represent the first materials to condense out of the hot solar nebula of the protoplanetary disc
They are probably the oldest materials in the solar system, dated to 4568.22Ma (used to date formation of solar system)
43
What minerals are present in CAIs?
The refractory minerals:
* Perovskite
* Melilite
* Anorthite
* Olivine
* Pyroxene
44
What is a chondrule?
A chondrule is a mm-scale spherical silicate grain found in chondritic meteorites.
45
What does the physical appearance of a chondrule tell us about conditions in the protoplanetary disc?
Chondrules have porphyritic textures that reflect rapid melting of disc material, followed by cooling over a period of hours at pressures \> 1 mbar
46
How old are chondrules?
Oldest are synchronous with CAIs
Youngest are 3Myr younger (and therefore ceased before the planets completed forming)
47
What textures do chondrules posses?
Igneous porphoritic textures
48
What do the porphoritic textures in chondrules reflect?
The melting and cooling of nebular solids before incorporation into the parent body
Rapid heating to 1000K, followed by cooling at
10-1000hr-1, at reletively high gas pressures ( \>1 mbar )
(Heat source not clear but maybe shock waves in turbulent protoplanetary disc)
49
Nebular shock waves are supportive of multiple events and mixing of chondrules. But what causes them?
Probably gravitational instabilities forming clumps and spirals
Modelling replicates heating and cooling rates
50
What evidence is there for asteroid differentiation?
Iron meteorites represent fragments of cores
Basaltic achondrites represent lava melting of asteroids
51
Explain how tungsten can be used to determine the timing of planetary differentiation
* This involves the separation of W and Hf during core formation.
* A planetesimal contains lithophile Hf and siderophile W.
* Upon differentiation, most Hf goes into the mantle and most W goes into the core.
* However, some of the Hf may be 182Hf, which decays to 182W with a half-life of 8.9 Myr.
* Hence, presence of excess 182W in the mantle indicates the former presence of 182Hf, meaning that differentiation occurred during the lifetime of 182Hf.
* The magnitude of the mantle 182W excess indicates the timing of core formation, relative to the 8.9 Myr half-life of 182Hf.
52
Briefly explain the effects of the T Tauri stage of the Sun on the protoplanetary disc
The intense solar wind during the T Tauri stage blows away dust and gas from the protoplanetary disc.
53
What are the stages of planet formation?
0. Condensation of nebular gas
1. Collisional growth (aka coagulation)
2. Gravitational instability
3. Runaway accretion (1000km scale)
4. Oligarchic accretion
5. Chaotic growth (7000kn across)
54
During chaotic growth, what happens to some forming planets?
Some fling into another solar system
Some collide
Some consumed by Sun
(At this stage, the 7000km across planets(?) are big enough to influence eachother gravitationally)
55
Outline six stages in the formation of a terrestrial planet, indicating the scale of material participating in each stage.
1. Condensation of nebular gas forms **micrometre-scale dust**
2. Collisional growth aka coagulation of dust forms **centimetre-scale grains**
3. Gravitational instability of the disc enables grains to coalesce into **1-10 km-scale planetesimals**;
4. Runaway accretion of planetesimals forms **1000 km planetesimals**
5. Oligarchic accretion of planetesimals forms **~5000 km embryos**
6. Chaotic scattering and collision of embryos forms **final planets**
56
Outline the two principle models of giant planet formation
**Core accretion method**: a 10 Earth mass icy embryo forms via terrestrial planet accretion processes; it has sufficient gravity to accrete gas directly from the protoplanetary disc.
The **disc instability model** holds that giant planets can form directly from knots and clumps of matter in the cold outer regions of a turbulent protoplanetary disc.
57
How does the study of exoplanets and their host stars inform the discussion on which of the core-accretion and disc instability models is more important?
Studies of exoplanets show that the probability of a star hosting a giant planet increases with the metallicity of the star. Hence, these materials must be significant in the process of the formation of a gas giant, hence supporting the core accretion model. Yet, direct imaging of exoplanets reveals giant planets in distant orbits, \>100 AU, where cool conditions favour the disc instability model.
58
Outline four features of solar system bodies that indicate that planetary migration has occurred in our solar system
* The LHB: cratering of the Moon dated to 500 Myr after the formation of the solar system
* The retrograde spin of Venus – implies a giant impact
* The existence of the Moon – implies a giant impact
* The tilt of planets such as Uranus – implies a giant impact
* The small size of Mars – prevented from growing further by migration of Jupiter
* The retrograde orbit of Triton – implies an origin as a captured Kuiper Belt object.
* Others examples could also be imagined…
59
Outline the geological evidence on the Moon for the Late Heavy Bombardment
Numerous Apollo impact melt samples and meteorites radiometrically date to around 3.8-4.1 Ga, indicating severe and intensive bombardment of the inner solar system at this time.
60
Explain the origin of the Late Heavy Bombardment
* Before the LHB, the giant planets are all in resonating orbits; these destabilise because of repeated resonant gravitational tugs at ~4.1Ga; the giants planets are thrown outwards (The Nice Model);
* the Kuiper Belt and Asteroid Belt are destabilised; impactors from these reservoirs are sent into inner solar system
61
What are the principal features of the 'dust coagulation' stage of planet formation?
* Disc material of micrometre-scale condensates and presolar ices stick together in low-velocity collisions
* Estimate of 2000 years to produce 10mm grains at 1AU (slower further out because of lower disc density)
* CAI's and chondrules may be relicts of this process
62
How do planetesimals get produced?
* The step from cm to km-scale is poorly understood
* The **disc instability model** offers a solution
* Individual grains have negligable gravity but a collection of them may not
* Disc becomes gravitationally unstable after contracting to \<100km thick
* Disc degrades into numerous turbulent knots that collapse further into planetesimals under gravity
63
What are the principal features of the 'runaway accretion' stage of planet formation?
* Planetesimals around 10km across are massive enough to have a meaningful gravitational field
* Pertubation of smaller planetesimals results in collisions via gravitational focusing
* More massive planetesimals have more potent gravitational focusing
* Hence, runaway accretion of a few large embryos
* This lasts 10-100kyr and ends around the 1000km diameter scale
64
What are the principal features of the 'oligarchic accretion' stage of planet formation?
* Occurs beyond about 1000km diameter
* Much lower accretion rates than runaway accretion
* Smaller oligarchs accrete more efficiently than large ones
* Results in around 100 embryos of Moon-Mars mass after a few hundred kyr
65
What are the principal features of the 'chaotic growth' stage of planet formation?
* Embryos are now massive enough to perturb each other gravitationally
* Orbits become chaotic; objects collide with eachother or are ejected over 10-100Myr (slowest process)
* Collisions can bring water into the inner solar system (inner solar system formed dry)
* Yields terrestrial planets
66
Outline the core accretion model.
* A 10 ME icy embryo forms via terrestrial planet processes
* Takes ~400kyr
* Assisted by proximity to the frost line
* Possible composition ice 4 : 1 rock
* It then accretes nebular gas directly
* Supported by studies of exoplanets that show that the probability of a star hosting a giant planet increases with the metallicity of the star
* Also supported by the large cores of our giants
67
Outline the disc instability model
* Instabilities in the disc form knots or clumps
* These collapse futher and experience runaway gravitational growth
* Controlled by disc thermodynamics
* Requires a rapidly cooling disc
* Favoured in the outer regions of discs, \>100AU
* But note that planets can migrate
68
Explain the Grand Tack hyprothesis.
* Mars and the asteroid belt are much less massive than models project
* Proto-Jupiter forms, creates an annular gap in the protoplanetary disc and migrates inwards via interactions with the disc
* Proto-Saturn did the same; however, proto-Saturn was catching up with proto-Jupiter
* Saturn entered 3:2 orbital resonance with Jupiter when Jupiter was at about 1.5AU
* Planetesimals in the inner solar system are scattered
* Under resonance, inwards migration halts and reverses
* The retreat of Jupiter and Saturn frees up space for the terrestrial planets to form
* The newly-formed Neptune and Uranus also enter into stable orbital resonances and also retreat
* The Grand Tack took \<1Myr and finished when the disc dispersed
69
What is the Nice Model?
* Around 4.1Ga, the orbital resonances of the giant planets collapsed, resulting in chaotic scattering
* the asteroid belt and cometary reservoirs were scattered
* neptune may have been scattered beyond uranus
* a fifth giant planet may have collided with Uranus, hence its obliquity
70
What consequences does the Nice Model have?
* Asteroids and comets are scattered throughout the inner solar system
* Asteroids are driven from asteroid belt into dynamically "sticky" / semi-stable orbits (takes a long time for them to escape it)
* These act as a long-term reservoir for large scale asteroid impacts on Earth
* global impact spherule beds on Earth indicate giant impacts throughout the Archean
* Seven in 3.47-3.23Ga
* Indicates no clear end to LHB, instead a gradual falloff
71
Intrepid space explorer Dave Cloister observes a 7000 km diameter planet on a course that will result in an oblique collision with an Earth-like planet. Predict the general consequences of the collision over the next 15 Myr or so.
This is a replica of the collision that formed the moon. Hence:
* The impactor core merges with the core of the target planet
* Mantle from both impactor and target might be ejected into orbit and accrete into a moon
* The target planet reforms into a hydrostatically equilibrated sphere
* With a hot silicate atmosphere and magma ocean;
* After about 1 kyr, the silicate atmosphere condenses
* The magma ocean lasts about 2 Myr before freezing
* The cooler surface allows water to condense and the first ocean forms
* Atmospheric carbon dioxide dissolves in the ocean and reacts with hot basaltic crust, yielding carbonates
* The super-greenhouse is slowly eliminated.
72
Outline three lines of evidence for habitable conditions on Earth in the Hadean and early Archaean
* 12C-enriched graphite in marine metasediments at Isua suggest life at 3.7 Ga
* Isua pillow basalts indicate the presence of liquid water at 3.8 Ga;
* Oxygen isotopes in Jack Hills zircons indicate liquid water in Hadean, 4.3 Ga;
73
Explain how carbonaceous chondrites might have contributed to the evolution of life on Earth
They contain abundant organic material, including amino acids and nucleobase, the building blocks of proteins and the genetic code. Delivery of this material to Earth in meteorites could have seeded the surface with compounds suitable for incorporation into a developing biochemical system.
74
What is the Faint Young Sun paradox?
The Faint Young Sun paradox describes the evidence for liquid water at the surface of the Earth during the Archaean despite the Sun being about 25% less luminous
75
What evidence suggests that a greenhouse gas is part of the solution to the Faint Young Sun paradox?
Oxygenation of the atmosphere around 2.4 Ga was immediately followed by the Huronian glaciation; this suggests that atmospheric oxygen destroyed a reducing greenhouse gas
76
Comment on the potential for i) methane, ii) carbon dioxide and iii) ammonia to be the greenhouse gas that is/may be part of the solution to the Faint Young Sun paradox?
* Methane is unlikely because is produces a cooling photochemical smog if the methane/carbon dioxide ratio exceeds 0.1
* Carbon dioxide of \<900 ppm are indicated by Banded Iron Formation chemistry; about 25× this abundance is required
* Ammonia is photochemically unstable and would react to nitrogen and hydrogen under UV light.
77
Why is zirconium silicate so useful to studies of the Hadean Earth?
Zircon is resistant to physical and chemical weathering and hence can survive geological time and metamorphism; grains of zircon are found in Archaean metasediments at Jack Hills, Australia;
zircon is formed in granitic melts hence granitic crust was forming during the Hadean; enrichment in 18O indicates aqueous alteration, hence water was stable near the surface at this time
78
Identify four unusual features of the Moon or Earth-Moon system that a model for the formation of the Moon must account for.
* Depletion of the Moon in water and iron
* high angular momentum of the Earth-Moon system
* the magma ocean of the Moon
* high mass relative to Earth
79
The standard model of the formation of the Moon has been criticised. Explain the grounds for criticism
Earth and Theia should have been composed of different materials having formed in different locations; if Moon is derived from Theia’s mantle, then we should see this difference. But lunar rocks are isotopically indistinguishable from Earth in O, Ti, Cr, W and K. Therefore, Moon is too Earth like to be Theia’s mantle.
80
Briefly describe three alternative models that are compatible with the angular momentum of the Earth-Moon system
* A trojan Theia made of similar materials because it formed at the same distance from the Sun as Earth
* a layered Moon, where Theia’s mantle is buried beneath Earth mantle
* a hit and run model where Theia’s mantle was ejected completely and the Moon formed from Earth mantle.
81
Explain the significance of the lunar highlands and KREEP basalts to models of the formation and evolution of the Moon
The lunar highlands are anorthositic flotation cumulates, representing material solidifying from a magma ocean.
KREEP is enriched in incompatible elements, K, REEs, P, U, Th; these indicate that the KREEP material come from a large magma reservoir that was slowly freezing and concentrating incompatible elements in the remaining melt.
82
Sketch the internal structure of the Moon, marking major subdivisions and their bulk compositions.
* Anorthositic highlands and basaltic maria (lunar crust)
* mafic, Fe-rich mantle with some frozen KREEP and;
* a small partial melt zone at its base
* small (350 km) Fe-FeS-C core
Recent reinterpretation of seismic data indicates a liquid outer core and a mantle melt zone
83
Comment on the relative size and structure of the lunar core, and explain the source of our understanding of it.
Recent reinterpretation of Apollo seismic data indicates the presence of a liquid outer core and solid inner core. Core is small because of iron depletion during giant impact.
84
Describe the nature of the lunar regolith and explain its origin
Regolith is a breccia of impact-processed rocks.
1. Surface is a few metres of fine jagged dust produced by micrometeorite impacts.
2. Lower is large-scale ejecta,
3. then disturbed crust,
4. then fractures crust.
85
Sketch the vertical structure of the atmosphere of Earth up to 150 km altitude, marking in important layer and trends in temperature and noting a significant processes operating in each layer
* Troposphere, temperature decreases with altitude;
* Stratiosphere: temperature increases with altitude
* Mesosphere: temperature decreases with altitude
* Thermosphere: temperature increases with altitude
86
Explain the reasons for the trends in temperature in each layer of the Earths atmosphere up to 150km altitude.
* Troposphere cools because solar heating is concentrated at ground level;
* Stratosphere warms because of absorption of UV
* Mesosphere cools because no significant absorption
* Thermosphere cools because of absorption of X-rays
87
Explain the concept of moment of inertia
The moment of inertia describes a body’s resistance to an angular acceleration
88
How does knowledge of a planet’s moment of inertia aid our understanding of its interior?
With knowledge of a planet’s moment of inertia (describes a body’s resistance to an angular acceleration), mass and radius we can calculate a moment of inertia factor:
α = CMR-2.
A planet of uniform density has α = 0.4; a value closer to 0 indicates a concentration of mass at the centre of the planet, such as an iron core
89
During a mapping project, you discover a circular structure containing brecciated rock. It is marked in the literature as a kimberlite pipe, but you suspect that it may be an impact structure. What geological formation might you search for to support an impact origin?
Shatter cones
90
During a mapping project, you discover a circular structure containing brecciated rock. It is marked in the literature as a kimberlite pipe, but you suspect that it may be an impact structure. Later, you have some thin sections made from some samples from the structure. Produce a labelled sketch of the appearance of quartz grains that have been subjected to impact shock.
Planar deformation features; two sets of lineations in quartz grains, at about 30° to each other
91
Sketch and label the likely final structure of the crater produced by a 500 m diameter impactor at Earth
Complex crater with central peak: subdued bowl, slumped terraces, breccia, melt lenses, fractured basement
92
Explain how iridium can be used to infer an impact event
Iridium is a siderophile and is rare in the silicate Earth as most was partitioned into the core; asteroids are enriched in siderophiles relative to the silicate Earth; hence, presence of a discrete iridium-rich layer indicates extraterrestrial material.
93
. Produce a sketch that illustrates how a transient crater evolves into a complex crater
Transient crater is deep and bowl-like;
final complex crater has faulted, slumped rims, a central peak or ring; an ejecta curtain outside and impact breccias and melt lenses inside
94
Identify and describe two controls on whether a crater takes a simple or complex form
Target lithology strength and impact speed
* a weaker lithology is more easily deformed into a complex structure,
* impact speed is a product of escape velocity and controls the energy of the impactor.
95
Identify and describe the controls on whether a crater takes a simple or complex form
Look it up. More than 2?
96
The shock of the formation of the Caloris basin on Mercury has been proposed to be linked with the antipodal chaotic terrain. Produce a sketch illustrating this process, labelling internal structures of relevance.
Surface waves, and pressure waves propagating though a large iron core, focused at antipode
97
How do we know that Mercury possesses a liquid outer core?
Mercury has a global magnetic field; radar observations indicate that tidal flexure of the crust is greater than would be expected for a fully solid planet, implying a fluid layer at depth.
98
The large core of Mercury has been proposed to result from loss of much of a more massive mantle. What geochemical evidence would such depletion leave behind?
Depletion in volatiles
99
The large core of Mercury has been proposed to result from loss of much of a more massive mantle. What measurements did the MESSENGER spacecraft make to investigate this, and what was the conclusion of these studies regarding the hypothesis of the loss of mantle?
X-ray and gamma-ray spectroscopy of surface materials; ratio of volatile K to refractory Th was similar to that of Earth; hence no great loss of volatiles apparent; hence no great loss of mantle seemed to have occurred.
100
Describe the fate of a hypothetical ancient Venusian ocean
* Warming Sun heats Venus;
* warmer atmosphere can hold more water vapour so more water evaporates;
* greenhouse effect increases from greater water vapour abundance;
* this cycles continues as a runaway greenhouse until the entire ocean has evaporated;
* water vapour is slowly destroyed by UV photolysis of water and Jeans escape of the hydrogen
101
Why might Venus be expected to possess a magnetic field comparable to that of Earth?
Earth and Venus have similar masses, bulk compositions, and both should have liquid iron cores
102
Outline a possible reason for the absence of a magnetic field on Venus, based on observations of the surface of Venus
* No features of plate tectonics are visible at the surface of Venus.
* Hence heat is lost only by hot spot volcanism and conduction.
* Hence, restricted heat flow on Venus from no plate tectonics results in hot mantle;
* hot mantle inhibits convective cooling of the core;
* no convection in core, hence no movement of conductive fluid and no magnetic field.
103
Produce a sketch that illustrates how radar observation of surface of Venus indicate that Venus lacks Earth-like plate tectonics
Sketch of crustal elevations against % surface area;
* Earth has a bimodal distribution with continents and ocean basins;
* Venus has a unimodal distribution.
104
Outline the mechanism of the loss of water from the atmosphere of Venus, and explain why this process is more efficient than on Earth
UV photodissociation of water followed by Jeans escape; Venus lacks an ozone layer to filter UV and its atmosphere is much hotter, aiding the transport of water vapour to higher levels
105
How would you measure the histories of water loss on Earth and Venus over geological time? Explain how your method works.
Measure D/H ratio of atmospheric water; H escapes preferentially over heavier D, hence enrichment in D is a guide to the extent of water loss via photodissociation and Jeans escape.
106
With sketches, demonstrate how shields, coronae, arachnoids and novae are related.
They represent different surface expressions of magma diapirism.
* Shield volcano: Lava erupted from magma chamber fed by rising magma diapirs
* Coronae (or arachnoid): Magma diapir uplifts surface but relatively little is erupted
* Nova: Magma diapir fractures surface with less uplift than corona
107
The Viking landers found no evidence of life or organic matter in their samples of martian soil. Explain why organic matter was expected to exist in the martian soil.
Long-term micrometeoritic infall delivers organic matter into the martian soil
108
The Viking landers found no evidence of life or organic matter in their samples of martian soil. Explain why the surface environment is not conducive to the preservation of organic matter or life.
The surface is exposed to ionising radiation in the form of solar UV and cosmic radiation, and also to oxidants produced by the solar UV flux. These destroy organic matter.
109
What are the constraints of the timing of the cessation of the ancient Mars magnetic field?
Magnetic stripes exist in the ancient southern crust; however, major impact basins are not magnetised, suggesting cessation of the field before the LHB at 4.1 Ga
110
Summarise the three main mineral alteration environments throughout Mars history
Noachian, neutral pH water, clays; Hesperian; acidic water, sulphates; Amazonian, arid, dry Fe oxides
111
What do the sediments in Gale crater tell us about the habitability of Early Hesperian Mars?
In Gale crater: Limited chemical weathering of sedimentary rocks indicates a frigid or arid climate; sequence of clastics with crossbedding indicates a fluviolacustrine environment; clays and absence of iron sulphates indicates near-neutral pH water; unit thickness suggest tens of millions of years of comparable conditions. Hence prolonged habitable environment
112
Outline the relationship between outflow channels and chaotic terrain on Mars
Many outflow channels appear to drain chaotic terrain; ice forms in crater; covered by sediment; heating melts ice, overburden collapses; water carves outflow
113
The atmosphere of Mars is enriched in deuterium relative to hydrogen, and also in 38Ar relative to 36Ar. Explain the processes responsible for these effects
UV photolysis of water at high altitudes yields hydrogen which escapes via Jeans escape. Deuterium escapes less easily than hydrogen, hence fractionation and an increase in D/H ratio. Ar is lost via sputtering resulting from the impact of the solar wind on the upper atmosphere; light Ar escapes more easily, producing an isotopic fractionation effect.
114
The atmosphere of Mars is enriched in deuterium relative to hydrogen, and also in 38Ar relative to 36Ar. Explain why Earth has not experienced these effects to the same extent as Mars
Earth is protected from UV photolysis by the ozone layer and by the stratospheric cold trap
Earth is protected from sputtering by its magnetic field
115
What properties of internal structure and composition distinguish an ice giant from a gas giant in our solar system?
Ice giants possess a lower hydrogen content, lack metallic hydrogen and possess oceans of volatiles like water, methane and ammonia
116
Outline two mechanisms by which the heat emitted by a gas giant may exceed the solar heat receive.
Kelvin-Helmholtz cooling in Jupiter: release of gravitational potential energy as Jupiter cools and shrinks; separation of helium from hydrogen: helium sinks and releases gravitational potential energy.
117
Describe the trends in composition and density among Jupiter’s Galilean moons, and explain the origin of these trends.
Moons become less dense further from Jupiter, reflecting a change from silicate-rich to ice-rich compositions
This reflects their formation from a Jovian mini-nebular – essentially a protoplanetary disc centred on Jupiter, with heat radiating from the forming Jupiter vaporising nearby volatiles
118
Explain the concept of tidal heating, with reference to Io.
* Io is tidally locked to Jupiter such that Io rotates once for every orbit it completes of Jupiter.
* The gravity of Jupiter distorts Io from a spherical shape to an ellipsoidal shape with tidal bulges.
* The orbit of Io is distorted from a circle into an ellipse by the gravity of the other Galilean moons – Io is a 4:2:1 orbital resonance with Europa and Ganymede.
* Hence, the orbital speed of Io is not constant but varies, being faster when Io is closer to Jupiter
* Hence, the tidal bulges of Io are not perfectly in position facing Jupiter throughout the orbit of Io, but instead attempt to shift from side to side.
* This repeated, regular distortion of the interior of Io generates heat
119
Explain the origin of the intrinsic magnetic fields of Jupiter and Neptune
In Jupiter, the intrinsic field is produced by convecting metallic hydrogen;
in Neptune, it is produced by convecting water with ammonia and methane
120
Explain the origin of the induced magnetic field of Europa.
What is the difference between an induced magnetic field and an intrinsic one?
Europa experiences a time-varying magnetic field as the off-centre intrinsic field of Jupiter sweeps over it. This induces an electric field in Europa by causing an electric current to flow in Europa’s brine ocean. This generates an induced magnetic field that is opposite in polarity to Jupiter’s field.
An induced field is produced by the influence of a varying magnetic field on a conducting fluid. This creates an electric field in the fluid and causes a current to flow, generating a *induced magnetic field.*
An intrinsic field is produced by a **geodynamo**, by a moving mass of conducting fluid (Ganymede has an intrinsic field produced by convecting liquid iron in its core).
All four Galilean moons have induced fields, formed by a magma ocean in Io and by brine oceans on the other moons.
121
Why does the material in Saturn’s rings not coalesce into a single moon?
Saturn’s rings lie inside Saturn’s Roche limit; a moon cannot form there because Saturn’s tidal forces would exceed the self-gravity of the moon and cause it to disintegrate.
122
Outline the evidence for a regional subsurface sea at the south pole of Enceladus
The south pole region has plumes of water ice and vapour, high heat flow rates and exhibits a surface depression
123
Outline the evidence for a global subsurface ocean on Enceladus
The ice crust of Enceladus move less in response to the tides of Saturn than predicted; this indicates that it is decoupled from the main mass of the moon in its interior; hence the sea must be global.
124
How is the global subsurface ocean on Enceladus maintained?
Ocean is maintained by tidal heating resulting from a 2:1 orbital resonance with Dione, and by the presence of dissolved materials such as NaCl, ammonia and methanol that depress the freezing point of the solution.
125
Describe the methane cycle on Titan, including references to its sources, sinks and geological features that it creates.
* Source of methane is the interior of Titan;
* sinks are photochemical destruction of methane via photodissociation and production of organic polymers (tholins);
* methane evaporates from the surface via solar heating and precipitates as rain;
* rainfall erodes dendritic drainages channels and creates the flood plains seen by Huygens;
* accumulation of methane forms seasonal rivers, lakes and seas near the poles.
126
Outline the evidence favouring a geyser origin for the dark streaks on Triton
* Darks streaks are aligned with modelled wind patterns;
* They are found at the subsolar point, hence maximum solar heating;
* Voyager 2 images show an association between plumes and dark streaks
* Nitrogen ice has a solid-state greenhouse effect capable of causing internal heating
* Mars has similar carbon dioxide geysers at its south polar cap
127
Outline the process by which the Sun produces 4He from 1H.
Two protons fuse to form a deuterium nucleus; deuterium fuses with a proton to form a 3He nucleus; two 3He nuclei fuse to yield a 4He nucleus and a proton
128
Why has the Sun slowly brightened since about 4.5 billion years ago?
Fusion produces helium from hydrogen in the core; helium is denser than hydrogen and hence the core density increases; the denser core contracts and heats up via releases of gravitational potential energy; hotter core fuses hydrogen more rapidly.
129
The Sun will eventually become a white dwarf. What two elements will the white dwarf mostly consist of?
Carbon and oxygen
130
Explain the origin of the Kirkwood Gaps in the Asteroid Belt
Asteroids in a Kirkwood Gap would be in orbital resonance with Jupiter, such as 3:1 resonance at 2.5 AU. The repeated gravitational influence of Jupiter would increase the eccentricity of the asteroid’s orbit, sending it on to a planet-crossing trajectory and thus removing it from the Belt.
131
Sketch the orbit of a short-period comet, making on the appearance of the comet at perihelion and aphelion.
Aphelion: located around the orbit of Jupiter, no coma or tail.
Perihelion: located at least close to Mars; coma around nucleus; plasma tail pointing away from the Sun; curved dust tail trailing behind the comet and away from the Sun
132
Describe two mechanisms by which a cometary nucleus may break up, and relate them to features observed on comet 67P.
Thermal stress of repeated heating cycles, as indicated by fracture networks on 67P; outbursts of dust and gas related to sinkhole formation.
133
Briefly outline the role of solar heating in surface process on Pluto
Pluto’s orbit varies from 30 to 49 AU; insolation varies by a factor of three; nitrogen ice evaporates, forms tenuous atmosphere and recondenses; contributes to nitrogen glaciers in Sputnik Planum.
134
Identify three features or structures that demonstrate that the nitrogen ice of Sputnik Planum is geologically active
Sublimation pits indicate evaporation of the ice; ovoid and polygonal cells form via solid-state convection; ice flow indicated by infilled craters and glaciers around water ice mountains
135
Explain the nature and origin of a chthonian planet
A chthonian planet is a silicate planet orbiting close to its star that formed by the hydrodynamic escape of hydrogen and helium from a Hot Jupiter, leaving only its silicate core
136
Sketch the variations in brightness of a star associated with the transit or a planet across its face
Explain how this sketch enables you to estimate the diameter of the planet relative to that of the star.
The % magnitude of dimming is the % proportion of the face of the star that is obscured by the planet.
137
Produce a sketch demonstrating how the transit method can be used to determine the atmospheric chemistry of a planet
* Starlight passes through the atmosphere of an exoplanet during transit
* Use spectrograph to examine spectrum of starlight passing through the atmosphere
* Compare it with spectrum of starlight not passing through the atmosphere
Compare spectrum of light that passed through atmosphere with light directly from star. Additional absorption lines result from atmospheric gases
Differences in absorption lines indicate the atmospheric chemistry.
138
Why might the detection of methane and ozone in the atmosphere of an exoplanet be interpreted as evidence of life?
Methane and oxygen are not stable together; one must be experiencing constant replenishment; on Earth, atmospheric methane is replenished by biology
139
Identify three other processes that could generate atmospheric methane
Interior outgassing (e.g. Titan); meteoritic and cometary infall; serpentinisation of ultramafic crust
140
Stars vary in spectral type from small, faint M-type stars to large, hot O-type stars, as shown below.
Why are small, dim M-type stars a poor target for searches for habitable exoplanets and extraterrestrial life?
They tend to experience giant flares that can irradiate a planet’s surface; a planet within the habitable zone is likely to be tidally locked to the star, presenting the same face to it, with potential for unpleasant climatic effects, such as freezing of the ocean and even atmosphere on the night side.
141
Stars vary in spectral type from small, faint M-type stars to large, hot O-type stars, as shown below.
Why are large, hot O- and B-type stars poor targets for searches for habitable exoplanets and extraterrestrial life?
Large, hot stars emit a lot of ionising UV and X-ray radiation that reduces the habitability of a planetary surface; they also have short lives and hence less opportunity for life to evolve
142
Describe three methods by which the existence of a planet may be inferred from its gravitational influence.
The gravity of the planet causes the star to regularly change position as it and the planet orbit their barycentre. Pulsar timing method: if the star is a pulsar, then the planet can be inferred by detecting variations in the timing of the radiation pulses. In the radial velocity method, a planet can be inferred by detecting cycles of variations in the radial velocity of the star, as measured by the Doppler shifts of spectral lines in the star’s spectrum. Or, use astrometry to measure regular changes in the position of star relative to a background reference of distant stars.
143
Explain how ultraviolet light can be responsible for atmospheric loss
UV light cleaves water, releasing hydrogen that undergoes Jean escape
144
Explain how ultraviolet light can be responsible for protecting against atmospheric loss
UV cleaves oxygen, resulting in the formation of stratospheric ozone that absorbs UV, heating the stratosphere and creates a temperature inversion that restricts the upwards transport of water to altitudes where it is vulnerable to photodissociation and Jeans escape of the hydrogen
145
The process of Jeans escape is normally restricted to low-density gases such as hydrogen and helium.
a. Identify two other gases whose depletion can be caused by Jeans escape
b. Explain the mechanism by which this occurs
c. For each gas, give an example of a solar system body on which this process is important
a. Water and methane
b. UV photodissociation of water/methane; subsequent Jean escape of the cleaved hydrogen
c. Water on Venus or Mars; methane on Titan
146
Outline three requirements for a planet or moon to possess a permanent global magnetic field
It must be a spinning body with a convecting layer of conductive fluid
147
Give three examples of planets or moons with a magnetic field produced by a different fluid medium
* Earth has an iron outer core;
* Uranus has a fluid water-ammonia mantle;
* Jupiter has a metallic hydrogen mantle
148
Why does the presence of sulphur in a planet’s core assist the formation of a global magnetic field?
Freezing of an outer core concentrates iron in the inner core and retain sulphur in the liquid outer core; low-density sulphur can accumulate at the core boundary and drive density-driven convection
Solar Systems
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