solar systems Flashcards
What is the best idea of how the solar system formed?
The nebular model
What uncertainties come with the nebular model?
e.g. angular momentum and the formation of the planets
What evidence supports the nebular model?
observational evidence from the Hubble telescope
Explain how to nebular model works.
- 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
What is a molecular cloud?
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
Outline the evidence that the collapse of a molecular cloud and the formation of the solar system were driven by a supernova
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.
What is the weight, size and density of a molecular cloud?
Typically a million solar masses
50 ly across
desnity of 109 particles per cubic metre
How many molecular clouds are there in the Milky Way and what form do they take?
2000 known
typically form a ring halfway between the Sun and the galactic centre
What determines if and how fast a molecular cloud will collapse?
- 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)
What triggers the collapse of a molecular cloud?
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
What does 26Al and 60Fe decay to and what are the half-lives of these decay products?
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)
What happens to a molecular cloud as it collapses?
- 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)
What is a protoplanetary disc?
- 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
What do the “empty” rings represent in this protoplanetary ring?
The locations of new born planets
Sketch a cross-section through a protoplanetary disc, labelling significant features
- 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.
What happens in the hot, gaseous inner disc of a protoplanetary disc?
- Material blown away from the proto-star by radiation pressure
- cooling allows solids to condense out of the vapour phase
What makes up the colder, outer disc of a protoplanetary disc?
- A mixture of gases and solid matter (dust);
- Ices, organic matter and hydrogen
(Organic matter comes from the molecular cloud)
Outline the significance of the frost line to planetary formation
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
Outline the evolution of a protoplanetary disc
- 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
Explain the concept of the condensation sequence with regard to the protoplanetary disc
The condensation sequence describes the sequence in which solid materials formed via condensation of the hot gas of the protoplanetary disc
What are the strengths of the nebular model?
- 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
What are the outstanding questions that negate the nebular model?
- 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
What is a Hot Jupiter?
A hot Jupiter is a gas giant orbiting very close to its star, heated by its star to over 1000 K
Why is Jupiter not a Hot Jupiter?
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.
What is a meteoroid?
A small piece of rock in space <10m or so
What is a meteorite?
A meteoroid after it has struck the ground
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
What is an asteroid?
An asteroid is bigger than 10m yet not big enough to have attained hydrostatic equilibrium (become spherical)
What is a bolide?
A large fireball meteor that explodes in the atmosphere.
What are the 3 main types of meteoroid/asteroid?
Irons
Stones
Stony irons
Stony meteorids/asteroids are divided into..?
Chondrites and achondrites
What are the types of chondrite?
Ordinary chondrites
Carbonaceous chondrites (look like coal and smell organic)
Enstatite chondrites (Fe silicates)
What are iron meteorites?
90% iron, 10% nickel
Fragments of the cores of differentiated planetesimals
What are stony irons?
Mixtures of iron and silicates
Mesosilicates - irregularly textured breccias
Pallasites - peridot olivine in iron-nickel
What are the characteristics of achondrites?
- Lack of chondrules
- Melted and/or differentiated
- Includes the martian and lunar meteorites
What are the characteristics of chondrites?
- Contain chondrules
What is the most common type of meteorite found on Earth?
Ordinary chondrite
And they are typically thermaly metamorphosed
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
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)
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
Are chondrites pristine samples of the disc?
–Unfortunately not
–Thermal metamorphism and/or aqueous alteration
–But some are better than others!
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)
What minerals are present in CAIs?
The refractory minerals:
- Perovskite
- Melilite
- Anorthite
- Olivine
- Pyroxene
What is a chondrule?
A chondrule is a mm-scale spherical silicate grain found in chondritic meteorites.
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
How old are chondrules?
Oldest are synchronous with CAIs
Youngest are 3Myr younger (and therefore ceased before the planets completed forming)
What textures do chondrules posses?
Igneous porphoritic textures
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)
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
What evidence is there for asteroid differentiation?
Iron meteorites represent fragments of cores
Basaltic achondrites represent lava melting of asteroids
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.
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.
What are the stages of planet formation?
- Condensation of nebular gas
- Collisional growth (aka coagulation)
- Gravitational instability
- Runaway accretion (1000km scale)
- Oligarchic accretion
- Chaotic growth (7000kn across)
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)
Outline six stages in the formation of a terrestrial planet, indicating the scale of material participating in each stage.
- Condensation of nebular gas forms micrometre-scale dust
- Collisional growth aka coagulation of dust forms centimetre-scale grains
- Gravitational instability of the disc enables grains to coalesce into 1-10 km-scale planetesimals;
- Runaway accretion of planetesimals forms 1000 km planetesimals
- Oligarchic accretion of planetesimals forms ~5000 km embryos
- Chaotic scattering and collision of embryos forms final planets
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.
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.
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…
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.