Our Planetary System & the Formation of the Solar System

Transcription

1 Our Planetary System & the Formation of the Solar System Chapters 7 & 8 Comparative Planetology We learn about the planets by comparing them and assessing their similarities and differences Similarities and differences help us understand solar system formation... tell us about the Earth... help us make sense of exo-planetary systems.... help us understand what the conditions for life are on other planets.... and allow us to understand trends and processes rather than memorize facts. What features of our solar solar system provide formation clues? The Sun, planets and large moon generally orbit and rotate in an organized way There are two major types of planets Asteroids and comets: numerous, and their composition varies with location in the solar system Exceptions... Inventory of the Solar System 1 Star 8 Planets + at least 5 dwarf planets 4 Planetary Ring Systems

2 Inventory of the Solar System > 100 Natural Satellites (i.e., moons) > 4000 Numbered Asteroids ~ 1012 comets Zodiacal Dust Cloud Solar Wind / Solar Magnetic Field 70,000 Kuiper Belt Objects (with diameters > 100 km) Jovian Planets Terrestrial Planets Mercury Earth Venus Mars Jupiter Saturn Uranus 1781 Neptune 1846

3 Density Density: Measure of the amount of mass contained in a given volume. Density is an indicator of the composition of a planet Density is not correlated with size Examples The Earth s density is 5.5 g / cm 3, but the density of the crust is g / cm 3. The core is comprised of iron & nickel compressed to abnormal densities The Jovian planets are very large, but have low densities. These planets are comprised mostly of hydrogen, helium, & methane (CH 4 ) Robotic missions: four types Flyby: a spacecraft flies by a world just once Orbiter: orbits the world it is studying, and collects long-term data Lander or probe: lands on the planet s surface or probes the atmosphere while descending through it Sample return mission: returns samples of target source to Earth for further study

4 Solar System: Major Characteristics Orbits of planets are co-planar Orbits of planets are nearly circular (exceptions Mercury, Pluto, & comets) Motion of Planets are prograde Planetary spins are prograde, with periods of hours (exceptions Venus, Uranus, & Pluto) Terrestrial planets (Mercury Mars) have refractory (bits of rocks) compositions, and the Jovian planets are gaseous Jupiter, Saturn, & Uranus resemble mini-solar systems (many satellites) Asteroids and comets are numerous, and their composition varies with location in the solar system Solar system is transparent (i.e., dust free) Nebular Theory Nebular Theory: Our solar system formed from the gravitational collapse of an interstellar cloud of gas theory credited to Immanuel Kant (1755 A.D.), and Pierre- Simon Laplace (~ 1795 A.D.) Where does the Solar System come from? It comes from gas clouds enriched by prior episodes of star formation (production of heavy elements) The Orion Nebula is an example of such enrichment Where does the Solar System come from? It comes from gas clouds enriched by prior episodes of star formation (production of heavy elements) The Orion Nebula is an example of such enrichment

5 The Orion Nebula More than 3000 stars are in this image amongst the gas and dust in the nebula What caused the orderly patterns of motion in our solar systems? Heating: as the nebula collapsed, gravitational potential energy kinetic energy heat. The Sun formed in the center Spinning: conservation of angular momentum ensured that everything didn t collapse into the center Flattening: random motion dampened out through collisions, leaving flattened rotating disk An example of a disk: β Pictoris Another Example disk star Central star has been blocked by a Coronagraph

6 Circumstellar disks (optical) Star Surrounding Gas!!!!?? Dust Disk Ionization of surrounding gas Jets: removal of mass reduces Angular Momentum (= mass x velocity x radius)

7 Circumstellar disks (optical) Circumstellar disks (optical) What an infrared telescope sees What an optical telescope sees Artist s conception of collapsing stellar disk More Examples

8 Four Types of Nebular Material Gas: what makes up planetary atmospheres Ice (Volatiles): molecules that are liquid or gaseous at moderate temperatures but form solids/crystals at low temperatures (e.g., Water H 2 O, Carbon dioxide CO 2, Methane CH 4 ) Rock: objects such as silicates that can be left behind after ice mixed with heavier elements are heated (e.g., silicates molecules of oxygen combined with either silicon, magnesium, or aluminum) Metal: material, such as iron, nickel, & magnesium that separate out from the rest of the material that make up rock when temperatures get extremely high Heat Why are there two major types of planets? planets formed out of material that was able to condense at particular distances from the Sun. The condensation of hydrogen, hydrogen compounds, rock and metal is temperature dependent Why are there two major types of planets? How did the terrestrial planets form? Hydrogen compounds can only condense beyond the frost line, which lies between the orbits of Mars and Jupiter grains stick together, forming planetesimals planetesimals attract each other gravitationally (accretion) Protoplanets form, sweeping up grains in their path

9 How did the Jovian planets form? Composition The composition of Jupiter and Saturn will reflect the materials that are available there. They build up 10 Mearth cores Which then gravitationally attract hydrogen and helium Their satellites and ring system form out of a surrounding disk Alternate theory: They formed from collapse (like the Sun) Solar System Formation Solar System Formation High resolution ALMA image of the star HL Tau Dust disk with dark rings FIG. 2. Panels(a),(b), and(c) show2.9, 1.3, and0.87mmalmacontinuum images of HL Tau. Panel (d) other panels, as well as an inset with an enlarged view of the inner 300 mas centered on the psf s peak (the othe (f) show the image and spectral index maps resulting from the combination of the 1.3 and 0.87 mm data. The s α/α error < 4. The synthesized beams are shown in the lower left of each panel, also see Table 1. The range o corresponds to 2 rms to 0.9 the image peak, using the values in Table 1. The colorscales for panels (a), (c) rms and image peak corresponding to each respective wavelength in Table 1. reconcile with a simple disk/outflow scenario, suggesting that the blue-shifted outflow has broken out of the parental core (Monin et al. 1996), or that there is another as yet unidenti Po The fitted position f

10 Solar System Formation What ended planetary formation? the clearing of gas and dust through radiation pressure from the Sun... and through streams of charged particles (solar wind) from the Sun The rings are carved out by orbiting planets What ended planetary formation? the clearing of gas and dust through radiation pressure from the Sun... and through streams of charged particles (solar wind) from the Sun Where did asteroids and comets come from? leftover planetesimals the result of fragmentation as protoplanets grow in size Note that many asteroids and comets crashed into planets...or were ejected to the outer solar system by planets

11 Creation of layers of the rocky parts of Planets Differentiation: The gravitational separation or segregation of different densities of material into different layers in the interior of a planet, as a result of heating The Process 1) E.g., the Earth was struck by large rocks in the early days of the solar system 2) Kinetic energy from these rocks was converted into heat 3) Central temperature rose, & the core of the planet became liquid 4) Denser material migrated to the center Atmospheres How does a planet obtain an atmosphere? - it forms with one (capture/primordial) -it produces one from the material in which the planet is made (outgassing) How does a planet hold an atmosphere? - must be massive enough - or the gas will escape - must be cool enough Why is the composition of atmospheres different for different planets? - Large planets: massive enough to capture hydrogen & helium early on in their formation - Small planets: outgassing (made up of what the planet formed with) Hydrogen Atom (atomic mass = 1) Exosphere (layer from which escape can occur) Argon Atom (atomic mass = 40) Atmosphere Exosphere: layer from which escape can occur k T ~ β m v 2 Mass Temperature Velocity For a fixed T, lighter atoms escape more readily than heavier atoms because they have higher velocities

12 Age-Dating Solidification Age: Time since the material became solid Gas Retention Age: A measure of the age of a rock, defined in terms of its ability to retain radioactive argon (which is the daughter product of potassium) Half-Life: Given a quantity of material, the half-life is the time which half the material will have decayed into the daughter product Examples - Radioactive Decay U-238 (92p +,146n) Pb-206 (82p +,124n) + (10p +,22n) K-40 (19p +,21n) Ar-40 (18p +,22n) The Decay Rates U billion years K billion years Radioactive Dating parent daughter Half-Life: Given a quantity of material, the half-life is the time which half the material will have decayed into the daughter product Examples - Radioactive Decay The Decay Rates U billion years K billion years Radioactive Dating U-238 (92p +,146n) Pb-206 (82p +,124n) + (10p +,22n) K-40 (19p +,21n) Ar-40 (18p +,22n) parent daughter Radioactive decay of Potassium-40 to Argon-40

13 Radioactive Decay Radioactive Decay To measure the age of the rock, Present amount Initial amount of Parent product We first determine λ in terms of the half-life time τ hl, And thus, Age of rock Inverse Fraction of Parent product left Radioactive Decay The number of daughter atoms after τ is, And thus, The ratio D τ / N τ can be measured, and τ hl is known from laboratory measurements. Age-dating (via U-238) of lunar rocks show the moon to be ~ 4.5 billion years old

14 Summary: Formation and Condensation of the Solar Nebula Stars form out of clouds of molecular gas & dust Collapse occurs when the gas is dense enough to collapse under its own weight Central parts of collapsing cloud become heated, & the shrinking nebulae begin to spin faster Angular Momentum = Mass x Velocity x Radius Results - center becomes star - spinning disk ultimately gives rise to planets - angular momentum decreased through mass loss (jets) Summary: Disk Evolution Temperature gradient develops in the disk - outer disk cools - inner disk is heated by proto-sun Grains, whose composition depends on the local temperature, begin to condense - stick together initially, building up planetesimals - planetesimals attract each other gravitationally (a process called accretion) - Protoplanets form, sweeping up grains in their path - As protoplanets grow in size, fragmentation becomes important for the production of meteoroids & asteroids (as well as for heating the interior of the planets)