EART160 Planetary Sciences Francis Nimmo

Course Overview Foundation class for Planetary Sciences pathway Introduction to formation and evolution of planetary bodies in this Solar System Focus on surfaces, interiors and atmospheres of planetary bodies, especially solid ones

Course Outline Week 1 – Introduction, highlights, missions, solar system formation, cosmochemistry Week 2 – Terrestrial planet surfaces (1) Week 3 – Terrestrial planet surfaces (2) Week 4 – Terrestrial planet interiors Week 5 – Midterm; Planetary atmospheres Week 6 – Orbital dynamics Week 7 –Giant planets & extra-solar planets Week 8 - Satellites Week 9 – Asteroids, Meteorites and Comets Week 10 – Recap. and putting it all together; Final

Logistics Website: Optional text – Hartmann, Moons & Planets, 5 th ed. Prerequisites – –One of: Math 11B or 19B; and –One of: Phys 6A or Phys 5A. WARNING: I am going to assume a good working knowledge of single-variable calculus and freshman physics. You will need to be able to set up and solve “word problems”. Don’t be under any illusions – this is a quantitative course. Grading – based on weekly homeworks (40%), midterm (20%), final (40%). Homeworks due on Fridays (not this week) Plagiarism – see Syllabus for policy (posted on web) Office hours – MWF 12:10-1:10 (A219 E&MS) or by appointment ( Questions? - Yes please!

Expectations Homework typically consists of 3 questions If it’s taking you more than 1 hour per question on average, you’ve got a problem – come and see me Late homework penalized by 10% per day Midterm/finals consist of short (compulsory) and long (pick from a list) questions Showing up and asking questions are usually routes to a good grade

Summer Research Opportunities There are many programs, usually paid, for summer undergraduate research positions in planetary science Most of the deadlines are within the next month! There is a list of some of these programs on the class website There are also going to be many planetary sciences talks this term – searching for a new professor!

This Week Introductory stuff Highlights Formation of the solar system and planets: What is the Solar System made of? How and how fast did the planets form? How have they evolved subsequently? [How typical is our Solar System?] Don’t hesitate to ask questions – it’s what I’m here for

Highlights (1) 1. The surface of Titan 2. New craters on Mars Why do we care? What is the fluid?

Highlights (2) 3. Subsurface oceans 4. An unexpected particle How do we know? How did it form?

Highlights (3) 5. Enceladus geysers 6. Direct imaging of exoplanets 250 km diameter What is the energy source? HR8799 Any Earths out there?

Mission Highlights Phoenix (Mars) Lunar frenzy Kaguya (Japan) Chandrayaan-1 (India) Chang’e (China) Mercury, the last unknown (MESSENGER)

Selected Missions MissionTargetDatesAgencyNotes Cassini/HuygensSaturn1997-NASA/ESA Doppler shift... M.E.R.Mars2003-NASA Still going... Mars ExpressMars2003-ESA First Mars radar MESSENGERMercury2004-NASA Third flyby RosettaComet2004-ESA Deep ImpactComet2005NASA What next? M.R.O.Mars2005-NASA 30cm resolution! New HorizonsPluto2006-NASA Jupiter flyby DawnVesta/Ceres2007-NASA Vesta 2011 L.R.O./GRAILMoon2009/2011NASA Lunar orbiters KeplerExoplanets2009NASA M.S.L.Mars2013(?)NASA $300m overrun BepiColomboMercury2013ESA

What does the Solar System consist of? The Sun 99.85% of the mass (78% H, 20% He) Nine Eight Planets Satellites A bunch of other junk (comets, asteroids, Kuiper Belt Objects etc.)

Where is everything? J S U N P 1 AU is the mean Sun-Earth distance = 150 million km Nearest star (Proxima Centauri) is 4.2 LY=265,000 AU KB Me V E Ma Note log scales! Inner solar system 5 AU 1.5 AU Outer solar system 30 AU Note logarithmic scales! Me V Ma E Gas giants Ice giants Terrestrial planets

Basic data Distance (AU) P orbital (yrs) P rotation (days) Mass (10 24 kg) Radius (km)  (g cm -3 ) Sun x Mercury Venus R Earth Mars Jupiter Saturn Uranus R Neptune Pluto R See e.g. Lodders and Fegley, Planetary Scientist’s Companion

Solar System Formation The basic characteristics of this Solar System – composition, mass distribution, angular momentum distribution – are mainly determined by the manner in which the solar system originally formed So to understand the subsequent evolution of the planets (and other objects), we need to understand how they formed

In the beginning... Elements are generated by nucleosynthesis within stars Heavier elements (up to Fe) are formed by fusion of lighter elements: H -> He -> C -> O Elements beyond Fe are produced by nuclei absorbing neutrons Elements are scattered during stellar explosions (supernovae) and form clouds of material (nebulae) ready to form the next generation of stars and planets Elemental abundance (log scale) From Albarede, Geochemistry: An introduction

Solar System Formation - Overview Some event (e.g. nearby supernova) triggers gravitational collapse of a cloud (nebula) of dust and gas As the nebula collapses, it forms a spinning disk (due to conservation of angular momentum) The collapse releases gravitational energy, which heats the centre; this central hot portion forms a star The outer, cooler particles suffer repeated collisions, building planet-sized bodies from dust grains (accretion) Young stellar activity (T-Tauri phase) blows off any remaining gas and leaves an embryonic solar system These argument suggest that the planets and the Sun should all have (more or less) the same composition Comets and meteorites are important because they are relatively pristine remnants of the original nebula

Jeans Collapse A perturbation will cause the density to increase locally Increased density -> increased gravity -> more material gets sucked in -> runaway process (Jeans collapse) Gravitational potential energy Collapsing cloud R M,  Thermal energy Equating these two and using M~  R 3 we get: Does this make sense? Example: R=60 light years T=50 K gives  crit ~ kg m -3 This is 6 atoms per c.c. (a few times the typical interstellar value) M=mass  =density k=Boltzmann’s constant  =atomic weight N=no. of atoms T=temperature (K) 

Sequence of events 1. Nebular disk formation 2. Initial coagulation (~10km, ~10 4 yrs) 3. Runaway growth (to Moon size, ~10 5 yrs) 4. Orderly growth (to Mars size, ~10 6 yrs), gas loss (?) 5. Late-stage collisions (~ yrs)

Accretion timescales (1) Planet density  Planetesimal Swarm, density  s R fR v orb Consider a protoplanet moving through a planetesimal swarm. We have where v is the relative velocity and f is a factor which arises because the gravitational cross-sectional area exceeds the real c.s.a. f is the Safronov number: Where does this come from? where v e is the escape velocity, G is the gravitational constant,  is the planet density. So:

Accretion timescales (2) Two end-members: –8G  R 2 << v 2 so dM/dt ~ R 2 which means all bodies increase in radius at same rate – orderly growth –8G  R 2 >> v 2 so dM/dt ~ R 4 which means largest bodies grow fastest – runaway growth –So beyond some critical size (~Moon-size), the largest bodies will grow fastest and accrete the bulk of the mass a, AU  s,g cm -2 n, s -1 , Myr 1102x x x ,000 Approximate timescales  to form an Earth-like planet. Here we are using f=10,  =5.5 g/cc. In practice, f will increase as R increases. Here  is the nebular density per unit area and n is 2  /orbital period. Note that forming Neptune is problematic! Growth timescale increases with increasing distance (why?):

Late-Stage Accretion Once each planet has swept up debris out of the area where its gravity dominates that of the Sun (its feeding zone, or Hill sphere), accretion slows down drastically Size of planets at this point is determined by the radius of the Hill sphere and local nebular density, ~ Mars-size at 1 AU Collisions now only occur because of mutual perturbations between planets, timescale ~ yrs Agnor et al. Icarus 1999

Complications 1) Timing of gas loss –Presence of gas tends to cause planets to spiral inwards, hence timing of gas loss is important –Since outer planets can accrete gas if large enough, the relative timescales of planetary growth and gas loss are important 2) “Snow line” –More solid material is available beyond the snow line, which allows planets to grow more rapidly 3) Jupiter formation –Jupiter is so massive that it significantly perturbs the nearby area e.g. it scattered so much material from the asteroid belt that a planet never formed there –It must have formed early, while the nebular gas was still present. How?

Timescale Summary Runaway growth Orderly growth Late-stage accretion (Giant impacts. Gas loss?) Dust grains ~Moon-size (planetesimal) ~Mars-size (embryo) ~Earth-size (planet) ~1 Myr ~0.1 Myr ~ Myr

Observations (1) Early stages of solar system formation can be imaged directly – dust disks have large surface area, radiate effectively in the infra-red Unfortunately, once planets form, the IR signal disappears, so until very recently we couldn’t detect planets (now we know of ~150) Timescale of clearing of nebula (~1-10 Myr) is known because young stellar ages are easy to determine from mass/luminosity relationship. This is a Hubble image of a young solar system. You can see the vertical green plasma jet which is guided by the star’s magnetic field. The white zones are gas and dust, being illuminated from inside by the young star. The dark central zone is where the dust is so optically thick that the light is not being transmitted. Thick disk

Observations (2) We can use the present- day observed planetary masses and compositions to reconstruct how much mass was there initially – the minimum mass solar nebula This gives us a constraint on the initial nebula conditions e.g. how rapidly did its density fall off with distance? The picture gets more complicated if the planets have moved... The observed change in planetary compositions with distance gives us another clue – silicates and iron close to the Sun, volatile elements more common further out

An Artist’s Impression The young Sun gas/dust nebula solid planetesimals

Cartoon of Nebular Processes Scale height increases radially (why?) Temperatures decrease radially – consequence of lower irradiation, and lower surface density and optical depth leading to more efficient cooling Polar jets Stellar magnetic field (sweeps innermost disk clear, reduces stellar spin rate) Disk cools by radiation Dust grains Infalling material Nebula disk (dust/gas) Hot, high  Cold, low 

What is the nebular composition? Why do we care? It will control what the planets are made of! How do we know? –Composition of the Sun (photosphere) –Primitive meteorites (see below) –(Remote sensing of other solar systems - not yet very useful) An important result is that the solar photosphere and the primitive meteorites give very similar answers: this gives us confidence that our estimates of nebular composition are correct

Solar photosphere Visible surface of the Sun Assumed to represent the bulk solar composition (is this a good assumption?) Compositions are obtained by spectroscopy Only source of information on the most volatile elements (which are depleted in meteorites): H,C,N,O Note sunspots (roughly Earth-size) 1.4 million km

Primitive Meteorites Meteorites fall to Earth and can be analyzed Radiometric dating techniques suggest that they formed during solar system formation (4.56 Gyr B.P.) Carbonaceous (CI) chondrites contain chondrules and do not appear to have been significantly altered 1cm chondrules They are also rich in volatile elements Compositions are very similar to Comet Halley, also assumed to be ancient, unaltered and volatile-rich

Meteorites vs. Photosphere Basaltic Volcanism Terrestrial Planets, 1981 This plot shows the striking similarity between meteoritic and photospheric compositions Note that volatiles (N,C,O) are enriched in photosphere relative to meteorites We can use this information to obtain a best-guess nebular composition

Nebular Composition Based on solar photosphere and chondrite compositions, we can come up with a best-guess at the nebular composition (here relative to 10 6 Si atoms): Data from Lodders and Fegley, Planetary Scientist’s Companion, CUP, 1998 This is for all elements with relative abundances > 10 5 atoms. Element HHeCNONeMgSiSArFe Log 10 (No. Atoms) Condens. Temp (K) Blue are volatile, red are refractory Most important refractory elements are Mg, Si, Fe, S

Planetary Compositions Which elements actually condense will depend on the local nebular conditions (temperature) E.g. volatile species will only be stable beyond a “snow line”. This is why the inner planets are rock-rich and the outer planets gas- and ice-rich The compounds formed from the elements will be determined by temperature (see next slide) The rates at which reactions occur are also governed by temperature. In the outer solar system, reaction rates may be so slow that the equilibrium condensation compounds are not produced

Three kinds of planets... Nebular material can be divided into “gas” (mainly H/He), “ice” (CH 4,H 2 O,NH 3 etc.) and “rock” (including metals) Planets tend to be dominated by one of these three end-members Proportions of gas/ice/rock are roughly 100/1/0.1 The compounds which actually condense will depend on the local nebular conditions (temperature) E.g. volatile species will only be stable beyond a “snow line”. This is why the inner planets are rock-rich and the outer planets gas- and ice-rich Gas-rich Ice-rich Rock-rich

Temperature and Condensation Temperature profiles in a young (T Tauri) stellar nebula, D’Alessio et al., A.J Nebular conditions can be used to predict what components of the solar nebula will be present as gases or solids: Condensation behaviour of most abundant elements of solar nebula e.g. C is stable as CO above 1000K, CH 4 above 60K, and then condenses to CH 4.6H 2 O. From Lissauer and DePater, Planetary Sciences Mid-plane Photosphere Earth Saturn

Terrestrial (silicate) planets Consist mainly of silicates ((Fe,Mg)SiO 4 ) and iron (plus FeS) Mercury is iron-rich, perhaps because it lost its mantle during a giant impact (more on this later) Volatile elements (H 2 O,CO 2 etc.) uncommon in the inner solar system because of the initially hot nebular conditions Some volatiles may have been supplied later by comets Satellites like Ganymede have similar structures but have an ice layer on top (volatiles are more common in the outer nebula) Mercury Venus Earth Moon Mars Ganymede Io

Gas and Ice Giants Jupiter and Saturn consist mainly of He/H with a rock- ice core of ~10 Earth masses Their cores grew fast enough that they captured the nebular gas before it was blown off Uranus and Neptune are primarily ices (CH 4,H 2 O,NH 3 etc.) covered with a thick He/H atmosphere Their cores grew more slowly and captured less gas Figure from Guillot, Physics Today, (2004). Sizes are to scale. Yellow is molecular hydrogen, red is metallic hydrogen, ices are blue, rock is grey. Note that ices are not just water ice, but also frozen methane, ammonia etc. 90% H/He 75% H/He 10% H/He

How old is the solar system? We date the solar system using the decay of long-lived radioactive nuclides e.g. 238 U- 206 Pb (4.47 Gyr), 235 U- 207 Pb (0.70 Gyr) These nuclides were formed during the supernova which supplied the elements making up the original nebula The oldest objects are certain meteorites, which have an age of 4550 Myr B.P. (see figure) Some meteorites once contained live 26 Al, which has a half-life of only 0.7 Myr. So these meteorites must have formed within a few Myr of 26 Al production (in the supernova). So the solar system itself is also 4550 Myr old Meteorite isochron (from Albarede, Geochemistry: An Introduction)

Summary Solar system formation involved collapse of a large gas cloud, triggered by a supernova (which also generated many of the elements) Solar system originally consisted of gas:ice:rock in ratio 100:1:0.1 (solar photosphere; primitive meteorites) Initial nebula was dense and hot near the sun, thinner, colder further out Inner planets are mainly rock; outer planets (beyond the snow line) also include ice and (if massive enough) gas Planets grow by collisions; Mars-sized bodies formed within ~1 Myr of solar system formation Late-stage accretion is slow and involved large impacts

Important Concepts Minimum mass solar nebula Stellar nucleosynthesis Solar photosphere Jeans collapse T-Tauri phase & gas loss CI chondrite Accretion Escape velocity Snow line Planetesimals Runaway growth Astronomical unit (AU)

End of Lecture

Forming Jupiters Individual gas giants probably form by gas accreting onto a pre-existing large solid planet How big does the initial solid planet have to be? Gravitational P.E. per unit mass of gas R Thermal energy per unit mass of gas Equating these two and using M~  R 3 we get: Does this make sense? M=mass  =density k=Boltzmann’s constant N=no. of atoms per kg T=temperature (K) M,  Solid core Gas Example:  =5000 kg m -3 T=1000 K gives M crit ~ 6x10 23 kg (=Earth) This is actually a bit low – real value is more like 8-10 M Earth