1Class 14Patterns & diversity in our Solar System Kinematics of the solar system Comparative planetologyI : The kinematics of the Solar System We have learned about the laws of physics (motion andgravity) which explain… Why planets move in elliptical orbits (with Sun at one ofthe foci of the ellipse), How the velocity of the planet changes as it orbits the Sun, How the period of the orbit depends on the semi-majoraxis of the orbit But there are other patterns in the kinematics that arenot explained by these basics physical laws… All planetary orbits are almost circular All planetary orbits lie in the same plane (within a fewdegrees) All planets orbit the Sun in the same sense Most of the planets (apart from Venus & Uranus) rotate inthe same sense as they orbit These patterns must be imprints of the way the solarsystem formed…2II : The basics of comparative planetology What kinds of comparisons can we make? Size and mass ⇒ density Properties of the surface Properties of the atmosphere Presence/properties of any satellites Magnetic fields3Size, mass and density… Radius r of a planet is trivially determined… ifwe measure the angular diameter of planet tobe θ, then its radius is r=θD/2 (D=distance toplanet from Earth) Mass M of a planet is determined by applyingNewton’s laws to either a satellite of the planetor a passing spacecraft Then the average density of the planet is Planets fall into two groups… Terrestrial (Earth-like) planets Small (diameters 4,880km - 12,756km) Close to Sun (0.39AU - 1.52AU) High density (3934 - 5515kg/m3) Contains significant amount of dense elements (e.g. iron) Have hard surfaces with relatively thin atmospheres Gas giant (Jupiter-like) planets Large (diameters 49,582km - 142,984km) Far from Sun (5.2AU - 30AU) Low density (687 - 1638 kg/m3) Composed primarily of light elements (H and He) Vast majority of planet is gaseous… no hard surface4A few other ingredients we need to know about… Asteroids Rocky objects orbiting Sun… most are between orbit of Marsand Jupiter (Asteroid Belt) Largest has diameter 1000km (Ceres); most are much smaller Some have orbits that cross planetary orbits (including Earth’s) Trans-Neptunian Objects (TNOs) Icy objects orbiting Sun beyond orbit of Neptune Most of them are in the Kuiper Belt (30-50AU) Largest one is Eris, most famous is Pluto Comets TNOs whose orbit has been disturbed, sending them plunginginwards towards Sun Ices start to vaporize when they are close to Sun433 Eros56Properties of the surfaces(Terrestrial planets only) Planets can be struck by asteroids or comets Collisions were very common in the early solar system But they occurs even now… All terrestrial planets and satellites display the marks ofthese impacts, craters Very different degree of cratering found on the differentplanets… for example The Moon and Mercury are very heavily cratered Earth and Venus show far fewer craters Mars is in-between Why the differences? Thought to be due to thepresence or absence of geological activity.78MercuryBarringer Crater (also called Meteor Crater)Arizona (Earth)9Yablochkina Crater on Venus(63km diameter) There is a pattern here… the largerterrestrial planets have fewer craters The lack of cratering indicates geologicalactivity… plate tectonics, volcanoes etc. In turn… geological activity indicates a hot,molten interior Makes sense… larger planets are able toretain their internal heat for longer than asmaller planet10Magnetic fields A planets magnetic field gives us moreinformation on the interior structure Earth’s magnetic field “dipole” form (same as bar magnetic) Due to currents flowing in the molteninterior of the Earth… forms a “dynamo” Lesson… the presence of a global (dipole)magnetic field signals the presence ofelectrical currents in a fluid interior11 Diversity of magnetic properties Mercury No global field (no fluid interior) But signs of “relic” magnetization in surface rocks Venus No global field (despite fluid interior!) Maybe rotates too slowly? Earth Moderate strength global field (fluid interior) Mars No global field (no fluid interior) Jupiter and Saturn Strong global field Currents generating field are produced in the“metallic hydrogen” in the core Uranus and Neptune Moderate global field Currents generating field are flowing in liquidwater containing dissolved