ASTRO - Notes

➔ Jupiter Saturn Conjunction ◆ A constellation is a region of the sky, within official borders set in 1928 by the IAU. ◆ Most official names of the 88 constellations come from antiquity. Some southern hemisphere constellations were named by European explorers in the 17th and 18th centuries. The patterns of stars have no physical significance. ➔ Constellations ◆ Most official names of the 88 constellations come from antiquity. The patterns of the stars have no physical significance! Stars that appear close together may lie at very different distances. ◆ Modern astronomers use constellations as useful general landmarks, but specify the exact location of objects by a system of celestial coordinates. ● North celestial pole, south celestial pole, celestial equator ● Ecliptic: the annual path of the Sun through the celestial sphere, which is just a projection of Earth’s orbital plane on the sky ➔ The Celestial Sphere ◆ North and south celestial poles. ◆ The sky above looks like a dome…a hemisphere… ◆ If we imagine the sky around the entire Earth, we have the celestial sphere. ◆ This is a 2-dimensional representation of the sky. There is no physical sphere up there. ◆ Because it represents our view from Earth, we place Earth in the center of this sphere ◆ ➔ Measuring the Sky ◆ We may use a term like “angular distance,” but we want an angle, not a distance! ◆ We measure the sky in angles, not distances. ◆ Full Circle = 360 degrees ◆ 1 degree = 60 arcmin ◆ 1 arcmin = 60 arcsec ◆ 3600 arcsec = 1 degree ◆ Note: The Moon is about 0.5 degrees, but so is the Sun! A larger object farther away can have the same angular size as a smaller object closer to us. ➔ MEASURING ANGLES IN THE SKY ◆ The moon is ½ degree. The Big Dipper is 5 degree. One finger is about 1 degree. A full hand is about 20 degrees. One fist is about 10 degrees. ◆ We say that the angular “distance” between these two stars in the Big Dipper is 5 degrees. ◆ There are about 9 “fists” between horizon and zenith. ◆ CONVENIENT: Held at arm’s length… ● Your fingernail is about 1 degree across ● Your fist is about 10 degrees across ● Your outstretched hand is about 20 degrees across

➔ Zenith: The point DIRECTLY ABOVE you ➔ Horizon: ALL POINTS 90 degrees from the zenith. ➔ Elevation: The angle above the horizon. ➔ Meridian: A line from the northern horizon point through the zenith and down to the southern horizon. ➔ To pinpoint a spot in the local sky… ◆ Once you specify elevation, you can use the meridian to identify the direction, and then you have your coordinates. So if I see a star 60 degrees above the horizon, then I know that I can use my meridian to determine the relative direction (let’s say SE), and then I can label this object as “Altitude: 60 degrees,” “direction: SE.” ◆ Elevation (or altitude) angle and direction along the horizon (or azimuth) ➔ Right Ascension and Declination ◆ Like latitude and longitude on the Earth ◆ Declination is the angle (above or below) the Celestial Equator - the projection of Earth’s equator onto the sky. ◆ Right Ascension : is measured along the Celestial Equator (increasing Eastward) from a reference point in the sky called the First Point of Aries ◆ Advantage : these coordinates don’t change as a star rises and sets. ◆ Disadvantage: You need to know your latitude and the local sidereal time. ➔ The Daily (diurnal) Motion ◆ As Earth rotates (W→E), the sky appears to us to rotate in the opposite direction (E→W). ◆ The sky appears to rotate around the N (or S) celestial poles. ◆ If you are standing at the poles, nothing rises or sets. ◆ If you are standing at the equator everything rises and sets 90 degrees to the horizon. ➔ Elevation angle of the celestial pole = your latitude ◆ All stars are at an angle LESS THAN your latitude away from: ● Your celestial pole never set (they are circumpolar) and from ● The other celestial pole never rise and cannot be seen by you. ◆ Other stars (and sun, moon, planets) rise in the Earth and set in the West at an angle = [90 degrees - your latitude] ➔ Solar and Sidereal Days ◆ ➔ Remember: the angular distance of the sun per hour. The sun is in the sky roughly 12 hours. This means that of the 360 degrees to be covered in those 24 hours, it must be 180 degrees visible/12 hours = 15 degrees one hour. Or… 360 degrees/24 hours = 15 degrees per hour. ➔ The moon is about 0.5, but so is the sun ◆ A larger object farther away can have the same angular size as a smaller object closer to us. ➔ Definitions: The local sky ◆ Zenith - the point directly above you

◆ New, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter ◆ The week comes from religion, the Book of Genesis. ➔ Why do we see the same face? ◆ Rotation period = orbital period ◆ Moon not rotating ◆ Moon rotating ➔ Why not eclipses every month? ◆ The Moon’s orbit tilted 5.2 degrees to the ecliptic plane. ◆ Crosses the ecliptic plane only at the two nodes. ◆ Eclipse possible only when full/new moon occur near those nodes - this is rare ◆ The orbital periods (of Earth and Moon) are NOT synchronized in any way. ➔ Review from Last Time ◆ Astronomy in the ancient world ◆ Ancient Greek astronomy, geocentric model of the solar system, parallax. ● Say there is a star between me and a nearby star. Take your thumb and move it to your face. Close one eye at a time. Does your thumb appear to move? What about when it’s further away, does it move as much? This is parallax. The midterm is for sure online. ◆ The Copernican Revolution and heliocentric model for the solar system. ◆ Copernicus, Tycho, Kepler, Galileo. ◆ Kepler’s Laws. ◆ The role of Galileo. ◆ The scientific method! ◆ We move slightly faster when we are closer to the sun, but it is not something we really perceive. ➔ Describing Motion ◆ Today we will discuss the laws that govern motion and energy in the universe. By understanding these laws, you will be able to make sense of the wide range of phenomena that we will encounter. ● In a previous lecture we learned that the ancient Greeks thought that it was natural for everything to be in a state of rest. Galileo’s Galilea ◆ Motion and Force ● Speed: rate at which object moves ○ Speed = distance/time (units of m/s) ○ Example: speed of 10 m/s ● Velocity: speed and direction ○ Example: 10 m/s, due east ● Acceleration: any change in velocity (either rate or direction) ○ Units must be speed/time (m/s^2) ○ Something that has a size and direction is called a vector ◆ The Acceleration of Gravity (G) ● All falling objects near the Earth’s surface accelerate (pick up speed) at the same rate (not counting friction of air resistance).

● We call this constant acceleration, g. The acceleration due to gravity. ● On Earth, g = 10 m/s^2. Speed increases 10 m/s^2 each second until hitting terminal velocity. Technically 9.8 m/s^2 ● Galileo showed that g is the same for falling objects, regardless of their mass; remember mass is the quantity of matter. ➔ Mass, Momentum, Force ◆ Mass: property of a body which determines its resistance to a change in its motion. The desire of a body to create gravity. Gravitational attraction is proportional to the mass of the two bodies. Mass is also the desire of an object to be moved or not (to be accelerated or not). ◆ Momentum: product of a body’s mass and velocity: M x V. ◆ Force: “push” or “pull”. More formally, a method for changing a body’s momentum. If the mass of the body stays constant, and mv changes, then the velocity changes: acceleration. Forces can be in balance (gravity vs. molecular forces holding your seat together). An imbalance, or net force, is required for a change in momentum. ◆ Normal force: the force of the floor is equal and opposite to the force of gravity. ➔ Angular Momentum ◆ Angular momentum: the momentum involved in spinning/circling: M x V x R ◆ Examples of force: gravitational, electrostatic (electromagnetic), frictional, tension, spring, technically nuclear as well (keeps your nuclei together. ➔ What are Newton’s Three Laws of Motion? ◆ Newton’s first law of motion: An object at rest or moving at constant velocity stays that way unless a net force acts to change its speed or direction. Inertia! ◆ Motion at constant speed is the normal state ◆ Newton’s second law of motion: the change in a body’s velocity due to an applied force is in the same direction as the force and proportional to it, but is inversely proportional to the body’s mass. Force = mass x acceleration. ● Force = the rate of change of momentum ◆ Newton’s third law of motion: for every force, there is always an equal and opposite reaction force. ● Action = reaction. ● Reaction forces act on different objects. ● For any force, there is always an equal and opposite reaction force. ● Is the force Earth exerts on you larger, smaller, or the same as the force you exert on it? ○ Earth and I exert equal and opposite forces on each other. ➔ How is Mass Different From Weight? ◆ Mass - the amount of matter in an object (m, units of kg) ◆ Weight - the force that acts upon an object (units of newtons, or kg m/s^2) ◆ On Earth, your weight is w = mg. When gravity is the ONLY force acting you are said to be in “free-fall.” ◆ You are “weightless” in free-fall! ➔ Conservation Laws

➔ Clues to the formation of the solar system: organized orbits/rotation of planets; terrestrial vs. jovian planets; the nature and locations of asteroids and comets; interesting exceptions to the patterns (motion of Venus and Uranus, the large size of Earth’s moon). ➔ A model for the formation of the solar system. ➔ The timescale of the formation of the solar system. ◆ Age of meteorites, sun, etc. Lecture 6: Overview of the Solar System ➔ Exam will have a mixture of multiple choice and true/false questions ➔ Exam will be based on the course material up through and including “The Solar System” ➔ This corresponds to lectures 1-6 ➔ The relevant reading from the textbook are given in the syllabus ➔ Review: ◆ Nature of light (wave, particle), the electromagnetic spectrum. ◆ Nature of matter on atomic scales. ◆ What do we learn from light (i.e. spectra)? ● Composition of matter, temperature of matter, motion of matter (along the line of sight) ◆ Telescopes: collecting area, angular resolution, refracting vs. reflecting. Multi-wavelength. ◆ Instrumentation: imaging and spectroscopy ◆ Telescopes in space. Particularly impressive telescope technologies: adaptive optics; interferometry. Both technologies provide increased angular resolution (i.e., sharper images). Molecules like Osof or Oxygen are pretty indicative of life. If the oxygen doesn’t have life, it will probably react with chemicals on the surface and change up. ➔ Mercury ◆ Made of metal and rock; large iron core. ◆ Desolate, cratered like our Moon; long, tall, steep cliffs ◆ Very hot and very cold; 425 C (day) -170 C (night) ◆ Mercury *day* = 58.6 Earth days. 3 Mercury *days*,every 2 Mercury *years* ➔ Venus ◆ Nearly identical in size to Earth; surface hidden by thick clouds. ◆ Hellish conditions due to an extreme greenhouse effect ◆ Even hotter than Mercury 470 C both day and night ◆ Atmospheric pressure is equivalent to 1 km deep in Earth’s oceans ◆ No oxygen, no water ◆ How did it end up so different from Earth? Greenhouse gas effects. ◆ Rotates backwards, and slowly, Sun rises in the West. ➔ Mars ◆ Looks almost Earth-like, but don’t be fooled - it would be hard to breathe there, with lots of harmful UV radiation ◆ Giant volcanoes, a huge canyon, polar caps, more… ◆ Water flowed in the distant past; could there have been life? ◆ Thin atmosphere of carbon dioxide (pressure like summit of Mt. Everest)

◆ 2 moons (Phobos and Deimos) ➔ Jupiter ◆ Much farther from Sun than inner planets (more than 2x distance to Mars) ◆ Also very different in composition; mostly H/He gas ball. ◆ No solid surface ◆ Gigantic for a planet. 300 x Earth’s mass; > 1,000 x Earth’s volume. ◆ Many moons, rings. ➔ Uranus ◆ Much smaller than Jupiter/Saturn, but still much larger than Earth ◆ Made of H/He gas, and hydrogen compounds (H2O2 NH3 CH4) ◆ Extreme axis tilt - nearly tipped on its “side” - makes extreme seasons during its 84 year-old orbit ◆ Moons also tipped in their orbits. ➔ Terrestrial planets are small, rocky, and close to the Sun ➔ Jovian planets are large, gas-rich, and far from the Sun ➔ What about Pluto? We will come back to Pluto ➔ Thousands of rocky asteroids lie between Mars and Jupiter ➔ A successful theory of solar system formation must allow for exceptions to general rules, such as Uranus’s odd tilt, orbit, Earth’s relatively large moon ➔ What theory best explains the Solar System? ➔ The Nebular Theory ◆ According to the nebular theory, our solar system formed from a giant cloud of interstellar gas (hydrogen), which also contained tiny solid grains of heavier elements. (Nebular = cloud) → orion nebula ➔ Conservation Laws ◆ Angular Momentum: ● In the product m X v X r, extended arms mean larger radius and smaller velocity of rotation. ● Bringing in her arms decreases her radius and therefore increases her rotational velocity. ◆ Energy: ● T = 0, v = 0 at the top of building ● t = 1, then t =2… v = 10 m/s, then v = 20 m/s… ➔ So, as clouds get compressed, it begins to spin faster and faster… there is a conservation of energy. ◆ As the cloud begins to fall on itself, it begins to compress, and it begins to get faster. Temperature is a result of this motion. Gravitational energy gets converted to thermal energy. ◆ We know this because we have taken pictures of other solar systems forming. ➔ The temperature was greatest in the core near the forming star and dropped with distance outwards in the disk. There was a critical distance called the FROST LINE. ◆ Inside the frost line: too hot for hydrogen compounds to form ices. Only rocks and metals condense. Outside the frost line: cold enough for ices to form; ice is

of Pluto, the planets divide clearly into two groups: terrestrial and jovian. (3) The solar system contains huge numbers of asteroids and comets. (4) There are some notable exceptions to these general patterns. ◆ What theory best explains the features of our solar system? ● The nebular theory, which holds that the solar system formed from the gravitational collapse of a great cloud of gas. ● Radiometric dating of meteorites sets the age of the solar system as 4.6 billion years. ◆ What caused the orderly patterns of motion in our solar system? ● A collapsing gas cloud naturally tends to heat up, spin faster, and flatten out as it shrinks in size. Thus, our solar system began as a spinning disk of gas. The orderly motions we observe today all came from the orderly motion of this spinning disk of gas. ◆ Why are there two types of planets? ● Planets formed around seeds Review Midterm 1 ➔ Mass of the hydrogen atoms: 10^-27 kg ➔ Age of the universe in seconds 10^17 sec ➔ If earth was a basketball ◆ The moon would be a tennis ball 30 ft.away ◆ Sun would be 2.2 miles away ◆ Pluto would be 80 miles away ◆ The nearest star would be 500,000 miles away. ➔ Light year ◆ The speed of light is 300,000 km/s ◆ There are about 31.5 million seconds in a year ◆ Alpha Centauri takes 4.2 years to travel going speed of light ➔ Universe ◆ Milky way contains 100 billion stars and is 100,000 light years across ◆ Solar system is about 28,000 light years from the center of the Milky Way ◆ Nearest galaxies (Magellic Clouds) are 200,000 light years away. ◆ Nearest spiral galaxy (Andromeda) is 2.3 million light years away) ◆ Most distant galaxies are about 10 billion ly away ◆ We see these galaxies as they were billions of years ago ➔ We are going about 1650 km/hour or 1000 mph at the equator ➔ Earth orbits revolves around the Sun once every year traveling at an avg speed of 30 km/s ◆ Earth’s average orbital speed is 108,000 km/hr ◆ Perihelion nearest point to the Sun in orbit is 147.1 million km and 152.1 million km is our aphelion: farthest point from the Sun in orbit ➔ Our Sun and solar system orbit around the center of the Milky Way Galaxy every 230 million years! This speed is at about 540,000 mph ➔ Mass-Energy ◆ Mass itself is a form of potential energy E = mc^2

◆ A small amount of mass can release a great deal of energy. ◆ 1 megaton H-bomb converts only 100 grams (3 ounces) of matter to energy. ◆ Also, concentrated energy can spontaneously turn into particles (for example, in machines called particle accelerators); m = E/C^2 ➔ The Force of Gravity ◆ One of Isaac Newton’s most important discoveries ◆ Masses attract each other through the force of gravity, Fg . ◆ The strength of the gravitational force, Fg , between two masses, M1 and M2 , is proportional to the product of M1 and M2 . ◆ The strength of the gravitational force, Fg , between two masses, M1 and M2 , is inversely proportional to the square of the distance between the two objects, d . ◆ What’s another force that falls off according to the inverse square of the distance? What’s a force that follows a different proportionality? ◆ What happens when th distance approaches infinity? ➔ WHAT DETERMINES THE STRENGTH OF GRAVITY? ◆ The Universal Law of Gravitation ◆ (1) Every mass attracts every other mass ◆ (2) Attraction is directly proportional to the product of their masses. ◆ (3) Attraction is inversely proportional to the square of the distance between their centers. ◆ Fg = G * (M1*M2)/d^2 ◆ Gravity is not a constant force; its value changes with distance ➔ What happens to the strength of the force is we double one of the masses? ◆ A. ½ as strong. B. 2 times stronger. C. 16 times stronger. D. ¼ as strong ➔ Escape Velocity ◆ If an object gains enough orbital energy, it may escape (change from a bound to an unbound orbit). ◆ Escape velocity from Earth = 11.1 km/s from sea level (about 40,200 km/hr; 25,000 mph or 7 miles per second). This velocity depends on the mass and radius of the Earth, not the mass of the object - i.e. Vesc = (2GMearth/Rearth)^½ ➔ Newton’s Version of Kepler’s 3rd Law ◆ If a small object orbits a larger one, and you measure the orbiting object’s orbital period AND average orbital radius ◆ THEN you can calculate the mass of the larger object ◆ Examples ( know these ): ● Calculate mass of Sun from Earth’s orbital period (1 year) and average distance (1 AU). ● Calculate mass of Earth from the orbital period and average distance of any orbiting satellite, including the Moon. ● Calculate mass of Jupiter from the orbital period and distance from Jupiter of one of its moons. ◆ Newton’s Version of Kepler’s 3rd Law ● p^2 = ((4pi^2)/G(M1+M2))*a^3 ● Where, p = orbital period; a = average orbital distance (between centers)

◆ Atomic Number = # of protons in nucleus ◆ Atomic Mass Number = # of protons + neutrons ➔ Isotopes ◆ Isotope: same # of protons but different # of neutrons. ◆ Molecules: consist of two or more atoms ◆ Ion: atom or molecule with net positive or negative charge (because # of protons and electrons doesn’t balance ◆ Different isotopes of the same element contain the same number of protons but different number of neutrons ➔ How do light and matter interact? ◆ Emission …photons are produced ◆ Absorption …photons are consumed ◆ Transmission …photons pass through freely ◆ Reflection or Scattering …photons are redirected, in the same (reflection) or random (scattering) directions ➔ What types of light spectra can we observe? ◆ Continuous spectrum ◆ Absorption line spectrum ◆ A plot of energy versus wavelength is a spectral energy distribution, or simply a spectrum ➔ How does light tell us what things are made of? ◆ Electrons in atoms have distinct energy levels . ◆ We say that the electron energy is quantized . ◆ You could think of this as orbital energy. ◆ Each chemical element, ion, molecule, has a unique set of energy levels . ◆ The energy levels are tiny, and given in special units called electron volts; 1 eV = 1.6 x 10^-19 joules ➔ Distinct energy levels in atoms lead to distinct emission or absorption lines ◆ The Unique Hydrogen energy levels and hydrogen spectra in emission and absorption. ➔ How does light tell us the temperatures of planets and stars? ◆ Thermal Radiation: ● Nearly all large or dense objects emit thermal radiation , including stars, planets, you… ◆ An object’s thermal radiation spectrum depends on only one property: its temperature ● Temperature represents average kinetic energy ● In a dense object, the many random bounces of photons means that the photons end up with energies that match the kinetic energies of atoms or molecules of the object. ● Because the photon's energies depend only on the object’s temperature, we call this light “ thermal radiation .”

● Electromagnetic radiation produced this way has a continuous spectrum of energy with a peak at one wavelength . That wavelength is determined only by the temperature of the object. ➔ Two Properties of Thermal Radiation ◆ (1) Hotter objects emit more light at all frequencies per unit area ◆ (2) Hotter objects emit photons with a higher average energy. ➔ Two Laws of Physics: ◆ Stefan-Boltzmann : Emitted intensity per square meter = constant * T^4 ◆ Wien’s Law : Wavelength of maximum emission = constant/T ➔ The Doppler Effect: ◆ Frequency changes with directional movement of the auditory source. Light waves do the same. ➔ Thought Question: ➔ I measured a line in the lab at 500.7nm. The same line in a star has a wavelength 502.8 nm. What can I say about this star? ◆ Longer wavelength means shift to red due to recession (moving away). ◆ The Doppler shift tells us only about the part of an object’s motion toward or away from us, also called the “line of sight” motion. The object could be moving partially across our line of sight. ➔ Doppler Effect Summary ◆ Motion toward or away from an observer causes a shift in the observed wavelength of light: ◆ Blueshift (shorter wavelength) → motion toward you ◆ Redshift (longer wavelength) → motion away from you ◆ V = change in wavelength / original wavelength * C (speed of light km/s) ◆ Greater shift → Greater speed ◆ 502.8 - 500.7 / * c → 2.1 / 500.7 * c = 0.00149 * 300,000 km/s = about 1.2 million m/s ◆ The Doppler effect tells us how fast an object is moving toward or away from us. ◆ Blushift: objects moving toward us. ◆ Redshift: objects moving away from us. ◆ Every kind of atom, ion, molecule, produces a unique set of spectral lines. ◆ We get temperature from the spectrum of thermal radiation. ➔ Collecting Light With Telescopes ◆ We collect more light with telescopes than with our eyes. And we have a stronger sharpness of things in the cosmos close together. ◆ Telescopes collect more light than our eyes → light-collecting area ● Scales as (Telescope Diameter)^2 ● 10-m telescope is 100 times better than a 1-m telescope ◆ Telescopes can see more detail than our eyes → angular resolution ● Smallest angular size resolved scales as 1/(Telescope Diameter) ● 10-m telescope can resolve 10 times finer detail than 1-m telescope ● Telescopes + instruments can detect light that is invisible to our eyes (e.g., infrared, ultraviolet)

◆ Adaptive optics and digital imaging ◆ Turbulence in the atmosphere blurs images and prevents telescopes from reaching their natural degree of angular resolution. ➔ Adaptive Optics and Digital Imaging ◆ Rapid changes (700 times per second) in the shape of a special mirror can compensate for atmospheric turbulence ● Without adaptive optics; single blurred object. ● With adaptive optics; resolved into 2 objects. ➔ The planets are tiny compared to the distance between them. Two groups: the inner 4 terrestrial planets and the outer 4 Jovian planets and pluto. ➔ Venus, Mercury, Earth, all the planets orbit on the same orbital plane, so they are NOT orbiting perpendicular to the ecliptic plane. ➔ If Ben Frank lit a candle at the moment of the declaration of independence, how far has it reached? ◆ 250 yr * 3e7 s/yr * 3e8 m/s = 2e18m ➔ Which of the following statements is true about a star and planet system? ◆ The star and planet move around a common center of gravity that is much closer to the star than to the planet. MIDTERM 2 PRACTICE - LECTURE 8 ➔ Jovian Planet Basics ◆ What are Jovian Planets made of? ● Hydrogen and helium ◆ What are jovian planets like on the inside? ◆ What is the weather like on Jovian planets ➔ Bigger than Earth, and with more mass than Earth ◆ Lower density, so different composition from Earth ◆ Rings. Numerous moons. Studied by space missions: Pioneer 10, 11; Voyager 1, 2,; Galileo; Cassini ➔ What are jovian planets made of? ◆ Jupiter and Saturn: almost all H and He, very little metal and rock (therefore less dense.) ◆ Uranus and Neptune: <50% H and He, the rest hydrogen compounds (water, methane, ammonia), with some metal and rock. ◆ Why are they different in composition? ➔ Jovian Planet formation ◆ Recall the conditions in the solar nebula ◆ Beyond the Frost Line, planetesimals could accumulate ICE ◆ Hydrogen compounds are more abundant than rock/metal so jovian planets got bigger. ◆ The gravity of such large cores pulled in hydrogen and helium from the nebula and so these objects acquired H/He atmospheres ➔ Jovian planet formation

◆ All the Jovian cores are very similar: ~10 x Earth masses ◆ The jovian differences are in the amount of H/He gas accumulated. ➔ Why did that amount differ? ➔ Differences in Jovian planet formation ◆ Timing : the planet that forms earliest captures the most hydrogen and helium has. Capture ceases after the first solar wind blew the leftover gas away. ◆ Location : the planet that forms in a denser part of the nebula forms its core first and grows bigger. ➔ More distant Jovian planets (Uranus, Neptune) started forming later than closer-in ones (Jupiter, Saturn). ➔ Since the solar wind blew out remaining gas at a certain point, Uranus/Neptune had less time to accumulate gas → ended up smaller. ➔ Jovian Planet Densities ◆ Stacking pillows ◆ Adding one more results in greater compression of the lower layers; the stack gets smaller. ◆ Jupiter and Saturn are nearly the same size (radius). More massive planets could even be smaller! ➔ What are jovian planets like on the inside? ◆ No solid surface. Layers and layers of gas under ever higher pressure and temperatures. ◆ Cores (~10 Earth masses) made of hydrogen compounds, metals & rock. ◆ The layers are different for the different planets. Why? ➔ Layers Differ in Phase ◆ Density of liquid water is ~ 1g/cm^3 ◆ Gaseous hydrogen: cold and very low density. ◆ Liquid hydrogen: caused by compression of overlying layers; much denser and hotter. ◆ Metallic hydrogen conducts electricity; it is not solid, but a very viscous fluid. ◆ Core is hydrogen compounds, metals, rocks. But not in a form you’d recognize… 10x the mass of Earth inside a volume the size of Earth. ➔ Why different? ◆ Less mass → less gravity → less compression. ◆ Mass controls everything. ◆ Boundaries of the layers are deeper in less massive jovian planets. ◆ The physical states of the less massive jovians are less extreme (could even be liquid). ➔ Rotation ◆ Jovian planets rotate rapidly (“day” is ~10 hours for Jupiter and Saturn, and 16-17 hours for Uranus and Neptune). ◆ Saturn is flattened by rotation! More so than Jupiter since Saturn’s surface gravity is weaker.

◆ Internal Structure and the Lithosphere ● The “lithosphere” is the cool rigid rock that forms a planet’s outer layer; the crust and some of the mantle. ◆ The lithosphere floats on the lo er layers. ● If the lithosphere is near the surface then the planet will be geologically active. ➔ Differentiation ◆ Layers ordered by density. ◆ Highest density on the bottom. ◆ Gravity sorts materials by density ➔ Differentiation converts gravitational potential energy to heat. ◆ As dense materials sink they convert gravitational energy to kinetic energy (heat). ◆ That’s definitely why the very dense core of Earth and other planets are hotter. ◆ This is what drives geological activity. ➔ Heat transfer drives geological activity ◆ As planet cools, internal heat must transfer to planetary surface. For Earth, most important heat transfer process is convection. ◆ Convection : hot molten rock rises, cool rock falls. ◆ 1 mantle convection cycle takes 100 million years on Earth (1 cm/year). ◆ Mantle convection: hot rock rises and cooler rock falls. ➔ Planet size matters ◆ A large terrestrial planet ◆ Is still warm inside (surface-area/volume ratio) ◆ Has a “convecting” mantle ◆ Has a thinner, weaker lithosphere ◆ Has molten rock nearer the surface. ● Which makes it more geologically active ● Earth and Venus should be geologically active, whereas Mercury and Moon should not. Mars is intermediate. ◆ Venus has too thin of crust, so the crust does not float, it breaks. ➔ Planet Size Matters ◆ Volume: total amount of heat contained in a planet ◆ Surface area: rate at which heat escapes a planet ◆ Ratio of surface-area-to-volume determines the time it takes for planet to cool off. ◆ Larger surface-area-to-volume → shorter cooling time. ◆ Smaller surface-area-to-volume → longer cooling time. ◆ Surface area relies on square of the radius. ➔ Surface-area-to-volume = surface area / volume

➔ Surface area of a sphere? 4piR^2 ➔ Volume of a sphere? 4/3piR^3 ➔ Surface-area-to-volume = 4piR^2 / (4/3piR^3) = 3/R ➔ This means that, as R gets bigger (i.e., in larger objects), surface-area-to-volume decreases. ◆ Since smaller surface-area-to-volume means longer cooling times, this means that larger objects take longer to cool off! ◆ This rule applies to the terrestrial planets. ➔ Mars vs. Earth ◆ 50% Earth’s radius, 10% Earth’s mass ◆ 1.5 A.U. from the Sun; so (1.5)^2 less heat and light ◆ Axis tilt about the same as Earth; seasons ◆ Similar rotation period (23.96 hours vs. 24.6 hours) ◆ Orbit is more elliptical than Earth; seasons more extreme in the South than the north. ◆ Very thin CO2 atmosphere: little greenhouse effect. ◆ Cold desert, despite warm red look due to soil color. ◆ So main difference is…Mars is smaller! ◆ Orbit is a little bit more elliptical than earth, so the seasons are more extreme. ➔ Why did Mars change? ◆ It was too small, it cooled off too quickly, lost its magnetic field and then the solar wind stripped away its atmosphere. Temps dropped and it “freeze dried” (3 billion years ago). Solar radiation wiped out atmosphere. ➔ Planetary Magnetic Fields ◆ Moving charged particles create magnetic fields. ◆ Magnetic fields are very important for many reasons: ● A planet’s interior can also create a magnetic field, if the core is electrically conducting (e.g. iron), convecting (molten), and rotating. ● Internal heat contributes to planetary magnetic field. ● Earth is only terrestrial planet with strong magnetic field. ◆ Dynamo effect ➔ Earth has both rotation and liquified metals in the interior, thus giving it that magnetic charge because its moving (rotating) and it has electrically conducting metals inside that are convecting. ➔ Mars has rotation, but not the liquified metals in the interior.