Summary of Our Solar System Earth, as viewed by the Voyager spacecraft

Studying the Solar System What does the solar system look like? What can we learn by comparing the planets to one another? What are the major features of the Sun and planets? How do we learn these things?

What does the solar system look like? Planets are tremendously exaggerated in size compared to their separations The inner, terrestrial planets are very different from the outer, jovian, ones. Accurate SS Model

The SUN rules the SOLAR SYSTEM Eight major planets with nearly circular orbits Most have moons, some very large All orbit the Sun moving in the same direction Pluto is smaller than the major planets and has a more elliptical orbit: with Eris, Ceres, “dwarf planets” Lots of asteroids and comets -- space debris; not much mass in them, though. Mass of Sun: 21030 kg Radius of Sun: 7105 km Luminosity: 41026 Watts Surface temperature: 5760 K

Planetary Orbits Planets all orbit in same direction and nearly in same plane Martian Orbit Applet Jovian Orbit Applet Pluto orbit applet

Thought Question How does the Earth-Sun distance compare with the Sun’s radius It’s about 10 times larger. It’s about 50 times larger. It’s about 200 times larger. It’s about 1000 times larger.

Thought Question How does the Earth-Sun distance compare with the Sun’s radius It’s about 10 times larger. It’s about 50 times larger. It’s about 200 times larger. It’s about 1000 times larger.

What can we learn by comparing the planets to one another?

Comparative Planetology We can learn more about a world like our Earth by studying it in context with other worlds in the solar system. Stay focused on processes common to multiple worlds instead of individual facts specific to a particular world: interiors, surfaces, atmospheres We can understand how planets formed and evolved better by comparing them The possibility of life elsewhere in our solar system or in other solar systems can be considered sensibly via comparisons Start here on 3/22

Comparing the planets reveals patterns among them Those patterns provide insights that help us understand our own planet

Some Major Features of the Sun and Planets Sun and planets to scale

Sun Over 99.9% of solar system’s mass: 1.9891030 kg These slides follow the planetary tour pages in ch. 7. Over 99.9% of solar system’s mass: 1.9891030 kg Made mostly of H/He ionized gas (plasma); radius: 6.96105 km Converts 4 million tons of mass into energy each second; powered by FUSION: 4 1H  4He + energy ; luminosity: 3.861026 W

Mercury Metal and rock; large iron core but tiny: 0.38 RE, 0.055 ME Desolate, cratered; long, tall, steep cliffs; no real atmosphere Very hot/very cold: 425°C (day), –170°C (night) at 0.39 AU Tides lock it into 3:2 resonance -- 3 sidereal days = 2 years

Venus Nearly identical in size to Earth (0.95 RE); surface hidden by clouds Hellish conditions due to an extreme greenhouse effect: Even hotter than Mercury: 470°C, day and night at 0.72 AU Were (are?) active volcanos; pressure of CO2 atm about 92 times ours

Earth Earth and its surprisingly large Moon to scale Average T = 17 C (290 K); modest greenhouse effect The only surface liquid water in the solar system An oasis for life: only O2 rich atmosphere Plate tectonics recycle the earth’s crust Significant magnetic field from liquid iron part of core

Mars Looks almost Earth-like, but 0.53 radius and 0.107 mass A very thin, CO2 dominated atmosphere: T = 220 K at 1.52 AU Giant volcanoes, a huge canyon, polar caps, more… Water flowed in distant past; could there have been life?

Jupiter Much farther from Sun than inner planets: 5.2 AU Mostly H/He; no solid surface 318 times more massive than Earth 11.2 times its radius Many moons, 4 very large, + rings 125 K, generates some of its own heat Very fast winds & big storms in atmosphere

Jupiter’s moons Io (shown here): active volcanoes (sulfurous) all over Are as interesting as some planets themselves, especially Jupiter’s four big Galilean moons Io (shown here): active volcanoes (sulfurous) all over Europa: very likely subsurface ocean Ganymede: largest moon in the entire solar system Callisto: a large, cratered “ice ball”

Saturn Giant and gaseous like Jupiter: 9.46 RE, 95.2 ME At 9.54 AU and T = 95 K (lots of 95s here!) Spectacular rings Many moons, including big cloudy Titan with hydrocarbon lakes

All Jovian Planets Have Rings & Moons Rings are NOT solid; they are made of many small chunks of ice and rock, each orbiting like a tiny moon. Artist’s conception

Current Robotic Mission to Saturn: Cassini Starte here on 10/13/09: Inset art shows the Huygens probe separated from the main spacecraft on its descent to Titan… Cassini mission arrived at Saturn in July 2004 (Launched in 1997) The Huygens probe landed on Titan in January 2005

Uranus Smaller than Jupiter/Saturn; much larger than Earth: 3.98 RE & 14.5 ME Made of H/He gas & hydrogen compounds (H2O, NH3, CH4) Extreme axis tilt -- nearly on its side, so very long seasons, 60 K at 19.2 AU Moons & rings

Neptune Similar to Uranus (except for axis tilt) but bluer 3.81 RE and 17.1 ME 30.1 AU and T=58K Many moons too (including big, retrograde orbiting Triton) Note: this is the same image as in the text but rotated 90° to show the axis tilt relative to the ecliptic plane as the horizontal on the page. (The orientation in the book chosen for its more dramatic effect.)

Pluto and other Dwarf Planets Much smaller than major planets: round via gravity but don’t dominate orbital zone. (Pluto: 0.18 RE and 0.0022 ME) Icy, comet-like composition -- smaller than our Moon Pluto’s main moon (Charon) is of similar size Pluto at 39.5 AU, Eris at 67.7 AU is a bit bigger

This important summary table may be worth some time in class to make sure students understand how to read it…

Thought Question What process created the elements from which the terrestrial planets were made? The Big Bang Nuclear fusion in stars Chemical processes in interstellar clouds Their origin is unknown

Thought Question What process created the elements from which the terrestrial planets were made? The Big Bang Nuclear fusion in stars Chemical processes in interstellar clouds Their origin is unknown

General Features of the Solar System What does the solar system look like? Planets orbit Sun in the same direction and in nearly the same plane. What can we learn by comparing the planets to one another? Comparative planetology looks for patterns among the planets. Those patterns give us insight into the general processes that govern planets Studying other worlds in this way tells us about our own Earth

Simplest Take-Away Points The major features of the Sun and planets Sun: Over 99.9% of the mass: provides light and heat Mercury: A hot rock Venus: Same size as Earth but much hotter Earth: Only planet with liquid water on surface Mars: Could have had liquid water in past Jupiter: A gaseous giant Saturn: Gaseous with spectacular rings Uranus: A gas giant with a highly tilted axis Neptune: Similar to Uranus but with normal axis Dwarf Planets: Most (like Pluto) are icy like comets

What features of our solar system provide clues to how it formed?

Motion of Large Bodies All large bodies in the solar system orbit the sun in the same direction and in nearly the same plane (ecliptic) Most also rotate in that “prograde” direction with modest tilts (<30o) Conservation of angular momentum

Two Main Planet Types Terrestrial planets are made of rocks & metals, are relatively small, and close to the Sun Jovian planets are gaseous, mostly H2 and He, much larger, and farther from Sun

Swarms of Smaller Bodies Too Many rocky asteroids and icy comets populate the solar system Most asteroids are in a “belt” between Mars and Jupiter Inner comets are in the Kuiper belt beyond Neptune Comet cores are now mostly in the huge Oort cloud

Notable Exceptions Several exceptions to the normal patterns need to be explained Earth’s big Moon Uranus’ sideways tilt Venus’ (& technically Uranus’) retrograde spins Triton’s retrograde orbit around Neptune

How did we finally learn the physical scale (1 AU = 1 How did we finally learn the physical scale (1 AU = 1.496108km) of the solar system? Edmund Halley suggested using parallax during transits of Venus in 1710 Tried in 1761 & 1769 and w/ photographs in 1874 and 1882

Transit of Venus Apparent position of Venus on Sun during transit depends on distances in solar system and your position on Earth Carefully measure times when transit starts and stops at different latitudes Transit of Venus: June 8, 2004

Measuring Distance to Venus Measure apparent position of Venus on Sun from two locations on Earth Earth’s radius is known so baseline on earth is known Use trigonometry to determine Venus’ distance from the distance between the two locations on Earth We know Venus’s orbit is at 0.723 AU so we can get the value of an AU

Summary of Key System Features What features of the solar system provide clues to how it formed? Motions of large bodies: All in same direction and plane Two main planet types: Terrestrial and jovian Swarms of small bodies: Asteroids and comets Notable exceptions: Rotation of Uranus, Earth’s large moon, etc.

Spacecraft Exploration of the Solar System Spacecraft are essentially robots with computers, rockets, cameras, & other instruments (spectrometers, magnetometers) Four Basic classes: Flybys (cheap), but fleeting -- mostly quick pix Orbiters (need more fuel to brake & enter orbit) but can make much longer observations Landers (really expensive & big risk of crash) but detailed measurements on the ground (or in atm) Sample return missions (most expensive: so far only manned Apollo & robotic Luna missions to the Moon)

How do robotic spacecraft get there? Often use a “gravitational slingshot” from a planet instead of fuel whenever possible to accelerate and change direction Of course rockets needed to get to earth’s escape velocity of 11.2 km/s Start here on 3/24/10

Flybys A flyby mission flies by a planet just once Cheaper than other mission types but have less time to gather data Voyagers 1 & 2 to outer planets Messenger to Mercury (as of 2009; will go into orbit)

Orbiters Go into orbit around another world in our Solar System More time to gather data but cannot obtain really detailed information about world’s surface; for example: Magellan orbit mapped Venus surface w/ radar in 1990 Mars Reconnaissance Orbiter took incredibly good photos of martian surface starting in 2006 Mars Express studies climate, geology (ESA, 2004) Galileo went around Jupiter & its moons (1995; sent probe into Jupiter’s atmosphere) Cassini orbits Saturn & its moons (2004; ESA’s Huygens probe to Titan) Chandrayaan (ISRO, 2008) and Lunar Reconnaissance Orbiter (2008) are recent missions orbiting the Moon

Probes or Landers Land on surface of another world (e.g. Spirit & Opportunity Rovers on Mars in 2004) Explore surface in detail with photos, mechanical & chemical analyses Surveyors to Moon, Veneras to Venus, Vikings to Mars in 1960s and 1970s are original landers; Galileo probed Jupiter Hayabusa (JAXA) landed on asteroid Itokawa in 2005 LCROSS hit the Moon on 10 Oct 2009

Sample Return Missions Land on surface of another world Gather samples by hand or claw Spacecraft designed to blast off other world and return to Earth The analysis of samples can be done in the most sophisticated way back on Earth -- and saved for superior methods to be used at later times Last 6 successful Apollo missions to Moon between 1969 and 1972 (and 2 unmanned Soviet Lunas in 1970 & 1976) are the only sample return missions so far if one exempts Stardust, which brought back material from the tail of Comet Wild 2 in 2006