The Solar System • 1 star • 8 planets • several dwarf planets • many moons • asteroids, comets, meteoroids
What is a Planet? Due to the discovery of additional solar system bodies in recent years, it became necessary to re-examine the term “planet”. At the August 2006 International Astronomical Union meeting in Prague, a new definition was passed. RESOLUTION 5A: A planet is a celestial body that (a) orbits the Sun, (b) has enough mass to form a spherical shape, and (c) has cleared the neighborhood around its orbit. A dwarf planet is a celestial body that (a) orbits the Sun, (b) has enough mass to form a spherical shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite. Thus, Pluto is now a Dwarf Planet mainly because it has not “cleared its neighborhood” sufficiently. There are other objects in similar orbits as Pluto - Neptune, Pluto’s moon Charon, Kuiper Belt objects.
The laws of planetary motion were determined by Johannes Kepler in 1609. The planets’ orbits obey these three laws based on the effects of gravity. The Sun’s gravitational pull dominates the motions of all planets. 1st Law 2nd Law 3rd Law
The distances to planets are known from Kepler’s Laws Sizes are determined from angular size and distance
Masses (and densities) - determined through observing the gravitational effect of the planet on some nearby object (moons, nearby planets, satellites) • Planets orbit the sun counter-clockwise as seen from the North Celestial Pole. • All planets are roughly in the same orbital plane EXCEPT Mercury (and the dwarf planet Pluto).
Terrestrial Planets Jovian Planets • Jupiter, Saturn, Uranus, and Neptune • Far from Sun • Large masses and radii • Gaseous surface • Low densities • Fast rotation • Strong magnetic field • Many rings • Many moons • Mercury, Venus, Earth and Mars • Close to Sun • Small masses, radii • Rocky, solid surfaces • High densities • Slow rotation • Weak magnetic field • No rings • Few moons
Comets Dirty snowballs - dust and rock in methane, ammonia and ice All light is reflected from the Sun - the comet makes no light of its own The nucleus is a few km in diameter Halley’s Comet in 1986 • Long period comets take up to 1 million years to orbit the Sun (may originate in the Oort cloud) • Short period comets orbit the Sun in 200 years or less (e.g. Halley’s comet) – likely originate in the Kuiper belt and were kicked into an eccentric orbit
Comet Temple 1 image obtained from Stardust satellite flyby on Feb 14, 2011 • Crater with a small mound in the center indicates cometary nucleus is fragile and weak. • Caused by impactor from Deep Impact mission in 2005 – found comet to be less icy and more dusty than expected...
Meteoroids – interplanetary rocky objects smaller than 100m (down to grain size). Consist mainly of iron and nickel with some carbon • called a meteor as it burns in the Earth’s atmosphere • if it makes it to the ground, it is a meteorite • Old objects that appear to be as old as the solar system based on radiometric dating Most meteor showers are the result of the Earth passing through the orbit of a comet which has left debris along its path Spring Meteor showers: Lyrids– Apr 21/22 Eta Aquarids– May 5/6
Asteroids - rocks with sizes greater than 100m across Most asteroids remain in the Asteroid belt between Mars and Jupiter but about 2000 have orbits that cross Earth’s path. Based on known Earth-crossing asteroid orbits, it is estimated that 3 asteroids impact Earth every 1 million years!
Asteroid Watch (Near Earth Object Program) http://neo.jpl.nasa.gov/ Asteroid 2005 YU55 to Approach Earth on November 8, 2011 Near-Earth asteroid 2005 YU55 (about 400m in size) will pass within 0.85 lunar distances from the Earth on November 8, 2011.
Asteroids range in size from 100m to ~1000km They are composed of carbon or iron and other rocky material. The Asteroid belt is a group of rocks that appear to have never joined to make a planet (as opposed to having once been a planet that was later destroyed). • Too little mass • Different chemical compositions • Planet formation probably effected by nearby Jupiter’s strong gravity
Formation of the Solar System Any theory to describe the formation of our Solar System must be consistent with these facts: 1. Each planet is isolated in space. 2. The orbits are nearly circular. 3. The orbits of the planets all lie in roughly the same plane 4. The direction the planets orbit around the Sun is the same as the Sun’s rotation on its axis. 5. The direction most planets orbit on their axes is the same as that for the Sun. 6. The direction of the planetary moon’s orbits is the same as that of the planet’s rotation. 7. The terrestrial planets are very different from the Jovian planets. 8. Asteroids are different from both types of planets. 9. Comets are icy fragments that don’t orbit in the ecliptic plane.
Nebular Theory for Solar System formation Our Sun and the planets began from a cloud of dust and gas (nebula) As the cloud contracts under its own gravity, the Sun is formed at the center. The cloud starts to spin and the smaller it contracts, the faster it spins. Conservation of angular momentum The cloud forms a flattened, pancake shape.
Condensation Theory for Planet Formation The gasin the flattened nebula would never eventually clump together to form planets. Interstellar dust (grain-size particles) lies between stars - remnants of old, dead stars. These dust grains form condensation nuclei - other atoms attach to them to start the “collapsing” process to form the planets in the gas cloud.
What happened next….. • Solar nebula contracts and flattens into a disk. • Condensation nuclei form clumps that grow into moon-size planetesimals. • Solar wind from star formation blows out the rest of the gas. • Planetesimals collide and grow. • Growing planetesimals form the planets over about 100 million years. • The more massive proto-planets are also able to sweep up large amounts of gas to become the Jovian planets.
Why the difference between inner and outer planets? TEMPERATURE! • Rocky inner planets: The type of the material that condensed out of the nebular cloud at these higher temperatures was rocky in nature. • Gaseous, Bigger outer planets: Both rock and gas could condense out of the cloud at lower temperatures where these planets formed. Why are they gaseous? - gas is present Why are they bigger? - accretion onto the planet starts sooner because they are further from the Sun, less effected by solar wind
Lunar Surface- lack of atmosphere and water preserves surface features • Maria – mantle material • “seas” - darker areas resulting from earlier lava flow • Basaltic, iron rich, high density (3300 kg/m3). • Highlands – crust material • elevated many km above maria • Aluminum rich, low density (2900 kg/m3).
Craters • Caused by meteoroid impacts • Pressure to the lunar surface heats the rock and deforms the ground • Explosion pushes rock layers up and out • The ejecta blanket surrounds the crater • The rate of cratering on the moon is determined from the known ages of the highland and maria regions. • The Moon (and solar system?) experienced a sharp drop in the rate of meteoritic bombardment about 3.9 billion years ago (end of accretion era) • The rate of cratering has been roughly constant since that time
Formation of the Moon The Moon has an overall composition and density quite difference from the Earth, but which resembles the material in the Earth’s mantle. This observation has led to the Impact Theory for the formation of the Moon. • Mars-sized body hit the molten Earth • Parts of the mantle blew off and later formed the moon • Earth had differentiated, so the mantle (from which the Moon formed) was already metal poor.
Mercury • Mercury is not tidally locked to the sun in the same way as the moon-earth system • Sun’s gravity and Mercury’s eccentric orbit brought it into semi-synchronous orbit • Mercury presents the same face to the sun every OTHER time around (rotational period is 59 days and orbital period is 88 days) These recent images from the MESSENGER satellite (Jan 2008) show a surface similar to the Moon • Old surface with no apparent plate tectonics • Craters flatter and have thinner ejecta rims than lunar craters due to higher gravity on Mercury than the moon • Almost non-existent atmosphere - high surface temperature and low escape velocity (drastic temperature changes 800F (day) to -280 F (night)
Venus • Rotation direction is retrograde (opposite that of other terrestrial planets)! • 243 day rotation period • Axis is almost exactly perpendicular to orbit • Why? Possibly hit by large body during formation altering spin direction • Much more massive atmosphere than Earth’s • Carbon dioxide (96.5%), Nitrogen (3.5%) • No H20 - the clouds are made of sulfuric acid • Fast moving (400km/hr!) clouds in upper atmosphere • Clear, still air at the surface • 730K temperature on the surface (due to greenhouse effect) Pioneer spacecraft image taken 1979
Venus reflected radio waves reveal the surface topology • Elevated “continents” make up 8% of the surface (25% on Earth) • Mostly rolling plains with some mountains (up to 14 km) • No tectonics • Volcanoes resurface the planet every ~300 million years • Shield volcanoes are the most common (like Hawaiian Islands) • A caldera (crater) is formed at the summit when the underlying lava withdraws • Venus is thought to still be volcanically active today
A few Soviet spacecraft have landed on Venus in 1970s • Survived only an hour before burning up • little evidence of erosion - young surface • rocks are basaltic and granite • Cloud cover makes Venus seem like a heavily overcast day on Earth all the time!
Mars • Slightly smaller than Earth with similar rotation period of 24.6 hours and inclination (24 degrees) • Very little atmosphere - 1/150 the pressure of Earth - CO2 (95.3%),nitrogen (2.7%), argon, oxygen, CO, water vapor • Two very small moons (Phobos and Deimos) • 1965 – Mariner 4, 6 and 7 “fly by” Mars • 1971 – Mariner 9 orbits and maps Mars in detail • 1976 – Viking 1 lands on Mars • 1997 - Pathfinder and Sojourner rovers - a highly successful mission which sent back lots of pictures of the Martian landscape (+ soil and atmosphere analyses) – revealed iron rich soil – Mars is “rusting” • 2004 – rovers Opportunity and Spirit are currently studying evidence for water on Mars • 2008 – Phoenix mission at north polar region
Mars Surface • Polar ice caps • frozen CO2 • Northern hemisphere • rolling volcanic planes • Like lunar maria but larger • Few craters – young (3 billion yrs old) • Southern hemisphere • heavily cratered highlands • Older (4 billion yrs old) Some interesting features: Tharsis Bulge - Roughly the size of North America, sits on the equator - 10km high, less heavily cratered (i.e. young surface (2-3 billion yrs)) Valles Marineris - extends one-fifth of the way around the planet at the equator, up to 120 km across and 7 km deep, the Grand Canyon would fit into one of its side "tributary" cracks, probably produced from stretching and cracking when Tharsis bulge formed
Mars Earth Volcanoes on Mars • Largest volcanoes in the solar system are here (Mars has a surface gravity only 40 percent that of Earth, and its volcanoes rise roughly 2.5 times as high) • Shield volcanoes • None appear currently active but eruptions occurred 100 million years ago Water on Mars About 4 billions years ago Mars had a thicker atmosphere, warmer surface, and liquid water • Runoff channels • Found in southern highlands • Extensive river systems (like Earth) • Carried water from highland to valleys • Outflow channels • Caused by flooding • Found at the equator • Formed about 3 billion years ago
Internal Structure of the Terrestrial Planets All terrestrial planets are likely to be differentiated like Earth The larger planets take longer to cool and therefore still have active mantles and liquid cores • Mercury • Interior dominated by large iron core (high density and magnetic field present) • Solid mantle prevents volcanoes and tectonics • As core formed and cooled, shrinking caused the surface to contract (scarps,fissures, etc)
*Venus and Earth are believed to have similar interiors • Venus • No magnetic field (slow rotation) • surface of Venus resembles that of the young Earth, at an age of perhaps a billion years • Never developed plate tectonics – possibly because of high surface temp and soft crust • Mars • Very weak magnetic field – non liquid or non metallic core? • large-scale tectonic activity almost started but was stifled by rapidly cooling outer layers
Space Craft Exploration of Jovian Planets • Voyager 1and 2 left Earth in 1977 and reached Jupiter in March and July of 1979 • Used Jupiter’s strong gravity to send them on to Saturn - gravity assist • Voyager 2 used Saturn’s gravity to propel it to Uranus and then on to Neptune • Studied planetary magnetic fields and analyzed multi-wavelength radiation • Both are now headed out into interstellar space • Galileo - launched in 1989 and reached Jupiter in December 1995 • Two components: atmospheric probe and orbiter • Probe descended into Jupiter’s atmosphere and orbiter went through moon system • Cassini mission to Saturn arrived June 30, 2004 • orbiter continues to orbit Saturn and its moons • Huygens probe launched from the orbiter January 14, 2005 to study Saturn’s moon Titan.
Jovian Planet Properties • Most of their mass is Hydrogen and Helium – light elements = low densities • High surface gravity allows their atmospheres to retain these light elements • Dense compact core at the center - no solid surface – gaseous atmosphere becomes denser (eventually liquid) at core • Differential Rotation – outer regions rotate at a different rate than the inner regions
Jovian planets - axis tilt and magnetic fields • All Jovian planets have strong magnetic fields - rapid rotation and liquid conductive cores or mantles • Uranus has the most inclined rotational axis (extreme seasons)
Jupiter’s Atmosphere • molecular hydrogen – 86% • helium – 14% • small amounts of methane, ammonia, and water vapor • Darker belts lie atop downward moving convective cells • Lighter zones are above upward moving cells • Belts are low-pressure, zones are high pressure • As on Earth, wind moves from high to low • Jupiter’s rotation causes wind patterns to move East/West along equator • Temperature difference between bands is main reason for color difference
Weather on Jupiter • Main weather feature – Great Red Spot! • swirling hurricane winds • has lasted over 300 years • diameter twice that of Earth • rotates with planet’s interior • the spot appears to be confined and powered by the zonal flow • smaller storms look like white ovals
Saturn’s Atmosphere • molecular hydrogen 92.4% • helium 7.4% • traces of methane and ammonia • Overall temperature is cooler than Jupiter • Cloud layer thickness is 3 times that of Jupiter (caused by lower surface gravity on Saturn) • Thicker clouds result in less varied visible colors • Computer enhanced image shows bands, oval storm systems, and turbulent flow patterns like those seen on Jupiter
Atmospheres of Uranus and Neptune • molecular hydrogen 84% • helium 14% • methane 2% (Uranus) 3% (Neptune) • abundance of methane gives these planets their blue color. Methane absorbs longer wavelength light (red) and reflects short wavelength light (blue) • Uranus • Few clouds in the cold upper atmosphere – featureless • Upper layer of haze blocks out the lower, warmer clouds • Neptune • Upper atmosphere is slightly warmer than Uranus (despite its further distance from Sun) • More visible features (thinner haze, less dense clouds lie higher)
Internal Structures – models that fit the data Metallic hydrogen is like liquid metal Uranus/Neptune Saturn Jupiter • Increasing temperature and pressure deeper in core • Jupiter bulges at radius (7% larger) • Saturn less asymmetric – larger core – same basic overall structure • Uranus/Neptune have a high density “slush” below cloud level - compressed water clouds
Pluto - and Charon • Discovered in 1930 • Charon (its moon) found in 1978 • 40 times the Earth’s distance from the Sun (40AU) • No spacecraft flybys but New Horizons mission launched in January 2006 will fly by in July 2015. • Only 20% the mass of our Moon • Similar in mass and size to Neptune’s large moon Triton • Probably formed in the Kuiper belt (comet birth place) • Highly inclined orbital plane HST image of Pluto
Eris Kuiper Belt Objects compared The discovery of Eris in 2005 showed that Pluto was not unique. These objects, along with Pluto, seem to be the largest of the Kuiper Belt objects.
Extrasolar Planets - planets around other stars…. • About 530 planets have been “detected” around 445 nearby stars. Most were identified by monitoring the star’s wobble due to gravitational attraction of the orbiting planet(s). • Kepler mission identified ~1200 candidate planets via the transit method. ~5 are earth-sized in habitable zone 51 Pegasi - the first detection of an extrasolar planet in 1994 Evidence of 3 planets orbiting Upsilon Andromedae! planetquest.jpl.nasa.gov