Announcement Chapter 12 Asteroids, Comets, and Dwarf Planets Mastering astronomy assignment 11 Due May 11, 8 am Read Chapters 12 and 13 Two Types of Solar System Debris Comet-Asteroid Similarities Size Smaller than the planets From objects under 1 km in size and up to a few thousand km across Populations Asteroids Comets Closer to the Sun Further from the Sun Largely composed of rock Largely made up of icy material What are asteroids like? A few large members - dwarf planets Countless small members Origin Leftover planetesimals from the formation of the solar system Asteroid Orbits Most asteroids orbit in a belt between Mars and Jupiter Largest is Ceres, diameter ~1,000 km 150,000 in catalogs Probably over a million with diameter >1 km All the asteroids in the solar system wouldn t add up to even a small terrestrial planet Trojan asteroids follow Jupiter s orbit The Asteroid Belt (shown in white) between the orbits of Mars and Jupiter Orbits of nearearth asteroids cross Earth s orbit 1
The possibility of successfully navigating an asteroid field Actually, NASA has sent many space probes into and through the Asteroid Belt Unlike in Star Wars, the Asteroid Belt is not that crowded Average separation between sizable asteroids is 10 million km! Why are there very few asteroids beyond Jupiter s orbit? A. There was no rocky material beyond Jupiter s orbit. B. The heaviest rocks sank towards the center of the solar system. C. Ice could form in the outer solar system. D. A passing star probably stripped away all of those asteroids, even if they were there at one time. Origin of Asteroid Belt The asteroid belt is not the remains of a destroyed terrestrial planet Rocky planetesimals between Mars and Jupiter did not accrete into a planet. Jupiter s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet. Which explanation for the belt seems the most plausible? A. The belt is where all the asteroids happened to form. B. The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter. C. The belt is where all the asteroids happened to survive. What are comets like? Comet Hyakutake (1996) Comet Hale-Bopp (1997) Formed beyond the frost line, comets are icy counterparts to asteroids. Nucleus of comet a dirty snowball Most comets do not have tails. Most comets remain perpetually frozen in the outer solar system. Only comets that enter the inner solar system grow tails. Comet McNaught 2
Nucleus of Comet A dirty snowball Source of material for comet s tail Deep Impact Mission to study nucleus of Comet Tempel 1 Projectile hit surface on July 4, 2005 Many telescopes studied aftermath of impact Anatomy of a Comet How do comets get their tails? Coma is atmosphere that comes from heated nucleus Plasma tail is gas escaping from coma, pushed by solar wind Dust tail is pushed by photons When a comet nears the Sun, its ices can sublimate into gas and carry off dust, creating a coma and long tails. If the Earth passed through the tail of a comet, what would happen? Where do comets come from? a) People would die from the gasses such as methane and ammonia b) It depends on if it was the gas tail or the dust tail c) Earth might be knocked out of its orbit or its axis might get tilted d) Nothing. Halley s comet did this and nothing happened 3
Where do comets come from? Only a tiny number of comets enter the inner solar system - most stay far from the Sun Oort cloud: On random orbits extending to about 50,000 AU Kuiper belt: On orderly orbits from 30-100 AU in disk of solar system How did they get there? Kuiper belt comets formed in the Kuiper belt: flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets. Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction. How big can a comet be? The Discovery of Pluto Discovered in 1930 by Clyde Tombaugh From the time of its discovery until 2006, Pluto was considered the ninth planet Map of Pluto, giving the highest resolution currently possible Pluto s Orbit Is Pluto a Planet? Pluto will never hit Neptune, even though their orbits cross, because of 3:2 orbital resonance Neptune orbits three times during the time Pluto orbits twice By far the smallest planet. Not a gas giant like other outer planets. Has an icy composition like a comet. Has a very elliptical, inclined orbit. Pluto has more in common with comets than with the eight major planets 4
Pluto is smaller than many moons in the Solar System! Pluto s orbit lies in the Kuiper Belt Sicne 1992, roughly a thousand small icy bodies beyond Neptune have been discovered Called the Kuiper Belt Pluto is located in this Kuiper Belt Orbit is typical of KBOs Composition is typical of KBOs Kuiper Belt Objects are denoted by white circles above Eris, the goddess of discord About 2400 km across, bigger than Pluto! Orbits the Sun in about 560 years, at an average distance of 68 AU If Pluto is a planet, what about Eris? The Original August 2006 Proposal A planet is a celestial body that (a) has sufficient mass so that it assumes a nearly round shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet Keck Telescope image of Eris and its moon Dysnomia 12 Planets? Pluto-Charon Binary Planets? Earth-Moon Eris When a moon orbits a planet, or a planet orbits a star, both bodies are actually orbiting around their center of mass 5
Hubble s view of Pluto & Moons Two Dozen Planets??? Santa Easterbunny The Final August 2006 Definition 1. A planet is a celestial body that a) is in orbit around the Sun b) has sufficient mass so that it assumes a nearly round shape c) has cleared the neighborhood around its orbit 2. A dwarf planet is a celestial body that meets (a) & (b) above, and c) has not cleared the neighborhood around its orbit d) is not a satellite of a planet 3. All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies" Clearing the Neighborhood Planets "sweep out" their orbital regions over time They do not share their orbital regions with other bodies of significant size Exceptions - satellites Dwarf planets share their orbital regions with other objects near their own size Plot of planet/dwarf planet mass, M, versus total aggregate mass in its orbital zone, m. Note that the planets greatly outmass the aggregates near their orbits, while dwarf planets do not. The Solar System has Nine Eight Planets Pluto is not a dog... Pluto, Ceres, & Eris are excluded from full planet-hood because they have not cleared the neighborhood...it s a dwarf! 6
Which definition of a planet would you vote for? What is a planet? a) What I learned in school (9) b) The initial 2006 proposal (12+) c) The final 2006 definition (8 + dwarf planets) d) Something else (?) Meteor Terminology Meteorite Impact Meteorite: A rock from space that falls through Earth s atmosphere Meteor: The bright trail left by a meteorite Chicago, March 26, 2003 Meteorite Types Primitive Meteorites 1) Primitive: Unchanged in composition since they first formed 4.6 billion years ago. 2) Processed: Younger, have experienced processes like volcanism or differentiation. 7
Processed Meteorites Meteorites from Moon and Mars A few meteorites arrive from the Moon and Mars Composition differs from the asteroid fragments. A cheap (but slow) way to acquire moon rocks and Mars rocks. A Mars rock found on Earth as a meteorite Today: Meteorites & Impacts Why do we have meteor showers? A celestial event where a large number of meteors are seen within a very short period of time The Cause of Meteor Showers Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet s orbit Meteors in a shower appear to emanate from the same area of sky because of Earth s motion through space 8
Have we ever witnessed a major impact? Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen. Tidal forces tore it apart during a previous encounter with Jupiter Impact plume from a fragment of comet SL9 rises high above Jupiter s surface Dusty debris at an impact site Artist s conception of SL9 impact Several impact sites 9
Did a meteorite impact kill the dinosaurs? Impact sites in infrared light Mass Extinction: large dip in number of species in the fossil record The most recent was 65 million years ago, ending the reign of the dinosaurs Was it caused by an impact? How would it have happened? Iridium Layer - evidence of an impact No dinosaur fossils in upper rock layers Thin layer containing the rare element iridium Likely Impact Site Geologists have found a large subsurface crater about 65 million years old in Mexico Dinosaur fossils in lower rock layers Consequences of an Impact Meteorite 10 km in size would send large amounts of debris into atmosphere. Debris would reduce sunlight reaching Earth s surface. Resulting climate change may have caused mass extinction. Comet or asteroid about 10 km in diameter approaches Earth 10
Facts about Impacts Asteroids and comets have hit the Earth. A major impact is only a matter of time: not IF but WHEN. Major impacts are very rare. Extinction level events ~ millions of years. Major damage ~ tens to hundreds of years. Tunguska, Siberia: June 30, 1908 A ~40 meter object disintegrated and exploded in the atmosphere 11
Frequency of Impacts Small impact happen almost daily. Impacts large enough to cause mass extinctions are many millions of years apart Meteor Crater, Arizona: 50,000 years ago (50 meter object) The asteroid with our name on it We haven t seen it yet. Apophis (270m): 2029 and 2036 Deflection is more probable with years of advance warning. Control is critical: breaking a big asteroid into a bunch of little asteroids is unlikely to help. We get less advance warning of a killer comet How do the jovian planets affect impact rates and life on Earth? Influence of Jovian Planets Influence of Jovian Planets Gravity of a jovian planet (especially Jupiter) can redirect a comet Jupiter has directed some comets toward Earth but has ejected many more into the Oort cloud. 12
Was Jupiter necessary for life on Earth? Impacts can extinguish life. But were they necessary for life as we know it? 13