Lecture 3: Overview of Asteroids and Meteorites Astro 202 Prof. Jim Bell (jfb8@cornell.edu) Spring 2008 But first... A few words about Referencing in science writing... (see http://astrosun2.astro.cornell.edu/academics/courses/a202/referencing.html) Paper 2 is Handed Out online REMINDER: Paper 1 due at beginning of class this Thursday, Jan. 30 Asteroids... Space junk...? Rosetta Stones...? or Harbingers of DOOM? Asteroids!"Asteroid" is Greek for "star-like"!asteroids are small, rocky objects that look star-like in telescopes, except they move across the sky like the planets do!the first asteroid was discovered in 1801 Ceres, about 940 km diameter!there may be 1,000,000+ >1 km diameter Astro202 3 Astro202 4
"Vermin of the skies" (!) Asteroids are discovered in telescopic images because they move at a different speed across the sky than the stars The Largest Asteroids!1 Ceres: Diam. = 940 km, a = 2.77 AU Discovered by Guiseppe Piazzi in 1801 Piazzi was searching for a "missing planet" between Mars and Jupiter!2 Pallas: D=540 km, a=2.77 AU, disc. 1802!4 Vesta: D=510 km, a=2.36 AU, disc. 1807!13 main belt asteroids have D > 250 km Astro202 5 Astro202 6 Where are the Asteroids?!Asteroids can be found throughout the solar system, but there are two main populations: The Main Belt between Mars and Jupiter The Kuiper Belt beyond Neptune's orbit!most of the asteroids with well-determined orbits are in the main belt!other smaller populations exist too Trojans, Apollos, Amors, Centaurs,... Astro202 7 The Main Asteroid Belt The main belt extends from about 2.2 to 3.3 AU Most of the orbits lie at or near (±10 to 20 ) the plane of the ecliptic More than 100,000 main belt asteroids have well-known orbital parameters The number has been recently increasing by a few thousand per year because of new telescopic search programs Astro202 Edge on 8
A snapshot of the locations of all known inner solar system asteroids as of today (Jan. 29, 2008) (outer circle is the orbit of Jupiter) Gaps in the Belt... Astronomer Daniel Kirkwood (1886) noticed that the main belt is not uniformly populated Asteroids "missing" from places where disturbances by Jupiter are strongest These places are where resonances with Jupiter's orbit occur May explain why no planet here: Jupiter only allowed small bodies to coalesce... Astro202 http://cfa-www.harvard.edu/iau/lists/innerplot.html 9 Astro202 10 Other Asteroid Groups In the news... Asteroid 2007 WD5: A Mars impactor?! Near-Earth Asteroids (NEAs) 436 Atens: orbits interior to Earth's 2489 Apollos: orbits that cross Earth's 2161 Amors: orbits exterior to Earth's (e.g. 433 Eros) Astro202 11 http://neo.jpl.nasa.gov/news/news151.html Tomorrow...
Other Asteroid Groups! Near-Earth Asteroids (NEAs) 436 Atens: orbits interior to Earth's 2489 Apollos: orbits that cross Earth's 2161 Amors: orbits exterior to Earth's (e.g. 433 Eros)!Trojans 2319 asteroids leading and trailing Jupiter by 60 There are even a few Mars (4) and Neptune (5) Trojans! Centaurs 67 asteroids found between Jupiter & Neptune! Trans-Neptunian or Kuiper Belt Objects More than 1000 large asteroids beyond Neptune! All these populations are incompletely counted Pluto Neptune A snapshot of the locations of all known outer solar system asteroids as of today (outer circle is the orbit of Neptune) Astronomers believe that there may be 100,000 or more Kuiper Belt Objects still to be discovered... The solar system does not end at Pluto! Astro202 13 Astro202 http://cfa-www.harvard.edu/iau/lists/outerplot.html 14 How Many Asteroids are There? Discovering Asteroids Number of Near-Earth Asteroids (a model) The number of asteroids of a given diameter D is proportional to 1/D 2 : a collisional distribution For example:!! 3 > 500 km!! 13 > 250 km!! hundreds > 100 km!! 10,000+ > 10 km (?) Total: >1,000,000 > 1 km (?) Most of the MASS is in the largest few asteroids Total mass of all asteroids is only ~5-10% the mass of the Moon! For many years, most discoveries were random! Within the past decade, dedicated surveys have begun in response to the possible threat of asteroid and comet impacts on the Earth NASA Spacewatch Program Air Force Near Earth Asteroid Tracking Program (NEAT) Lowell Observatory Near Earth Object Survey (LONEOS) Lincoln (MIT) Near Earth Asteroid Research Project (LINEAR) Catalina (Arizona) Sky Survey! Hundreds of thousands of objects discovered, many are Near-Earth Asteroids (NEAs)... Astro202 15 Astro202 16
Asteroid Physical Properties! Many diagnostic properties can be determined by telescopes: Albedo (% of reflected sunlight) darkest are 1% to 2%, brightest are 40% to 50% Spin period (length of an asteroid "day") Shape (from radar & variations in the light curve) Astronomers believe that dedicated search programs started in the last decade have located about 80%-90% of the >1 km Potentially Hazardous Asteroids (PHAs) that could pose a threat to life on Earth... http://neo.jpl.nasa.gov/stats Astro202 18 Astro202 17 Some asteroid sizes have also been measured directly... (from NSS Chapter 25) Asteroid Composition! Spectroscopy and radar observations of asteroids reveal four main classes: C Type: dark, "carbon rich", albedo ~5% S Type: brighter, "stony", albedo ~20% D Type: dark, very red spectra M Type: rare "metallic" type, very radar bright! C,S,D Types probably all/mostly primitive (?)! M Types and other anomalous classes (like Vesta) from differentiated parent bodies Astro202 20
(controversial!) Composition varies systematically with distance from the Sun Implies that asteroids preserve the conditions of planetary formation in the early solar system But primitive vs. differentiated issue is controversial! Astro202 21 Asteroid Geology! 4 asteroids visited up close by spacecraft 951 Gaspra: Galileo flyby in 1991 243 Ida: Galileo flyby in 1993 253 Mathilde: NEAR flyby in 1997 433 Eros: NEAR orbital mission in 2000-2001! Also: Spacecraft images of Martian moons Phobos and Deimos: captured asteroids?! Abundant evidence for impacts, and surprising evidence for erosion and tectonism on these bodies Astro202 22 951 Gaspra S Type 16 x 11 x 10 km 243 Ida S Type 60 x 25 x 19 km 253 Mathilde C Type 66 x 48 x 46 km (large!) Many large craters Density only 1.3 g/cm 3 May be 50% "empty" Astro202 23 Dactyl Ida's moon! D = 1.5 km 433 Eros S Type 31 x 13 x 13 km Many large craters Density about 2.6 g/cm 3 Geology! Astro202 24
Tectonic ridges: Eros appears to have been compressed and possibly even twisted during its history... Fine-grained regolith appears to be moving downhill on Eros: erosion! Last image from NEAR before its landing: very finegrained regolith! Astro202 Eros movies from NEAR: craters, boulders, & more! Astro202 26 Are many asteroids Rubble Piles? (from NSS Chapter 25) Meteorites!Samples from the Early Solar System!Age, origin and evolution!classification!parent Bodies: Asteroids & Planets Astro202 28
Meteorites are... Small pieces of asteroids that have impacted the Earth and that provide us with direct information on the original building blocks of the solar system!samples from the Early Solar System!Age, origin and evolution!classification!their Parent Bodies are Asteroids & Planets Astro202 29 Meteorites "I would rather believe that two Yankee professors would lie rather than stones would fall from heaven."!! Thomas Jefferson! Free "Mini Sample Return Missions"! Do rocks really fall from the Sky? Astro202 30 Definitions Fall or Find?! A meteoroid is any object which orbits the Sun on a collision course with another planetary body! A meteor is a bright streak of light in the sky, sometimes called a "shooting star" or "falling star", produced as atmospheric friction burns up the outer layers of a meteoroid. A large, bright meteor is called a fireball.! A meteorite is a fragment of one planetary body that lands on another planetary body.! Meteoroid Meteor Meteorite Astro202 31! A fall is a meteorite that was actually observed falling and then was collected. (Thus, usually not very weathered or altered by Earth's wind, water, and rain)! A find is a meteorite that was not seen to fall but was found on the ground some time later (Antarctic finds may have fallen 10,000 to 700,000 years ago)! Only about 10% of meteorites are falls (skewed by the more than 20,000 Antarctic finds in the last 10 years) Astro202 32
Meteorite Taxonomy!Stony (most common type) Some "primitive", others differentiated Compositions like the mantles & crusts of the terrestrial planets!stony -Iron (rare--easily weathered, eroded) Contain roughly equal mixtures of rock and metal Remnants of the mantles of previously-formed parent bodies?!iron (less rare) Dense metallic objects % occurrence of meteorite types Class! Falls! Finds! Antarctic Primitive Stones! 88%! 51%! 85% Differen. Stones! 8! 1! 12 Irons! 3! 42! 2 Stony-Irons! 1! 5! 1 Large % of finds because they are harder to weather away Remnants of the cores of previously-formed parent bodies Astro202 33 Types of Stony Meteorites! Ordinary Chondrites: most common meteorite type, some contain chondrules or condensed grains from the primitive solar nebula, heated to various levels! Carbonaceous Chondrites: are the most primitive type of stony meteorites (completely unprocessed), many contain chondrules, and are possibly samples of the original "starting material" for planets and asteroids! Achondrites: are the most heavily processed stony meteorites, and have undergone differentiation and igneous processes on larger parent bodies. Processing has destroyed/melted the chondrules Astro202 34 Chondrules Millimeter-sized spherical deposits scattered throughout a dark background "matrix" Found only in the most primitive chondrites Spherical shape: implies that these deposits were once molten droplets The oldest stuff in the solar system! (4.6 b.y. +) Chondrules may have been condensed from the original gaseous solar nebula and then melted, but their origin is controversial Composition of Chondrites Each dot is a particular element Y-axis: The Sun X-axis: Chondrites The slope of the line is 1 (correlated!) Chondrites are made of the same "stuff" as the Sun! Astro202 35 Astro202 36
Where do Meteorites Come From?! Most are pieces of parent bodies in the asteroid belt or near-earth space that were either broken up or destroyed by impacts! Some may be pieces of comets! A very few are pieces of the Moon! A very few are pieces of other planets (Mars) Recent discovery: the planets "trade" rocks! Astro202 37 Meteorites come from Parent Bodies... Some must have been Primitive or Unprocessed rocky and metallic mix Most Chondrites came from this kind of Parent Body But some must have been Differentiated or Processed lower density rock metal Achondrites, Stony-Irons, Irons came from this kind of Parent Body after it was smashed up! Differentiated Meteorites! Irons Contain Fe-Ni alloys & Fe-S inclusions Characteristic cooling patterns ("Widmanstätten") Cooling rate allows estimate of parent body size! (rate! 1/size) Typical parent body sizes: 70 to 200 km radius (asteroidal) Pieces of the cores of differentiated parent bodies! Stony-Irons Mixtures of Fe-Ni and silicate rocks Pieces of the boundary between the rocky mantle and metallic core of Meteorite Origins!Evidence for asteroidal parent bodies: Spectra of many meteorites resemble spectra of asteroid surfaces Orbits of some fireballs can be traced back into the asteroid belt Theoretical modeling shows that there are some gravitational "conveyor belts" (resonances) between the asteroid belt and the inner planets Astro202 39 Astro202 40
Asteroid & Meteorite Connections Asteroid & Meteorite Connections!Some meteorites are samples of asteroids...!they provide direct information on the early conditions of the solar system!they provide direct information on the interiors of asteroids and planets!some meteorites have been found to be related to specific asteroids or even planets Astro202 41! Asteroid spectra (black lines) similar to certain meteorite spectra (red lines)! General trends: Asteroid Meteorite C types Carbonaceous chondrites S types Ordinary chondrites Anomalous achondrites M types Irons V types Achondrites Astro202 42 Meteorites probe different asteroid source regions... (controversial!) Meteorites from the Moon!! Impacts can eject rocks off the Moon at speeds > escape velocity--some of these rocks fall to Earth! More than 50 meteorites (out of >20,000) have been identified as Moon rocks from their chemistry and isotopes! Most are breccias from the highlands! A few are mare basalt (volcanic) samples! These rocks are free (but random) samples of different places on the Moon than those sampled by Apollo missions Provide additional (new) information on lunar composition May represent more "typical" lunar crust than the Apollo samples? Astro202 43 Astro202 44
Meteorites from Mars!!Impacts eject rocks off larger planets too!~30 meteorites have been identified as coming from Mars, based on composition, isotopes, and trapped gas bubbles!samples of Mars! (but random & deep)!provide new data on crustal composition!some meteoriticists believe that one of the Mars rocks contains chemical and fossil evidence for life on Mars! Extremely Controversial! No Scientific Consensus! Summary! Asteroids are small, rocky, primitive objects thought to be remnants from the formation of the solar system! There are probably more than 1 million asteroids > 1 km in size in the solar system, but their total mass is only a small fraction of the mass of the Moon! Remote sensing shows that not all asteroids are alike Different compositional classes (C, S, D, M) Some asteroids have had complex internal histories! Spacecraft observations show that asteroids have interesting and complex geologic histories: impacts, tectonics, erosion Astro202 45 Astro202 46 Summary! Meteorites are rocks that fall from space Most are samples of asteroids, and provide a way to remotely study the asteroid belt Most are primitive (unprocessed) rocks, formed from the building blocks of the planets! Many meteorites must have come from differentiated parent bodies that no longer exist The only surviving record of the solar system's birth?! A few meteorites are from the Moon and Mars and provide new and controversial data! Next Time Overview of the Giant Planets Reading: NSS 15; PBD 4 Astro202 47