Asteroids, Meteorites, and Comets

Transcription

1 Asteroids, Meteorites, and Comets

2 The Search for the Missing Planet Bode s Law relates the sizes of planetary orbits Astronomers noticed a missing planet if this rule of thumb was correct. Body Actual distance (A.U.) Mercury Venus Earth Mars Bode's Law 2.8 Jupiter Saturn Uranus To find the mean distances of the planets, beginning with the following simple sequence of numbers: Add 4 to each number: Then divide by 10: The resulting sequence is very close to the distribution of mean distances of the planets from the Sun:

3 The Discovery of Ceres Discovered on 1 January 1801 by Giuseppe Piazzi Semi-major axis = AU Orbital period = 4.60 yr Mass = Earths = Moons Now classified as a minor planet, or asteroid. Also classified as a dwarf planet. Diameter comparis on of the dwarf planet asteroid Ceres with the Moon and Earth. NASA's Hubble Space Telescope color image of Ceres, the largest object in the asteroid belt.

4 How do we find asteroids? Asteroids appear to move relative to background stars Path of 1 Ceres in late 2012 As an asteroid moves along its orbit it will produce an elongated trail in an image taken with a telescope following the background of "fixed" stars, which are not moving in this way.

5 Small, irregular objects, mostly in the apparent gap between the orbits of Mars and Jupiter. Thousands of asteroids with accurately determined orbits known today. Asteroids pose virtually no hazard to space navigation. The average distance between them is 10 6 km The Asteroid Belt Sizes and shapes of the largest asteroids, compared to the moon

6 The Asteroid Belt As seen from the side As seen from above This is where we find the majority of asteroids

7 The Asteroid Belt Jupiter s gravity likely prevented the formation of a planet between Mars and Jupiter. Most of the planetesimal were kicked out of this region, leaving the objects that now make up the asteroid belt. Jupiter s gravity also shapes the asteroid belt Kirkwood gaps

8 Kirkwood Gaps The distribution of asteroids in the main belt is NOT uniform. Gaps form at specific regions where there are whole number ratios (1/3, 2/5, 3/7, etc.) of Jupiter s orbital period. Mean motion resonances Where else have we seen resonances phenomena?

9 Picture of Ida (with its tiny moon) from Galileo. Asteroids Picture of the asteroid Gaspra from the Galileo spacecraft

10 Studying Asteroids 15 asteroids have by visited by spacecraft Radar can be used to determine the shapes of asteroids that are too small to resolve with telescopes Optical telesopes can be used to infer an asteroid s shape and rotation period Computer model of asteroid (216) Kleopatra, based on radar analysis. Motion of near-earth object 2001 FE90 showing brightness variation over two rotations on 6/26/2009.

11 Non-Belt Asteroids Asteroids with elliptical orbits, reaching into the inner solar system. Not all asteroids orbit within the asteroid belt! Trojans: Sharing stable orbits along the orbit of Jupiter: Trapped in the Lagrangian points of Jupiter. Some potentially colliding with Mars or Earth.

12 Near Earth Objects (NEOs) Objects that cross Mars orbit or orbit entirely within the orbit of Mars are called near- Earth objects. These objects occasionally pass very close to Earth, and can potentially pose a threat. Orbits of near Earth asteroids wider than.88 miles are in yellow; comets in blue. Paul Chodas / JPL / NASA

13 NEOs As of February 21, 2013, 9738 Near-Earth objects have been discovered. Some 863 of these NEOs are asteroids with a diameter of approximately 1 kilometer or larger. Also, 1379 of these NEOs have been classified as Potentially Hazardous Asteroids (PHAs).

14 Meteor Crater, Arizona Evidence of Impacts on Earth Wolfe creek crater, Western Australia Pingualuit Crater, Quebec Nördlinger Ries, Germany

15 The K-T event Sixty-five million years ago about 70% of all species then living on Earth disappeared (including dinosaurs) within a very short period. K-T boundary clay contains iridium, an element common in iron-rich minerals like asteroids and meteorites 10 km,

16 Impacts in Modern Times 2013 Russian meteor event Tunguska River in Siberia, Russia in square kilometers affected Airburst of large meteoroid or comet (100 meters in size) 5 10 kilometers above the surface of the Earth. largest impact event on or near Earth in modern times.

17 Meteoroid = small body in space Meteor = meteoroid colliding with Earth and producing a visible light trace in the sky Meteorite = meteor that survives the plunge through the atmosphere to strike the ground... Some Nomenclature Sizes from microscopic dust to a few centimeters. Estimated 300 tons of meteoritic material falls on Earth each day. Typically impact onto the atmosphere with km/s ( 30 times faster than a rifle bullet).

18 Analysis of Meteorites 3 broad categories: Iron meteorites Stony meteorites (95%) (4%) Stony-Iron meteorites (1%)

19 What Does a Meteorite Look Like? Every year between 30,000 and 80,000 meteorites larger than 20g in mass fall from space to Earth. Willamette Meteorite Goose Lake meteorite (iron) Selection bias: Iron meteorites are easy to recognize as meteorites (heavy, dense lumps of iron-nickel steel) thus, more likely to be found and collected.

20 The Origins of Meteorites Probably formed in the solar nebula, ~ 4.6 billion years ago. Almost certainly not from comets (in contrast to meteors in meteor showers!). Probably fragments of stony-iron planetesimals Allende meteorite Some melted by heat produced by 26 Al decay (half-life ~ 715,000 yr). 26 Al possibly provided by a nearby supernova, just a few 100,000 years before formation of the solar system (triggering formation of our sun?)

21 The Origins of Meteorites (2) Planetesimals cool and differentiate (if they are large enough) Collisions eject material from different depths with different compositions and temperatures. Meteorites can not have been broken up from planetesimals very long ago so remains of planetesimals should still exist. Asteroids

22 Asteroid Collisions

23 Carbonaceous Chondrites A rare type of stony meteorite which contains large amounts of the magnesium-rich minerals and a variety of organic compounds, including amino acids. No evidence of melting (many were never heated above 50 C) Primitive and undifferentiated meteorites Most of them contain water or minerals that have been altered in the presence of water, and some of them contain larger amounts of carbon as well as organic compounds.

24 Comets Comet Halley, 1986 Comet McNaught C/2009 R1 was visible on June 6, CREDIT: Michael Jäger Comet Holmes, 2007 Hale-Bopp, 1997

25 Comets Small objects made of frozen gases, rock and dust, "dirty snowballs the nucleus When close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. Coma is surrounded by a giant hydrogen envelope. Breakup of comets lead to meteor showers on Earth. Orbital periods from 20 years to 100,000 years or more Short period Intermediate period Long period Halley's Comet becomes visible to the unaided eye about every 76 years as it nears the sun. Image credit: Lick Observatory

26 Eccentric orbits

27 The Comet Nucleus Comet Hartley, as imaged by NASA's EPOXI spacecraft. (Credit: NASA) Composite image of the nucleus of Comet Halley produced from 68 original photographs taken by the Halley Multicolour Camera on board the Giotto spacecraft on March 13 and 14, 1986.

28 Structure of a Comet

29 The Hydrogen Envelope Surrounding every moderately active comet is a sparse but extensive envelope of neutral hydrogen atoms. The hydrogen is liberated when ultraviolet radiation from the Sun splits the water vapor molecules released from the nucleus of the comet into the constituent components, oxygen and hydrogen. Up to 10 million km is size Not visible to the human eye Visible in UV light The hydrogen cloud surrounding comet Hale Bopp in 1997 far exceeds the comet s visible tail (inset). Although not visible from the Earth, the hydrogen envelope is enormous, completely dwarfing the Sun which is shown as the yellow dot in the lower right corner. Credit: SOHO/SWAN (ESA & NASA) & J.T.T. Mdkinen et al.

30 Two Types of Tails Ion tail: Ionized gas pushed away from the comet by the solar wind. Pointing straight away from the sun. The ion tail glows blue by emitting light when elections re-combine with electrically charged ions to make uncharged molecules. Dust tail: Dust set free from vaporizing ice in the comet; carried away from the comet by the sun s radiation pressure. Lagging behind the comet along its trajectory The dust tail of a comet appears whitishyellow, because its microscopic dust particles reflect sunlight.

31 Evolution of Comets Comets lose 0.5% to 1% of its ice each perihelion passage Over time comets lose all their ice leaving nothing but rubble and dust called a meteoritic swarm These particles orbit in the comet s orbit but now spread out along it. Gravitational interactions with planets can tidally disrupt comets (Shoemaker-Levy 9) A NASA Hubble Space Telescope (HST) image of comet Shoemaker-Levy 9, taken on May 17, 1994, with the Wide Field Planetary Camera 2 (WFPC2) in wide field mode.

32 Short-Period Comets Orbital periods of less than 20 years, also called Jupiter-family comets Orbit more-or-less in the ecliptic plane in the same direction as the planets. Example: Comet Tempel 1 and Hartley 2 These comets could not have formed here Where do they come from? Kuiper Belt?

33 Intermediate-Period Comets Orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees As of 2012, only 64 Halley-type comets have been observed Probably originate in an outer icy asteroid belt, the Kuiper Belt, beyond Neptune. The orbit of Halley's Comet is pretty typical. It has an eccentricity of.967, meaning an extremely elongated ellipse with the Sun very close to one end.

34 The Origin of Long-Period Comets Long period comets are believed to originate in the Oort cloud: Spherical cloud of several trillion icy bodies, ~ 10, ,000 AU from the sun. Highly eccentric orbits and periods ranging from 200 years to thousands or even millions of years Oort Cloud Gravitational influence of occasional passing stars may perturb some orbits and draw them towards the inner solar system. Interactions with planets may perturb orbits further, capturing comets in short-period orbits.

35 Meteoroid Orbits Meteoroids contributing to a meteor shower are debris particles, orbiting in the path of a comet. Spread out all along the orbit of the comet. Comet may still exist or have been destroyed. Only a few sporadic meteors are not associated with comet orbits.

36 Meteor Showers Meteor showers are mostly caused by the trails of dust and debris left in the wake of a comet. When Earth passes through this material we see a meteor shower. Meteors are observed to radiate from one point in the night sky called the radiant. Zenith hourly rate varies between several meteors per hour to over 100 per hour depending on the shower.