In the Beginning. The Origin of the Solar System

Transcription

1 In the Beginning The Origin of the Solar System The Early Earth Impacts of Extraterrestrial Objects The Origin of the Solar System Nebular Hypothesis The Sun and the planets that orbit around it began as a Nebula (an immense cloud of gas and dust in space). Star Birth Clouds: Hubble Telescope image of Eagle Nebula. Image is approximately 1 light year across. Within the Nebula the pressure of the gases act outwards to cause it to expand while gravitational forces (forces that pull bodies towards each other) act to cause the Nebula to collapse onto itself. Gravity prevails and the Nebula collapses and begins to spin. As the diameter of the Nebula is reduced, the rate of spin increases. Due to the interaction of the pressure and gravitational forces, as the Nebula spins it becomes flatter and forms a broad disk over time as the Nebula continues to collapse. As the density of the centre of the disk increases it heats up to form the Protosun. Within the cloud swirling eddies develop drawing matter towards their centres to form the Protoplanets. 1

2 As the Protosun becomes even hotter the gases are driven out of the inner planets of the Solar System. The Nebular Hypothesis is attractive because it explains many features of the Solar System: The orbits of the planets lie in a plane with the sun at its centre (the plane of the early disk-shaped Nebula). The protoplanets become solid planets and continue their orbit, governed by the initial spin of the swirling nebula. The planets all orbit around the Sun in the same direction (inherited from the spin of the nebula which caused the orbital motion of the protoplanets). The planets mostly rotate in the same direction; their axes of rotation are nearly perpendicular to the orbital plane. The direction of rotated is inherited from the direction of spin of the eddies in the spinning nebula. The Planets of the Solar System The outcome for our Solar System: The Inner Planets: Earth-like planets: metallic cores, dominated by silicon and oxygen compounds Name Diameter (km) % of Earth in brackets Distance from Sun (AU) 1 Mercury 4,880 (38%) Venus 12,103 (95%) Earth 12, NASA Mars 6,787 (53%) Astronomical Units: 1 unit is the distance from Earth to the Sun, 149,597,900 km. 2

3 The Outer Planets: Gas Giants (in the case of the first four) with thick atmospheres that thicken and become hotter towards their rocky or icy cores. Name Diameter (km) % of Earth in brackets Distance from Sun (AU) Jupiter 143,800 (1,127%) 5.2 Saturn 120,660 (946%) 9.5 Uranus 51,120 (401%) 19.2 Neptune 49,500 (388%) 30.1 Pluto 2,284 (18%) 39.4 Why do the outer planets diminish in diameter as they are farther from the sun? The thickness of the disk thins away from the sun so the the diameter of possible planets becomes smaller away from the sun. Inner planets lost their early atmospheres so that they are much smaller than the outer planets. Orbits of the planets of the Solar System Solar System Oddities Venus, Uranus and Pluto do not rotate in the same direction as the other planets. Venus s rotational axis is at right angles to the plane of the planets (the ecliptic plane) but is in the opposite direction. Uranus rotates about an axis that is almost parallel to the plane of the planets. Modern thinking is that both planets were rotated by major collisions early in their history. In addition to rotating in the opposite direction to most planets, Pluto has a strongly eccentric orbit (it is elliptical rather than circular and not in the same plane as the other planets). Pluto s orbit crosses Neptune s orbit so that at times it is closer to the sun than Neptune. Most recently this was the case from January 1979 through February Planetary update: A proposed tenth planet was photographed on October 21, 2003, but remained undiscovered until January 8, Named 2003UB313 at present a more fitting name will likely be assigned in future. Lila? After the discoverer s baby daughter. Many believe that Pluto is not a true planet but is a large body that was captured by the Sun s gravitational attraction. 3

4 Estimated to be least as big as Pluto Three times farther away from the Sun than Pluto. The distance from the sun varies from 38 to 97 AU. Time for one orbit of the sun is 560 Earth years. The early history of the Earth Accumulation to form the planet: a three-stage process that was completed by about 4.56 Billion years ago. Stage 1: Dust grains in the Nebula begin to stick together forming larger particles that grow into large, discrete objects with their own gravitational field. Check out the discoverer s web site for some really interesting stories about such a discovery: Stage 1 accretion of small particles to form larger objects. Stage 2: Planetary embryos came together during a phase of relatively rapid accretion (collisions forming larger objects). Created tens of objects larger than the moon. This stage took approximately 1 millions years to complete. The gravitational attraction of the large objects pulls smaller objects into them and they grow into planetary embryos (up to a few hundred kilometres in diameter). 4

5 Stage 3: Large objects from Stage 2 are attracted to each other due to their gravity; colliding and forming larger objects (the actual formation of the planets). Stage 3 was complete after 50 to 100 million years. Solar wind (particles emitted from the Sun) drove off the gases that made up the early atmosphere of the Inner Planets. The Earth evolved its own atmosphere later in its history. How did the Moon form? The Lunar orbit suggests that it was captured from debris ejected from the Earth during a stage 3 impact. Lunar orbit if it was a captured planetary embryo. Lunar orbit if it was a generated by debris from Earth. During Stage 3 the Moon came into existence. The circular orbit suggests that the Moon was derived from the Earth. Towards the end of Stage 3 the Earth had a Magma Ocean; melting due the energy released from frequent large collisions. One or possibly two, oblique collisions ejected a ring of molten debris into orbit about the Earth. Once in orbit the molten debris accreted (came together) in a manner similar to the accretion of the planets. It is estimated that the Moon was pretty much complete after a decade following the causal collisions with Earth. Initially the moon was a molten mass but it cooled to form a solid crust when impact frequency and magnitude diminished. Just following completion of Lunar construction it was in an orbit about the Earth that was about 15 time closer than the current orbit. A billion years later the orbit was 4 times closer than today; it has been moving away from the Earth ever since it formed. View of the Moon over Brighton today. View of the Moon over Brighton 4.5 Billion years ago. 5

6 Origin of the moon. Comets, Asteroids and Meteoroids The Movie Peekskill Meteorite The Peekskill Meteorite Ever since the formation of the Earth it has been bombarded by debris from space. Debris in space comes in a variety of forms: Comet: a mixture of ices, both water and frozen gases (carbon dioxide, methane, ammonia) and dust. Often called a dirty snowball. Material that was not incorporated into planets when the solar system was formed (most comets are 4.6 billion years old, or so). 6

7 Most comets have elliptical orbits about the sun that often take them well beyond Pluto. Anatomy of a Comet Nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids. Coma: dense cloud of water, carbon dioxide and other neutral gases derived from the nucleus. Dust tail: up to 10 million km long composed of smokesized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye. A comets is visible only when its orbits take it near the Sun. Ultraviolet light from the Sun causes the comet to fluoresce and some gases to escape. After several hundreds of passes by the Sun a comet loses its gas and ice and only a rocky object remains. Comet Hale-Bopp in Photo by John Laborde Where do comets come from? Answer: the Kuiper Belt and the Oort Cloud. Both are regions of space around our Solar system that, combined, contain up to trillions of small, icy bodies that become comets when their orbits are disturbed. Kuiper Belt: a disc-shaped region past the orbit of Neptune, 30 to 100 AU from the Sun. Oort Cloud: a huge spherical cloud of many billions of icy bodies, surrounding the outer limits of the Solar System and extending approximately 3 light years (about 30 trillion kilometers) from the Sun. 7

8 Gravitational interaction with the outer planets can disturb the orbit of icy bodies, sending them on their elliptical comet orbits. On July 4, 2005, JPL s Deep Impact Mission was complete when its impactor collided with Comet Temple 1. The purpose of the mission was to observe the formation of a crater and to analyze the debris that was ejected to determine the comet s composition. The following movies and images are from the Deep Impact home page at: Here s a QuickTime animation of the mission. Here is the Quicktime movie of the impactor s approach to Temple 1 s nucleus. 1 minute before impact. 5 minutes before impact before impact. Impact 20 seconds before impact. 8

9 By analyzing the dust ejected from Temple with a spectrometer, molecules of material making up the comet are identified. Asteroids: small (metres to less than 1000 kilometres), dense objects that orbit the Sun. Made up of inner solar system material that was not formed into planets and masses of planetary material produced by major collisions. Largest: 1 Ceres which is 933 kilometres in diameter. (most are smaller than 300 km) The first asteroid ever discovered (1801). This spectra shows water, hydrocarbons, carbon dioxide and carbon monoxide. Asteroid Ida is large enough to have its own satellite in orbit around it. Eros Eros s giant gouge.a past collision? Asteroid Eros Orbit: 172,800,000 km from the sun Size: 33 x 13 x 13 km 9

10 Eros rotates with a clumsy wobble NEAR-Shoemaker spacecraft landed on Eros on Monday, February 12, metres above the surface 54 metres Near-Shoemaker landing on Eros 700 metres above the surface 33 metres 250 metres above the surface 12 metres 10

11 120 metres above the surface 6 metres Asteroids are classified according to how much light that they reflect. Albedo: the proportion of incoming light that is reflected from a surface. Albedo = 1, perfect reflector; Albedo=0.01, reflects very little incoming light. Classes of Asteroids: C-type, > 75% of known asteroids: extremely dark (albedo 0.03); approximately the same chemical composition as the Sun minus gases. S-type, 17%: relatively bright (albedo ); metallic nickel-iron mixed with iron- and magnesium-silicates. M-type, most of the rest: bright (albedo ); pure nickel-iron. Most asteroids are in orbit within the Asteroid Belt Meteoroid: a piece of stone or metal that travels in space (smaller than an asteroid, from dust size to a metre or so) Meteor: a meteoroid that falls towards the Earth, heating up due to friction and glowing as it crosses the sky. Meteorite: a meteor that lands on the Earth s surface. Bolide: a large, particularly bright meteor that often explodes (syn. fireball). It is estimated that between 20,000 and 100,000 tons of material from space collides with Earth each year.much of this burns up in the atmosphere. 11

12 World s largest meteorites Hoba West Meteorite, 60 metric tons, Namibia The Canadian Meteorites Site (University of Calgary) The Risk of Space Objects to Humans Today there is considerable concern about space objects colliding with Earth, despite the paucity of recorded strikes directly on humans: Ancient records from China indicate that people have been killed by falling meteorites; no such deaths are known from modern times. A meteorite killed a dog when it fell in Egypt in Elizabeth Hodges, of Sylacauga, Alabama, was given a terrible bruise on the side by a falling meteorite in A young boy was struck on the head by a meteorite that had been slowed down by the leaves of a banana plant in Uganda in So why the concern today? Three events over the 20 th Century heightened interest in evaluating the risk of impacts of space objects on Earth. The K/T boundary impact, the Tunguska Event and the Comet SL-9 impact on Jupiter. 1. Recognition that a major impact led to the extinction of the dinosaurs and much other life prompted detailed studies of the effects of such an impact and smaller impacts. 2. The Tunguska Event was an atmospheric explosion of an asteroid in Such events happen in our own time! 12

13 On June 30, 1908, at 7:30am a 15 megaton blast was felt over a large area of Siberia (the Hiroshima nuclear explosion was about 0.02 megatons; 750 X Hiroshima). The blast was an airburst (explosion) of a 60 m diameter asteroid. The explosion was heard in London England. Over 60,000 trees were flattened over an are of 800 mi 2 3. In 1992, Comet Shoemaker-Levy 9 (SL-9) passed near to Jupiter when it broke up into at least 21 separate fragments, up to 2 km in diameter, dispersed over several million kilometres along its orbit. Between 16 July 1994 and 22 July 1994 the fragments impacted the upper atmosphere of Jupiter. The first collision of two solar system bodies ever witnessed. The first fragment struck Jupiter with energy equal to about 225,000 megatons of TNT creating plume which rose about 1000 km above the planet. A later fragment struck with an estimated energy equal to 6,000,000 megatons of TNT (about 600 times the estimated arsenal of the world). The fireball rose about 3000 km above the surface of the planet. 13

14 These three events illustrated that: Such impacts were possible and not just the stuff of SciFi. We could see it happen (with the aid of space telescopes). Major impacts can have a devastating effect on all life on Earth. Even minor impacts on Earth (Tunguska) that have taken place in recent record could kill millions and cause billions of dollars in damage. Governments and insurance companies developed concern regarding the risks and costs of such events. In light of these three 20 th Century events researchers have focused on several questions: What has been the frequency of impacts with Earth? How many objects are close enough to Earth to pose a risk? What happens when an object of a given size arrives at Earth? How do we assign a level of risk to space objects? What does the geologic record tell us about major impacts (the past is the key to the present)? What is the frequency of impacts with Earth? Based on estimates of modern objects and the geological record of impacts worldwide. These are average values; large events can happen at any time! Zhamanshin Impact (Siberia): 13.5 km crater, 1 km diameter object. Average time between impacts can be resolved for smaller regions of Earth to evaluate human risk: For the Tunguska-class impacts: Average interval between impacts for total Earth: 300 years Average interval between impacts for populated areas of Earth: 3,000 years (given the population distribution on Earth; about 10% of Earth surface area is populated). Average interval between impacts for world urban areas (0.3% of Earth surface): 100,000 years Average interval between impacts for U.S. urban areas (0.03 of Earth surface): 1,000,000 years How many objects are close enough to Earth to pose a risk? Defined as: Near Earth Objects (NEOs) Comets with orbits within 1.3 AU of the sun and with orbital periods (time to complete one orbit) of less than 200 years. Asteroids those whose orbits are within 1.3 AU of the Sun. Potentially hazardous asteroids (PHA): asteroids that are larger than 110 metres in diameter with orbits that bring them within 0.05 AU of Earth. Source: 14

15 The NEO Program (within NASA) aims to find 90% of all NEOs within 10 years. The program began in A part of NASA s Spaceguard Effort to avoid catastrophic impacts The NEO Program s budget is 10.5 million dollars/year Data largely from photographs from wide-field telescopes. Overlays of sequential photos allow identification of moving objects. Estimated size distribution of NEOs Size Estimated number of near-earth objects >30 m >50,000,000 >100 m >320,000 Lincoln Near-Earth Asteroid Research Telescope >500 m >9,200 >1,000 m >2,100 >2,000 m >400 Source: Thomas Grollman; NASA provides details on PHA orbits at : Awesome Asteroids: Introduction To Asteroids, Small Bodies In Out Solar System Orbits, dates and other data are available for particularly close approaches at: From the Near Earth Objects Program 15

16 What happens when a meteoroid or asteroid reaches the Earth? Asteroids approach the Earth at speeds of 15 to 25 km/sec (54,000-90,000 km/hr). Comets can approach the Earth at speeds up to 70 km/sec (252,000 km/hr). Depending on the mass (volume X density) the atmosphere may slow the object down to about 200 km/hr. The energy released upon impact is the Kinetic Energy of the object. Kinetic Energy = E = ½ mv 2 Where m is the mass of the object and V is its velocity. As the mass (size) of the object increases so does the Kinetic Energy Double the mass leads to a doubling of the Kinetic Energy The Kinetic Energy increases with the square of the velocity. When velocity increases by a factor of two (i.e., it doubles) the Kinetic Energy increases by a factor of 4 (four times the value). The available Kinetic Energy determines the effect of the impact because that Energy is released upon impact. Specific effects at impact can include: Heat wave: travels kms per second; due to the release of energy from the asteroid as it explodes in the air or impacts on the Earth s surface. Incineration of the area close to the event; start fires on the ground around the site. Pressure wave: shock wave in the air followed by winds >200km/hr; results from the compression of the air due to an explosion or impact. Shock wave knocks down buildings/trees; winds cause hurricane-like devastation; winds may blow out fires. The pressure wave of the exploding Tunguska object knocked down over 60,000 trees. Impact crater formation: impact displaces crustal material, ejecting it into the atmosphere leaving a large crater on the surface (1 km diameter asteroid produces a 20 to 20 km diameter crater). Total devastation at the crater site; climate change due to fine debris in the atmosphere. Wolf Creek Australia.The crater is 875 m across and the rim rises ~25 meters above the surrounding plains and the crater floor is ~50 meters below the rim. 300,000 yrs old. Rain of small rocks and dust: material ejected from a crater (both asteroid and Earth material) or produced as an object explodes in the air can travel for thousands of kilometres. Large debris falls relatively close to the impact whereas dust is carried in the atmosphere for years. Secondary damage to anything remaining in the region around the impact or explosion; global cooling due to the reduction in sunlight caused by the atmospheric dust. Earthquake: much of the energy released as shock waves through the Earth; magnitude 12 earthquakes are possible. Surface shock waves can devastate the landscape (including buildings) hundreds of killometres from the impact site. 16

17 Tsunamis: when an asteroid impacts on a large water body (e.g., the ocean) a wave is generated that travels very quickly over the water surface, steepening and flowing onshore along coasts. Wave speeds have been recorded at almost 800 kilometres per hour (generated by earthquakes, not asteroids). At the shoreline waves can reach over 100 metres in height and wash out buildings for kilometres from the shore. Grollman described the types of damages as evaluated by the insurance industry: Type I Asteroid: ranging from 0-30 m in diameter; 10,000 50,000/year. Type II Asteroid: 50 m diameter; every 250 yrs. Type III Asteroid. 1 km diameter; every 100,000 yrs. Type IV Asteroid. 10 km diameter; every 50 million yrs. Type I: <30 metre diameter: Normally explodes before impact into dust and small fragments. On March 27, 2003, such an explosion took place and the fragments (the size of tennis balls) crashed into several houses. Fragments cause damage but no risk of heat, shock, earthquakes, etc. Type II: 50 metre diameter: Explodes in the air. Over land the heat wave starts fires within several kilometres below the explosion. Heat and pressure wave causes extensive damage within 25 or 30 kilometres of explosion. Diminishing damage from pressure wave and winds to almost 100 kilometres. Damage can exceed that of a major Earthquake metre tsunamis can cause extensive damage to large coastal cities (e.g., Vancouver, San Francisco, Tokyo if the Pacific receives the impact). Type III: 1 km diameter: Objects of this size impact the surface; a 1 km object would create a 20 to 30 km diameter crater. Very heavy damage for 500 km around the impact site due to heat and pressure wave. Major earthquake would add to extensive damage. Forest fires rage across the entire continent due to extensive heat wave and falling hot debris. Local climatic change would have an effect on fauna and flora for decades to come. An impact at sea would send masses of water upwards to 10 km. Tsunamis would make landfall as waves hundreds of metres high. Los Angeles, Tokyo, Hong Kong, Miama..destroyed except for concrete-reinforced ruins. 17

18 Type IV: 10 km diameter (K/T impact): Impact crater: 300 km in diameter. Entire continent destroyed. Falling masses of molten rock would start forest fires world-wide. Magnitude 12 earthquake would just add to the devastation. Auxiliary damage as nuclear power plants are destroyed. Global climate change due to dust in the atmosphere. Global food supply jeopardized. How do we assign a level of risk to space objects? Level of risk depends on the frequency of the event causing risk and its scale (how much damage or # people affected). Car accidents: high frequency, very small scale (affects a few people per accident): High frequency results in fairly high risk. Airplane crashes: low frequency, moderate scale (may affect hundreds): Low risk due to the low frequency. Asteroid impacts: very low frequency, potentially very large scale (may affect billions): Low risk due to the low frequency. The risk of being killed by an 1.5 km diameter asteroid impact has been determined to be approximately equal to the risk of being killed in an airplane crash or by an electric shock! Torino Scale: a measure of the Risk posed by a given asteroid or comet. Based on the probability of collision (closeness of passing by Earth) and the Kinetic Energy upon impact (size and speed). The following link shows the estimated risk posed by a large number of asteroids: 18

19 How much warning will we have? Here s the answer from David Morrison of NASA: With so many of even the larger NEOs remaining undiscovered, the most likely warning today would be zero -- the first indication of a collision would be the flash of light and the shaking of the ground as it hit. In contrast, if the current surveys actually discover a NEO on a collision course, we would expect many decades of warning. Any NEO that is going to hit the Earth will swing near our planet many times before it hits, and it should be discovered by comprehensive sky searches like Spaceguard. In almost all cases, we will either have a long lead time or none at all. Impact craters on Earth The formation of impact craters A crater is generally 20 to 30 times the diameter of the object that creates it. Three stages to crater formation: 1. Contact/compression Stage. How they form. Crater anatomy. Major craters on Earth and elsewhere. 2. Excavation Stage. 3. Modification Stage. 1. Contact/compression Stage. A very brief stage (fraction of a second) when pressure and temperature due to the impact are intense. Rock in immediate contact is vaporized, surrounding rock melts due to the high temperatures. Rock adjacent to the impact is displaced upwards and other material is ejected. Spalled material is derived from the impacting object. 2. Excavation Stage This is the stage when material is ejected upwards, away from the impact site. Rock beneath the impact is compressed into a short-lived transient crater. The Chixulub crater would have taken less than 2 minutes to complete this stage. 19

20 3. Modification Stage. Anatomy of a crater The crater walls slump into the crater. For craters >4km diameter a peak rises up in the centre as rock that was compressed and pushed downwards lifts upward. Central up lift reaches 10% of the crater diameter. The uplift event takes a few minutes. Earth s moon: craters in craters. A crater chain on the moon. A crater chain on Jupiter s moon Ganymede. 20

21 Martian complex crater Impact Crater Images The Geological Survey of Canada (GSC) maintains a list of all major impact sites world-wide at: Meteor Crater Arizona Impact crater in Namibia. The rim of the crater rises 160 metres above the surrounding country-side. Rim diameter: 1.2 km diameter Age:49,000 years Rim diameter: 3.4 km Age: 1.4 ± 0.1 million years New Quebec Crater Brent Ontario Rim diameter: 3.4km Age: 1.4 million years Rim diameter: 3.8 km Age: 400 million years 21

22 Sudbury Impact Structure Holleford, Ontario Rim diameter: 250 km diameter Age: 1900 million years Rim diameter: 2.35 km Age: 550million years Is the Hudson Bay bight an impact crater? The GSC says no for now but one geologist is gathering evidence for a 2 billion year old event on a scale that is almost unimaginable. Erosion and plate tectonics have reduced the number of preserved craters on Earth Wednesday, September 29, :35:28 h EDT The 4.8 km long Asteroid 4179 Toutatis was within 1.55 million km of Earth Toutatis has not approached as close to Earth since the 12 th Century AD. It will not come as close again for another 500 years. 22

23 What space object has posed the greatest risk? 2002 NT7? A 2 km diameter asteroid with an impact velocity of 28 km/s Initially thought to be incoming January 19, NT7 was the first space object to be assigned a high risk of impact when it was first discovered. With further observations it was removed from the list of threatening objects. June 14, 2002 Asteroid 2002 MN passed within the moon s orbit of the Earth (within 120,000 km of Earth). About 1/3 the distance from the Earth to the moon. Asteroid and Comet Defense? Identify all Near Earth Objects and determine the probability of a collisions with Earth The size of a football field ( m in diameter) traveling 37,000 km/hr Point: close approaches have always happened but we have just begun to be able to see them coming. Two approaches to dealing with high probability collisions with sufficient lead time: 1. Land astronauts on the object, drill into it and plant nuclear bombs. The explosions should break the object up into smaller, harmless pieces. 2. Detonate nuclear weapons at selected locations in nearby space. The blasts will nudge the object in the opposite direction, sending it off course for collision with Earth. 23