University:
Glendale Community College
Course:
GLG110 | Environmental Geology
Academic year:

2021
Views:

252

Pages:

5
Author:

Jadyn Palmer

Environmental Geology CHAPTER 12 IMPACT OF EXTRATERRESTRIAL OBJECTS • Case History: The Tunguska Event -‐ Aerial Blast & Largest Impact Event in recorded human history. On June 30, 1908, blue-‐white fireball exploded over Tunguska River Valley with the blasting force of 10 megatons of TNT, 10 H-‐ bombs. It left NO impact crater. Estimated size of the asteroid 20–50 m in diameter. 2000 km2 of forest flattened and burned. At least 1,000,000-‐km2 area heard the explosion. Loss of lives in millions if it had occurred over a major city. It could happen again; We narrowly missed a similar hit in 2004. • Asteroids -‐ Size: Ranging from 10 m (32 ft) to 1000 km (621 mi). They are composed of rocky or metallic material, or rock–metal mixtures. Most of them are located in asteroid belt between Mars and Jupiter. Some cross Earth’s orbit due to the collision of asteroids. • Comets -‐ Originate from the Oort Cloud, deep in the solar system, about 50,000 times the distance between Earth and the Sun; many are in the Kuiper Belt. They sometimes intersect Earth’s orbit. Size: Ranging from a few meters to a few hundred kilometers. Composition: A rocky core surrounded by ice and covered by carbon-‐rich dust. The “ice” may be water ice, carbon dioxide, carbon monoxide, methane, ammonia. The “dust” is silicate minerals, metallic iron and nickel specks, and carbon compounds. Earth’s water is thought to have come mostly from comets • Meteoroids – Meteors – Meteorites : Origin: Small, fragmented rocky, icy or carbonaceous space particles from the disintegration of asteroids. Size: Ranging from dust-‐size to a few meters. They are called Meteoroids when they are in space. Meteors are burning meteoroids due to frictional heat from their entrance to Earth’s atmosphere. A meteor shower is a large number of meteors. They are called meteorites when they are found on Earth’s surface. • Early Evolution of the Earth -‐ Since Earth formed it has been bombarded by meteoroids, asteroids, or comets (MACs). Impact of the MACs contributed to the formation and makeup of Earth. Earth’s evolution greatly affected by the impacts—both destruction and construction effects. It is by accretion, or “sweeping up” of space debris, via impacts, that Earth is thought to have grown from the nebula in the first place. • Impact Craters and Erosion -‐ If an impact hits the surface at “hypervelocity” (i.e., it is still moving kms/sec), it will produce an explosion crater, known as an impact crater. Meteoroids/comets enter Earth’s atmosphere with velocities of 12 to 72 km/sec! Most impact craters on Earth destroyed (erased) by tectonics, erosion and volcanism • • • • • • The Moon-‐There are many obvious impact craters on the moon-‐ retained for billions of years . Impact cratering was initially intense, but has dropped to a much lower frequency over the past 3.8 billion years or so. The Moon has no atmosphere and craters are not eroded and significant volcanism ended over 3 billion years ago and tectonism/faulting appears to have been a very minor process on the Moon. Aerial Bursts and Impacts -‐ MACs travels at high speed: 26,000 to 156,000 mph, and undergo remarkable changes when entering Earth’s atmosphere, causing aerial bursts in the atmosphere when disintegrated in the air, and direct impacts on collision with Earth. Impact Craters-‐The most direct and obvious evidence of extraterrestrial impacts. Earth has few obvious impact craters (Meteor Crater in Arizona is the most famous). Careful study has revealed the presence of over 150 impact structures, most of which have been heavily eroded. Impact cratering is one of the four basic types of geologic processes that occur on planets. Eosion,/gradation, volcanism, and tectonism are the other 3. • Meteors are almost instantly vaporized upon impact, as are some rocks (others melt). Intense pressures produce “shocked” minerals, distinctive to impact events (and nuclear blasts) and highly fractured/brecciated rock layers beneath crater. Material in ground is compressed, then bounces back and flies out of growing crater. Material thrown out of crater is called ejecta and an ejecta blanket is layer of fragments produced by falling to Earth of ejecta around crater, which is thickest at rim and thins rapidly away from crater, and produces “inverted layering” with top layers thrown out and landing first, deeper layers excavated and landing later. Barringer Impact Crater -‐ 50,000-‐year-‐old Barringer Crater in Arizona, the most famous meteor crater in the U.S. It is 1.2 km (.75 mi) in diameter and 590 ft in depth bowl-‐shaped depression with upraised ejection rim (850 ft). It resulted from the impact of a small asteroid, 25–100 m in diameter Impact Craters • Craters involved with extremely high temp, velocity, and pressure; different from craters from other terrestrial geologic processes • Shock metamorphism: Forming high-‐pressure modified minerals such as quartz or materials of mixed partially melted rocks and impacting objects • Other associated processes: Vaporization, melting, and ejection of materials • Happened before and likely to happen again Barringer Crater and others on Earth Remotely studied craters on the moon and Mars Impact on Jupiter Comet Shoemaker-‐Levy 9 orbited Jupiter and became 21 fragments In 1994, the family of fragmented “string of pearls” entered Jupiter’s atmosphere and exploded Ø Energy released: Greater than that of the entire stockpile of nuclear weapons on Earth • • • Ø Ø • • Mass Extinction -‐ Massive and sudden loss of plants and animals relative to the number of new species being added. Extinction-‐scale events occur every ~100 million years. Some (but not all) mass extinction events correspond in time with dated impact structures. Several hypotheses suggested: Rapid climate change, plate tectonics, extremely large volcanic events, and impacts of extraterrestrial objects. Six major mass extinctions during the last 540 Million years • • • First: ~443 MYA, near end of the Ordovician period, extinction of 100 families and associated species Second: 350 MYA, near end of the Devonian, death of 70% of all marine invertebrates Third: 245 MYA, end of the Permian Ø 95% of all marine species Ø Possibly caused by more than a single catastrophe Ø Likely spanned for a 7-‐million-‐year period • Fourth & Most Famous: Around the K-‐T (Creatceous – Tertiary) boundary, 65 MYA Ø Resulting from a giant asteroid (diameter of 10 km) impact in the vicinity of the Yucatan Peninsula Ø Louis and Walter Alvarez (1980) – Best evidence (short of actual crater) was enhanced iridium in clay layer at boundary – No crater was identified at that time Ø Extinction of about 70% of all genera, such as dominant species of dinosaurs Ø Setting stage for the evolution of mammals, including humans • Chicxulub – Buried Impact Crater -‐ Found in Yucatan of Mexico in 1991 – ~180 km diameter…. Formed at the K-‐T boundary • 10 km (6mi) diameter asteroid • 100 million megaton blast (10,000x entire world’s nuclear arsenal) • Dust in atmosphere / Acid rains / Forest fires • Mega-‐tsunami (3000 ft high!) • Food chain collapsed • • • • • ~40 to 100 million year recurrence interval for event of this size • Fifth: Mass extinction -‐ Near end of the Eocene, 35 MYA Ø Likely resulted from cooling and glaciation Ø Recently linked to an impact event (Chesapeake Bay) Sixth: Mass Extinction (last): Near end of the Pleistocene to present Ø Ongoing mass extinction of mammals, reptiles, amphibians, birds, fish, and plants Ø Climate change Ø human activities Ø Pleistocene megafauna extinctions and disappearance of Clovis (human) culture – poss. link to an impact almost 13,000 years ago Environmental Risk of Impacts -‐ Depending upon both the probabilities and the consequences of the impacts. Ø Catastrophic impact risks: Aerial blasts or direct ground impact Ø Impact area: Regional or global scale, mass extinction Ø Smaller event about every 1000 years, and larger event to occur every few tens of thousands of years Ø Impact risks for humanity Ø Tunguska-‐like event every 1,000 years or so Ø With current city distribution, massive impact catastrophe should hit a city, on average, every few tens of thousands of years Ø Such an event could kill millions almost instantaneously – averages out to hundreds of deaths/year, similar to storms and floods in U.S. (our number one threat!) Ø Chicxulub-‐type event would likely wipe out humanity entirely Ø Sequence of Events – Catastrophic Impact Ø Est. relationship between energy of impacts and frequency of events Minimizing the Impact Hazard -‐Identify and Categorize: Goal….to Identify all potentially hazardous near-‐Earth objects (NEOs) – including asteroids and space junk, like spent Apollo rocket stages, etc and categorize the comets and asteroids crossing Earth’s orbit • Spacewatch and NEAT -‐ Spacewatch program, developed in 1981, has expanded its observations to the entire solar system. NEAT (near-‐earth asteroid tracking) system, support by NASA, focuses on the size and distribution of NEOs. How many are out there? There are about 20 million NEOs, 4% with the ability to penetrate and excavate a crater; another 40% to penetrate the atmosphere and pose a threat. Extrapolations from sky surveys suggest there are about 1,500 objects of 1 km or more in diameter are in Earth-‐ crossing orbits. Around 135,000 Earth-‐crossing asteroids with a diameter of 100 m or less. • Minimizing the Impact Hazard…… considerations and possibilities • Diverting Objects and Evacuation • Designing an approach to divert large objects away • Designing a plan for early evacuation for smaller impact events, if precise location can be predicted • Comets have orbits that take them very far from the Sun, and may cross Earth’s path • Such objects are far harder to predict/see and would give far less lead time to prepare and potentially nudge out of Earth’s path