Meteoroids, Meteors, and Meteorites (Part 8)
Small, Rocky Debris Peppers the Solar System
Meteoroids, Meteors, and Meteorites
Meteoroids: Rocky and metallic debris smaller than asteroids, scattered throughout the solar system.
Generally no more than about across; most are smaller than .
Originate from asteroid collisions, fragments from evaporating comets, and material that never coalesced with larger bodies.
Example: Asteroid Braille ( long) was blasted off the -diameter asteroid Vesta.
Meteors: Meteoroids that are being vaporized as they move through Earth's atmosphere.
They compress and heat gases, causing the surrounding air to glow, creating a visible trail.
Meteorites: Remnants of meteoroids that survive the fiery descent and land intact on Earth.
Meteors, Fireballs, and Bolides
Passing meteoroids are often pulled by gravity toward Earth.
As they move through the atmosphere, they compress and heat gases, causing the surrounding air to glow.
This glowing gas, along with vaporized pieces of the infalling matter, creates a trail, turning the meteoroid into a meteor.
Common names for these streaks of light include:
Shooting stars: Not actual stars of any kind or dying stars.
Fireballs: Meteors at least as bright as Venus.
Bolides: Bright meteors that explode in the air.
Notable Meteoritic Events
Chelyabinsk Bolide Event (February 15, 2013)
Estimated size: () across.
Moved faster than the speed of sound, creating a sonic boom that shattered numerous windows.
Exploded at an altitude of approximately () above Lake Cherbarkul.
Caused about 1600 injuries, primarily from shattered windows, but no fatalities.
Tunguska Event (June 30, 1908)
Location: Tunguska region of Siberia, Russia.
Blast comparable to a nuclear detonation of several megatons.
Audible over away; injected millions of tons of dust into the atmosphere.
1927 Expedition Findings:
Trees had been seared and felled radially outward in an area about in diameter.
No clear evidence of a crater; trees at "ground zero" were left standing upright but stripped of branches and leaves.
Debate on Cause:
Initially assumed to be a small comet, but recent arguments suggest a small asteroid or large meteoroid.
Consistent with an asteroid about () in diameter, entering Earth’s atmosphere at a shallow angle, moving at (), and exploding in the air.
The resulting shock wave caused damage without creating a crater.
Meteorites and Impact Craters
Most meteors vaporize completely before striking Earth, with their dust settling to the ground.
Impact Craters: Formed when pieces of a meteor (meteorites) survive descent and strike Earth.
Nearly 200 known impact craters on Earth, but thousands more have been eroded or drawn into Earth by tectonic plates.
Existing craters are all less than 500 million years old due to Earth's surface reshaping.
Meteor (Barringer) Crater, Arizona:
One of Earth’s least-weathered impact craters.
Measures across and deep.
Formed approximately 50,000 years ago when an iron-rich meteoroid (about across) struck at ().
The blast was like the detonation of a 20-megaton hydrogen bomb.
Meteor Showers
Frequency: On a typical clear night, one meteor can be seen about every 10 minutes.
Cause: Occur predictably when Earth passes through the orbit of debris left behind by a comet.
Approximately 30 meteor showers are visible each year.
Naming: Named after the constellation from which the meteors appear to radiate (e.g., Leonids from Leo Major).
More than one meteor can be seen each minute at the peak of prodigious showers like the Perseids.
Most meteor showers are best seen after midnight, except for the Lyrids.
Lunar Impacts during Showers:
The Moon's thin atmosphere provides no protection, leading to numerous meteoroid impacts during showers.
The Leonid meteor shower of 1999 provided opportunities to observe contemporary impacts on the Moon, some bright enough to be seen with the naked eye.
A meteorite striking the Moon generates an impact as powerful as pounds of TNT, with the resulting cloud momentarily reaching temperatures between and (hotter than the surface of the Sun).
Meteorites Are Space Debris That Land Intact
Definition and Origin
Meteorites are remnants of meteoroids that survive atmospheric entry and land intact on Earth.
They originate from various sources:
Ejected from planets, moons, asteroids, and comets by collisions (e.g., SNC meteorites from Mars).
Lunar meteorites: ejected from the Moon (e.g., during Mare Imbrium impact basin formation).
Meteorites from asteroid Vesta: show signs of molten rock, ejected by impact creating a () wide, () deep crater.
Some are debris that were never part of larger bodies.
Dating the Solar System
Meteorites are crucial for determining the age of the solar system.
Measurements of radioactive elements in meteorites indicate their formation time.
The oldest known meteorites solidified about billion years ago.
Impacts on Earth as early as billion years ago have been identified.
The solar system is estimated to be at least billion years old (approximately billion years often used).
Historical and Practical Significance
Early civilizations (Chinese, Indian, Islamic, Greek, Roman) attributed special significance to "rocks from heaven."
Infalling space debris increases Earth's mass by nearly tons per day on average.
Classification of Meteorites
Meteorites are classified into three main types: stones (stony), stony irons, and irons.
Stony Meteorites
Often resemble ordinary rocks.
Some are covered with a dark fusion crust, formed when the outer layer melts during atmospheric descent.
May contain tiny flecks of iron when cut and polished.
Iron Meteorites
Very dense; look noticeably different from ordinary rocks.
Can contain 10% to 20% nickel by weight.
Iron is a common rock-forming element and constituent of asteroids/meteoroids.
Iridium is common in iron-rich meteorites but rare on Earth's surface rocks; it helps determine the rate at which meteoritic material has been deposited over the ages.
High iron content allows detection with metal detectors; more likely to be noticed and thus dominate museum collections.
Widmanstätten patterns: Unique structure of long nickel-iron crystals, visible after cutting, polishing, and acid treatment.
Discovered by Count Alois von Widmanstätten in 1808.
Formed by slow cooling of molten metal over many millions of years; not found in "meteorwrongs" (counterfeit meteorites).
Stony-Iron Meteorites
Consist of roughly equal amounts of rock (e.g., olivine) and iron.
Considered the most exotic type of space debris on Earth.
Formation and Diverse Origins
Many meteorites were once parts of asteroids.
Asteroid interiors underwent differentiation due to heat from impacts and radioactive decay:
Iron sank to the center (forming cores).
Lighter rock floated to the surface (forming crusts).
Iron meteorites are fragments of asteroid cores.
Stony-iron meteorites come from the boundary regions between iron cores and stony crusts.
Stony meteorites have varied origins:
Some from outer layers of asteroids.
Ordinary chondrites: Show no evidence of melting; considered primordial material from which the solar system formed.
Carbonaceous chondrites: Rare chondrites containing small glass-rich beads called chondrules.
Contain complex carbon compounds (simple sugars, glycerin, amino acids).
Can have up to 20\text{%} water bound in minerals.
Unheated, suggesting they retain primordial volatile components.
Asteroid Mathilde has a similar spectrum, suggesting it is also likely composed of this primordial material.
Suspected to have played a role in the origin of life on Earth due to amino acid content.
Meteorite Falls vs. Finds
Falls: Meteorites that are observed impacting Earth.
Finds: Meteorites discovered on Earth, not necessarily observed falling.
Stony meteorites account for about 94\text{%} of all material that falls on Earth, but most are overlooked because they resemble ordinary rocks.
Finds percentages differ from falls percentages.
Accurate percentages of impacts are determined by carefully surveying areas in which only meteorites land, such as snow/ice-covered regions (e.g., Antarctica) or deserts, using tools like metal detectors.