Ch 18 Asteroid & Comet Impacts

Evidence for Past Impacts: Ejecta

- Big ejecta deposits - Microscopic ejecta - glass spherules and shocked quartz grains

Comet Orbits

- Comets highly eccentric - so can come closer to inner solar system - Accelerates closer to the sun

Long Term Effects of Complex Craters

- Huge tsunamis - Massive amounts of dust and debris in atmosphere - Global firestorm and acid rains - Several years of darkness and cold climate - Collapse of photosynthesis and base of food chain - Extinctions

Kuiper Belt

- Lies in plane of solar system - 30-50 AU from Sun - Contains millions of comets - Short periods of 200 years or less - E.g. Halley's comet

Potentially Hazard Asteroids - PHAs

- Minimum distance of 0.05 AU or less - Larger than 140m in diameter

Planetary Defense

- Need to detect and warn - Then prevent or mitigate - All under efforts of NASA

Asteroid Characteristics

- Over 500K identified in belt - Most over 3km wide - Orbital speeds of 25 km/s - Orbits perturbed inward by collisions between other asteroids or gravity of nearby planets

Kinetic Energy on Impact

- Some energy transferred to surface; creates shock waves - Some energy converted into heat, vaporising impactor and target and meltign rocks

Comet Glowing Tails

- Spray of water, dust and volatiles to form tails when hit by solar wind - Tail points away from sun - Eventually lose their water - become rocky and look with asteroids - Travel faster, 60-70 km/s - much more damage

Evidence of Past Impacts: Iridium

- Thin dark layer of clay with lots of elements (iridium, platinum, nickel) that are abundant in iron meteorites - Coincides with change in fauna and flora - Radiometric dating

Oort Cloud

- Vast spherical region, 50,000 AU from sun - Extends more than 100,000 AU - Contains trillions of comets - Long periods of 200 years or more

Amors

- Within asteroid belt - 2912 bodies - Cross Mar's orbit, not Earth's

Atens

- Within belt - 663 bodies - Entirely within Earth's orbit

Apollos

- Within the asteroid belt - 3678 known bodies - Cross Earth's orbits

most asteroid impacts:

-are the result of oblique arrival 5-15 degrees from earths horizontal. -break up in the atmosphere into multiple fragments that retain 50% of its original speed

Kinetic Energy Formula

0.5 x mass x (speed)2 - Asteroids are slower but bigger mass - Comets are faster but smaller mass

What to do with PHA?

1) Blast asteroid with nuclear weapon - deflect it 2) Attach a rocket to asteroid to deflect orbit. gravity tractor or kinetic impactor to knock off course 3) Change amount of heat radiated from one side to change its orbit by painting one side white- Yarkovsky effect

3 Stages of Crater Formation

1) Contact and compression 2) Excavation 3) Modification and collapse - either small or large

Type of Meteorites

1) Iron - 6% - nickel-iron alloys, same density as Earth core, magnetic 2) Stony-iron - <1% - Mg and Fe 3) Chondrite - 93% - Mg-Fe, same density as Earth mantle 4) Achondrite - <1% - similar to basalt in oceanic crust

A large impact's immediate effect:

1. The fireball or ejecta from the impact would ignite fires within hundreds of kilometers of the impact site. 2. A heavy plume of smoke would linger for years in the atmosphere. 3. Sulfate aerosols and water would be added to the atmosphere as well. 4. large portion of the ozone layer would be destroyed leaving us vulnerable to UV rays 5. dust blown into stratosphere would cause many harmful effects and block out sun

Most space rocks originate from:

Asteroid belt between mars and jupiter

Manicouagan Crater

Chain of blocks from one larger rock - they all line up when you put them back together in original tectonic plate conditions

Perihelion

Closest point to Sun

Large modification

Compression and rebound create central high and rim forming complex craters. Proximal and distal ejecta and dust and aerosols capable of circulating globe.

Comets

Consists of a solid nucleus of a rock and ice mixture surrounded by a coma of dust and gas. The tail is a mixture of water, other volatiles, and dust that the solar wind sprays away from the direction of the sun. left over from formation of outer solar system

Why do meteors explode in air?

Due to compression and heat by friction with the air

large impact craters

First (1), a transient crater is excavated and compressed, and the base of the cavity melts. Second (2), the base of the transient cavity rebounds as excavation continues. Then (3), the raised rim of the transient crater and central uplift both collapse to form a larger and shallower crater basin partly filled with inward-facing scarps, large blocks, smaller fragments, and melt rocks. The final crater is much broader and shallower than the initial transient crater. Some large craters on the moon have this central uplift (see photo).

Open Impact Crater Formation

First (1), a transient crater* is excavated, compressed, and fractured; the base of the cavity melts; and the rim is raised. Then (2) the ejecta blanket* spreads around the cavity, and the part of the rim slumps back into the cavity. Finally (3), fallback material* partly fills the cavity, along with some melt-rich material.

Aphelion

Furthest point from Sun

Impact Risk of Meteorite

Larger impacts less frequently. 1m diameter every year Impact of 1.5-2km diameter could kill 1/4 Earth population

Glass Spherules

Molten rocks blasted into air as a spray of droplets. most obvious sign of impact

Shocked Quartz Grains

Multiple sets of thinly spaced deformation planes imposed by high intensity impact

Meteor

Object that enters Earth's atmosphere; forms a streak of light in the sky - shooting star, fireball. Small meteors burn up in upper atmosphere, large ones becomes incandescent on outside.

Meteorite

Objects that survive entry through the atmosphere and collide with earth. Move faster than speed of sound so we generally dont hear them

Recording Impacts

Only on land - don't know about ones in ocean

Near Earth Object (NEO) Observation Program

Responsible for finding, tracking and characterising NEOs asteroids and comets that come within 0.3AU or 74.9 million km of Earth's orbit catalog anything larger than 100m in diameter

Asteroids

Rocky or metallic fragments in space that orbit the sun

Small modification

Small impacts form simple craters

Meteoroid

Small object - less than 10m - moving in space; debris of asteroids and comets

Tektites

Small pherules or blobs of glass formed by meteorite impacts from ejected material in the atmosphere falling back to earth

strewn field

The elongated area on the ground where scattered meteorite fragments from an exploded (meteor) are found.

Asteroid Belt

The region of the solar system between the orbits of Mars and Jupiter, where many asteroids are found. Initially thought to an exploded planet (impossible from Jupiter's gravity); actually made from Nebular Hypothesis.

Torino Scale

a system that allows scientists to rate the hazard level of an object moving toward Earth No hazard, normal, merits attention, threatening, certain collision

Iron Meteorite Characteristics

angular, typically black, extremely hard and unbreakable, show intersecting sets of parallel lines showing nickel iron structure

Meteorite types from most to least common

chondrite, iron, stony iron, achondrite

comet impact probability

comets that have orbits outside our solar system but become visible when they pass close to the sun or Earth. About 10% of impacts on Earth and the moon are caused by comets.

Evidence for Past Impacts: Shatter Cones

cone shaped, Radiating fractures caused by high pressure of impact. form mostly in fine grained massive rocks. The apex points up toward the shock source. originate from imperfections on surface of rock

evidence of past past collisions is evident by:

energy of the impacts

T/F: Impact crater shape is dependent upon angle of object

false, high velocity causes explosion on contact and creates a round hole regardless of the angle. chicxulub impact was likely the one that killed dinos

t or f: predicting asteroid impacts is easy and possible

false, there is no way to be certain. we can only estimate the odds

Most asteroids are land:

in the ocean

Impact of a modest-sized asteroid 1.5 to 2 km in diameter

is thought to be enough to kill perhaps one-quarter of the people on Earth and threaten civilization as we know it

Impacts of asteroids

kinetic energy converted to heat and vaporization, melts rock, excavates a crater and blasts out rock and molten glass. Results in huge fireball

Comets have ___________ density and travel at ___________ speeds compared to asteroids

low, faster

Comet tails are the result of

melting and evaporation of ices from the comet core when it comes into contact with our suns solar wind. The tail points away from the sun.

record of impact is preserved by:

shape of final crater and minute particles/ chemical traces in sedimentary rocks deposited around the time of the impact

T or F Impacts can cause other hazards such as tsunamis, earthquakes, and volcanic eruptions

true

t or f: there is massive fallout after a large impact similar to that of a eruption

true

The odds of an undiscovered one striking Earth in the next 200 years

very small. But it is quite clear that it will eventually happen—we just do not know when. We can neither predict the impact of a small but deadly rock nor see it coming before being struck. solution= dont think about it

fusion coating

very thin layer of dark glass formed when friction against earths atmosphere heats it above its melting temp. Meteorite coatings are sharply bounded and very different from their interiors

The chance that Earth will be struck by a civilization-ending asteroid next year is greater than

winning the lottery