Comet; Asteroid and Meteorites

COMET

A comet is a small solar system body bigger than a meteoroid[citation needed] that, when close enough to the Sun, exhibits a visible coma (fuzzy "atmosphere"), and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, ranging from a few hundred metres to tens of kilometres across.

Comet Swarm Delivered Earth's Oceans?

A barrage of comets may have delivered Earth's oceans around 3.85 billion years ago, a new study suggests.

Scientists have long suspected that Earth and its near neighbors were walloped by tens of thousands of impactors during an ancient event known as the Late Heavy Bombardment.
This pummeling disfigured the moon, leaving behind massive craters that are still visible, preserved for millennia in the moon's airless environment. But it's been unclear whether the impactors were icy comets or rocky asteroids.

Now, based on levels of a certain metal in ancient Earth rocks, a team led by Uffe Jorgensen of the Niels Bohr Institute in Denmark says comets were the culprits.
Whether Earth had oceans before any comets arrived has been intensely debated, Jorgensen noted.
Some experts say enough water could have existed from the moment Earth formed, while others argue that the young planet's heat would have vaporized any liquids.
"It's the kind of subject that can make scientists fight physically with one another," Jorgensen said.
His team thinks early Earth was just too hot to retain large bodies of water. But by the time of the Late Heavy Bombardment, things had cooled down, allowing meltwater from the flurry of comets to become the world's first seas.
"We may sip a piece of the impactors every time we drink a glass of water," the study authors write in their paper, which will be published in an upcoming issue of the journal Icarus.

Comets' Metal

Jorgensen and colleagues arrived at this conclusion after measuring the levels of iridium in surface and near-surface rocks from Greenland—some of the oldest known rocks in the world, dating back to the time of the bombardment.
Iridium is a scarce metal on Earth, but it's relatively common in comets and asteroids.
According to the team's calculations, iridium levels in the rocks around an asteroid impact should be about 18,000 parts per trillion. A comet impact, meanwhile, should leave behind only about 130 parts per trillion. That's because comets would carry less metal, since they're mostly made of loosely packed water ice with some rocky debris.
Comets also strike Earth at higher speeds, because of their longer orbits around the sun.
As a result, "the explosion formed by a comet is more violent than from an asteroid, and the amount of material—including iridium—thrown back into space is larger," Jorgensen said.
The team found that the Greenland rocks contained about 150 parts per trillion of iridium, supporting the idea that comets were the main players in the Late Heavy Bombardment.
All that ice from the comet swarm then thawed to create a global ocean more than half a mile (about a kilometer) deep, the team calculates.
The moon, meanwhile, lacks an ocean because its gravity is much weaker than Earth's, so most if not all of the debris from a comet strike would be thrown back into space, Jorgensen said.
But Nicolas Dauphas, a geophysicist at the University of Chicago, isn't yet convinced that the bombardment featured comets, not asteroids.
The new study, he said, relies on too many estimates—such as the predicted amount of iridium deposited following an impact.
"I am afraid [they have] stretched their conclusions too far," Dauphas said.

Accidental Life?

Chandra Wickramasinghe, an astrobiologist at Cardiff University in the U.K. not involved in the new study, also supports the theory of an ancient comet bombardment.
And he thinks it's possible that comets seeded Earth not only with water but with life.
According to some controversial studies, the oldest evidence for life on Earth dates back to about 3.85 billion years ago, around the time of the Late Heavy Bombardment, he noted.

Asteroid

Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are smaller than planets but larger than meteoroids. The term "asteroid" has historically been applied primarily to minor planets of the inner Solar System, as the outer Solar System was poorly known when it came into common usage. The distinction between asteroids and comets is made on visual appearance: Comets show a perceptible coma while asteroids do not.

Asteroid Impact

There have been many Asteroid Impact videos but not one is as beautiful and scary as this one. If you realise this could indeed happen some day it makes you wonder will mankind be on outer space in time or will we all die when this happens? Check out this HD CGI footage by Discovery Channel about an Asteroid the size of the moon hitting planet earth.

Meteorites

A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids. When it enters the atmosphere, impact pressure causes the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting/falling star. The term bolide refers to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface.

More generally, a meteorite on the surface of any celestial body is a natural object that has come from elsewhere in space. Meteorites have been found on the Moon and Mars.

It's amazing what a rover can find laying by the side of the road. The Mars Exploration Rover Opportunity has found a rock that apparently is another meteorite. Less than three weeks ago, Opportunity drove away from a larger meteorite called "Block Island" that the rover examined for six weeks. Now, this new meteorite, dubbed "Shelter Island," is another fairly big rock, about 47 centimeters (18.8 inches) long, that fell from the skies. Block Island is about 60 centimeters (2 feet) across and was just 700 meters (about 2,300 feet) away from this latest meteorite find. At first look, the two meteorites look to be of a similar makeup; Opportunity found that Block Island was is made of nickel and iron.

Meteorites that are recovered after being observed as they transited the atmosphere or impacted the Earth are called falls. All other meteorites are known as finds. As of mid-2006, there are approximately 1,050 witnessed falls having specimens in the world's collections. In contrast, there are over 31,000 well-documented meteorite finds.

Meteorites have traditionally been divided into three broad categories: stony meteorites are rocks, mainly composed of silicate minerals; iron meteorites are largely composed of metallic iron-nickel; and, stony-iron meteorites contain large amounts of both metallic and rocky material. Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy.

Meteor Crater