Comets and Meteorites, Water in - river, oceans, temperature ...

Comets and Meteorites, Water in
An understanding of the earliest composition of the solar system is
derived from analyses of the Sun, comets, and the little-altered
carbonaceous chondrites
. Most of these measurements have come from analyzing the spectra of light
that originates from the Sun or that is reflected by bodies such as comets
and
asteroids
, the presumed source of most
meteorites
. Many chemical analyses of meteorites have been made in laboratories on
Earth and, increasingly, from spacecraft that encounter comets, asteroids,
and the Sun's atmosphere.
Comets comprise a huge reservoir of water in Earth's solar
system. Shown here is Halley's Comet, which crossed
Earth's orbit in 1986 but was barely visible to the naked eye
in much of the world.
Solar Components of Water
Because the great mass of matter in the solar system (nearly 99.9
percent) resides in the Sun, solar hydrogen (H), oxygen (O), and the
water molecule (H
2
0) must be considered when discussing
extraterrestrial
water.
The Sun, with an average surface temperature of about 6,000 kelvin
(approximately 5,727°C or 10,340°F), would seem to be a poor
place to look for water. However, scientists know that the constituents
of water are enormously abundant in the Sun: hydrogen is its most
abundant component, and oxygen also is a major component. Although tiny
in amount, traces of the water molecule have been reported in cooler
parts of the Sun's atmosphere.
Comets
Earth's solar system is believed to be about 4.6 billion years
old, based in part on the age-dating of many
chondrites
. The Sun is slightly older. Comets, consisting of ice and grains of
minerals and rocks—the "dirty snowballs" of
astronomer Fred Whipple—are thought to be the oldest, least
altered of any of the components of the solar system. They contain
mainly water ice, but other icy components have been measured as well,
such as carbon monoxide, carbon dioxide, sulfur, hydrogen sulfide,
methane, and hydrogen cyanide. (The term "water ice" is
not redundant because in space, some gases can change to ice on the
surface of planetary bodies.) Comets may have rocky cores as well as
interspersed grains of material. The interspersed granular character
extends to comet tails, which form by
ablation
as comets approach the Sun and the ice heats up.
These materials are minor components, however, compared to water ice.
Comets thus account for a very significant fraction of water in the
solar system: although small, there are many of them. The
Oort Cloud
of comets and the more recently described
Kuiper Belt
of comets account for untold numbers of comets and therefore comprise a
huge reservoir of water. Many planetary scientists believe that cometary
impacts early in Earth's history could have supplied much of the
water for its oceans.
Meteorites
Though small in amount compared to cometary water, meteoritic and
asteroidal water is important in understanding water-rich Earth. Earth
accumulated early in the history of the solar system by the sweeping up
and accumulating of enormous amounts of debris as the early Earth
orbited the Sun. These materials contributed their contained water to
the growing Earth.
As Earth grew, it evolved into a core, an intermediate
mantle
, and an external crust. Water played a critical part, especially
through the process of melting of the mantle and crust, with resulting
volcanism
and other
igneous
activity. Owing to volcanism, water and other
volatile
components were expelled to the surface, forming, along with possibly
significant additions of cometary water, Earth's
hydrosphere
.
Meteorites fall to Earth continuously and are studied intensively for
the information they provide about the earliest solar system and Earth.
Until the return of rocks from the Moon, meteorites were the only known
samples of extraterrestrial materials. Most meteorites are believed to
come from the
asteroid belt
, which contains numerous rocky objects that orbit the Sun, mainly
between the orbits of Mars and Jupiter. Some asteroids may have
originated as comets, with icy exteriors that were vaporized by the Sun,
leaving the rocky core.
Meteorites and asteroids have been extensively studied for their
chemical composition, including water. Carbonaceous chondrites, which
are the most fundamental and least modified type of meteorites, and are
chemically most like the composition of the Sun, have several percent of
water, primarily chemically combined in their mineral structures. The
amount of contained water may decrease or increase as meteorites are
altered, such as through
metamorphism
, melting, or
weathering
or impacts.
Bibliography
LIFE'S TWO REQUIREMENTS
Where water is found on Earth, life is also found, even in the most
extreme physical and chemical environments. Although there is no direct
evidence that life was transported to Earth by way of comets or
meteorites, cometary and meteoritic transport certainly delivered the
components required for the development of life: water and organic
(carbon-containing) compounds.
The most common surficial features of most planets and moons are impact
craters, so water and organic compounds also have been transported to
all other solar system bodies via the impacts of comets and asteroids.
Where heat or other sources of energy exist, along with water and
organic compounds, the potential for the development of life is high.