DEFINITION OF FORMATION OF THE EARTH AND SOLAR SYSTEM

Understanding the origin of the Earth, planets, Sun, and other bodies

in the solar system is a fundamental yet complex problem that

has intrigued scientists and philosophers for centuries. Most of the

records from the earliest history of the Earth have been lost to tectonic

reworking and erosion, so most of what we know about the

formation of the Earth and solar system comes from the study of

meteorites, the Earth’s moon, and observations of the other planets

and interstellar gas clouds.

The solar system displays many general trends with increasing

distance from the Sun, and systematic changes like these imply

that the planets were not captured gravitationally by the Sun but

rather formed from a single event that occurred about 4.6 billion

years ago. The nebular theory for the origin of the solar system

suggests that a large spinning cloud of dust and gas formed and

began to collapse under its own gravitational attraction. As it collapsed,

it began to spin faster to conserve angular momentum

(much as ice skaters spin faster when they pull their arms in to

their chests), and eventually formed a disk. Collisions between particles

in the disk formed protoplanets and a protosun, which then

had larger gravitational fields than surrounding particles, and

began to sweep up and accrete loose particles.

The condensation theory states that particles of interstellar

dust (many of which formed in older supernova) act as condensation

nuclei that grow through accretion of other particles to form small

planetesimals that then have a greater gravitational field that attracts

and accretes other planetesimals and dust. Some collisions cause

accretion, other collisions are hard and cause fragmentation and

breaking up of the colliding bodies. The Jovan planets became so

large that their gravitational fields were able to attract and accrete

even free hydrogen and helium in the solar nebula.

The main differences among the planets with distance from

the Sun are explained by this condensation theory, since the temperature

of the solar nebula would have decreased away from the

center where the Sun formed. The temperature determines which

materials condense out of the nebula, so the composition of the

planets was determined by the temperature at their position of formation

in the nebula. The inner terrestrial planets are made of

rocky and metallic material because high temperatures near the

center of the nebula only allowed the rocky and metallic material to

condense from the nebula. Farther out, water and ammonia ices

also condensed out of the nebula, because temperatures were

cooler at greater distances from the early Sun.

Early in the evolution of the solar system, the Sun was in a TTauri

stage and possessed a strong solar wind that blew away

most gases from the solar nebula, including the early atmospheres

of the inner planets. Gravitational dynamics caused many of the

early planetesimals to orbit in the Oort Cloud, where most comets

and many meteorites are found. Some of these bodies have eccentric

orbits that occasionally bring them into the inner solar system,

and it is thought that collisions with comets and smaller molecules

brought the present atmospheres and oceans to Earth and the

other terrestrial planets. Thus air and water, some of the basic

building blocks of life, were added to the planet after it formed,

being thrown in from the deep space of the Oort Cloud.

elements, and is quite depleted in these volatile elements relative

to the Sun, it is thought the primary atmosphere has been

lost to space.

Gases are presently escaping from the Earth during volcanic

eruptions and are also being released by weathering of

surface rocks. The secondary atmosphere was most likely

produced from degassing of the mantle by volcanic eruptions,

and perhaps also by cometary impact. Gases released from

volcanic eruptions include N, S, CO2, and H2O, closely

matching the suite of volatiles that comprise the present

atmosphere and oceans. However, there was no or little free

oxygen in the early atmosphere, as oxygen was not produced

until later, by photosynthetic life.

The early atmosphere was dense, with H20, CO2, S, N,

HCl. The mixture of gases in the early atmosphere would

have made greenhouse conditions similar to that presently

existing on Venus. However, since the early Sun during the

Hadean Era was approximately 25 percent less luminous

than today, the atmospheric greenhouse served to keep temperatures

close to their present range, where water is stable,

and life can form and exist. As the Earth cooled, water

vapor condensed to make rain that chemically weathered

igneous crust, making sediments. Gases dissolved in the rain

made acids, including carbonic acid (H2CO3), nitric acid

(HNO3), sulfuric acid (H2SO4), and hydrochloric acid

(HCl). These acids were neutralized by minerals (which are

bases) that became sediments, and chemical cycling began.

These waters plus dissolved components became the early

hydrosphere, and chemical reactions gradually began changing

the composition of atmosphere, getting close to the

dawn of life.

It is of great intellectual interest to speculate on the origin

of life. In the context of the Hadean, when life most likely

arose, we are forced to consider different options for the

initial trigger of life. It is quite possible that life came to

Earth on late accreting planetesimals (comets) as complex

organic compounds, or perhaps it came from interplanetary

dust. If true, this would show how life got to Earth, but not

how, when, where, or why it originated. Life may also have

originated on Earth, in the deep sea near a hydrothermal

vent, or in shallow pools with the right chemical mixture. To

start, life probably needed an energy source, such as light-

ning, or perhaps submarine hydrothermal vents, to convert

simple organic compounds into building blocks of life

(RNA- ribonucleic acid) and amino acids.

See also LIFE’S ORIGINS AND EARLY EVOLUTION; METEOR;

PLATE TECTONICS.

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