Astronomy - Planet Earth
Mass: 5.976 X 1024 kg (1 Earth)
Radius (equatorial): 6378 km
Mean density: 5.515 g cm-3
Distance from Sun: 149 600 000 km
Rotational period: 0.99727 days
Orbital period: 365.256 days
Escape velocity: 11.18 km s-1
Surface temperature: 15°C (mean)
Atmospheric composition: nitrogen (77%), oxygen (21%), other (2%)
Our planet Earth is the only planet known to harbor life. There are many intertwining factors why life thrives on the Earth both in terms of quantity and variety. Being the third planet, it is situated at a mean distance of just short of
150 000 000 km from the Sun, a distance that is neither too cold nor too hot for water to exist, an essential ingredient for living organisms. Earth's thick atmosphere helps to regulate the temperature that prevent excessive heat from baking the surface while burns most meteorites before they hit the
Earth. At night the Earth's molten nickel-iron core warms the surface and prevent excessive cooling. The molten core also generates strong magnetosphere and, together with the atmosphere, shield the Earth from nearly all harmful radiation coming from the Sun (and other stars).
The Earth's atmosphere consists of 77% nitrogen and 21% oxygen. The former is chemically inert that helps to stablize the atmosphere. Without it, the chemically reactive oxygen will react with many materials, leading to spontaneous combustion.
In addition, the tiny amount of carbon dioxide (~0.03%) is acting as a moderate green house agent. The gas retains heat so that the average surface temperature is maintained at a comfortable 15°C. Otherwise, the temperature will easily drop to around -20°C and ocean would freeze.
Geologically speaking, the Earth's surface is very young, about 500 000 000 years. Erosion and tectonic processes destroy most of the traces of earlier geologic surface history. Nowadays, it is very difficult to find rocks older than 3 billion years old, about 1.5 billion years short of the Earth's age. About 71% of the Earth surface is covered with water. Earth is the only planet where water can exist in liquid form. The presence of large amount of water also helps to regulate the temperature. The liquid water is also responsible for a variety of
geochemical processes such as erosion, migration and cyclization of minerals and elements. Oxygen accounts for about 47% of Earth's crust by weight, while silicon and aluminium comprise of 27% and 8% respectively. Complex cyclization processes (carbon cycle, nitrogen cycle etc) between atmosphere and surface ensure supplies of biologically important elements are properly maintained, often involving waters as intermediaries.
Earth has only one satellite, the Moon. Interesting gravitational effects between the Earth and Moon result in the tides. The gravitaional interaction also causes the Moon to rotate synchronously, that is, it is locked in phase with its orbit so that the same side is always facing the Earth. In addition, Earth is known to have 'companions' which have complicated orbital relationship with the Earth. One example is the Asteroid 3753 Cruithne.
With our very presence on Earth we do not need to study Earth using remote spacecrafts. Nevertheless, we have learn a great deal about our Home by using artificial satellites. Global images taken at various wavelengths with unprecedented details have taught us a lot in areas such as human impacts on Earth's ecology and environment.
One of the most requested picture of the Earth, the image was taken from Apollo 17, on December 1972, as the spacecraft is travelling to the Moon. The image shows Saudi Arabia and Africa partially shrouded by clouds. Huge masses of clouds is seen to cover most of the southern
hemisphere. This picture also shows for the first time the Antartica south polar ice cap.
View of Earth from Moon, taken by Apollo 8 astronauts in December 1968, the first humans to circumnavigate the Moon. The Earth is about 5 degree above the Lunar horizon.
Visible image of Earth, taken from Polar mission, showing for the first time simultaneously the northern and southern lights (auroras borealis and australis). The auroras appeared during a Sun storm on October 2001. The images confirm the three-century old theory that auroras in the nothern and southern hemispheres are nearly mirror images.
Credit: Polar: GSFC/NASA.
Venus express, launched in 8 November 2005, took this infrared view of Earth at 3.5 million km using VIRTIS instrument. The planet appears almost 'fully illuminated' with no difference from day and night regions since only the thermal radiance is seen.
The Antarctica region, being colder, produces a weaker signal and appears as dark blue.
Credit: ESA/VIRTIS Team.
Views of Moon taken by Galileo. Left picture shows Orientale Basin and the right picture shows the rotated Moon, with the Orientale now on the Western limb.
Credit: Galileo: NASA
POLAR Mission - Exploring Through the Solar Cycle. A mission to study Earth's aurora regions in ultraviolet, visible and X-ray.
SeaWIFS - Study and collect data on oceans.
Clementine Mission - Map lunar surface, in a wide range of wavelengths- from ultraviolet to infrared red. Mission ended due to malfunction in June 1994.
Lunar Prospector Mission - Low polar orbit investigation of the Moon, especially for any possible sign of water at the polar regions. Controlled crash to surface on July 1999.