The Atmosphere

Earth is surrounded by a relatively thin atmosphere (commonly called air) consisting of a mixture of gases, primarily molecular nitrogen (78 percent) and molecular oxygen (21 percent). Also present are much smaller amounts of gases such as argon (nearly 1 percent), water vapour (averaging 1 percent but highly variable in time and location), carbon dioxide (0.037 percent 370 parts per million and presently rising), methane (0.00015 percent 1.5 parts per million ), and others, along with minute...

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Meteorite crater surrounded by rays of ejected material on Mercury, in a photograph taken by the Messenger probe, Jan. 14,2008. A chain of craters crosses the centre of the rayed crater. NASA Johns Hopkins University Applied Physics Laboratory Carnegie Institution of Washington few hundred metres in diameter. Interspersed among the larger craters are relatively flat, less-cratered regions termed intercrater plains. These are similar to but much more pervasive than the light-coloured plains that...

Transient Atmospheric Phenomena

Early telescopic observers noted instances in which Martian surface features were temporarily obscured. They observed both white and yellow obscurations that were correctly interpreted as due to condensed gas and dust, respectively. Telescopic observers also noted periodic disappearances of all dark markings, usually around southern summer. Again they were correctly interpreted as the result of global dust storms. Spacecraft observations have confirmed that hazes, clouds, and fogs commonly veil...

THE HyDrosphere

Earth's hydrosphere is a discontinuous layer of water at or near the planet's surface it includes all liquid and frozen surface waters, groundwater held in soil and rock, and atmospheric water vapour. Unique within the solar system, the hydrosphere is essential to all life as it is presently understood. Almost 71 percent of Earth's surface is covered by saltwater oceans, with a volume of about 1.4 billion cubic km (336 million cubic miles) and an average temperature of about 4 C (39.2 F), not...

Internal Activity of the Past and Present

The idea that the lunar crust is the product of differentiation in an ancient magma ocean is supported to some extent by compositional data, which show that lightweight rocks, containing such minerals as plagioclase, rose while denser materials, such as pyroxene and olivine, sank to become the source regions for the later radioactive heating episode that resulted in the outflows of mare basalts. Whether or not there ever was a uniform global ocean of molten rock, it is clear that the Moon's...

The Kant Laplace Nebular Hypothesis

Kant's central idea was that the solar system began as a cloud of dispersed particles. He assumed that the mutual gravitational attractions of the particles caused them to start moving and colliding, at which point chemical forces kept them bonded together. As some of these aggregates became larger than others, they grew still more rapidly, ultimately forming the planets. Because Kant was highly versed in neither physics nor mathematics, he did not recognize the intrinsic limitations of his...

Differentiation into Inner and Outer Planets

At this stage the individual accreting objects in the disk show differences in their growth and composition that depend on their distances from the hot central mass. Close to the nascent Sun, temperatures are too high for water to condense from gaseous form to ice, but, at the distance of present-day Jupiter (778 million kilometres km , 5.2 astronomical units AU , or 483 million miles) and beyond, water ice can form. The significance of this difference is related to the availability of water to...

Small Scale Features

On a small-to-microscopic scale, the properties of the lunar surface are governed by a combination of phenomena impact effects due to the arrival, at speeds up to tens of kilometres per second, of meteor-itic material ranging in size down to fractions of a micrometre bombardment by solar-wind, cosmic-ray, and solar-flare particles ionizing radiation and temperature extremes. Subject to no meteorological effects and unprotected by a substantial atmosphere, the uppermost surface reaches almost...

Caloris Basin and Surrounding Region

The ramparts of the Caloris impact basin span a diameter of about 1,550 km 960 miles . Estimates of its size from the part of Caloris seen by Mariner 10 were considerably smaller. Its interior is occupied by smooth plains that are extensively ridged and fractured in a prominent radial and concentric pattern. The largest ridges are a few hundred kilometres long, about 3 km 2 miles wide, and less than 300 metres 1,000 feet high. More than 200 fractures that are comparable to the ridges in size...