Frequency of Impacts

Because there are far fewer large NEOs and long-period comets in space than smaller ones, the chances of a collision decrease rapidly with increasing size. The impact-hazard community primarily scientists with an interest in the issue has defined a global catastrophe to be an impact that leads to the death of one-fourth Meteor Crater (or Barringer Crater), Arizona, U.S., a pit 1.2 km (0.75 mile) in diameter excavated about 50,000 years ago by the explosive impact of an object with the...

The Corona

Another important set of unknown lines, revealed during an eclipse, come from the corona, and so its source element was called coronium. In 1940 the source of the lines had been identified as weak magnetic dipole transitions in various highly ionized atoms such as iron X (iron with nine electrons missing), iron XIV, and calcium XV, which can exist only if the coronal temperature is about 1 million K. These lines can only be emitted in a high vacuum. The strongest are from iron, which had...

Transits Of Mercury And Venus

A transit of Mercury or Venus across the face of the Sun, as seen from Earth, occurs at inferior conjunction, when the planet lies between the Sun and Earth. Because the orbits of both planets, like the Moon's orbit, are inclined to the ecliptic, these planets usually pass above or below the Sun. Also like the Moon's orbit, each planet's orbit intersects the ecliptic plane in two points called nodes if inferior conjunction occurs at a time when the planet is near a node, a transit of the Sun...

Energy Generation and Transport

The energy radiated by the Sun is produced during the conversion of hydrogen (H) atoms to helium (He). The Sun is at least 90 percent hydrogen by number of atoms, so the fuel is readily available. Since one hydrogen atom weighs 1.0078 atomic mass units, and a single helium atom weighs 4.0026, the conversion of four hydrogen atoms to one helium atom yields 0.0294 mass unit, which are all converted to energy, 6.8 million electron volts (MeV), in the form of gamma (y) rays or the kinetic energy of...

The Solar Atmosphere

Although there are no fires on the surface of the Sun, the photosphere seethes and roils, displaying the effects of the underlying convection. Photons flowing from below, trapped by the underlying layers, finally escape. This produces a dramatic drop in temperature and density. The temperature at the visible surface is about 5,800 K (5,500 C, 10,000 F) but drops to a minimum about 4,000 K (3,700 C, 6,700 F) at approximately 500 km (300 miles) above the photosphere. The density, about 10-7 g cm3...

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...