MERCURY

As the innermost planet in the solar system, Mercury is difficult to observe from the Earth because it rises and sets within two hours of the sun. Consequently, little was known about Mercury until the Mariner 10 spacecraft visited it in 1974–75.
Planetary Data
With a diameter of approximately 3,032 miles (4,879 kilometers), Mercury is not much larger than the Earth's moon. It is the second smallest planet in the solar system, after pluto. Along with the earth,venus and mars , Mercury is one of the inner, terrestrial planets, which are the dense, rocky bodies close to theSun. Like Venus, it is called an inferior planet, because its orbit is smaller in diameter than the Earth's.
Mercury can be seen from the Earth without a telescope. Since it is never more than 28 degrees from the sun, it appears as both a “morning” star just before sunrise and an “evening” star just after sunset. Because its orbit is inside the Earth's, it displays phases like those of the moon. When the planet lies approximately between the Earth and sun, it looks like a thin crescent to an observer on the Earth. It appears as a half disk when at its farthest from the sun and as a full disk when at the opposite side of the sun from the Earth.
The planet was named after the Roman god Mercury, the counterpart of the Greek god Hermes. Like Hermes, the swift-footed messenger of the gods, the planet Mercury is known for its speed. It completes its orbit around the sun in only about 88 days at an average rate of about 30 miles (48 kilometers) per second, the fastest of the nine planets.
Mercury moves much more slowly around its axis, taking almost 59 Earth days to complete one rotation. However, the time between one sunrise and the next is 176 Earth days, because Mercury rotates on its axis three times for every two revolutions around the sun. After one rotation, Mercury has completed two thirds of its orbit around the sun, so that the sun is in a different place in Mercury's sky. It takes three rotations, or two Mercurial years, for the sun to reappear in the same place in the planet's sky.
This characteristic, combined with Mercury's highly elliptical orbit, creates effects unusual by Earth standards. Mercury's greatest distance from the sun, or aphelion, is about 43.4 million miles (69.8 million kilometers). At that point in the planet's orbit, an observer on Mercury would see the sun appear more than twice as large as it does from the Earth. At Mercury's closest approach, or perihelion, when it is only some 28.6 million miles (46.0 million kilometers) from the sun, the sun would appear almost four times as large as it does from the Earth. Furthermore, the sun would not seem to move steadily across the sky. Its apparent speed would change depending on the viewer's location on the planet and on the planet's distance from the sun. Sometimes the sun would even appear to reverse its course.
Temperatures on Mercury vary widely. The planet's proximity to the sun makes it a fiery-hot world, with surface temperatures reaching about 755° F (400° C) at “noon.” Because Mercury lacks a thick atmosphere to trap heat, however, the planet cools to about −280° F (−175° C) just before “dawn.” Mercury's spin axis is vertical relative to the sun, unlike the Earth's, which is tilted almost 24°. The sun remains overhead at Mercury's equator year-round, so the planet does not have Earth-like seasons.

Surface and Interior
Mercury's surface has several different types of terrain. Planetary scientists can estimate the age of a surface by the number of impact craters on it; in general, the older the surface, the more craters it has. Some regions on Mercury are heavily cratered. They are probably very old surfaces formed about 4 billion years ago. Between these regions are areas of gently rolling plains that may have been smoothed by volcanic lava flows or by accumulated deposits of fine material ejected from impacts. These plains are also old enough to have accumulated a large number of impact craters. Elsewhere on the planet are smooth, flat plains with few craters. These plains are probably younger and volcanic in origin. Sometime between the formation of the intercrater plains and the formation of the smooth plains, the whole planet may have shrunk as it cooled, with the crust buckling and forming the long, steep cliffs called scarps.
Hilly and lineated terrain covers the side of Mercury opposite the Caloris impact basin. A patch of …
The largest impact basin on Mercury, Caloris, is about 800 miles (1,300 kilometers) across and is surrounded by mountains that rise to heights of about 1.2 miles (2 kilometers). It was probably created from the impact of a large meteorite when mercurry was forming. On the opposite side of the planet from Caloris is an area of hilly, lineated terrain that probably resulted from seismic waves caused by the same impact.
Like other airless, solid bodies in the solar system, the entire surface of Mercury is covered with a layer of rubble called regolith. Regolith is composed of material, ranging from dust to boulders, that was scattered when impact craters formed. Subsequent impacts in turn broke up and redistributed this debris.
Mercury is very dense and has a magnetic field that is about 1 percent as strong as the Earth's. This suggests the existence of a core composed of iron and nickel and constituting about 40 percent of the planet's volume. The surface gravity is about one third as strong as the Earth's. A very thin atmosphere of hydrogen, helium, potassium, and sulfur surrounds the planet. Mercury's total atmospheric pressure is about 500 billion times less than that of the Earth. Radar images taken of Mercury in 1991 show what are considered to be large patches of water ice at the planet's north pole.

Exploration
Much of the information known about Mercury comes from images and data transmitted by the Mariner 10 spacecraft, the first to visit the planet. In November 1973 the National Aeoronautics and Space Administration launched the craft toward Venus for the initial leg of its mission. After photographing Venus, Mariner 10 became the first spacecraft to use a “gravity assist,” drawing on the planet's gravitational field to boost its speed and divert its course toward Mercury. It captured the first close-up photographs of Mercury in March 1974, flying within about 435 miles (700 kilometers) of the planet's surface. After entering into orbit around the sun, Mariner 10 encountered Mercury twice more. Its final and closest pass, in March 1975, brought it to within 200 miles (325 kilometers) of the surface.
Mariner 10's orbital trajectory allowed it to photograph only one side of Mercury. Subsequently, low-resolution radar images taken from the Earth showed the planet's other hemisphere to have a similar terrain. At the beginning of the 21st century, NASA and the European Space Agency were each planning missions to the innermost planet.

THE SUN
Although the sun is a rather ordinary star, it is very important to the inhabitants of the Earth. The sun is the source of virtually all of the Earth's energy. Yet the Earth receives only half a billionth of the energy that leaves the sun. Because the sun's energy is so intense, there are some real dangers in studying it. The intense heat of the sun's rays can destroy the retinal cells, causing blindness. For this reason, the sun should never be viewed directly. Furthermore, there is no safe way to view the sun through an ordinary telescope. Smoked glass and dark glasses give no protection from the great concentration of heat and light. The only safe way to study the sun is to project its image through a pinhole or a telescope onto a white screen.

The Sun's Position in the Universe
The average distance of the sun from the Earth, arbitrarily called one astronomical unit by astronomers, is 149,597,870 kilometers (92,958,350 miles). The sun's radius is about 432,500 miles, or 109.3 times the radius of the Earth, giving the sun a volume of about 1,306,000 times the volume of the Earth. It has been calculated that the sun's mass, or quantity of matter, is some 333,400 times as great as the Earth's mass.
A ray of light traveling from the sun at about 186,282 miles per second takes about 8 minutes 19 seconds to reach the Earth. Light from those other suns, the stars, takes much longer to reach the Earth. Light from the next nearest star, Alpha Centauri, takes more than four years to arrive, and light from the center of our galaxy, the Milky Way, takes many thousands of years. Because the sun is so near, it seems much larger than the other stars. They are visible on Earth only as bright points, even when viewed with the most powerful telescopes.
Stars vary greatly in size and in color. They range from giant stars, which are much larger than the sun, to dwarf stars, which can be much smaller than the sun. In color they range from whitish blue stars with very high surface temperatures (more than 30,000 Kelvin, or 53,500° F) to relatively cool red stars (less than 3,500 K, or 5,840° F). The sun is a yellow dwarf star, a kind that is common in the Milky Way, and has a surface temperature of about 5,800 K (10,000° F). (The Kelvin temperature scale uses degrees of the same size as Celsius, or centigrade, degrees, but it is numbered from absolute zero, which is –273.15° C.)

Studying the Sun
The telescope has been used in solar studies since 1610. With the telescope, scientists could describe the sun's appearance, watch the movement of sunspots, and measure the sun's rotation. The solar tower telescope, a special vertical telescope, was invented for use in solar studies. Its long focal length can give very large images of the sun (to more than 30 inches in diameter). The coronagraph, another special telescope, is used to examine the sun's atmosphere. The instrument blocks the direct light from the sun's disk and allows its dim outer atmosphere, called the corona, to be viewed.
When a ray of the sun's light, which appears white, is passed through a prism or a diffraction grating, it spreads out into a series of colors called a spectrum. Scientists analyze this spectrum to determine what chemicals make up the sun as well as their abundance, location, and physical states.
In 1814 Joseph von Fraunhofer began a thorough study of the sun's spectrum. He found that it was crossed by many dark lines, now called absorption lines or Fraunhofer lines. Meanwhile, other scientists had been studying the light emitted and absorbed by elements in the gaseous state when they were heated in the laboratory. They discovered that each element always produced a set of bright emission lines associated with that element alone. The dark solar line that Fraunhofer had called D was shown to have the same position in the spectrum as the brilliant line that sodium gave off when it was heated in the laboratory.
Scientists now believe that the dark bands represent elements in the sun's atmosphere. The lines are dark because the elements in the sun's atmosphere absorb the bright lines given off by the same element on the sun's disk.
Linking the lines of the spectrum with the elements that emit or absorb them provided a way to study the composition of the sun's surface. Almost all elements known on Earth have been shown to exist on the sun. Studies of the solar spectrum have revealed that hydrogen makes up some 92 percent of the sun's atmosphere and helium about 8 percent. Carbon, nitrogen, oxygen, sodium, and other elements are also present.
The spectroheliograph and the birefringent filter are also used to study the sun's atmosphere. Both of these instruments can limit the light that passes through them to a very small range of wavelengths, such as the red light emitted by hydrogen or the violet light of calcium.
In 1942 it was discovered that, besides the known kinds of solar radiation such as light and X rays, the sun also emitted radio waves. One cause of these radio waves is the thermal motion of atoms in the sun's atmosphere. Studies with radio telescopes have shown that radio waves are emitted by a sphere larger than the visible atmosphere of the sun, evidence that its atmosphere extends farther than can be seen.
On the ground, the effectiveness of any telescope is limited because the Earth's atmosphere absorbs much of the sun's radiation. With advances in space science, rockets and artificial satellites were launched above the terrestrial atmosphere. They were equipped with instruments that began to fill in the gaps in the solar spectrum.

The Sun's Violent Core
The sun looks like a burning sphere. In fact, it is often pictured as a circle with flames surrounding it. But the sun is actually too hot for an Earth-type chemical reaction like burning to occur on its surface. Besides, if burning produced its energy, it would have run out of fuel many millions of years ago.
Various theories have been advanced to explain the sun's tremendous energy output. One said that all the bits of matter in the sun were exerting gravitational attraction on each other and causing the sun to shrink and become more tightly packed. This process, called gravitational contraction, does occur in some stars and can release a great deal of energy. However, gravitational contraction could produce energy for only 50 million years at most, while the sun's age must be at least as great as the Earth's age of 41/2 billion years.
Atomic theory finally provided an explanation. Scientists now agree that thermonuclear reactions are the source of solar energy. Albert Einstein's theoretical calculations showed that a small amount of mass could be converted to a great amount of energy. The vast amount of matter in the sun could provide fuel for billions of years of atomic reactions. The sun's core is believed to be a superhot, extremely dense mass of atomic nuclei and electrons. Its temperature is calculated to be about 15,000,000 K (27,000,000° F). Under these conditions, nuclei can collide and fuse into new and heavier nuclei. This is a type of thermonuclear reaction called a fusion reaction. During such a reaction some of the mass of the nuclei changes to energy. Two specific processes, the carbon cycle and the proton-proton reaction, occur most often.

The Photosphere and Sunspots
The sun's surface, called the photosphere (sphere of light), is the lowest visible layer of the sun. The temperature of the photosphere ranges from 7,500 K (13,000° F) at the base to 4,700 K (8,000° F) at the top. Its average temperature is 5,800 K (10,000° F). The photosphere has a definite texture. Many small, luminous grains are separated by dark areas that look like nets or canals. High-altitude photographs of the photosphere's granulation show that the grains are hundreds of miles in diameter. They are continually forming and disappearing. According to one hypothesis, the grains are the tops of gas columns that ascend and descend through the photosphere.
The uniformity of the granulation indicates that a relatively calm condition exists at the solar surface. This is periodically subject to violent disturbances. Generally, these disturbances appear as darker points, called pores, on the more luminous background of the photosphere. The pores usually grow rapidly in number and in size to form a large single sunspot or a group of sunspots.
Early observers noticed that the sunspots seemed to drift. Galileo deduced that the drifting was due to the rotation of the solar globe. The observed rotation is completed in about 27 days, a time that also includes the Earth's movements. The actual period of rotation varies with the latitude of the sun. It is 25 days at the sun's equator and 27.4 days at a latitude of 40°. This lag occurs because the sun rotates as a gas, not as a solid body.
Typical sunspots have a dark, circular center, called the umbra, surrounded by a lighter area, the penumbra. Rays issuing from the center of the umbra form the penumbra. Sunspots vary greatly in size but are always small compared to the size of the sun. When they appear in groups, they may extend over thousands of miles. The darkness of the umbras is a sign that the sunspots are cooler than the photosphere. The umbras appear to be some 2,000 K (3,100° F) cooler than the photosphere. Furthermore, when they approach the sun's edge, the umbras appear to be lower than the photosphere as well.
Regular observations of sunspots have been made from 1750 to the present. They reveal that the spots appear and disappear in a definite cycle, and that they are limited to the two zones of the sun contained between latitudes 40° and 5° of its northern and southern hemispheres. Their cycle lasts an average of 11 years. At the beginning of a cycle a few spots appear at around 35° latitudes. Then they rapidly increase in number, reaching a maximum in the course of around five years. At the same time they move slowly toward the equator. During the next six years their number decreases while they continue to approach the equator. There the cycle ends, and at the same time another cycle starts immediately.
The American astronomer George E. Hale observed that certain photographs of sunspots showed structures that seemed to follow magnetic lines of force. Often a pair of sunspots appeared to form the north and south poles of a magnetic field. Hale was finally able to establish that sunspots are indeed seats of magnetic fields. In addition, it was discovered that from one 11-year cycle to the next, a total reversal, or flux, of the sunspots' polarity occurs in the two solar hemispheres, so that a magnetic cycle of sunspots lasts 22 years. Using a new imaging technique in 1992, astronomers were able to show that the sun's surface is covered with relatively small, magnetically active regions, called magnetic flux tubes, and that these regions are related to the intense magnetic activity of sunspots. While spots on stars throughout the universe are thought to account for about 90 percent of the universe's magnetic flux, magnetic flux tubes account for the remaining 10 percent.

The Chromosphere and Solar Corona
The layer above the photosphere is called the chromosphere (sphere of color) because of its reddish color, visible during total eclipses of the sun. The lower chromosphere absorbs some of the light that is emitted from the photosphere, causing the dark absorption lines of the sun's spectrum. This absorption occurs because the lower chromosphere is cooler than the photosphere. Much of the sun's “weather” takes place in the chromosphere. When the chromosphere is viewed under hydrogen light or under the violet light of calcium, bright areas called plages appear. The plages are usually located above or near sunspots and may be extensions of bright patches—faculae—that occur on the photosphere near sunspots.
A far more violent phenomenon is the solar flare, a sudden chromospheric eruption from a plage area. The solar flare may emit high energy radiation and highly energetic charged particles. Solar flares usually form very rapidly, reaching their maximum brilliance within minutes, after which they slowly die out. Very strong solar flares may emit X rays, radio waves, and swarms of charged particles. These enormous spurts of energy could be very dangerous for space travelers above the protection of the Earth's atmosphere because such fast-moving radiation can penetrate spaceship walls and damage body cells.
Early astronomers noticed huge red loops and streamers that appeared around the black disk of solar eclipses. These streamers were called prominences. Later, the long, dark, threadlike regions that had been known as filaments were shown to be prominences when they approached the sun's edge. Filaments, or prominences, are scattered on the sun's surface. Like most solar phenomena, they are not well understood. There may be several kinds of prominences. Quiescent prominences may maintain their shape for months, while prominences in active regions associated with sunspots may be short-lived. Long prominences stretch for over a hundred thousand miles and are usually a few thousand miles wide. They appear to form from glowing matter that falls steadily from the corona into the chromosphere, somewhat as rain condenses in the Earth's sky. But the prominence itself may rise, sometimes exploding upward at speeds of a thousand miles a second.
The chromosphere is surrounded by the solar corona, a faintly luminous outer atmosphere. As this atmosphere is thousands of times dimmer than the sun's disk, it is usually invisible. Before the invention of the coronagraph, it could be seen only when the sun was totally eclipsed by the moon. When the photosphere is blocked out, the corona appears as a silvery halo with long arcs and streamers. The arcs are usually visible above disturbed regions, especially where prominences are present. When sunspots are at a minimum, the corona has long streamers along the equator with shorter rays at the poles. This changes when sunspots are at a maximum. The corona then appears almost circular, with streamers distributed uniformly around the disk.
For a long time many of the emission lines of the corona's spectrum could not be identified with terrestrial elements. They were once believed to be emitted by “coronium”, an element not found on Earth. Advanced atomic research has shown that these lines can be produced when iron, nickel, and calcium gases have a very low density and a very high temperature.
Today it is known that the corona consists of a form of matter called plasma—a very hot gas composed of a sort of soup of charged particles corona has a temperature of about 2,000,000 K (3,600,000° F), yet it is not dense enough to produce much heat. A meteor traveling through the corona does not burn up, as commonly happens in the Earth's cooler but denser atmosphere. It was once believed that the atomic collisions within the photosphere and their resulting shock waves were responsible for the high temperatures of the corona. However, it is now known that solar magnetic fields generate the energy that heats the corona.