Formation Of Solar System

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The solar system began to form from a spinning solar nebula about 5 billion years ago, 9 billion years after the universe started expanding from nothing in the big bang some 14 billion years before the present. This solar nebula consisted of a mass of gas, dust, and fragments that began spinning faster as gravitational forces caused the material to collapse on itself. Temperatures ranged from extremely hot in inner parts of the solar nebula to cold in the outer reaches. Planets began accreting by accumulating more and more dust and small fragments to form the bigger

PHYSICAL PROPERTIES OF OBJECTS IN THE SOLAR SYSTEM

Object

Orbital Distance (AU)

Mass (earths)

Diameter (Earths)

Rotational Period (Days)

(- Means Retrograde)

Orbital Period (Years)

Density (Earths)

Surface Gravity (Earths)

moons

Sun

0.0

332,000

109.2

25.8

1.42

28

Mercury

0.39

0.06

0.38

59

0.24

0.98

0.38

0

Venus

0.72

0.81

0.95

-243

0.62

0.95

0.90

0

Earth

1.0

1.00

1.00

1.00

1.0

1.00

1.00

1

Mars

1.5

0.11

0.53

1.03

1.9

0.71

0.38

2

Ceres asteroid

2.8

0.0002

0.07

0.38

4.7

0.38

0.03

0

Jupiter

5.2

317.8

11.2

0.42

11.9

0.24

2.34

63

Saturn

9.5

95.2

9.5

0.44

29.5

0.12

1.16

60

Uranus

19.2

14.5

4.0

-0.69

83.7

0.23

1.15

27

Neptune

30.1

17.2

3.9

0.72

163.7

0.30

1.19

13

Pluto (dwarf planet)

39.5

0.002

0.18

-6.40

248.0

0.37

0.04

3

Eris (dwarf planet discovered June 2007)

67.7

0.002

0.18

~8

557

?

?

1

planetesimals that began rotating around the large mass accumulating as the Sun at the center of the disk. These early planetesimals grew into protoplan-ets, still experiencing many impacts with large asteroids and comets by 4.56 billion years ago. As the main planets formed, they differentiated into core-mantle-crust systems, and in the late bombardment period from about 4.5-3.5 billion years ago, these planets suffered many impacts with large asteroids and comets. Several large, differentiated bodies in what is now the asteroid belt were destroyed by large impacts, forming billions of fragments that now form the bulk of meteorites that hit the Earth. The inner planets are made dominantly of silicate minerals and are called the rocky or terrestrial planets, whereas the outer planets are mainly gaseous, often called the Jovian planets after the largest body, Jupiter. Comets come from further out in the solar system, most from beyond the orbit of Neptune in a region called the Oort Cloud.

See also asteroid; astronomy; comet; Earth; Galilei, Galileo; Jupiter; Mars; Mercury; Neptune; origin and evolution of the Earth and solar system; Pluto; Saturn; Uranus; Venus.

FURTHER READING

Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. Upper Saddle River, N.J.: Addison-Wesley, 2007. Cloud, Preston. Oasis in Space. New York: W.W. Norton, 1988.

Comins, Neil F. Discovering the Universe. 8th ed. New

York: W. H. Freeman, 2008. Condie, Kent C., and Robert E. Sloan. Origin and Evolution of Earth, Principles of Historical Geology. upper Saddle River, N.J.: Prentice Hall, 1997. Snow, Theodore P. Essentials of the Dynamic Universe: An Introduction to Astronomy. St. Paul, Minn.: West, 1984.

Sorby, Henry Clifton (1826-1908) British Geologist, Biologist, Microscopist, Metallurgist Henry Sorby was a well-known British scientist whose most influential scientific work was done from 1849-64. His work was based on the application of the microscope to geology and metallurgy. In these two fields, his work included simple quantitative observation and the building and meticulous use of new experimental equipment and interpretation based on the application of elementary physicochem-ical principles to complex natural phenomena. His goal was to "apply experimental physics to the study of rocks." Sorby's most famous achievement was the development of the basic techniques of petrography by using the polarizing microscope to study the structure of thin rock sections.

Henry Sorby was born in Woodbourne near Sheffield in Yorkshire on May 10, 1826, and died March 9, 1908. He attended the Sheffield Collegiate School, where he developed a keen interest in natural sciences, especially geography, and began to study some of the excavated valleys of Yorkshire. In these studies he became interested in the older geological periods, sedimentary layers, and the formation of structures during the deformation of these rocks. He began working on sedimentary rocks, and by 1851 he was involved in a debate on the origin of slaty cleavage. His paper "On the Origin of Slaty Cleavage" in 1853 showed that cleavage was a result of the reorientation of particles of mica accompanying the deformation flow of the deposit under aniso-tropic pressure. Basically Sorby demonstrated that when a rock is flattened by deformation, the flat mica grains tend to rotate so that most of them are close to parallel, forming the planar structure called slaty cleavage in the rocks. This work was followed by the publication in 1858 of an important memoir in the Quarterly Journal of the Geological Society of London, which Sorby titled "On the Microscopical Structure of Crystals." He went on to study organisms in limestone and discussed the significance of microorganisms in chalk. Sorby then moved from slate to schist and metamorphic rocks in general. His paper on liquid inclusions in crystals, both natural and artificial, was very important since later study of these inclusions yielded information about the pressure and temperature conditions during the deformation and metamorphism of these and other rocks. His use of the microscope helped him to find abundant smaller inclusions within the microcrystals of many metamorphic rocks.

Henry Sorby was a member of the Royal Microscopic Society and was awarded the Wollaston Medal by the Geological Society of London in 1869, the highest award given by that society. In 1882 he was elected president of Firth College in Sheffield. The International Association of Sedimentologists and the Yorkshire Geological Society both have Sorby Medals, named after Henry, and award these to individuals with outstanding achievements in geology.

See also sedimentary rock, sedimentation; structural geology.

FuRTHER READING

Sorby, Henry C. "On the Application of Quantitative Methods to the Study of the Structure and History of Rocks." Quarterly Journal of the Geological Society 64 (1908): 171-233.

-. "On the Origin of Slaty Cleavage." Edinburgh

New Philosophical Journal 55 (1853): 137-148.

-. "On the Theory of the Origin of Slaty Cleavage."

Philosophical Magazine 12 (1856): 127-129.

South American geology south America has a diverse and long geological history. The Andean Mountain chain on the western side of the continent has been active for a couple of hundred million years since before the breakup of Gondwana, with most activity in the Mesozoic-Cenozoic-Tertiary recent times, and it is still one of the world's most active continental margin arcs. These ranges are drained by the Amazon and Paraná River systems, which cut through other ranges including the Pampean, Austral, and Tandilia ranges. In the north, the Caribbean oceanic plateau, formed by the Caribbean mountain system and the Andean Cordillera in the northwest, is in an oblique collision with the south American continent. The southern boundary of south America is marked by the Magellan mountain system and the Falkland Plateau to the southeast.

The core of the south American continent consists of the Precambrian Guiana and Brazilian shields and the Río de la Plata craton. The Guiana shield is bordered, in subsurface strata, on the north and northeast by gently folded Paleozoic strata, and by metamorphic belts in the south and northwest. The Brazilian shield is bordered in the north along the Amazon basin by gently folded

South America Map

Landsat satellite image of South America

(Bill Howe/Alamy)

Paleozoic strata, and by Paleozoic and Mesozoic strata in the east.

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