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«INTRODUCTION There are many ways to define the solar system. For instance, it might be sufficient for some purposes to define it as consisting of ...»

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The Solar System or

Cosmic Chemistry: Planetary Diversity

Do Nine Planets a

Baseball Team Make?

APPENDIX A

INTRODUCTION

There are many ways to define the solar

system. For instance, it might be

sufficient for some purposes to define it

as consisting of those objects subject to

the sun's gravitational field. Or it might

be defined in terms of the reach of the

solar wind. In either case it would be

clear that the solar system extends

beyond the outermost planet. What is intended here is a discussion of what might best be called the central region of the solar system, the part that extends only out to the edge of the planetary system, which is what most individuals think of when the term solar system is mentioned.

This region of space is, to be sure, quite empty on the average. However, located NASA in certain regions are many objects of fascination, study, and speculation. Most The solar system: Note black lines for terrestrial of these objects orbit around the sun in a plane close to the sun's equatorial plane, and they comprise only a small portion (less than 0.15 %) of the total mass of the solar system. The most massive and obvious of the solar system objects are the nine planets, which circle the sun in almost circular orbits ranging in radius from 0.4 AU to 40 AU (Note:1 AU is the average distance from the sun to the Earth, 1.5 x 1011 meters). The diameters of these objects range enormously from a few thousand km to more than 100,000 km and some of them have orbiting satellites and encircling rings consisting of dust and ice. Some of the satellites are quite large, in some cases as large as the smallest of the planets!

Between the orbits of Mars and Jupiter a large family of smaller bodies is found, having diameters ranging from a few hundreds of km. These asteroids also orbit the sun in roughly circular paths. Last, as unique solid objects, there are the comets, which are smaller yet and which move in elliptical orbits that usually are highly inclined relative to the plane of the Earth's orbit. Finally distributed throughout are micron-sized dust particles and the solar wind, a plasma consisting largely of electrons and protons.

In the remainder of this article, the planets will be discussed. This discussion will not be comprehensive or exhaustive because there are many excellent, well-illustrated books (see reference list) that present beautifully detailed descriptions of the planetary system. In this text, the goal is to present only selected, salient features of the planets, especially as they relate to planetary diversity.

It is convenient to organize one's thoughts about the solar system around the rocky, terrestrial inner planets (Mercury, Venus, Earth, and Mars) and the large, cold and gaseous outer planets (Jupiter, Saturn, Uranus, and Neptune). The ninth planet, Pluto, does not fit conveniently into either category and some workers in the field do not consider it to be a planet at all!

–  –  –

atoms contained in the rocks. The molten iron percolated down into the interior to form a metallic core. During this time, tidal forces from the sun acting on the molten components resulted in the planet's changing its shape to a sphere, the thin crust cracking to accommodate the changes. Lava may have welled up through the cracks and cooled to form the flat plains that now cover the majority of the planet's surface. Then some 3,850 million years ago a catastrophic collision with a large asteroid shook the entire planet to its core, sending out shock waves that rearranged the entire appearance. Subsequently the planet started to cool and settle down into a slightly shrunken, wrinkled, and cratered planet that has continued to be assaulted by a diminishing barrage of small rock fragments.

Of great interest is Mercury's density of 5.43 g cm-3, which was measured accurately by Mariner 10 during three passes in the mid-70's. If we allow for the fact that Mercury is small with low compressional forces, the case can be made that it has the highest average density of any of the planets. Earth is often listed as having a higher density, but Earth is much larger than Mercury, and the average density is rendered artificially large by the compressional forces that compact the interior.

The "uncompressed" densities of Mercury and Earth are about 5.3 and 4.45 g cm-3, respectively. The high density of Mercury suggests that half or more of its volume consists of an iron core surrounded by lighter mantle and surface rocks.

Measurements of Mercury's magnetism by Mariner are consistent with this view.

Since Mercury is thought to have been formed in much the same way as the other rocky, terrestrial planets were formed, why is it so laden with iron? There is no plausible fractionation process for the products of the solar nebula that can account for this iron content. Perhaps the most widely (but not universally) accepted explanation is that originally, Mercury was considerably larger than it is today, but that at some point much of the low density mantle and surface rock was blasted away either by large asteroids or through the impact of another small planet.

Mercury has the largest range of hot and cold temperatures in the solar system. Why do you think this is so? Hydrogen, helium, oxygen, sodium, and potassium have been detected in the atmosphere, although the concentration of these materials is very low. There is some evidence of sub-surface ice at both the poles, where temperatures may be as low as 100 Kelvins.





Mercury is indeed a desolate and frightening place that is continuously blasted by the intense radiation of the sun and the solar wind. Nevertheless, over the past few years the importance of Mercury has begun to be realized in the world of planetary science because its surface more closely preserves the ancient history of planet formation than do the surfaces of the other terrestrial planets, which have been modified by recent erosion, volcanic and tectonic activity.

Venus

The second closest planetary relative of the Earth presents a vastly different face to the casual observer than does Mercury.

Venus is always totally obscured by clouds and all that the Pioneer Venus spacecraft (1978) ever observed were cloud tops.

Nevertheless, various probes dropped through the massive atmosphere have provided information about its composition, temperature, and pressure. And several instruments actually have landed on the surface providing initial tantalizing

–  –  –

What evolves from the data obtained to date is a picture of a rocky world not greatly different in size from Earth, but with a surface pressure 90 times that of Earth's and a temperature of almost 750 Kelvins. It is indeed a foreboding place. The surface of Venus is covered with myriad volcanoes of all sizes, enormous lava flows, and related, but very unusual structures that have no counterparts on Earth.

While Venus and Earth are of much the same size and composition, their surfaces have rather different histories. The Earth's internal heat emerges as distinct lines of volcanoes NASA located along cracks that divide the planet's surface into several plates. On Venus, where there may indeed be similar plates, the crust is thin and flexible allowing magma to well up and create a plethora of volcanoes—perhaps as many as 100,000 of them. Thinly superimposed on the magma-covered surface are craters left from meteorite impacts. Since there are many fewer impact craters per unit surface area on Venus than on Mercury, it may be concluded that the surface of Venus has undergone more or less continuous renewal from volcanic eruptions that may continue even to this day. In fact the Pioneer Venus craft obtained information in 1978 that suggested there had been a recent injection of sulfur dioxide into the atmosphere. This oxide, which eventually is converted chemically into sulfuric acid, could have arisen from a volcanic eruption. As a result of the extensive volcanism that has afflicted Venus for much of its life, significant concentrations of corrosive sulfuric acid have accumulated in its atmosphere.

Since Venus has an atmosphere, it also has weather, complete with high winds and squalls. This weather, like Earth's, is driven by the energy of the sun, but here the comparison ends because the Venusian clouds are composed mostly of carbon dioxide garnished with sulfuric and hydrofluoric acids, as well as with myriad other molecules. This composition creates a gigantic greenhouse effect that raises the temperature of the surface enormously and makes Venus the hottest planet in the system, even though it is almost twice as far from the sun as is Mercury. At first glance the seemingly large amount of carbon dioxide might be a surprise; however, both carbon and oxygen are common in the universe and Earth's stores of this material are not unlike those of Venus's. On Earth, however, the carbon dioxide is locked up in the form of solid carbonate minerals (limestones) or is dissolved in the oceans instead of floating in the atmosphere.

A more difficult and puzzling problem is the apparent lack of appreciable water on Venus. Clearly liquid water could not exist on the scorching surface, but we might expect to find it in the atmosphere. So far little has been detected. Perhaps the water present long ago was split into its component hydrogen and oxygen molecules by the action of sunlight or through the catalytic reaction with hot surface rocks.

The rotation of Venus about its own axis is of note because Venus rotates more slowly than any other planet (turning once on its axis in 243 Earth days) and it rotates in a backwards sense relative to the rotation of Earth and the other planets. That is to say, the sun appears to rise in the west and set in the east on Venus. The reason for this is the subject of controversy.

This leaves us with the final question of why Venus is so different from Earth. At one time when the sun was younger and less bright, was the surface of Venus a cool, watery swamp having an atmosphere like that of Earth's? Did the increasingly hot sun boil away Venusian liquid water reserves into the atmosphere, creating an ever-increasing greenhouse effect? Did the carbonate rocks then decompose in the intense heat to release carbon dioxide, providing ever more greenhouse heating and the atmosphere as it exists today? Or did Venus never contain appreciable reserves of water? If not, why not?

Was some fractionation mechanism at work in the early days that discriminated against the deposition of water on Venus?

These and related questions remain unanswered.

Earth

While Earth can claim to be distinctive because it is the largest and most massive of the terrestrial planets, it is the presence of a large satellite, the moon, which clearly distinguishes it from the other planets in this class. Any discussion of Earth must include comments about the moon, and it is important to recognize that reasonably large samples of moon rock have been returned to Earth for study. These rocks proved to contain surprisingly few volatile materials, in particular water. They were bone-dry and different from typical rocks found on the surface of Earth. This suggests, but does not prove, that the moon

–  –  –

The Earth on the other hand has remained alive and geologically active, in part because of its large size. It still stores heat in its interior that originally arose from the collision of the asteroid-sized bodies that coalesced to form the planet and it contains, as well, stores of radioactive elements that are constantly decaying with the release of thermal energy. As a consequence the central core is hot (around 5000 Kelvins). It furthermore is thought to consist largely of solid, highly compressed iron, some of which could have come from the "planet" that collided with the Earth. Surrounding the core is a layer of liquid metallic iron that circulates as a result of the planet's rotation and/or thermal convection currents, giving rise to the strongest magnetic field found among the terrestrial planets. Above the liquid layer and extending to the thin crust is the semi-liquid mantle, consisting of molten rock. This molten rock sometimes breaks through the crust and a volcano is created.

The Earth also is distinguished by the extensive amount of tectonic activity found in the crust, where large plates float and move on the underlying semi-liquid mantle. This tectonic activity coupled with erosion and volcanism has fundamentally altered the surface of the Earth from what it was like during the early days after it was formed.

The erosion of the Earth's surface can be traced in large measure to the presence of copious amounts of liquid water, which again makes this planet unique among the terrestrial set. It also is the case that Earth is the only planet where water is found in three different physical states: gas, liquid, and solid. The origin of the water is not clear, though there is some evidence that water may have come from meteorites. It is clear that the presence of ample amounts of liquid water might be the most important aspect of Earth's character, since life could not have originated and survived without the water.

Finally, Earth is surrounded by a thin veneer of an all-important atmosphere consisting primarily of nitrogen and oxygen.

The most outstanding aspect of the atmosphere is the high concentration of oxygen, but one should note as well the low concentration of carbon dioxide relative to that found in other planetary atmospheres. The presence of an atmosphere and magnetosphere, which protect Earth's inhabitants from the constant bombardment of energetic materials and radiation from outer space, also play a vital role in making it possible for Earth to support life.

–  –  –

At one point early in its history Mars probably looked like today's Mercury or the Earth's moon—it was covered with impact craters. However, in contrast to Earth, the red planet today probably is extinct, all of its internal energy having been dissipated long ago. Mars is circled by two small satellites, Phobos and Deimos, both of which seem to have a different origin than their host planet.



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