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NA IONAL AENUNAU I Ic ; AND :,DACE ADMINISTRATION 'fHE NOW FRONTILI{ LINKING EARTH AND PLANETS (NASA-C'- 14 2 ) U ,5 ll PLAT. (J-t Prop il:ilon ib.) p Fi g -p 9.25 ,:SC:L 01B ''",c; 63/141 '" ^ entis and Mercury as Planets

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S JET PROPULSION LABORATORY ISSUE NUMBER TWO CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CALIFORNIA PUBLIC EDUCATIONAL SERVICES OFFICE EVOLUTION OF THE PLANETARY into Earth's atmosphere to break water vapor SYSTEM into o x ygen and hydrogen. This is importam because the hydrogen would bubble off into Planets of the solar system probably formed , pace like steam from a saucepan arid grad four to five billion years ago when hosts of ually Earth would have lost its oceans in this ,mall rocky particles and clouds of gases way -ollected together by their own gravity. Gravity appears to be a universal property of A phutographic mosaic of the Earth, taken natter, as a result of which every particle, by Mariner 10, is shown in Figure 1. -io matter how small, attracts every other. Thus, left to themselves in space, individual THE CHARAC fERISTICS OF :)articles lend a gas consists of particles. nolecules) tend to collect together into large Masses. Venus is very similar to Earth in site and mass its rhanieter rs 7,520 miles compared So after the Sun condensed front pri- with Earth's 7,920, its mass and density are tieval nebula, planets of different sites and slightl', less than those of Earth. Physically )robably different composition originated the two plari:;ts -ire almost twins, but they from concentrations of matter present at seem to have groan up quite differently. various distances from the Sun. Arid electric and magnetic fields in the gas forced these Venus, too, is Earth's nearest neighbor in 'ondensing planets into o. pits around the space after the Moon. Its closest approach is central Sun and spun them on their own axes 26 million m' ompared with 34 million like tops. miles for Mars. Figure 1. A mosaic view of the Earth taken by Mariner 10 on its way to Venus and The larger craters ore Mi-s, the Moon, and But a telescope reveals virtually no details on Venus are thought to be gouged by filling the bright disc of the planet. Some obser• Mercury. Such pictures were used by the engineers and scientists to test the space- bodies during the final stages of planetary vers Have recorded faint arid elusive markings, craft's cameras in space. These pictures were accretion. as the process of falling together is visible in the near , ill-defined dark termed. Smaller ones represent a continuing shadows and aright patches seemingly be- taken when Mariner 10 was 120,000 miles front but much lower rate of bombardment by iraviitrJ v^ry much as cloud systerns might solar system debris. behave. quantities of heat that might have been the The absence of surface features or persis- While Mariner 10, now on its way to Venus, cause of extensive volcanism to generate the tent cloud features made it difficult to deter- is not designed to find out anything directly dense . mine the period of rotation of Venus on its about life on Venus, .he scientific informa- axis. Wildly varying estimates were made, tion about conditions on Venus may be im. Venus has been the target for several earlier ranging from a 24 hour day like that of Earth portant to biologists seeking understanding space missions: two successful flybys were to a day equal to the Venusian year. The of why Earth spawned lite and Venus did made by earlier Mariners, and Soviet Veneta question was riot answered until recently not. It was life in the pr imitive atmosphere spacecraft flew by, orbited, and landed cap- Men radio waves penetrated the thick clouds of Earth which helped to produce the sules on the surface. These probes confirmed of the brilliant planet. Surprisingly, it was oxygen-rich atmosphere that in turn keeps a a high surface to iperature of around 4750C found that Venus rotates in 243 days in the lid on the oc-any and stops them leaking and a pressure at the base of Venus' atmos opposite diraction to that of the Earth. This off into space. Oxygen, converted to ozone phere about equal to that at a depth of 400 is slightly longer than the Venus year-the it high altitude by the Sun's rays, prevents fathoms in Earth's oceans. Genera„y Venus time the planet takes to revolve around the solar ultraviolet from penetrating low enough is a hot dry planet with only slight traces of Sun, which is 225 Earth days. Because of water vapor in its atmosphere of 95 per- the rotation and revolution in opposite di- cent carbon dioxide. There are also traces of Cover The planet Venus as it has been rections, the day on Venus is only 127 Earth oxygen, nitrogen, and inert gases, it is ohs.?rved from Earth through the technique days. But these figures pose the question of believed. of radar mapping, in which a radar signal is how Venus might have slowed down from bounced off the surface of the planet. The rotating as the other planets do and started The clouds of Venus, which, according to re- three light areas marked with Greek letters to rotate in the opposite direction. One of cent spectroscopic observations, may have a (a, 13, and h) are rough sections that may be several theories is that Venus captured a large topmost layer of concentrated sulfuric acrd mountains, but could also be craters or Moon like body moving in the opposite di- droplets, are about 18 to 25 miles thick, and bouldor field's. The mapping was done by rection to Venus' spin and that this body their tops may be about 90 miles above the the Jet Propulsion Laboratory (JPL), using crashed onto the surface of Venus. The surface, compared with Earth's highest clouds the NASA/JPL deep space antennas at Gold- impact would have stopped Venus from rota- of six miles or so. Below the Venus clouds stone, California. ting and would also have released tremendous is a clear atmosphere, while the cloud em- 1

selves probabl y consist of stacked layers of atmosphere build ap such high pressures? One theory suggests that water rs flapped in different composition, for example, carbon How could bodies plunge throerfll this dense volurninous polar cap', of Venus, another, dioxide ice at some levels and water ice at ahnospheie to produce craters detected by that all the water was lost because water ;ethers. riidar? vapor could rise in the atmosphere of Venus and be broken down by sunlight into oxygen Cloud features seen in ultraviolet light appear Answers to these and other questions about and hydrogen. The hydrogen !riled off into to move around the planet in only four days. Venus will help us understand how planets space. But what happened to the oxygen ? It There are also some large scale ulr and-down evo ve and why there is an Eden like Earth does not aplwar to be left in the atnwsphere. pulsations of the cloud layers. while nearby planets are so inhospitable. Understandinghow the atmosphere of Venus On the other hand, the major differences An •iteresting theoretical aspect of the dense became, or stayed, inimical to life may help among the terrestrial plane. s may have arisen but clear atmosphere below the clouds of us prevent our own atmosphere front to;;ause these planets former; at different dis- Venus is that if a student could stanc r on the wrong too, either from natural or man- tances from the Sun aril thus consisted of surface and look around, he would appear to made causes. An Important question today different rrlaterials in the first inst,.nre. For be standing in the bottom of a vast bowl. is whether pollution might tugger drastic example, Mercury might have formed from Looking into the distance in each direction, change s in Earth's atmosphere which could materials rich in iron, whereas Venus formed he would see a blurred, ruddy red landscape change our planet into another Venus. ;rom silicate-rich materials. Arid Earth may F curving upwards towards the cloud layers as have accreted in a region in wfuch there were though he were inside a hollow planet: the At the distance of Venus from the Sun itt water containing substances while Venus did atmosphere, acting like a grant lens, v )uld temperature would ue expected r j be abcat not. Calculations have shown that the pri- bend the light rays upwards in an eftrct GOoC on the average if it we a an aide-, meval nebula mryht have separated in this similar to a mirage appearing on a hot road body. Why the , ) is it so ,such hotter, es- way at vanous distances from the Sun. surface in summer. oecially when its clouds reflect a large part of the solar radiation falling upon them? This strong bending of light might turn night The answer seems to lie in the atmosphere .t THE CHARACTERISTICS OF intoa dull day oil night side of the planet, carbon dioxide. This gas acts, with wa,.:r MERCURY and this could explain the ashen light of vapor, as a one way transmitter of mcom.ng Venus. Astrononers have claimed that they Mercury, 3000 miles in diameter, is probably heat energy from the Sun. It opens a dour see this faint glow on the dark side of the the smallest planet in the solar system (Fig- to let the energy in and slams it shut when planet when it is turned towards Earth. But ure 2). The outermost planet, Pluto, might the energy tries to go out again. The planet this ashen light, first seen in 1643, is nowa- possibly be smaller. Mercury is halfway heats up like the inside of an automobile in days believed to be an auroral glow, like the between the Moon and Mars in size. Even sunshine with its windows closed. northerr, and southern lights over Earth's two satellites of the planet Jupiter are as large as Mercury, though much less dense. Polar region. Since Venus does not have a By contrast, the atmosphere of Earth has stro% , its auroras can occur only 0.03 percent of carbon dioxide. On 01 over the planet, whereas on Earth the Venus, carbon dioxide ha, remained free in !vong magnetic field causes the aurora- the atmosphere, whereas on Earth in the producing particles from the Sun to stream presence of much water it has reacted with towards the polar regions. One of the space- minerals to form large deposits of carbonates. craft did observe the glow over the night side Photosynthesis in plants has also extracted of Venus, and the existence of an electrically carbon from the atmosphere. excited upper atmosphere, the , has also been confit med by spacecraft. Another mystery about Venus is why it does not have much water. One suggestion is that 4 Study of the atmosphere of Venus is im- the water is still entrapped in molten rock portant because the whole balance between beneath a plastic surface which resists frac- heat coming in and heat going out from the tures. In the formation of Venus, high tem- planet is bound up with atmospheric struc peratures drove off carbon dioxide but al• ture and composition. This heat budget of lowed water to remain in solution in the energy income and spending could explain rocks. As the planet cooled, a plastic crust why Venus is today such a vastly different formed of sufficient thickness to prevent planet from Earth despite the similarities in fractures and the consequent escape of water size. The great mystery about Venus is how to form oceans. a planet about the same size as Earth, which Figure 2 A photograph of Mercury taken at might have supported oceans in the past as However, the fact that the planet has not the Pic du Midi observatory in France. This Earth floes today, developed so differently outgassed suggests little differentiation or, at is the best type of picture that can be taken during its evolution. Why did the planet the least, an inactive mantle. If the planet from Earth, but Mariner 10 shoe dd sent! lose all its water? Why did the surface were to outgas, the crust would not be likely back Mercury photographs as good as the temperature rise so high? How did the to keep the lid on this outgassing. picture of Earth in Figure 1. At oite :.me it w,is thoright that the closeness a kind, n it is temporarily held by Mercury Student Project two of the planet to the Suit it to turn one before streaming off again Into space. hemisphere eternally sunwards, just as the From the textbooks now make a list of the The surface temperature of Mercury is Moon turns one hemisphere towards Earth. unknowns about Mercury and Venus, their thought to range from about 3250C at local However, radio astronomers discovered very surface features, their atmosphere, their noon to -125 0C at local midnight, but these recontly (1965) that Mercury rotates on its physical characteristics, their origin, their temperatures vary considerably with the po axis I11 58 days. Coupled with the planet's evolution. Check with a later pamphlet in sitien of Mercury on its orbit as the planet 88 day period of revolution around the Sun, this series es to what instruments are car- moves in and out from the Sun. this gives Mercur,1 a solar day of 176 Earth ried by M,iriner 10 and what they might ie veal. Identify those present unknowns that days. So one day of Mercury occupies two Mercury's surface is exposed to the fierce might be solved by Mariner 10. Later check years of Mercury time. Arid because the erosion of the as well as to solar the actual results frorn the flybys and see planet follows an elliptical path around the heat and light. As with Earth's Moon, the which of the problems are solved. Sun with its speed in orbit changing as a solar wind probably changes the composition consequence, the path of the Sun through of the outer surfaces of rocks and surface Student t'ro; yet Three Mercury's sky is quite erratic. Some parts of soils. Solar radiation can vary between five Mercury experience a double dawn, for e, and ten times that received by Earth be For individual student or as a classroom pro- ample, in which the Sun rises, then slips ba,,k tween aphelion and perihelion of Mercury ject. Using the map of the orbits of E, rth, Wow the horizon, to rise again later. (most distant and closest parts of its orbit to Venus, and Mercury prepares) as a project in the Sun). Imagine a day in Earth's desert Through a large telescope the planet pre connection with the first pamphlet of this with ten suns shining at once in the noon cents a yellowish color broken by ine gray- series, mark a position for Earth with a cir- sky That approaches what it may be like ish patches. Mercury is believed to be cle on its orbit. Then on the orbits of on Mercury when the planet is at perihelion. cratered and much like Earth's Moon. The Venus aril Mercury draw circles, approxi- grayish areas may be the same as the lunar Key questions about this tiny plane' concern mately to scale (that is, the circle represent maria, the great gray plains of the Moon. its rotation, density, and surface molding. Ing Venus should be about the same size as Mercury, like the Moon, does not appear to Why does Mercury rotate three times while that for Earth and about 211, times the di- have an appreciable atmosphere, and it is a making two revolutions around the Sun? ameter of Mercury) at the correct positions poor reflector of light. But its density is Might this have produced some unusual Sur for eastern arid western elongation, superior believed to be much more than that of the face features? One theory suggests that the conjunction (far side of the Sun) and inferior Moon, possibly a little greater than the heat from the Sun at perihelion may have conjunction (between Earth and Sun) and Earth's. This is unexplained. It leads to the produced two opposing bulges on the planet half way between each of these positions, anomaly that Mercury's surface gravity is which now keep Mercury locked into this that is, eight positions for each orbit. greater than that of the bigger planet Mars. peculiar rhythm of rotation and revolution. A 100 pound student would weigh 40 pounds Now draw a set of circles showing the rely on Mercury, compared with 38 pounds on Why does Mercury have the highest density live sizes of each planet as seen from Earth Mars and 17 pounds on the Moon. On a;nong the planets arise their satellites ? Does for each of the eight positions. On each of Venus the same student would weigh 91 it have a central core ofiron? Arid what these comparative circles draw the phase of pounds. processes have shaped the surface of the in- the planet, that is, the part that is in dark• nermost planet? It is unlikely that Mercury can possess all ness as seen from Earth and the part illumi- nated; for example, fully illuminated like the mosphere, though some astronomers say they Mariner 10 is expected to provide much full Moon, half illuminated like a quarter have seen veihngs of surface detail at times, valuable information hearing upon these Moon, gibbous, and crescent shaped. You Ili the form of a whitish haze. Despite Mer- questions. cury's greater surface gravity over that of the Will have to calculate the sire of the planet Moon, Mercury iscloser co the Sun and there STUDENT INVOI—VEMENT at each of these phases from the rule that fore much hotter than the Moon: molecules if the planet is twice as far from the Earth can be heated to move at greater velocities Stud. in Feorect One it will appear half as large (Ili diameter) and and thereby reach escape velocity. Thus, so on. F^:?' '^ • s•'ut astronomical textbooks make gases that may have come from the rocks as four :Ist5 as follows- Note how much smaller Mercury always ap a result of volcanic activity will, over mil- pears to an observer on Earth even wh-n lioitsof years, have escaped from the planet's (1) Those things about Venus that are close to the Earth, and how both planets gravitational grasp and rocketeu a, molecules similar to those on Earth. turn their dark hemispheres towards us when into space. (2) Those things that are different. (3) Those things about Mercury that are they are at the very closest. Mercury does, however, collect plasma from similar to those ore the Moon. the Sun, the steady stream of particles flung Thus you understand why we. know so Iitile (4) Those things that are different. out by the Sun Into interplanetary space and about these planets and why a spacecraft referred to as the solar wind. Some of this Make a drawing or paint , ng of you. ;mpres flying by them can add greatly to man's plasma m.ght indeed form an atmosphere of Sion of the surface of M. rcury. knowledge of these neighboring worlds.

P73-219 .IPL 654 4.5M 1 . 74 PRINTED IN U.S.A.

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