magellan the unveiling of venus

8/8/2019 Magellan the Unveiling of Venus

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e r7nveiling of Venus


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ORIGINAL PAGECOLOR PHOTOGRAPH

fa l l the planets in the solarsystem, Venus is the mostlike our own Earth in size, mass, anddistance from the Sun. Themotionsof our planetary "tlvin" were knownto the ancients, and its apparent .changes in shape, similar to thephases of the Moon, were firststudied by Galileo more than fourcenturiesago. In the modern era,Venus has been one of the mostvisited planets in the solar system-20 spacecraft from the Soviet Unionand the UnitedStates have been sentthere since the early 1960s. Venus'sulfur-yellow clouds have beenprobed, its atmosphericstructure andcomposition have been measured,and automatic landers havephotographed portions of itslandscape and chemically analyzedits rocks.! Yet, for all our fascination withVenus, we have only a sketchy,1 general knowledge of the planet'ssurface. While the faces of the other"terrestrial" planets-Earth, Mars,and the lighted sides of Mercury andthe Moon-have long since beenmapped, details of Venus' face arestill largely unknown, due to theplanet's dense, constant cloud cover.j

Theclouds prevent us fromphotographing the solid surface fromI space with conventionalcameras.

Since the early 1960s,scientists haveused radar to counter this problem.Unlike visible light, radar wavespenetrate the Venusianclouds andreflect off the solid planet back toEarth. With the help of computerprocessing, these radar reflectionscan be turned into pictures of theVenusiansurface. Earth-based radarL

imaging is thus extremely valuable,

but it also is limited-Venus alwaysshows the same hemisphere to uswhen it is near enough in its orbit forhigh-resolution study, so only afraction of theplanet can be exploredfrom Earth.Therefore, in the late 1970sand early1980s, the UnitedStates and theSoviet Union sent the Pioneer Venusand Veneraspacecraft, respectively,to study Venus more closely and toimage its surface with radar. Thesemissions have answered many of ourquestions about Venus' atmosphereand large-scalesurface features.However, many more questionsremain unanswered about the extentto which Venus' surface has beenshaped by volcanoes, plate tectonics,impact craters, and water and winderosion.Tohelp answer these remainingquestions, NASAplans to launch anew radar imaging spacecraft,Magellan (named for the 16thcenturyexplorerFerdinand Magellan), fromthe space shuttleAtlantis in April1989. Arriving at Venus in August1990,Magellan will spend eightmonths mapping most of the planetat a resolution (a measure of thesmallest objects that can be seen inits map) nearly ten times better thanthat of any previous spacecraftviewsof the surface. More than any othersingle mission, Magellan is expectedto unveil the secrets of the Venusianpast, just as Mariner 9 revealed theunsuspectedrichness of Martiangeology in 1972. In 1990, for thefirst time, we will really come to knowthe face of our planetary "twin." y


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fii@@@plo BlllJ&@t lap4 6~~~~ i~~ i*@~hI l l

like highlands (Aphrodite and Ishtar), hilly plains, large volcano-like mountains, and flat lowlands. However, as important

as this radar map is, it. showt, enlfy I ; l rp~-Sal@ra~m.Rie klllx.and wall-, wa#xfnmid Esrm flolmrt4hsMling aletililt

of Yenmian gcology-are as Tfet u n c b r t ~ d . . ive years after the Pioneer mission, in 1983, the Soviet Union tent .hro Venara s p a w m f l ta mp Ven~mat a molrrfion of appm-b 2 2 4 k l l ~m~ b r o1.2 tw 2.4 milas]. ksam ef

the nature of their orbits around the planet, the spacecraft were able to map only about 25 percent of the surfa~ near

*henn,r& pole. In mmparim,~ ~ ~ I l r nil l map TO te !M pemlent d ho planel at a re td l~ t i en ryin~gram 25a tra T56

A b y slture of the #(ellan rplcesnlt is fh8 econony and relatire s im~pl i~ l i y1 its design. TO s m osts, SW.R

radar sensl This one instrument, how eve^ ill perform three important


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ORIGINAL PAGE00LOR PHOTOGRAPH

TheMagellan spacecrafi isprepared fortesting inDenver, Colorado,at theMartin

Marietta AstronauticsGrorcp,prime contractorfor the spacecrafi. Most of Magellan's major

parts are wrapped in reftctfvewhite thermalblankets to maintain temperature control.

m e spacecrajt's radar sensor was buiB by theHughes Aircrafi Company of El Segundo, Califor


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ORIGINAS PAGECOLOR WOTOGRAPH

How synthetic aperture radar 7

ORBITALGROUNDTRACK

LINES OF EQUAL(SAR) works: Any point in the

radar map image can be locatedby using two measurements-the

distance to the point (determined\by the time it takes or the radar

signals to return to MageZCan),and the amount of Doppler shzp 7

in the signal (a shifl in requencycaused by the spacecrafls motion

along its orbit). However, two'7widely separatedpoints (such as 7A and B shown here) have the

same delay and Doppler shzp.Consequently, to avoid confusion

between these two Doints.Magelhank SAR k t e u u l will be

L I LLUM ~ATE D D I S TANCEAREA

pointed to the lefl of the orbital 7ground track to illuminate only

one of thesepossible points(A as shown here;B will have

already been mapped in aprevious orbit's swath). The

resulting radar map will 1therefore show onlyfeatures

to the lefl of the ground track.Theportion of the Venusian

surface shown here is aradar image rom the Soviet

Venera mission


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ermine

map image. Through this technique, data w il l be collected by the radar instrument and radioed back to Earth,where images of the Venusian surface will be constructed by computers at the Jet Propulsion Laboratory. In these images,

i t wil l be passible to distinguish features as small as 250 meters (820 feet) for the equatorial regions of the planet (where

magellan wil l pass closest to the surface) and about 750 meters (2,461 feet) near the poles. By comparison, the best

existing ground-based and spacecraft maps of Venus show no features smaller than 2,000 meters (6,562 feet).APRIL 1W9 TO AUGUST 1990: FROM EARTH TO VENUS

In late April 1989, Magellan wi ll be carried into low Earth orbit by the space shuttle Atlantis. After several revolutions

around Earth, the spacecraft, with an Inertial Upper Stage (IUS) booster attached to its base, will be deployed from Atlantis'

cargo bay into its own orbit. After two-thirds of another revolution around Earth, the IUS w il l fire and propel the Magellanspacecraft toward Venus. The IUS w il l then be jettisoned. . he launch period will begin on April 28 and will lastfor 30 days. During this launch period, Venus wil l be approximately 255 milli on kilometers (158 mill ion miles) from Earth.

After launch, i t wi ll take just under 15 months for Magellan to reach its destination. Three adjustment maneuvers along

the way wi l l keep the spacecraft on time^and on target for its rendezvous with Venus. . hen Magellan arrives atVenus in early August 1990, a solid rocket motor attached to the spacecraft wi ll fire to plaoe Magellan in orbit around then.planet. After a few adjustment maneuvers, thb spacecraft wil l be in an elliptica l orbit, with its lowest point at an altitude

af 24H Idtamat~rrc lM miles) rbamBe planet's U~ ~ I EGnd its hlghrwt paint d 8,- kilometem (4,MOmilas). The time


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MARSORBIT

trajectory shows the orbitsof the our innerplanets,as weld as

SUN

This depictionof

thepositions of Earth and Venusat thetimes of the spacecrafls launch and ofits arrival at Venus. Magellan wig eravel an eUipticalflightpath one andone-halftimesaround the Sun before Lintercepting Venus. A traditionaltrajectory to Venuswould useapproximatelyone-half of a revolutionaround the Sun; however, shuttle launchscheduling has dictated the longertrajectory or Magellan.

VERNALEQUINOX


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T M k m p ~ p h ~ill PU lays. I he arriwal did-e art Yenus, August 10, 1990, will place Ma~grllann Venus

oirbil rprpRarimWt@tb@e W~IIR~ l lper i~ romjuR1Gfjran the passage of the planet behind the Sun as seen h m

Earth). During superior conjunction, radio interference from the Sun's atmosphere wi ll make it impossible to communicatearrm ma ~ p e ~ ~ m f tnd tn mtduIE1;WW rawr I I M ~ ~ ~ ~ Q .he nsultant gap in mapping cowrage can be filled in during

7 -Cn

subq~writ 43-1.1 m p p i q = ~ i r c l i n g the planet every three hours and nine minutes, Magellan will pass

51-34 to Wle ~ ; ~ r J l m d r ~ust north m i B e Equator, at 10 degrees Venus latitude, and wil l then move dawn arwlld the south

amrn~d e pllla~ta~n wid, l o ~ q .Eemwe ad this ellipti681 orbit, Magellan will be close enough to the surfdlcaw u ~ d ~ dappimp op@rationror an$ abo miannfes out of each three-hour orbita l period. The rest of fhe time will

3.

be spent t ramnit l ing th.c ntddrr%edtaw data frbm the just-mmpleted mavping pass, receiv ing telemetry instructiorrs from

"*h and calibrating tha spacecraft's attitude control system with reference stan. W During rnzipping operations,


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ORIGINAL PAGE'COLOR.PHOTOGRAPH

(D ) ATTITUDECONTROLCALIBRATION

The3.15-hour ellipticalorbit ofMagelhawill be divided into distinctphases. U%en

PLAYBACKOFDATATO EARTH/(

the spacecrafl isclosest to Venus,the antennawtl2point at theplanet and tbe radar wiU map

the surface (A), aIt.em&&gbetweennorth andsouth swaths on successivepasses. AEer radar

operations are compkte4 the spacecrafl wiU turntopoint its antenna toward Earth (B) so that data

canbe transmitted (C). APer calibrationof tbespacecrafls atMtude control subsystem (D) a

another dataplayback, MageUun will turnattentiononce again to the surface.

,.I N NORTHI MAPPING

SPACECRAFTCLOSESTTOVENUS(1o0 N)


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receiver. By measuring the time it takes for the signal to return, the altimeter wi l l determine the distance to the pointdirectly below the spacecraft, and so will construct a topographic profile of the planet in much the same way that sonar

is used on board ships to profile the ocean floor. By mission's end, the Magellan altimeter experiment wi ll have produced

a topographic map showing height variations as small as 30 meters (98 feet) for the entire mapped part of the planet.

Sever31 additional types of information wi ll be collected by Magellan. When the dish antenna is pointing down atVenus, it w il l also be used to measure the amount of natural thermal emissions, from which temperature variations on

the planet's surface can be determined. Analysis of the way in which the radar signals are reflected wi ll yield data on

the electrical conductivity and roughness of the Venusian surface. After each mapping pass, tha spacecraft will

recede from Venus and the tape recorders wi l l be rewound in preparation for data transmission. Because the same

._5 antenna used for mapping wil l also be used for radio communications, the spacecraft must reorient itself to point the

antenna toward Earth. The transmissions will be received by the large antennas of NASA's Deep Space Network located

at ~ W I O D E Sites amornd L e world, th&n r e l q e d tatk Jet Propulsion Laboratory in Pasadena, Califolrnb. While Magellam

is iln nd lo mrnmnicatitunwith bm, lrclcis nuezmrnmeW earn be m d e oS the slight m n g w n the spacecraft's orbitald

~ m a t i o ~ ~ .hlwe timy mtiorrs, which are pmdumd by w r i a t i m in Veniws"pr;roitati~nill field, wil l provide important cluesd.

about the nature of the planet's interior. ~ f t i rts "call Rome" b mmpleted, the spacecraft wi l l maneuver into positionI ' :T7, -' I - .

t i : I - *to begin anlother mapping pass and w il l a~ a i n oint down toward the surface. . ince Venus will be rotating slowly >


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mill w e YI L.obtain* by us in,^ mery other math, an alternating p a l r n of northern and smuthen mapping passes.

minutes. On the next orbit, mapping wi ll start 4.7 minutes later than on the previous orbit and wil l stop 4.7 minutes farther

south. W Magellan's inclined orbit and its limited time for transmitting data to Earth make i t impossible to obtain

full coverage of both poles during the course of one 243-day mapping cycle. Scientists and mission designers thereforefaced a difficult choice: whether to fully map the northern or the southern hemisphere. Bemuse the large "continent"

of Ishtar, which extends into high northern latitudes, seems to have a number of significant geologic provinces, i t was

aecided to provide full coverage of the northern hemisphere. Mapping of the low southern hemisphere, which does not

have as high a scientific priority, will extend to about 70 degrees south latitude. . hus, eight times each day, for243 day, Magellan will take radar images of the Venusian surface. At the end of the primary mission, almost 90 percent

of the planet will have been mapped. The image strips will be combined by computers on Earth into photomosaic images

m r i n g arge regions o_f the Yenmian surfam.

THE PtA#ET VENUS

Earth and Venus have many similar characteristics, such as size, density, and the presence of atmospheres. However,c,t h w also show important differences. ~ l t h o i g h oth planets are most likely made of the same type of silioate rock and

probabty hare similar interiors, Venus does not appear to have a magnetic field as Earth does. Velius is clmer to thle

12T W E L V E- - - .-


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ORDINAL PAGEWLOR PHOTOGRAW

Sweltering under aperpetual cloud cover, Venus

reveals no surface detailseuen in ultraviolet light, but a

radar instrumenton the PioneerVenus Orbiter revealfd the large-scale

geography of the planet for the first time. Blue areas representthe Venusian lowlands, wbile highlands

are sbbwn in green, yellow, and red


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t#-&3uds laced with sulfuric acid d r o p l e l h i s th iWtmosphe-.

. .-,...,, < -1. =. ' .:- >< < - -& r. - > -.:- -e =c. . 8 - I

b i a t i o n between-thesolid surface and the atmospher& #ar from b

furnace, where surface temperatures reach 4E10 degrees Celsius (900 degrees Fahrenheit) and the atmospheric pressur

sites are basaltic. Ib Ula rocks on Earth's ocea'n floor or the rocks that are formed from oozing volcanic lava flows&

Venus' large-scale geography has been disclosed by radar studies from Earth, by the Pioneer Venus Orbiter in 1978

and by the. Soviet Venera 15 and 16 missions in 1983. Most of the planet consists of either rolling upland plain; , f" ,yrn .-us,:ws.r w -Z .5 2m - 3 -.w;"'".GC-=.akV. - %-xF?

(apparently composed of older crustal rock) or smooth lowland areas. There are two major "continents," or elevated

plateaus-Aphrodite, named for the Greek equivalallt of the goddess Venus, and Ishtar, named for the Babylonian

equivalent-that appear to be younger geologically. lshtar is about the size of Australia; Aphrodite is about twice as large,

.":m:h--F-


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Aportion of the elevated"continent"of Zshtar Tewa(about the size ofAustralia) is shown in thiscomputer-processedPioneer Venusimage.At the center is MountMaxwell (also calledMaxwell Montes), which is10,800 meters (35,400feet)high, more than a miletaUer than Mount Everest.There is some evidence thatthis huge mountain is anactive volcano. TheLakshmiplateau, rising 4to 5 kilometers (2.5 to 3.1miles) above the mean levelof Venus, is bordered bymountain ranges to thenorth and northwest.Thisplateau is thought toconsist of thin lavasoverlying an upla@edsection of older crust.Soviet Veneraradar:datasuggest that the depressioncalled Colette is a co@sedvolcanic crater. OnZshtar'ssouthernmnk are the Utand VestaRupes (clans),which descend to vastlowlands.

ORIGINAL PAGECOLOR P H O T O G W

. .I.t . 9I . . .

Aphrodite Terra, Venus'largest elevated landmas(about the size of SouthAmerica), has two majormountain regions onopposite sides of the"continent." AphroditeC also has the lowest

APHRODITETERRA elevations on Venus-inthe trenches of the DianaChasma,which may be arip valley caused by themovement of two blocksof crust awayfrom eachother.


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ion years ago, the planet8 of the solar syst

terial and gas revolving around the Sut~. deavier elements like iron and silicon remained.in the s n;= -- ?J-&., - f;.?


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This glimpse of the Venusian surfacewas taken by one of the SovietVenera landers. The reddishappearance of the rocks k due to thereddish color of the thickatmosphere. The shabby rocks, whichare probably volcanic in origin,would appear neutral gray innatural sunlight. The rectangularcolor bar at the bottom of the photois a part of the lander.


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Volcanoes

One of the most important tasks for Maaellan during its mapping mission will be to take an inventory of volcanic cratersL A ~ J-.+#%and other volcanic features on Venus solinat scientists can reconstruct the planet's geologic history. Ground-based and

Venera radar images have shown the existence of volcanic craters on the lshtar plateau. Variations in the concentratioa'of sulfur dioxide in the atmosphere, detected by Pioneer Venus, suggest 'that Venus may be volcanically ahlve. By

counting how many volcanoes are on Venus' surface and identifying where and what kind they are, Magellan will provide

oata on the planet's internal processes. The high-resolution radar images wil l allow us to discriminate betwegn

individual overlapping lava flows so as to determine the sequence of volcanic events that have helped shape the surface,--p-&.rshapes of these volcanic flows, scientists car mati! rudgments about th G cmposition oS the

lava and thus obtain further clues about the nature of the planet's interior and the thickness of the crust. W Earliep

@al&mff data laam&am gal the gravitational field of Venus is stronger w e r the planet" elevated [email protected] .

that these topographic features are related to the interior structur@, Magellan's high-resolution gravity survey, constructeQ.by precise measmmlenk of fie q m t e W s orb1iW mutiom, will prolride detgik about this important currelation L8Weel

g d t y and topography.


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The improvement in resolutionexpected from the Magellan data

is illustrated in these images ofthe Mount Saint Helens region of

Washington,which are simulutionsderivedfrom the radar imaging data

acquired by the Seasat oceanographicsatellite. The stin-active volcano does

MAGELLAN

not show at the Pioneer Venus resolutionAlthough the eature is visible at theVenera resolution, it is not possible to tell

whether it is a volcano or a meteoriteimpact crater.

ORlQlMLPAGECBI-QR PHOTOGRAPH


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L ebwita innact cnbn also appear in abrmdance in radar l m a g r o( Venus. Swek waters am more~plmitifulhavi .on%m? -sm

,: t h ~ empact szgn from volcanic c m h n , to ca~wnthow many are stil l

exist. It is important to establish Venus' impact cratering record, since the more cratered a surface is, the older i t must

be. Earth's surface is relatively young-looking and uncratend. Although meteorites have struck our planet in the past,

most impact craters have been erased by wind and water erosion and by the constant motion of tectonic plates through

time. The surface of Earth is a slate that has been drawn on, wiped clean, redrawn, and rewiped over mill ions o fcenturies. Venus, on the other hand, appears to retain evidence of a comparatively distant past. Magellan's global

inventory of impact craters will have much to tell scientists about the history of the planet and the ages of different

geologic provinces. The rate of surface cratering may also provide information on how dense the planet's atmosphere

has been through time. H At the best resolutions obtained to date, i t is unclear whether many of the circular featuresseen on Venus a n he scars of old impacts, collapsed remnants of volcanic craters, or domes of rock somehow warped

upward by tectonic forces. Magellan's high-resolution radar images wi ll clear up the mystery. If these images show large

stretches of old, cratered terrain, i t would argue against tectonic motion in those regions, because crustal movements

would destroy old craters. It would also indicate that the processes of erosion proceed much more slowly on Venus than


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ORIGINAL PAG

This computer-generatedphotoshows t f ~ a tonly very general conclusions about the geology of a planet-

Venus or Earth -can be drawn rom radar imageswith the resolution obtained by Pioneer Venus. The images

from the higher-resolutionMageUan radar will give-i Iscfentists a better unlterstanding of processes that have

shaped the Venusian surface and interior.


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. -. -Sm'ikind d m#&lqm@llKM@3td@ntJyL@t OW pmVWDh LmUI, i@lob111*1~1

appear In soviet radar images of '-?tar eatures are most probably ~ a I ~ e d-'the aompressiorr gr eX&n$~on

of the crust. Magellan will reveal the details of these features, allowing I

wark. I t haa been proposed that the high bullace temperatu~~an Venus play a part in the distortion of the crust, and

Magellan wil l provide new data to test that theoq. Large rif l valleys such as Devana C h m a n Beta Regio will be studied

to see whether they were formed by volcanic processes or by tectonic motions.

Water and Wnd

Another critical question about Venus is whether i t once had water on its surface. lodern-day ratios of deuterium to

hydrogen in Wenu' atmosphere (measured by descending atmospheric probes) suggest that at some point in the past there

was more water in the planet's atmosphere. Mapellan wil l look for evidence of ancient marine terraces, river channels

and deltas, or other geologic features that might point to the existence of ancient oceans. Such discoveries would have

profound implicatians for tbe evolulion of the planet's atmosphere as well as of its surface. W Although surface wind

speeds on Venus are believed to be lower than on Earth, there may be large windblown dunes on the surface that would

be midlcmtL igh-resolution Magellan inxqgj~:The sizes and shapes of such dunes would allow s ~ j mg rgmn -- . .

the wind behavior on Venus.


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ORIGINAL PAGECOLOR PHOTOGRAPH

During its primary 243-day mission,Magellan win acquire more digital imaging

data than aUprevious US. lanetary missionscombined. Zbese data will be a legacyfor future investigators of the veiledplunet, just

as thefindings from the expedition ofFerdinand Magellan, the Project's namesake,were or the then-fiture explorers of Earth.


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ORIGINAL PAGE*gxnoR PHOT.OGRAPH


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J e Soviet Venera 15and 16spacecraft mapped less thanone-third of the Venusiansurface athigh resolution. Over the course ofone Venusian day (243 Earth days),the Magellan spacecraft will mapmost of the surface with detail thatexceeds that of these best previousradar images. The resultant mapswill reveal the traces (if they exist) ofmany fundamentalplanetaly forces:volcanism, wind, water, andmeteorite impacts-in short, all theprocesses that determinea planet'shistory and shape its face. By givingus this new information, the Magellanmission will help to tell us whyVenus, our planetary "twin," s at thesame time so much a stranger.The International Science TeamRadar lnvestigation GroupGordon H. Peltengill (Principal Investigator), Massachuselts nstitute of TechnologyRaymondE Arvidson, Washington UniversityVictor R. Baker, University of ArizonaJoseph H. Binsack, Massachuselts lnstitute of TechnologyJoseph M. Boyce, National Aeronautics and Space AdministrationDonald B. Campbell, Cornell UniversityMerton E. Davies, RAND CorporationCharles Elachi, Jet Propulsion Laboratoa California nstitute of TechnologyJohn E. Guest, University College London, EnglandJames W. Head, 111 Brown UniversityWilliam M. Kaula, National Oceanographic and Atmospheric AdministrationKurl L. Lambeck, The Australian National University, AustraliaFranz W. Leberl, Vexcel corporationHarold MacDonald, University of ArkansasHarold Masurshy, U.S. Geological SurveyDaniel P. McKenzie, Bullard Laboratories, EnglandBarry E. Parsons, University of Oxford, EnglandRoger J. Phillips, Southern Methodist UniversityR. Keith Raney, Canada Centre for Remote Sensing, CanadaR. Stephen Saunders, Jet Propulsion Laboratory, California Institute of TechnologyGerald Schaber, U.S. Geological SurveyGerald Schubert, University of California, Los AngelesLaurence A. Soderblom, U.S. Geological SurveySean C. Solomon, Massachuselts Institute of TechnologyH. Ray Stanley, National Aeronautics and Space AdministrationManik Talwani, Geotechnology Research lnstituteG. Leonard Tyer, Stanford UniversityJohn A. Wood, Smithsonian Astrophysical ObservatoryGravity Investigation GroupMichel Lefebvre (Principal Investigator), Centre National #Etudes Spatiales, FranceWill iam L. Sjogren (Principal Investigator), Jet Propulsion Laboratory, California nstitute of TechnologyMohan Ananda, Aerospace corporationGeorges Balmino, Centre National #Etudes Spatiales, FranceNicole Borderies, Centre National #Etudes Spatiales, FranceBernard Moynot, Centre National #Etudes Spatiales, France


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magellan the unveiling of venus

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