essed firsthand so far by only 12 humans； the lunar landing crews of Apollos 11； 12； and 14…17。
A full list of the satellites in the Solar System； known as of this writing； is given in the acpanying table。 （Nearly half of them were discovered by Voyager。） All have black skies—except Titan of Saturn and perhaps Triton of Neptune； which are big enough to have atmospheres。 And all asteroids as well。
Venus has about 90 times more air than Earth。 It isn't mainly oxygen and nitrogen as here—it's carbon dioxide。 But carbon dioxide doesn't absorb visible light either。 What would the sky look like from the surface of Venus if Venus had no clouds。 With so much atmosphere in the way； not only are violet and blue waves scattered； but all the other colors as well…green yellow； orange； red。 The air is so thick； though； that hardly any blue light makes it to the ground； it's scattered back to space by successive bounces higher up。 Thus； the light that does reach the ground should be strongly reddened…like an Earth sunset all over the sky。 Further； sulfur in the high clouds will stain the sky yellow。 Pictures taken by the Soviet Venera landers confirm that the skies of Venus are a kind of yellow…orange。
SIXTY…TWO WORLDS FOR THE THIRD MILLENNIUM： KNOWN MOONS OF THE PLANETS （AND ONE ASTEROID）—
LISTED IN ORDER OF DISTANCE FROM THEIR PLANET
EARTH； 1
MARS； 2
IDA； 1
JUPITER； 16
SATURN； 18
URANUS； 15
NEPTUNE； 8
PLUT0； 1
Moon
Phobos
Dactyl
Metis
Pan
Cordelia
Naiad
Charon
Deimos
Adrastea
Atlas
Ophelia
Thalassa
Amalthea
Prometheus
Bianca
Despina
Thebe
Pandora
Cressida
Galatea
to
Epimetheus
Desdemona
Larissa
Europa
Janus
Juliet
Proteus
Ganymede
Mimas
Portia
Triton
Callisto
Enceladus
Rosalind
Nereid
Leda
Tethys
Belinda
Himalia
Telesto
Puck
Lysithea
Calypso
Miranda
Elara
Diane
Ariel
Ananke
Helene
Umbriel
Carme
Rhea
Titania
Pasiphae
Titan
Oberon
Sinope
Hyperion
Iapetus
Phoebe
Mars is a different story。 It is a smaller world than Earth； with a much thinner atmosphere。 The pressure at the surface of Mars is； in fact； about the same as the altitude in the Earth's stratosphere to which Simons rose。 So we might expect the Martian sky to be black or purple…black。 The first color picture from the surface of Mars was obtained in July 1976 by the American Viking 1 lander—the first spacecraft to touch down successfully on the surface of the Red Planet。 The digital data were dutifully radioed from Mars back to Earth； and the color picture assembled by puter。 To the surprise of all the scientists and nobody else； that first image； released to the press； showed the Martian sky to be a fortable； homey blue—impossible for a planet with so insubstantial an atmosphere。 Something had gone wrong。
The picture on your color television set is a mixture of three monochrome images； each in a different color of light—red； green； and blue。 You can see this method of color positing in video projection systems； which project separate beams of red； green； and blue light to generate a full…color picture （including yellows）。 To get the right color； your set needs to mix or balance these three monochrome images correctly。 If you turn up the intensity of； say； blue； the picture will appear too blue。 Any picture returned from space requires a similar color balance。 Considerable discretion is sometimes left to the puter analysts in deciding this balance。 The hiking analysts were not planetary astronomers； and with this first color picture from Mars they simply mixed the colors until it looked 〃right。〃 We are so conditioned by our experience on Earth that 〃right；〃 of course； means a blue sky。 The color of the picture was soon corrected—using color calibration standards placed for this very purpose on board the spacecraft—and the resulting posite showed no blue sky at all； rather it was something between ochre and pink。 Not blue； but hardly purple…black either。
This is the right color of the Martian sky。 Much of the surface of Mars is desert—and red because the sands are rusk。 There are occasional violent sandstorms that lift fine particles from the surface high into the atmosphere。 It takes a long time for them to fall out； and before the sky has fully cleaned itself； there's always another sandstorm。 Global or near…global sandstorms occur almost every Martian year。 Since rusty particles are always suspended in this sky； future generations of humans； born and living out their lives on Mars； will consider that salmon color to be as natural and familiar as we consider our homey blue。 From a single glance at the daytime sky； they'll probably be able to tell how long it's been since the last big sandstorm。
The planets in the outer Solar System—Jupiter； Saturn； Uranus； and Neptune— are of a different sort。 These are huge worlds with giant atmospheres made mainly of hydrogen and helium。 Their solid surfaces are so deep inside that no sunlight penetrates there at all。 Down there； the sky is black； with no prospect of a sunrise—not ever。 The perpetual starless night is perhaps illuminated on occasion by a bolt of lightning。 But higher in the atmosphere； where the sunlight reaches； a much more beautiful vista awaits。
On Jupiter； above a high…altitude haze layer posed of ammonia （rather than water） ice particles； the sky is almost black。 Farther down； in the blue sky region； are multicolored clouds—in various shades of yellow…brown； and of unknown position。 （The candidate materials include sulfur； phosphorus； and plex organic molecules。） Even farther down； the sky will appear red…brown； except that the clouds there are of varying thicknesses； and where they are thin； you might see a patch of blue。 Still deeper； we gradually return to perpetual night。 Something similar is true on Saturn； but the colors there are more muted。
Uranus and especially Neptune have an uncanny； austere blue color through which clouds—some of them a little whiter—are carried by high…speed winds。 Sunlight reaches a paratively clean atmosphere posed mainly of hydrogen and helium but also rich in methane。 Long paths of methane absorb yellow and especially red light and let the green and blue filter through。 A thin hydrocarbon haze removes a little blue。 There may be a depth where the sky is greenish。
Conventional wisdom holds that the absorption by methane and the Rayleigh scattering of sunlight by the deep atmosphere together account for the blue colors on Uranus and Neptune。 But analysis of Voyager data by Kevin Baines of JPL seems to show that these causes are insufficient。 Apparently very deep—maybe in the vicinity of hypothesized clouds of hydrogen sulfide—there is an abundant blue substance。 So far no one has been able to figure out what it might be。 Blue materials are very rare in Nature。 As always happens in science； the old mysteries are dispelled only to be replaced by new ones。 Sooner or later we'll find out the answer to this one； too。
ALL WORLDS WITH NONBLACK SKIES have atmospheres。 If you're standing on the surface and there's an atmosphere thick enough to see； there's probably a way to fly through it。 We're now sending our instruments to fly in the variously colored skies of other worlds。 Someday we will go ourselves。
Parachutes have already been used in the atmospheres of Venus and Mars； and are planned for Jupiter and Titan。 In 1985 two French…Soviet balloons sailed through the yellow skies of Venus。 The Vega 9 balloon； about 4 meters across； dangled an instrument package 13 meters below。 The balloon inflated in the night hemisphere； floated about 54 kilometers above the surface； and transmitted data for almost two Earth days before its batteries failed。 In that time it traveled 11；600 kilometers （nearly 7；000 miles） over the surface of Venus； far below。 The Vega 2 balloon had an almost identical profile。 The atmosphere of Venus has also b