Detail of Phobos photo- 

 graphed at a distance of 75 

 miles by Viking Orbiter I in 

 1977. The largest crater 

 shown here Is 1 .2 km across. 



Courtesy NASA 



ing in continuous storms. Down deep in their interiors 

 there might be solid rock, but no one knows. They could 

 just as easily be gaseous. 



These outer planets, however, are rich with moons. 

 Jupiter has 16, Saturn more than 17, Uranus 15, and 

 Neptune a minumum of 3. There is a hint that even 

 little Pluto has at least one moon. Among the inner 

 planets, as already mentioned, the Earth is unusual for 

 having a moon at all, and especially for having a moon 

 that is so large relative to the size of Earth itself. If you 

 add up all the masses of the moons of Jupiter, for exam- 

 ple, they total up to only a fraction of one percent of 

 the mass of Jupiter itself. The same is true for each of 

 the other giant outer planets. Earth's moon, however, 

 is over one percent of the mass of the Earth. The origin 

 of Earth's moon is a fascinating story that has emerged 

 from over fifteen years of analysis of the samples that 

 were returned to Earth during the U.S. program of 

 landings on the lunar surface — the Apollo Program. 

 That's another story. 



The two moons of Mars are very small compared to 

 the mass of Mars. Deimos and Phobos are not spheri- 

 cal. Each is lumpy and irregular. Phobos looks like a 



giant Idaho potato. Its dimensions are approximately 

 17 by 13 by 12 miles (27 by 21 by 19 kilometers). Their 

 orbits are also peculiar — peculiar, that is, in contrast to 



the orbit of our own moon. Our moon makes a full 

 rotation around the Earth about every 28 days. So it 

 takes almost a calendar month for the moon to go 

 through all of its phases from full moon to new moon 

 and back to full moon again. Our moon rises in the east 

 and sets in the west. Each night it rises a little later than 

 the night before because it has moved l/28th of its way 

 along its eastward orbit around the Earth. So we are 

 used to a moon that rises and sets in the general manner 

 of the sun, once a day. 



For a person standing on the surface of Mars the two 

 little moons offer a bizarre ballet of risings and settings. 

 A day on Mars is just a bit longer than a day on Earth — 

 24 hours and 39 minutes. Deimos takes 6 hours longer 

 to go around Mars than Mars takes to go through a full 

 Martian day. This means that Deimos moves sluggishly 

 through the sky and, after rising, takes more than two 

 days to set. Then it takes two more days before one 

 would see it rise again. In the meantime, Phobos whips 

 around Mars in a lively 7 hours and 39 minutes. On 

 Mars, a person would see Phobos rise in the west and set 

 in the east, twice a day. 



The U.S. Viking program, initiated in 1975, sent 

 rockets to circle Mars, photograph its surface, measure 

 its physical and chemical properties, and drop landers 

 onto the surface. In the process, they obtained pictures 



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