"Until Prof.Hall,at Washington,discovered the two satellites in 1877,"he continued,"Mars was supposed to be without moons.
The outer one,Deimos,is but six miles in diameter,and revolves about its primary in thirty hours and eighteen minutes,at a distance of fourteen thousand six hundred miles.As it takes but little longer to complete a revolution than Mars does to rotate on its axis,it remains in the Martial sky one hundred and thirty-two hours between rising and setting,passing through all the phases from new moon to full and back again four times;that is,it swings four times around Mars before going below the horizon.It is one of the smallest bodies discovered with a telescope.The inner one,Phobos,is considerably larger,having a diameter of about twenty miles.It is but twenty-seven hundred miles from Mars's surface,and completes its revolution in seven hours and thirty-eight minutes,which is shorter than any other known period,Jupiter's nearest moon being the next,with eleven hours and fifty-nine minutes.It thus revolves in less than a third of the time Mars takes to rotate,and must consequently rise in the west and set in the east,as it is continually running ahead of the surface of the planet,though the sun and all the other stars rise and set on Mars in the same way as on the earth."When about fifteen thousand miles from Mars,they sighted Deimos directly ahead,and saw that they should pass on its left--i.e.,behind--for it was moving across them.The sun poured directly upon it,making it appear full and showing all its features.
There were small unevennesses on the surface,apparently seventy or a hundred feet high,which were the nearest approach to mountains,and they ran in ridges or chains.There were also unmistakable signs of volcanic action,the craters being large compared with the size of the planet,but shallow.They saw no signs of water,and the blackness of the shadows convinced them there was no air.They secured two instantaneous photographs of the little satellite as the Callisto swept by,and resumed their inspection of Mars.They noticed red and brownish patches on the peaks that had that morning turned white,from which they concluded that the show had begun to melt under the warm spring sun.This strengthened the belief they had already formed,that on account of its twenty-seven and a half degrees inclination the changes in temperature on Mars must be great and sudden.So interested were they with this,that they did not at first see a large and bright body moving rapidly on a course that converged with theirs.
"We must be ready to repel boarders,"said Bearwarden,observing it for the first time and fixing his glass upon it."That must be Phobos."Not ten miles off they beheld Mars's inner moon,and though their own speed caused them to overtake and rush by it like a whirlwind,the satellite's rapid motion in its orbit,in a course temporarily almost parallel with theirs,served to give them a chance the better to examine it.Here the mountain ranges were considerably more conspicuous than on Deimos,and there were boulders and loose stones upon their slopes,which looked as if there might at some time have been frost and water on its surface;but it was all dry now,neither was there any air.The evidences of volcanic action were also plainly visible,while a noticeable flattening at the poles showed that the little body had once rotated rapidly on its axis,though whether it did so still they had not time to ascertain.When abreast of it they were less than two miles distant,and they secured several instantaneous impressions,which they put aside to develop later.
As the radius of Phobos's circle was far shorter than that of the parabolic curve they were making,it began to draw away,and was rapidly left behind.Applying the full apergetic force to Mars and the larger moon,they shot away like an arrow,having had their speed increased by the planet's attraction while approaching it,and subsequently by repulsion.
"Either of those,"said Bearwarden,looking back at the little satellites,"would be a nice yacht for a man to explore space on.
