te known in
the solar system is one of Saturn's--Hyperion; almost 800 miles in
diameter. The inner one goes all round Mars in 71/2 hours. This is
Phobos' month. Mars turns on his axis in 24 hours and 40 minutes,
so that people in Mars would see the rise of Phobos twice in the
course of a day and night; lie would apparently cross the sky

177

going against the other satellite; that is, he would move
apparently from west to east.

We may at least assume as probable that other satellites have
been gathered by Mars in the past from the army of asteroids.

Some of the satellites so picked up would be direct: that is,
would move round the planet in the direction of his axial
rotation. Others, on the chances, would be retrograde: that is,
would move against his axial rotation. They would describe orbits
making the same various angles with the ecliptic as do the
asteroids; and we may be sure they would be of the same varying
dimensions.

We go on to inquire what would be the consequence to Mars of such
captures.

A satellite captured in this manner is very likely to be pulled
into the Planet. This is a probable end of a satellite in any
case. It will probably be the end of our satellite too. The
satellite Phobos is indeed believed to be about to take this very
plunge into his planet. But in the case when the satellite picked
up happens to be rotating round the planet in the opposite
direction to the axial rotation of the planet, it is pretty
certain that its career as a satellite will be a brief one. The
reasons for this I cannot now give. If, then, Mars picked up
satellites he is very sure to have absorbed them sooner or later.
Sooner if they happened to be retrograde satellites, later if
direct satellites. His present satellites are recent additions.
They are direct.

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The path of an expiring satellite will be a slow spiral described
round the planet. The spiral will at last, after many years,
bring the satellite down upon the surface of the primary. Its
final approach will be accelerated if the planet possesses an
atmosphere, as Mars probably does. A satellite of the dimensions
of Phobos--that is 36 miles in diameter--would hardly survive more
than 30 to 60 years within seventy miles of Mars' surface. It
will then be rotating round Mars in an hour and forty minutes,
moving, in fact, at the rate of 2.2 miles per second. In the
course of this 30 or 60 years it will, therefore, get round
perhaps 200,000 time