Philips Atlas of the Universe

ATLAS OF THE UNIVERSE

Satellites of Mars

ATLAS OF THE UNIVERSE

M

ars has two satellites, Phobos and Deimos, both

discovered in 1877 by Asaph Hall, using the large

refractor at Washington Observatory; previous searches,

by William Herschel and Heinrich D’Arrest, had been un-

successful. Both are very small, and are not easy telescopic

objects, because they are so close to Mars. It is interesting

to recall that Jonathan Swift, in his Voyage to Laputa (one

of Gulliver’s Travels), had described how astronomers on

the curious flying island had discovered two Martian satel-

lites, one of which revolved round the planet in a time

less than that of the planet’s axial rotation – as Phobos

actually does. However, Swift’s reasoning was not strictly

scientific. If the Earth had one moon and Jupiter four, how

could Mars possibly manage with less than two?

Phobos moves at less than 6000 kilometres (3700

miles) above the planet’s surface, and the orbital period is

only 7 hours 39 minutes, so that to an observer on Mars

Phobos would rise in the east and set in the east 4^1 ⁄ 2 hours

later, during which time it would go though more than half

its cycle of phases from new to full; the interval between

successive risings would be no more than 11 hours. Yet

Phobos would be of little use as a source of illumination at

night. From Mars it would have an apparent diameter of

less than half that of our Moon seen from Earth, and would

give little more light than Venus does to us; an observer

at a latitude of more than 69° would never see it at all, and

for long periods it would be eclipsed by Mars’ shadow. It

would transit the Sun 1300 times in every Martian year,

taking 19 seconds to cross the solar disk.

Phobos is a dark, irregularly shaped body, with a

longest diameter of 27 kilometres (17 miles). Its surface is

coated with a ‘dusty’ regolith, and spacecraft images show

that it is cratered; the largest crater, 10 kilometres (6 miles)

across, is named Stickney in honour of Asaph Hall’s wife

(this was her maiden name, and it was she who urged her

husband to continue hunting for satellites when he was on

the verge of giving up). Other craters, one of which is

named after Hall, are around 5 kilometres (3 miles) across;

there are also ridges, hills, and strange parallel grooves

inclined to the equator at 30°. These grooves are from

100 to 200 metres (330 to 660 feet) wide, and 10 to

20 metres (33 to 66 feet) deep. It has been calculated that

Deimos

Phobos

Planetary orbit

 Phobos.The larger

satellite of Mars; like

Deimos, it is a dark grey

colour with a reflectivity of

around 5 per cent. Its density

indicates that it is made of

carbonaceous chondritic

material similar to that of

some asteroids.

▼ The satellite orbits.

Both Phobos and Deimos

are very close to Mars;

Phobos moves in a circular

orbit 9270 km (5800 miles)

from the centre of Mars.

Deimos orbits at 23,400 km

(14,500 miles).

 Map of Phobos.Stickney

is much the largest crater;

it lies close to the sub-Mars

point. From Stickney there

extends a well-marked ridge,

Kepler Dorsum. Of the other

main craters, Roche lies in

the north and Hall in the

south.

–70°

–60°

–30°

0°

270° 240° 210° 180° 150° 120° 90° 60° 30° 0° 330° 300° 270°

30°

60°

70°

0°

0°

180°

180°

270°

225° 135°

225° 135°

315° 45°

315° 45°

90°

270° 90° Roche

Wendell

Sharpless

Todd

D’Arrest

Stickney

Kepler

Dorsum

Sub-

Mars

Point

N

S

Hall

Roche

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