ATLAS OF THE UNIVERSE
Missions to Mars
M
ariner 4 bypassed Mars on 14 July 1965, at a range
of 9789 kilometres (6080 miles). It carried cameras,
and the images showed that Mars is a cratered world
rather than having a smooth, gently undulating surface, as
had been generally believed. Other probes followed,
mainly American; even now the Russians have had
very little luck with their Martian spacecraft. Then, on 13
November 1971, Mariner 9 entered a closed orbit, and
for the next eleven months sent back splendid pictures of
much of the surface. The minimum distance from Mars
was 1640 kilometres (1020 miles), over 7000 images were
received, and contact was not lost until October 1972.
Mariner 9 changed many of our ideas about Mars.
First the atmosphere turned out to be much thinner than
anticipated. It had been estimated that the ground pressure
should be about 87 millibars, equivalent to the pressure
in the Earth’s air at rather less than twice the height of
Everest, and that the main constituent was likely to be
nitrogen; in fact the pressure is below 10 millibars every-
where – so that it corresponds to what we regard as a
reasonably good laboratory vacuum – and most of the
atmosphere is made of carbon dioxide, with only small
amounts of nitrogen and other gases.
It is now known that the polar caps are made chiefly
of water ice, with a variable coating of carbon dioxide ice.
The caps are not identical; the carbon dioxide layer is
thicker in the south, and never vanishes completely.
During the southern winter, which is colder than that of
the northern, carbon dioxide condenses out of the atmos-
phere on to the polar cap, and there is a temporary fall in
atmospheric pressure.
The pictures of the surface were dramatic; naturally, it
had not been realized that all the earlier probes had sur-
veyed the least interesting areas of the planet. For the first
time we could examine the giant volcanoes, such as the
majestic Olympus Mons, which has an altitude of 25 kilo-
metres (15 miles) – three times the height of Everest –
with a 600-kilometre (375-mile) base and an 85-kilometre
(50-mile) caldera at its summit. It was found that there are
two marked bulges in the Martian crust, those of Tharsisand Elysium, and it is here that most of the volcanoes lie,
though there are minor volcanoes elsewhere. Tharsis is
the major feature; along it lie the volcanoes of Ascraeus
Mons, Arsia Mons and Pavonis Mons, with Olympus
Mons not far away. All these had been seen by Earth-
based observers, but there had been no way of finding out
just what they were; Olympus Mons had been known as
Nix Olympia, the Olympic Snow, as it sometimes shows
up telescopically as a white patch. North of Tharsis lies
Alba Patera, only a few kilometres high but more
than 2400 kilometres (1500 miles) across. The Elysium
bulge is smaller than that of Tharsis, and the volcanoes
are lower.
The two hemispheres of Mars are not alike. In general
the southern part of the planet is the higher, more heavily
cratered and more ancient, though it does contain two
deep and well formed basins, Hellas and Argyre. The
northern hemisphere is lower, younger and less cratered,
though it does contain part of the Tharsis bulge.
The Mariner Valley (Valles Marineris), just south of
the equator, has a total length of 4500 kilometres (2800
miles), with a maximum width of 600 kilometres (375
miles); the deepest part of the floor is 7 kilometres
(4 miles) below the rim. There are complex systems, such
as Noctis Labyrinthus (once taken for a lake, and named
Noctis Lacus), with canyons from 10 to 20 kilometres
(6 to 12 miles) wide making up the pattern which has led
to the nickname of the Chandelier. There are features
which can hardly be anything other than old riverbeds, so
that in the past Mars must have had a warmer climate
and a denser atmosphere than it does now; and of course
there are the craters, which are everywhere and some of
which are more than 400 kilometres (250 miles) across.
Here and there we find ‘islands’, and there is strong
evidence of past flash-flooding.
Whether there is any active vulcanism going on now
on Mars is a matter for debate. There is a crust, probably
between 15 and 20 kilometres (9 to 12 miles) deep, which
overlies a mantle. A magnetic field has been detected but
is very weak.▲ Olympus Mons,the
highest volcano on Mars;
it is 25 km (15 miles) above
the outer surface, and has
a base measuring 600 km
(375 miles). It is crowned by
an 85-km (53-mile) caldera.
It is a huge shield volcano,
far larger and more massive
than terrestrial shield
volcanoes such as Mauna
Kea and Mauna Loa. This
view is a composite of
images obtained from the
Viking orbiter. Mars in winter,
photographed by Viking
Lander 2. In the shadows
of rocks and boulders a
white condensate can be
seen, either water ice or
frozen carbon dioxide (dry
ice), or a combination of the
two precipitated on to the
ground as snow or frost.
Or it could have come to
the surface from below,
by cryopumping. Several
small trenches can be seen
in the centre foreground;
they were dug by the
lander’s soil-sampler arm
as it gathered material for
Viking soil experiments.
The soil scoop’s cover
lies to the right of the
trenches, where it was
dropped when the lander’s
mission began. Most rocks
in the scene are
approximately 50 cm (19–20
inches) across.C Atl of Univ Phil'03stp 2/4/03 3:04 pm Page 78