ATLAS OF THE UNIVERSE
Mars
M
ars, the first planet beyond the orbit of the Earth, has
always been of special interest, because until relative-
ly recently it was thought that life might exist there. Less
than a century ago, there was even a prize (the Guzman
Prize) offered in France to be given to the first man to
establish contact with beings on another world – Mars
being specifically excluded as being too easy!
Mars is considerably smaller and less dense than the
Earth, and in size it is intermediate between the Earth and
the Moon. The escape velocity of 5 kilometres per second
(3.1 miles per second) is high enough to hold down a thin
atmosphere, but even before the Space Age it had become
clear that the atmosphere is not dense enough to support
advanced Earth-type life; neither could oceans exist on
the surface. The axial tilt is much the same as ours, so
that the seasons are of similar type even though they are
much longer. The orbital period is 687 days. The axial
rotation period, easily measured from observations of the
surface markings, is 24 hours 37 minutes 22.6 seconds, so
that a Martian ‘year’ contains 668 Martian days or ‘sols’.
The orbit of Mars is decidedly eccentric. The distance
from the Sun ranges between 249 million and 207 million
kilometres (between 155 million and 129 million miles),
and this has a definite effect upon Martian climate. As
with Earth, perihelion occurs during southern summer,
so that on Mars the southern summers are shorter and
warmer than those of the north, while the winters are
longer and colder.
At its nearest to us, Mars may come within 59 million
kilometres (36 million miles) of the Earth, closer than any
other planet apart from Venus. Small telescopes will thenshow considerable surface detail. First there are the polar
ice-caps, which vary with the seasons; at its greatest extent
the southern cap may extend down to latitude 50°, though
at minimum it becomes very small. Because of the more
extreme climate in the southern hemisphere, the variations
in the size of the cap are greater than those in the north.
The dark areas are permanent, though minor variations
occur; as long ago as 1659 the most conspicuous dark
feature, the rather V-shaped patch now known as the Syrtis
Major, was recorded by the Dutch astronomer Christiaan
Huygens. Originally it was assumed that the dark areas
were seas, while the ochre tracts which cover the rest of the
planet represented dry land. When it was found that the
atmospheric pressure is too low for liquid water, it was
believed that the dark areas were old sea-beds filled with
vegetation. This view was generally accepted up to the
time of the first fly-by made by Mariner 4, in 1965.
There are various bright areas, of which the most
prominent is Hellas, in the southern part of the planet. At
times it is so bright that it has been mistaken for an extra
polar cap, and it was once thought to be a snow-covered
plateau, though it is now known to be a deep basin.
In general the Martian atmosphere is transparent, but
clouds can be seen in it, and there are occasional dust-
storms which may spread over most of the planet, hiding
the surface features completely. What apparently happens
is that if the windspeed exceeds 50 to 100 metres per sec-
ond (160 to 320 feet per second), tiny grains of surface
material are whipped up and given a ‘skipping’ motion,
known technically as saltation. When they strike the sur-
face they force still smaller grains into the atmosphere,
where they remain suspended for weeks. Widespread dust-
storms are commonest when Mars is near perihelion, and
the surface winds are at their strongest.
The first reasonably reliable maps of Mars date back
to the 1860s. The various features were named, mainly
after astronomers; the old maps show Mädler Land,
Lassell Land, Beer Continent and so on. (The latter name
honoured Wilhelm Beer, a German pioneer of lunar and
planetary observation.) Then, in 1877, G. V. Schiaparelli
produced a more detailed map and renamed the features,
so that, for example, the most prominent dark marking
on Mars, the V-shaped feature drawn by Huygens so long
ago, and formerly known as the Kaiser Sea, was renamed
Syrtis Major. It is Schiaparelli’s nomenclature, modified
and extended, which we use today.
Schiaparelli also drew strange, artificial-looking lines
across the ochre deserts, which he called canali or
channels; inevitably this was translated as ‘canals’ and the
suggestion was made that the features might be artificial
waterways. This view was championed by Percival
Lowell, who built the great observatory at Flagstaff inThe rotation of Mars.
A series of images taken by
Charles Capen with the
Lowell refractor. The V-
shaped Syrtis Major appears
on the left-hand picture; the
extreme right-hand picture
shows the Meridiani Sinus.
The south polar cap is much
in evidence. The main
markings shown on these
pictures can be seen with
moderate-sized telescopes
when Mars is well placed. Marsphotographed by
Charles Capen with the
61-cm (24 inch) Lowell
refractor. The south polar
cap, at top, is prominent.
The very dark feature to
the left is Meridiani Sinus,
which includes the small
crater which makes the
zero for Martian longitude.
To the right of Meridiani
is the dark mass which
includes Margaritifer
Sinus and Aurorae Sinus.
(According to the new
nomenclature, ‘Sinus’
has become ‘Planum’.)C Atl of Univ Phil'03stp 2/4/03 3:03 pm Page 76