Mars Past, Present & Future
A talk given during Astronomy Week 2003 by Brian Sheen
The word Planet comes from the Greek meaning "Wandering
Star". Of all the planets in the sky Mars is the one to which this
term applies the most. Because it has such a strange motion it has been
studied for thousands of years, although until the arrival of the
telescope with little effect. At its best only Venus can be brighter, as a
red object Mars stands alone, the Romans naming it after the god of war,
the rate at which it travels east, it brightness variations even at
opposition attracted much attention from the astronomers of old.
The Greeks including Ptolemy studied the stars, Sun and Moon with a
view to measuring the passage of time. They found Mars to be a serious
hindrance in this respect. They were forced to propose Earth centred
orbits for the planets with sub circles, deferents, to account for
retrograde motion Eudoxus was a leading figure in this work which
culminated in the Ptolemaic Solar System which stood unchallenged, to any
great extent until Copernicus in 16th Century.
In the 17th Century Kepler also found Mars trouble some as its very
elliptical orbit was hard to define with certainty. By the late 18th
Century William Herschel had turned his telescopes on the planet and
decided that it was the most Earth like of all the planets, smaller than
Earth to be sure, but it has polar caps that changed with the seasons and
fixed surface markings.
The early 19th Century books had little to say about the red planet,
Ferguson merely mentioning that the variations in size indicated a
variation in distance from Earth and therefore an elliptical orbit.
The Telescope era.
By the second half of the 19th century a number of interesting facts
had been determined due to the improvement in the telescope and the
micrometers attached to them. The fact is that August the 26th represents
the closest approach of Mars to the Earth and when this corresponds to
opposition then the planet appears at its brightest. 11th Aug. 1877 was
the occasion of a very favourable apparition followed by those of 1892,
1909 and 1924 amongst others. Very poor oppositions occur in February when
the orbital separation is at its greatest.
Parallax is a favourite tool of astronomers when seeking to determine
the distance of fairly close objects. Ascension Island, on the equator,
was chosen by David Gill as a suitable site in 1877. By measuring the
angular distance between Mars and nearby stars throughout the night
arrived at a fairly accurate figure and from Kepler's Law the distance to
the Sun and hence the scale of the Solar System.
The mass of the Earth was also determined by making use of an
occultation of a star in 1672. Then recording the transit of Mars relative
to the same star some 200 years (1877) later, the time delay due to the
presence of the Earth allowed its mass to be established to less than 1%.
Also in this period, the first maps of the planet were drawn and
measurements made of the polar caps and their changes through the season.
The fact that Mars rotates in some 40 minutes longer than Earth means that
it does not present the same face to the Earth throughout the night and
features can be seen coming into view on one side and vanishing from the
other. Its inclination to the plane of the ecliptic at 24 degrees is
similar to Earth's. Today computer programmes allow amateur astronomers to
be sure which part of Mars is visible at the time they are observing.
The opposition of 1877 allowed the American astronomer Asaph Hall to
discover the two moons, which were called Phobos and Deimos, Fear and
Terror, the attendants of Mars in Greek mythology, in Homer's Iliad. A
translation reads, "Mars spake, and called Dismay and Rout to yoke
his steeds, and he did on his harness sheen." The news was flashed by
telegraph to Plymouth just in time to be announced to a meeting of the
British Association. The tiny moons are now believed to be captured
asteroids. Typically Newcombe claimed the discovery for himself as
Director of the Washington Observatory where Hall worked. Incidentally
Phobos has a large crater called Stickney after Hall's wife's maiden name.
The infamous canals were first seen by Schiaparelli in 1877 he actually
called them channels but were translated canals and were studied
assiduously by Percival Lowell at Flagstaff Observatory from 1894 onwards.
Of course today we realise that they were only tricks of the light.
The space age
Our knowledge of Mars increased only slowly until the first Space
Probes and Landers arrived in 1970. Then of course it increased again
exponentially until the present day. The Mariner Probes were the first
sending back good quality pictures, Mariner 9 being the most successful.
The instrumentation allowed the atmosphere, surface features, including
polar caps, craters, channels and volcanoes to be studied in detail. Two
Viking Landers arrived in the mid 70's and set about trying to discover if
life was or had been present on the planet. The instrumentation produced
some odd results but in the end they were put down to a chemical reaction.
Mars Global Surveyor was launched in 1996 and arrived in September 1997
and carried out the most detailed survey of the red planet in history. The
images are of such quality that the theory has been advanced that the
whole surface of Mars must have been covered in water to a significant
Pathfinder was launched a little later but arrived ahead of Surveyor,
after bouncing around on the surface for a while it opened up to release
its Rover named Sojourner, many excellent pictures were obtained and
chemical analysis carried out.
The opposition of 1877 sparked a flurry of interest in Mars and the
same has happened this time around. The first to arrive will be ESA's Mars
Express carrying Beagle 2, the first British Lander. It will land in
Isidis Planitia and carries and range of analytical equipment, the most
significant of which is a miniature mass spectrometer. Colin Pillenger
hopes that it will give firm evidence of the existence for the first time,
of life outside planet Earth.
Strange to say one of the best bits of Mars science has come from
meteorite ALH84001 that landed in Antarctica, when examined under a SEM
produced images that resembled fossilised bacteria. However most
scientists remain sceptical.
The Orbiter will image the surface of the planet and produce a map of
the mineral composition of the surface.
The Americans arrive a little later, Jan 2004, with twin Rovers Spirit
and Opportunity they will be producing stereo images and using
spectrometers to determine the composition of the Martian Surface.
See it for your self
Although we here on Earth cannot hope to equal the quality of the
images produced by the various Probes it does not stop amateurs the world
over from trying to take their own pictures. Sky and Telescope has
produced a number of features on its web site which are well worth
studying. It will tell you exactly which part of Mars is visible at the
moment, how to use an occulting bar to image the two satellites. Access
www.skyandtelescope.com and browse the site, not forgetting to check under
the heading planets in addition to the specific Mars pages.
Position, the fourth rock from the Sun, Diameter 6800 kms, length of
day 24 hrs, 37 mins., length of year 687 days, inclination to orbit, 25
Books Refs.Dicks, D.R. Early Greek astronomy to Aristotle, pub Cornell
1985 Hoskin.M, Cambridge Illustrated History of Astronomy, pub CUP, 1997
Ferguson. J - Astronomy, pub A.Strahan, London, 1803 Ball,R.,The story of
the heavens, pub Cassell, 1893 Rudaux.L. & de Vaucouleurs G.,
Encyclopedia of Astronomy, Batchworth Press, 1959 Moore P. Atlas of the
Solar System, pub. Chancellor Press, 1997 Moore P. Mars, pub. Cassell,
1998 Lee.W. To rise from Earth, pub Blandford, 2000. Ridpath.J &
Woodruff. J, Astronomical Dictionary. Pub. Phillips 1995 Moore P.
Astronomy Encyclopedia. Pub. Phillips 2002
Journals Astronomy & Geophysics Aug. 2003 Vol. 44 Issue 4,
Blackwell Publishing. Astronomy Now Aug. 2003, Pole Star Publications
Web Sites. www.skyandtelescope.com for a range of information about
viewing Mars. www.beagle2.com for all the information about the British
Lander, it links to the Mars Express site.