Mars Past, Present & Future

A talk given during Astronomy Week 2003 by Brian Sheen

Introduction

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.

Early Work

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 depth.

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.

Basic data.

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 degrees.

Sources

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.