During late Summer of 2003 sky watchers will have noticed the bright orange 'star' low in the south east gradually increasing in brightness until on August 28th it will outshine everything else in the sky apart from the Sun and Moon. This object is, of course, not a star but the planet Mars coming into opposition, its closest approach to the Earth. As Mars takes about two years to orbit the Sun, oppositions occur with a similar frequency, but not all oppositions bring Mars as close to us as this one. Mars' orbit is quite eccentric compared to the Earth's and because of this it is only occasionally that both planets line up at their closest separation. This year's opposition is not the closest possible but it is not far short of it. On August 28th Mars will be closer to us than it has been during the last 60,000 years! This year's opposition will bring the Red Planet to within .37 AU (Astronomical Units- the Earth-Sun distance) or about 34.4 million miles. It is for this reason that the planet appears so bright, attaining a magnitude of -2.9 on August 28th at which time it will present a disk of some 25 arc/seconds in diameter, big enough to reveal considerable detail in quite modest telescopes.
It was this mysterious periodic fluctuation in brightness coupled with the red hue of the planet, suggesting a drop of blood, that earned Mars its designation as the' God of War' by the ancients. Its perplexing apparent motions against the background stars, moving first in one direction, then halting, then appearing to move backwards in loops, first gave the clue to philosophers like Copernicus and Galileo, that the Sun was the centre of the Solar System and our Earth was merely another planet. Later the detailed observations of Mars' positions made by the Danish astronomer Tycho Brahe, enabled the great Johann Kepler to formulate his famous laws of planetary motion.
But Mars' main fascination for generations of astronomers lies in the fact that it is the only other planet in the solar system on which we can actually examine the surface.
The first person to record detail on the surface of Mars was Christian Huygens in 1659. He made a drawing which showed a dark wedge shaped feature, now called the Syrtis Major. By making further observations and making timings, Huygens was able to determine Mars' rotation period at about 24 hours, similar to the Earths'. This was later refined to 24 hours 40 minutes by Cassini in 1666. Today's value is 24 hours 37 minutes. Cassini also recorded the presence of the Martian polar caps.
Over the years much detail was recorded on Mars' surface, enough to produce fairly accurate maps. Many of the dark features were permanent, but subtle changes in their shape and intensity were observed. This led some to believe that the dark marking represented areas of primitive vegetation such as lichens and mosses. The polar caps diminished in size during the Martian Summers and clouds were also observed, indicating the presence of an atmosphere.
However, Mars' main claim to fame lay in the observations made by G.V. Schiaparelli during the opposition of 1877. Schiaparelli made drawing showing strange linear features which he called 'canali' which is Italian for channels. Naturally this was mistranslated as 'canals' and began the myth of a Mars inhabited by intelligent beings busy constructing planet wide waterways to convey melt water from the polar caps to irrigate the deserts.
The myth was consolidated by the American astronomer Percival Lowell who was utterly convinced that Mars was inhabited and that the 'canals' were real. There is no doubt that linear features can be detected on Mars, but they are a kind of optical illusion, produced when small features are poorly resolved. The eye and the camera tend to 'join up the dots' to produce streaks. I have both observed and photographed many linear feature on Mars but unfortunately they do not correspond to features on images secured by space probes.
Most of our knowledge concerning Mars has come from space probes. The first in 1965 was the American Mariner 4. This probe showed that the Martian surface was peppered with impact craters. These results were confirmed by Mariner 6 & 7. In 1971 and 1972 Mariner 9 completed numerous orbits of the planet and took thousands of images. These revealed features undreamed of. Apart from the ubiquitous craters, there were vast systems of canyons and gigantic shield type volcanoes. The largest of these Olympus Mons is 27 km (17 miles) high with a base diameter of over 700 km (435 miles). There were also features which resembled dried up river beds and other apparently 'water cut' features. However, no water flows on Mars today as the very low atmospheric pressure would instantly vaporise it. Water does exist on Mars today but it is frozen in the polar caps and may exist deep beneath the surface as a kind of permafrost.
Mars is an unsymetrical planet. The northern hemisphere is mainly low lying craterless plains or desert, while the southern hemisphere is mainly well cratered highland terrain. The lack of craters in the north suggests that the surface is younger and that some resurfacing has taken place, perhaps during bouts of past volcanism. Perhaps during these events sufficient heat was released for water to flow albeit briefly.
Later space missions by the USA put a lander onto the surface. The images revealed a rock strewn desert beneath a pale pink sky, the colour due to fine particle of iron oxide dust whipped up by winds and suspended in the thin atmosphere which is mainly composed of 95% carbon dioxide, the other 5% consisting of nitrogen and other gases. The surface temperature was shown to be in the order of -100 C. The lander scooped up samples of the Martian soil which were analysed by on board equipment but no signs of organic life were detected.
There was a certain amount of excitement recently when the microscopic examination of a meteorite believed to have originated on Mars and found here on Earth in Antarctica, revealed traces of what some scientists believe to be fossil bacteria. If it proves to be correct it is a significant discovery, but at present the results are by no means conclusive.
For the past five years the Mars Global Surveyor probe has been engaged in thoroughly mapping the planet and it was joined a year ago by Mars Odyssey which is now busy analysing the Martian mineralogy. Now we eagerly await the most recent mission to the Red Planet - Beagle II due to rendezvous with Mars at Christmas. If all goes well we may at last get an answer to the perennial question "Is there life on Mars?".
The various space missions to Mars, have to some extent, removed much of the mystery surrounding the Red Planet but many of us were disappointed to see so many impact craters. Gone are the vast tracts of lowly vegetation fed by the melting ice caps, once imagined by certain astronomers. But these have been replaced by mysterious canyons and volcanoes of unimaginable proportions. When we see the fantastic images relayed by the surface landers, we can imagine ourselves lifting up some of those rocks to see what might be hiding beneath them.
Mars still has the power to fuel our imaginations and I guess it will continue to do so until we actually set foot on its ruddy surface.
HSS National Astronomy Week Page
Last updated by Julie Atkinson 18-Aug-2003