Mars

Mars is the outermost of the terrestrial planets. Its diameter is only half of that of the Earth. Seen through a telescope, Mars seems to be a reddish disk with dark spots and white polar caps. The polar caps wax and wane with the Martian seasons, indicating that they are composed of ice. Darker areas were suspected to be vegetation. At the end of the 19th century, an Italian astronomer, Giovanni Schiaparelli claimed that there are canals on Mars.

In the United States, the famous planetary astronomer Percival Lowell studied the canals and even published books on the topic. Martians were also very popular in science fiction literature. Now the canals are known to be nonexistent, an optical illusion when the obscure details at the limit of visibility seem to form straight lines, canals. Finally, the first clear pictures by Mariner 4 in 1965 buried even the most optimistic hopes concerning life on Mars. Later spacecraft revealed more details of the planet.

Mars is a superior planet, which means that it is most easily observable when it is closest to the Earth, i. e. during opposition, when the planet is above the horizon all night long.

The rotation axis of Mars is tilted 25° to the ecliptic, about the same amount as the Earth's axis. A Martian day is only half an hour longer than a terrestrial day. Mars' orbit is significantly elliptical, resulting in temperature variations of about 30 °C at the subsolar point between the aphelion and perihelion. This has a major influence on the climate. Huge dust storms are occasionally seen on Mars (Fig. 7.33). Usually the storms begin when Mars is at the perihelion. Heating of the surface gives rise to large temperature differences that in turn cause strong winds. The wind-driven dust absorbs more heat and finally the whole planet is covered by a dust storm where the wind speeds exceed 100 m/s.

The atmosphere of Mars is mainly composed of carbon dioxide (95%). It contains only 2% nitrogen and 0.1-0.4% oxygen. The atmosphere is very dry: if all the moisture were condensed on the surface, the water layer would be thinner than 0.1 mm. Even the minor

7.13 Mars

169

Fig. 7.33. Two pictures of Mars, taken by the Mars Global late winter south polar cap. The view from July shows the same Surveyor in June and July 2001. The view from June (left) regions, but most of the details are hidden by dust storms and shows the Tharsis volcanic region, Valles Marineris and the haze. (NASA/JPL/Malin Space Science Systems)

Fig. 7.33. Two pictures of Mars, taken by the Mars Global late winter south polar cap. The view from July shows the same Surveyor in June and July 2001. The view from June (left) regions, but most of the details are hidden by dust storms and shows the Tharsis volcanic region, Valles Marineris and the haze. (NASA/JPL/Malin Space Science Systems)

Fig. 7.34. A topographic shade map of Mars made from the and the Valles Marineris canyon system that is more than Mars Global Surveyor data. The most prominent features 3000 km long and up to 8 km deep. (MOLA Science are the large shield volcanoes in the northern hemisphere Team/NASA)

amount of water vapour is sufficient to occasionally form some thin clouds or haze.

The air pressure is only 5-8 mbar. A part of the atmosphere has escaped but it is probable that Mars never had a thick atmosphere. The primordial atmosphere of Mars was, however, somewhat similar to that of the Earth. Almost all of its carbon dioxide was used up to form carbonate rocks. Because there are no plate tectonics on Mars, the carbon dioxide was not recycled back into the atmosphere as on the Earth. Therefore, the greenhouse effect on Mars is significantly smaller than on the Earth.

Craters were already found in the first pictures. The southern hemisphere is especially marked by craters, indicating that the original surface is still visible there. The largest impacts, Hellas and Argyre are about 2000 km in diameter. On the other hand, the northern hemisphere has an abundance of large lava basins and volcanoes (Fig. 7.34). The surface is younger than in the south ern hemisphere. The largest volcano, Olympus Mons (Fig. 7.35), protrudes more than 20 km above the surrounding terrain. The diameter at the bottom is about 600 km.

There are no active volcanoes on Mars. The marelike plains on Mars are of the same age as the Lunar maria, about 3 x 109 years old. Volcanism in the highland and mare-like plains stopped at that time, but the giant shield volcanoes are much younger, possibly 1-2 x 109 years. The youngest lava flows on Olympus Mons are possibly less than 100 million years old. Mars shows no sign of plate tectonics. It has no mountain chains, nor any global patterns of volcanism.

There are also several canyons, the largest of which is Valles Marineris (Fig. 7.34). Its length is 5000 km, width 200 km, and depth about 6 km. Compared with Valles Marineris, the Grand Canyon is merely a scratch on the surface.

"75

KflP

Fig. 7.35a-c. Volcanoes, impact craters and rivers. (a) Mars Global Surveyor wide-angle view of Olympus Mons in April 1998. (b) Small impact craters and sand dunes wuth a resolution of 1.5 m per pixel. The picture covers a 1.5 km wide portion of Isidis Planitia. (c) Three major valley systems east of the Hellas plains. These valleys have probably been formed by large outbursts of liquid water but the age of the valleys is unknown. The valleys are all roughly 1 km deep and 10-40 km wide. The picture covers an area approximately 800 km across. (Mars Global Surveyor, 2000) (NASA/JPL/Malin Space Science Systems)

Fig. 7.35a-c. Volcanoes, impact craters and rivers. (a) Mars Global Surveyor wide-angle view of Olympus Mons in April 1998. (b) Small impact craters and sand dunes wuth a resolution of 1.5 m per pixel. The picture covers a 1.5 km wide portion of Isidis Planitia. (c) Three major valley systems east of the Hellas plains. These valleys have probably been formed by large outbursts of liquid water but the age of the valleys is unknown. The valleys are all roughly 1 km deep and 10-40 km wide. The picture covers an area approximately 800 km across. (Mars Global Surveyor, 2000) (NASA/JPL/Malin Space Science Systems)

7.14 Jupiter

171

Fig. 7.36. The 360 degree panorama was taken by the Mars Pathfinder Lander in 1997. The Sojourner rover is visible near the centre of the panorama, in front of the ramp. (NASA/JPL)

Ancient riverbeds (Fig. 7.35), too small to be seen from the Earth, were also discovered by spacecraft. Rivers were probably formed soon after the formation of Mars itself, when there was a great deal of water and the atmospheric pressure and temperature were higher. At present, the temperature and air pressure on Mars are too low for free water to exist, although there have been speculations on warm weather cycles in the more recent history of the planet. The mean temperature is now below -50 °C and, on a warm summer day, the temperature can rise close to zero near the equator. Most of the water is contained in kilometres deep permafrost below the surface and in the polar caps. The theory was confirmed in 2002, when the Mars Odyssey spacecraft detected a large supply of subsurface water ice of a wide area near the south pole. The ice is mixed into the soil a meter below the surface. Two rovers, Spirit and Opportunity, operating on the Mars discovered in 2004 minerals like hematite and goethite which proved the existence of liquid water on the surface of Mars. The period when the liquid water existed is unknown.

The polar caps are composed both of water and carbon dioxide ice. The northern cap is almost season-independent, extending down to latitude 70°. On the other hand, the southern cap, which reaches to the latitude - 60° in the southern winter, disappears almost totally during the summer. The southern cap consists mostly of CO2 ice. The permanent parts are of ordinary water ice, since the temperature, -73 °C, is too high for CO2 ice. The water ice layers can be hundreds of metres thick.

The dark areas are not vegetation, but loose dust, moved around by strong winds. These winds raise the dust high into the atmosphere, colouring the Martian sky red. The Mars landers have revealed a reddish re-golithic surface, scattered with boulders (Fig. 7.36). The red colour is caused mainly by iron oxide, rust; already in the 1950's, the existence of limonite (2FeO3 3 H2O) was deduced from polarization measurements. The on-site analysis showed that the soil consists of 13% iron and 21% silicon. The abundance of sulphur was found to be ten times that found on the Earth.

The interior of Mars is not well known. Mars has probably a dense core approximately 1700 km in radius, a molten rocky mantle which is denser than the Earth's mantle and a thin crust. The crust is 80 km thick in the southern hemisphere but only about 35 km thick in the northern one. The low mean density compared with other terrestrial planets may indicate that in addition to iron the core contains a relatively large fraction of sulphur .

The Mars Global Surveyor confirmed in 1997 a weak magnetic field. It is probably a remnant of an earlier global field that has since disappeared. This has important implications for the structure of Mars' interior. There are no electric currents creating a magnetic field and therefore the core may be (at least partially) solid.

Three biological experiments of the Viking landers in 1976 searched for signs of life. No organic compounds were found - however, the biological tests did give some unexpected results. A closer look at the results indicated no life, but some uncommon chemical reactions.

Mars has two moons, Phobos and Deimos (Fig. 7.37). The size of Phobos is roughly 27 km x 21 km x 19 km, and the orbital period around Mars is only 7 h 39 min. In the Martian sky, Phobos rises in the west and sets in the east. Deimos is smaller. Its diameter is 15 km x 12 km x 11km. There are craters on both moons. Polarimetric and photometric results show that they are composed of material resembling carbonaceous chondrite meteorites.

7. The Solar System

172

Fig. 7.37. Phobos (left) and Deimos, the two moons of Mars. They can be captured asteroids. (NASA)
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment