Wednesday, 8 February 2012

Mars – Red Planet

Mars Data
Mass: 6.42 x 1023 kg or 0.11 of Earth’s
Diameter: 6794 km or 0.53 of Earth’s
Surface gravity: 0.38 gee
Axial tilt: 25.2°
Mean surface temperature: -23 Celsius
Rotation period: 24.62 hours
Orbital period: 1.88 years
Inclination of orbit to ecliptic: 1.8°
Orbital eccentricity: 0.093
Distance from the Sun: 1.38–1.66 AU
Sunlight strength: 0.36–0.52 of Earth’s
Satellites: 2
Largest satellite: Phobos, diameter 27 km

As we head away from the Sun we have one more stop before leaving the realm of the inner planets: the Red Planet, Mars. Only 53 per cent the diameter of Earth, Mars is a midsize terrestrial world, accompanied by two small moons. Impact craters dominate its southern hemisphere, but extensive volcanism has significantly modified the north. Red sand, rich in particles of rusted iron, covers the frigid surface, blown by huge, global storms. Meanwhile, the polar regions sport extensive ice caps – though they are made of frozen carbon dioxide as well as water. Carbon dioxide is also the primary gas in the thin Martian atmospher, as it is on Venus. But Mars has lost most of its atmosphere, and its water is frozen at the poles or embedded in the ground as permafrost. Now, the Red Planet is a cold, hostile desert.

Physical Overview

Mars, more than any other planet, has long been the subject of much human fascination. Nineteenth-century astronomers perpetuated the vision that Mars was covered in lush vegetation, irrigated by a clever network of canals – evidence, they thought, of intelligent Martians. The planet even has the same axial tilt as the Earth, a very comparable rotation period of 24.62 hours, and dramatic seasons. Thus the romantic view of Martian life persisted well into the twentieth century. But when the first probes reached Mars in the 1960s and 1970s, the truth was finally revealed. Mars is not alive. It is dead, and looks as if it has been that way for a long time. No conclusive evidence for life there, either now or in the past, has ever been found. Instead, the planet in some way resembles parts of Mercury, the Moon and Venus.

Like all these worlds, Mars has roughly two different terrains. The highlands, which dominate the southern regions, are heavily cratered. In the north are rolling lowland plains, which have few craters and are more recent than the highlands. One exception to the general low altitude of the northern territories is the Tharsis rise. This huge bulge rises some 10 kilometres above the mean level of the terrain in the north, and runs 8000 kilometres across the planet. Astronomers suspect that Tharsis was created either when magma welled up under the planet’s crust and pushed it skywards, or when the magma flowed onto the surface itself in multiple episodes and solidified. Possibly both mechanisms were at work. Evidence for this volcanic origin is strong. A thong of massive shield volcanoes crowns Tharsis. Three of them lie in a line, and one – the 600-kilometre-wide Olympus Mons – is off to one side. These volcanoes are enormous, evidence that Mars, like all terrestrial planets except Earth, lacks plate tectonics. The magma beneath the stationary crust simply oozed onto the surface and piled up. On Earth, by contrast, this pile-up cannot happen because the continents never stay in one place. Also, mountains are absent on Mars – further evidence for a stationary crust. But there are some tectonic features. The largest and most awesome is Valles Marineris. This is a truly enormous canyon, south of the Tharsis bulge. It is 8 kilometres deep and 4500 kilometres long – the Earth’s Grand Canyon, by comparison, is little more than a scratch. Valles Marineris may have been created with the Tharsis uprising, the equatorial crust literally ripped apart as magma pushed its way up further north.

The lack of plate motion on Mars could mean that its crust is thicker than Earth’s. The planet cooled more quickly than Earth, being smaller, and so its crust did not fracture with the impacts from space. Underneath the crust, Mars is similar to Venus. It has a thick, rocky mantle and a metal-rich core. And, like Venus, Mars has almost no detectable magnetic field.

Atmosphere and Climate

Like Earth, Mars has polar caps. As well as water ice, they include significant quantities of frozen carbon dioxide: dry ice. But because Mars’ orbit is quite eccentric – its average distance from the Sun of 229 million kilometres (1.52 AU) varies by 11 per cent – the polar caps change dramatically with the seasons. The planet’s closest approach to the Sun brings a swift summer in the southern hemisphere and sees the gradual melting of the southern polar cap. Temperatures in the summer can reach 22 Celsius at southern midlatitudes, and a sweltering 37 Celsius at the subsolar point – but this is very exceptional. Mars is usually very, very cold. And when furthest from the Sun and moving at its slowest, the planet plunges into a deep and extended freeze, _125 Celsius at the south pole. Carbon dioxide – which is the principal component of the Martian atmosphere – then condenses out in the sky and falls to the ground as snow, and the south polar cap gradually creeps across the surface of the planet to reclaim the ground coverage lost in the summer. The water in the polar caps, however, remains frozen at all times; Mars never gets warm enough at the poles to melt water. And even if the water did melt, it would evaporate straight away because Mars’ atmosphere is exceedingly thin. It is 100 times thinner than our atmosphere, the pressure at its surface equivalent to that at an altitude five times higher than Everest on Earth. Without a spacesuit, you’d last less than a minute in the cold, dry semi-vacuum that hugs the hostile Martian surface.

Evolution of Mars

Mars’ atmosphere was not always this thin, though. There is lots of evidence that liquid water once flowed on Mars, cutting canyons and riverbeds as it does on Earth today. The Red Planet’s atmosphere must once have been much denser. The reasons for its unfortunate transformation are many. But two of the main culprits are the planet’s diminutive size and its lack of magnetism.

Mars is very small compared with its neighbours Earth and Venus, which are 9.3 and 7.6 times more massive respectively than the Red Planet. Why it ended up with so little mass may have something to do with its position in the Solar System. Mars is found near the inside edge of the asteroid belt. The asteroids are leftovers from the planet-building process. As we shall see, they were unable to form a planet because of the gravitational field of nearby Jupiter, the next planet from the Sun after Mars. It is possible that Jupiter’s disruptive influence was felt even where Mars was forming. The giant’s gravity ejected many planetesimals out of the plane of the Solar System, leaving Mars to mop up the scraps. Still, Mars was massive enough to hold onto the atmosphere that it gradually outgassed as it cooled – but only just. Because the Red Planet is so small, much of its original atmospheric gas has slowly escaped. Impacts would no doubt have heated and stirred up the atmosphere during the heavy bombardment phase. Hot gases have faster-moving molecules than cooler gases, so as the atmosphere warmed up it gradually slipped from Mars’ weak gravity and leaked away into space. Moreover, because Mars lacks a magnetic field – and may have done in the past – it has no protection from the steady flow of particles from the Sun, the solar wind. Earth’s magnetic field deflects the solar wind. But on Mars the wind brushes up to the planet and gradually strips it of gas – up to 45 000 tonnes are lost every year like this.

Gradually, as Mars’ atmosphere grew thinner, it also grew colder because of the reduced greenhouse effect. Its atmospheric water was destroyed by the Sun’s ultraviolet light. Its surface water either froze solid at the poles or, when the pressure got too low, evaporated and joined the atmosphere where it too was destroyed by ultraviolet light. The rest of Mars’ water might have seeped into the soil where it still exists in a layer of permafrost. Liquid water may not have flowed on Mars now for 2500–3500 million years. (New research, however, shows that liquid water could exist in small quantities on Mars in very low-altitude regions.) Its volcanoes have also stopped erupting – or so we think. Today, Mars is cold and hostile, probably inactive, and it has not changed in billions of years. If life does exist there, it is certainly not complex. Perhaps it never was.

The Dogs of Mars: Phobos and Deimos

Before we leave Mars for the rubble of the asteroid belt, let’s meet a couple of the belt’s former members: the Martian satellites Phobos and Deimos. Mars’ two moons are nothing like ours. They are little more than pebbles in comparison. The larger, Phobos, is only 27 kilometres along its longest axis, about half the size of London. It is irregularly shaped, not spherical, because its gravity is too weak to pull it into a ball. Phobos is the satellite closer to Mars. It orbits 6000 kilometres above the red sands where it swings around the planet in 7.6 hours – less than a Martian day in fact. The other, Deimos, is smaller still, about half the size of its cousin, similarly shaped, and more than 2.5 times further out.  Both satellites have exceedingly dark surfaces that reflect just 2 per cent of incident sunlight. They are
also heavily cratered.
Phobos and Deimos are not Martian natives but are most likely asteroids. They broke free of their orbits in the belt – assisted by Jupiter’s gravity – and headed sunwards where Mars captured them at different times. As to when this happened, though, nobody can tell. The events may date to the beginning of the Solar System, or could have happened much more recently.

Source :
Mark A. Garlick. The Story Of The Solar System. University Press: Cambridge. 2002.



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