|ESA Science & Technology||30-Jun-2005 04:03:26|
Mars vs Earth
Last Update: 07 Jul 2003Like the other planets in the Solar System, the Earth and Mars are thought to have condensed about 4.6 billion years ago from the solar nebula, a giant cloud of hot gas swirling around the young Sun. The evolution of the two planetary neighbours has been driven ever since by the loss of heat produced by radioactive decay within their interiors. "Mars is an attractive planet because many of the processes that happened on Earth, happened there," says David Stevenson, Professor of planetary sciences at the California Institute of Technology.
The two planets share many similarities - both have hard crusts, dense cores and are made from the same materials, though in different proportions - but they are also very different. Today, Earth is a dynamic place, teeming with life. Mars is relatively static and lifeless - although it's just possible that primitive life exists below the surface.
It is not surprising that the evolution of the two planets has resulted in these very different outcomes. Mars is one and a half times as far away from the Sun as Earth: it's also much smaller. "Size matters," says Stevenson. "Mars is only one tenth the mass of Earth, which means it has lower gravity and lower pressures. It's not surprising that it lost a lot of volatiles - atmospheric gases and water - through impacts and high energy processes early on." Because of its smaller size, Mars has also cooled more rapidly than Earth which accounts for its present relatively static state.
The Earth's surface is still continually changing. Plate tectonics is one of the major forces that sees to that. New crust forms at seafloor spreading vents and old crust is swallowed up into the Earth's interior at subduction zones. Plate tectonics may have played a role in shaping the Martian surface during the planet's first 500 million years, but large parts of the Martian crust have been undisturbed by such major transforming forces. "The lack of plate tectonics on Mars limited the planet's ability to recycle material. Four billion year old rocks are fairly common there," says Stevenson.
Most of the southern hemisphere of Mars consists of such ancient crust. (Only small isolated pockets of 4 billion-year-old rocks still exist on Earth.) Planetologists date the surface of rocky planets from the number, size and degradation of impact craters: the higher the density of craters the older the crust. The southern crust of Mars is scarred by many impacts, indicating that it has not been reformed since the impacts were made. On the edges of this ancient crust are the largest volcanoes in the Solar System. Their great size suggests that they have been allowed to grow for billions of years undisturbed by major crustal recycling. The northern crust of Mars is far less scarred than in the south, suggesting a younger age. The processes that formed this young, lowland region, but left the south undisturbed, remain a major unknown about Mars.
Water has also played a major role in shaping Mars, just as it has on Earth. But whereas two thirds of the Earth's surface is still covered by water, there's no liquid water on the Martian surface today. Features resembling ancient river valleys, lake beds and glacial deposits suggest that water did once flow freely on Mars, but more than 3.8 billion years ago. It disappeared relatively suddenly, but no-one knows where or how. The Earth's magnetic field protects the atmosphere from the eroding effects of the solar wind. Mars has no global magnetic field, so the atmosphere is prey to the continuous stream of charged particles flying out from the Sun. The lack of a magnetic field suggests that the circulation of molten iron in the planet's core is too sluggish to generate a dynamo. But recent observations by NASA's Mars Global Surveyor spacecraft have revealed evidence for a powerful magnetic field in the first few million years of Mars's history. What made it turn off when it did? So three of the major processes that have shaped the Earth's environment - plate tectonics, a global magnetic field and flowing water - may also have shaped that of Mars. But they did so early in the planet's history. For 3.8 billion years or so, they have had little, if any effect. What happened to turn these processes off? And could the same thing happen to the Earth? These are questions for future space missions.
Mars isn't completely static, however. Three other processes show recent or current activity: volcanism, tectonics and erosion of surface features by the atmosphere. There's evidence of lava flows from the major volcanoes as little as a few million years ago. That's so recent in geological time that volcanic activity may not have completely died out. Movement of magma underneath the Martian crust also continues to deform the surface gently as the planet cools. Rocks are susceptible to erosion by winds in the Martian atmosphere, just as they are by the movement of air on Earth. But as the Martian atmosphere is much thinner than the Earth's, the process is much slower. Sand dunes, especially at the poles, however, show this is a significant force. So Martian evolution continues. As life requires an energy source and a dynamic environment in which to flourish, it's just possible that it could find a niche somewhere within Mars.
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