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Cicerone 2-2004

Our changing solar system

For billions of years, our solar system has been undergoing dramatic changes. The planets that once had water in abundance are now either frozen wastelands or blazing furnaces.

By Knut Jørgen Røed Ødegaard

Even our own planet has changed, but surprisingly little. Over the course of some hundred million years, however, Earth will also undergo some rapid changes. By studying Venus, we can see what happens when the greenhouse effect spirals out of control: The planet was most likely once a tropical paradise before increasing solar heat and the greenhouse effect caused all the water to evaporate and created the worst conditions on any planet in our solar system.

Our solar system, including the sun and Earth, came into existence about 4.6 billion years ago. The young sun shone as much as 40 percent weaker than today’s middle-aged sun does. When we talk about global warming today, we are talking about how only a few tenths of a percent change in the solar radiation can change the climate on the Earth dramatically. This makes it interesting to consider how the Earth and its life forms have managed to survive the truly incredible changes that have taken place over the eons. During this time, incredibly dramatic changes have taken place on Venus and Mars. The innermost planet, Mercury, has always been a lifeless, scorching, stone desert. Let’s then take a look at how the planets Venus, Earth, and Mars have evolved.

Venus

Today Venus has an abysmal climate. The temperature on the surface is over 460 degrees, the pressure is over 90 times greater than it is here on the Earth’s surface, and the atmosphere is highly corrosive. High altitude regions are covered in a frost of condensed lead!

In terms of size, however, Venus is our sister planet. It is almost just as big as the Earth, but is an average of 41 million kilometers closer to the sun. Recent scientific findings indicate that Venus used to have a completely different climate than it has today.

Venus appears to have been covered by large oceans for at least 600 million years. Recent findings indicate that the planet may have had oceans for several billions of years. The water was created by water-rich asteroids and comets that bombarded the surface and gases that sprayed out of the many volcanoes. Venus can thus have been a virtual tropical paradise – and the conditions for primitive life were certainly met. If life did develop, it had several billion years to do so and could have progressed beyond the most primitive organisms.

As the sun’s radiation increased, however, the climate began to deteriorate. Increased temperatures trapped more moisture in the atmosphere, and water vapor is a well-known and powerful greenhouse gas. The water vapor retained more and more of the sun’s heat, increasing the temperature further. This in turn caused increased water evaporation, and the planet entered a vicious circle.

Soon the oceans began to boil, and the high temperatures caused large amounts of other greenhouse gases to leak out of the bedrock. Violent volcanic eruptions most certainly added to the mix. The disappearance of the water meant that a powerful mechanism for removing CO2 from the atmosphere also disappeared.

The solar radiation split the water molecules. The very lightweight hydrogen atoms were released to outer space, and without them water could not be formed again. Today the atmosphere consists of 96.5 percent CO2. The sun’s radiation will continue to increase slowly, and Venus will never again return to being a “blue” planet with liquid water.

There is a slight possibility that some remnants of life may have survived and now live high up in the atmosphere where the conditions are somewhat similar to those on Earth.

The Earth

Our own planet has been partly covered by oceans for billions of years. Up until about 650 million years ago, there were several episodes of intense ice coverage. In some cases, only small ocean areas were ice-free. If these had also been frozen over, it is possible that our planet’s climate would always have been completely different than the life-giving climate we are used to.

During the last 650 million years, however, there were extended periods when the temperatures on Earth were significantly higher than they are today. The placement of the continents and astronomical conditions has meant that at certain times the Earth has been virtually free of ice – even in the polar regions. During the last few million years there have been periodic freezes. These ice ages are caused by a combination of continent placement, which hinders the free flow of warm water over the poles, and astronomical conditions such as the shape of the Earth’s orbit, the time of the year when Earth is closest to the sun, and the angle of the Earth’s axis.

Greenhouse gases in the atmosphere, such as CO2 and water (H2O) cause a rather dramatic increase in the mean temperature of the Earth. Without the natural greenhouse effect, the mean temperature of the Earth’s surface would have been 19 degrees below zero (Celsius). Now the average is 14 degrees above zero. The natural greenhouse effect thus warms the Earth by about an average of 33 degrees.

Warm future

The sun’s radiation increases very slowly but surely as more and more of the hydrogen fuel in the sun’s core is converted to helium. Thus in the very long term, the temperature on Earth will increase. Higher temperatures mean increased evaporation from the world’s oceans. Water vapor is a strong greenhouse gas that very effectively retains the warmth that comes from the sun. We see this clearly in the differences in night temperatures between when the sky is cloudy and when it is clear.

Increased amounts of water vapor and an intensified greenhouse effect cause the temperatures to increase even more. The result is even greater evaporation, and in about 500 million years the process will spin out of control, as it did a long time ago on Venus. Then all life will be extinguished, the atmosphere will become hostile, and all liquid water will disappear.

The sun’s radiation will just keep increasing at a continually greater rate. In about five billion years, when the sun’s core is completely transformed to helium, the heat from this new source of energy will cause the outer parts of the sun to swell as it transforms into a red giant. The innermost planets, Mercury and Venus, will be devoured. Earth, too, may also be engulfed, but we do not know for sure because our planet will lie just at the limit, and several different competing physical processes will determine whether we are spared or destroyed. In the best case scenario, Earth will become a lump of flowing lava without an atmosphere or water!

Mars

The red planet Mars is located even farther away from the sun than Earth. The solar radiation is thus weaker, and the temperatures considerably cooler. The average temperature is 53 degrees below zero, while the day temperature at the equator can reach 25 degrees above. Today the atmosphere on Mars is far too thin for liquid water to exist on the surface. Any water that was created would immediately evaporate or freeze. The thin atmosphere can only to a limited degree retain heat, and night temperatures below minus 100 are not unusual.

But there are clear signs that Mars was once a “blue” planet with large amounts of liquid water. Pictures from the surface show dry riverbeds, traces of large, shallow lakes, and signs of erosion in many areas. Many scientists believe that there could have been enough water on Mars to cover the entire planet by an average of 100–1000 meters! Some scientists claim it is possible to explain the signs of water in other ways, but they are in the clear minority.

Periodically habitable

Recent findings show that Mars must have been completely different before. The atmosphere must have been denser, and the temperatures higher. A denser atmosphere with a lot of water vapor caused a greenhouse effect that kept the temperatures much higher than they are today.

There are two possibilities: Either Mars was much warmer and habitable for a long period up to some time many million years ago, or Mars has only been habitable for shorter periods intermittently.

In the first case, many different processes – such as greenhouse gases from larger volcanic eruptions – could have kept Mars warm for long periods of time. In the second case, it is possible that large asteroids could have collided with the surface causing warm periods that lasted up to several million years.

Brighter future

The future of Mars, however, has a brighter outlook. The ever-increasing solar radiation will increase the temperature, and gases will be released and create a denser atmosphere. Eventually Mars is likely to become a wet and blue planet. At that time, Earth will have become uninhabitable, and our descendants will have perhaps escaped to Mars.

Perhaps humans will make Mars habitable sooner, as early as a few centuries from now. There are future visions of extracting gases from the bedrock on Mars and thus making the atmosphere denser. Greenhouse gases will increase the temperatures – and in this case most people would agree that greenhouse emissions are a good thing! The transformation will take several hundred years.

When the Sun, in about five billion years, becomes a red giant, it will be too hot – even on Mars.

Last updated: 27.05.2004

Our sun as a red giant rises over a decimated landscape five billion years from now. (Illustration: Einar Bordewich)Our sun as a red giant rises over a decimated landscape five billion years from now. (Illustration: Einar Bordewich)
The planet Venus is swathed in enormous, dense clouds. Special instruments reveal cloud formations that otherwise appear to be a shapeless cloud cover. (photo: NASA/Pioneer Venus)The planet Venus is swathed in enormous, dense clouds. Special instruments reveal cloud formations that otherwise appear to be a shapeless cloud cover. (photo: NASA/Pioneer Venus)
Our own planet is currently an almost perfect place for life: The temperature is just right, and there is an oxygen-rich atmosphere with much water vapor. Large parts of the surface are covered with liquid water. (photo: NASA)Our own planet is currently an almost perfect place for life: The temperature is just right, and there is an oxygen-rich atmosphere with much water vapor. Large parts of the surface are covered with liquid water. (photo: NASA)
In the blue areas, there are large concentrations of ice underground. The blue areas near the equator are caused by water that is chemically bound to the bedrock. (photo: NASA)In the blue areas, there are large concentrations of ice underground. The blue areas near the equator are caused by water that is chemically bound to the bedrock. (photo: NASA)
The red planet Mars photographed in August 2003 was record close to Earth. (photo: STScI/NASA)The red planet Mars photographed in August 2003 was record close to Earth. (photo: STScI/NASA)
Knut Jørgen Røed Ødegaard,

is a research fellow at the Department of Astrophysics, University of Oslo. (k.j.r.odegaard@astro.uio.no)

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