The table below gives information concerning the planets. Use the reading passage to help you fill in the table. Some of the boxes have been filled in for you.
Model Atmosphere Shows Signs of Life
Life could exist on planets that are much farther from their stars than the Earth is from the Sun. Planetary scientists at NASA’s Ames Research Center in California have developed models for climates on other planets that make the existence of life in other solar systems much more likely.
The scientists, James Kasting, Brian Toon and James Pollack, have increased by more than 75 million kilometres the distance an Earth-sized planet may be from a star and yet support life. The atmosphere on such a planet could be unlikely to support human life, but it could support some forms of life, according to the researchers. “We’re saying that these planets may be habitable, but not necessarily inhabited,” Kasting, an atmospheric physicist, told New Scientist.
The models suggest that the concentrations of carbon dioxide in the atmosphere would be enough to warm planets and prevent water from freezing as far away as the orbit of Mars from the Sun — an average distance of 228 million kilometres. The average distance of Earth from the Sun is 150 million kilometres.
In the past, scientists doubted that “civilizations" could exist in other solar systems because the chance of an Earth-like planet evolving is slim. Michael Hart of NASA's Goddard Space Flight Center in Maryland has calculated that if the Earth were 5 per cent closer to the Sun, its oceans would evaporate, or if it were 1 per cent farther away, glaciations would take over. Hart’s work suggests that only a narrow range of orbits — within a few million kilometres either side of the Earth’s orbit of the Sun — would provide a habitable environment in another solar system.
But the scientists at Ames have pushed the range of orbits for Earth-like planets by more than 50 per cent. Habitable planets could be found from 0.95 to 1.5 astronomical units, they say. One astronomical unit (AU) is the distance from the Earth to the Sun. 1.5 AU is about the distance from the Sun to Mars.
The work at Ames might also help to explain what planetary scientists call the “Goldilocks paradox’’. Earth and its two neighbouring planets, Mars and Venus, were formed at the same time, about 4.6 billion years ago, from the same ingredients, including water, carbon dioxide and nitrogen. But only Earth developed life. Mars is too cold. Venus is too hot, but Earth is just right, like Goldilock’s porridge.
The scientists say that earlier models, including Hart’s, failed to include the stabilizing effect of the exchange of carbon dioxide between the atmosphere and the silicate rocks in the Earth’s crust. This exchange helps to regulate atmospheric and surface temperatures. Exchange of C02 also regulated temperatures in the early history of Venus and Mars before Venus became too hot and Mars became too cold.
In its first few hundred million years, Venus may have had a thin atmosphere and hot oceans, like the Earth, that were capable of supporting life. The scientists have developed a new model called “wet greenhouse" to explain how Venus evolved into its current hell-like state where clouds of sulphuric add fill the sky and surface temperatures reach 460C. The model challenges the generally accepted “runaway greenhouse" model that Fred Hoyle first suggested in the 1950s.
According to the "runaway greenhouse", Venus has always had a heavy atmosphere which traps the Sun's heat In primordial times, the atmosphere was a mix of water vapour and C02. The water vapour would have risen to the upper atmosphere, well above the level at which water condenses. Ultraviolet radiation from the magnetically active young Sun would have split the water molecules into hydrogen and oxygen and the hydrogen would have escaped into space. The result would be a dry planet, with no oceans and high concentrations of C02 in the atmosphere.
But, say Hasting and his colleagues, there are two problems with this model. Where is the oxygen stored and why is there no surplus water? Escape of hydrogen from the stratosphere would have ceased when the level of water vapour in the lower atmosphere reached about 20 per cent of saturation. The stratosphere would have become dry but water should have remained in the Venusians atmosphere.
By adding a mechanism to recycle C02 on early Venus as happens today on Earth — the two objections can be met. The water carried by the lumps of rock from which Venus formed could well have condensed out to form oceans with at least and tenth of the volume of those currently on Earth. The oceans were hot, about 100°C but the pressure of the atmosphere would have prevented excessive evaporation.
An exchange of C02 occurred for several hundred million years between the atmosphere and carbonate rocks on the surface of the planet. The process, known as "weathering”, reduced the density of the Venusian atmosphere to about that of the Earth. The thinner atmosphere allowed most of the hydrogen to escape. The remainder escaped more slowly over the next few million years, so that Venus now has about 100,000 times less water than Earth. According to Hasting and his colleagues, the early interaction between the atmosphere and the surface rocks may also have locked up most of the planet’s oxygen.
After the water was gone, exchange of C02 would have ceased and the level of C02 in the atmosphere would have risen causing a severe “greenhouse” effect. Clouds of sulphuric acid would also have evolved. On Mars, according to the model, carbonates were recycled for about a billion years but eventually, the exchange broke down because the planet was too small to retain its internal heat The C02 has become trapped in carbonate rocks. The thin atmosphere caused the planet’s surface to freeze.
List the three elements that were formed on the Earth 4.6 billion years ago.
,
What helps regulate atmospheric exchange and surface temperature is the exchange of CO2 between the atmosphere and in the earth's crust.
The effect of “weathering” on Venus is reducing the density of the atmosphere to about that of the earth.
| Distance from the sun | Effects of distance on planets | Periods of the exchange of co2 occurs | Reason of breakdown of co2 exchange | Surface temperature | |
| Earth | constantly | ************ | ********* | ||
| Mars | in its first | too small to retain its | freezing | ||
| Venus | no infomation given | in its first | was almost gone | degree celsius |