These three planets and Mercury are collectively known as the terrestrial (rocky) planets. Mercury is lost in the Sun's glare much of the time and so is difficult to observe from the Earth. Only one space mission has visited Mercury, mapping less than half of its surface. Mercury seems to be very much like our Moon, heavily cratered and with no atmosphere. Because of this and its close proximity to the Sun and high surface temperatures, it was not considered for a visit in "Habitable Zone."
So, why isn't Venus a great place for humans?
At first glance Venus may seem ideal. But its thick atmosphere makes it deadly in several ways. The atmosphere of Venus contains chemicals that combine to spell disaster: CO2SO4 (sulfuric acid), commonly used on Earth as battery acid. The CO2 in the thick atmosphere has caused (and may have been created by) a runaway greenhouse effect, making the temperature at the surface of Venus a lead-melting 750 Kelvin (485º C or 890º F). The pressure caused by the thick atmosphere is 90 times that of the Earth's pressure at sea level. You would have to go down one kilometer in the ocean to experience what that pressure feels like!
Venus was the first planet to be visited by spacecraft from Earth. The Soviet Venera series of spacecraft landed on the surface in the 1970s and operated for up to 2 hours - enough time to do brief experiments on the composition of the soil and atmosphere and relay a few photos back to the Earth. Then, either the high atmospheric pressure crushed the landers, or their delicate electronics melted.
Since we couldn't view the surface of Venus from orbit or from Earth, and the lifetime of landing craft was so limited by the harsh conditions, we had to resort to another method to map the surface of Venus. In the 1990s, the Magellan spacecraft created detailed radar maps of Venus, mapping 98% of its surface at a resolution better than 100 meters. When the mapping mission was complete, the spacecraft was sent into the atmosphere to gather data. From this mapping we discovered that the surface of Venus is not very cratered - this is because meteors are unlikely to get through Venus's thick atmosphere without disintegrating.
|Images of the Venusian surface like that shown above have been constructed in recent years by digitally merging distant photographs from height-sensitive radar. This view shows a 100-km-wide nova (nova are circular hills with star-shaped fractures). This view is a close-up that shows its fractures to be grabens or fault bound depressions. Novae may represent an intermediate stage in coronae formation. Coronae are roughly circular, volcanic features believed to form over hot upwellings of magma within the Venusian mantle.|
Venus is not a very attractive place for humans to live. A century ago people wanted to believe that the planet named (rather ironically) for the Goddess of Love was a wonderful place, like a cloudy, moist rainforest, but we now know better. Venus is not for us.
So, why isn't Mars a great place for humans?
Mars is a much more hospitable place than Venus is, but still has some drawbacks. Its thin atmosphere is very dry and cold. Daytime highs in the summer almost make it up to 0º C. The mean temperature on Mars is -60º C, just a bit warmer than the coldest recorded temperatures on Earth. Mars's atmospheric pressure is less than 1% of that of the Earth's at sea level. There is no ozone layer to protect against ultraviolet radiation. You would need a space suit to survive.
Mars lacks a magnetic field (as does Venus). This may have contributed to the loss of Mars's early atmosphere. The lack of a magnetic field allows "solar wind scavenging," the removal of ions in the upper atmosphere by the solar wind. The observed rates of escape for oxygen ions suggest that the solar wind scavenging process has the potential to remove all of Mars's present inventory of atmospheric oxygen over the next 100 million years. Earth's magnetic field and atmosphere protect it from solar energetic particles released during solar flares and against cosmic radiation. Mars, with its thin atmosphere and lack of magnetic field, has no such protection.
Frequent summer dust storms occur on Mars. A desert dust storm on the Earth may cover several hundred square kilometers, but dust storms on Mars are often global in nature, covering much of the surface of the planet. The surface area of Mars is roughly equal to the land area of the continents of the Earth. Imagine a dust storm covering all the continents at once! Dust storms are global on Mars because it is a desert planet. With no water to damp out the dust, the storm is self-sustaining. The temperature during Martian dust storms typically rises by 20º C. The sunlight-absorbing dust can substantially heat the dry, thin atmosphere - raising winds and, of course, more dust. This doesn't happen on the Earth, because the temperature of Earth's atmosphere is regulated by the latent heat of water vapor.
Earth's day is 24 hours long and it takes 365.24 days to orbit the Sun. The tilt of Earth's axis of rotation induces seasons and there are polar ice caps on both poles. Earth's atmosphere is mainly N2, O2, and H2O. These particular gases make it extremely likely that there is life on the Earth: the amount of free oxygen implies that it is being generated somehow, probably by plants.
The oceans of the Earth act as a heat reservoir, storing and moving heat and gases from the tropics to the polar regions and back again. They tend to distribute the heat around the planet, keeping the equatorial regions from becoming too hot and the polar regions from becoming too cold. The water vapor and CO2 present in Earth's atmosphere does this job, too. The atmosphere also contains sufficient levels of ozone to protect any life-forms from intense solar ultraviolet radiation. Studies of the atmosphere in the past few years have shown that the ozone is being depleted at high altitudes over the polar regions. If this trend continues, the surface of the Earth could receive dangerous levels of UV radiation, causing trouble for any life on the planet. The Earth also has a well-developed magnetic field, which protects it from dangerous solar particle events and cosmic rays.
The extremes of surface temperatures on the Earth range from -89.2º C in the south polar region to 56.7º C in a desert region on one of the larger continents in the northern hemisphere. The global mean temperature of the Earth is 14º C, pleasantly above the freezing point of water.
The Earth is clearly habitable for human life, where Mars and Venus are not.
If the Earth were located in Mars's orbit, it would have 12,000 times more CO2 in its atmosphere. The thick atmosphere of CO2 would keep the Earth warm enough for liquid water and life. Mars is cold and arid not because it is too far distant from the Sun, but because it's too small to retain its atmosphere.
Recently, meteorologists have found that the clouds of "dry ice" - frozen CO2 - that would form in the upper atmosphere of such a distant high-CO2 world would actually have a further warming effect, so that it might be possible for Earth to be comfortable for life even if it were twice as far from the Sun as it is - as far out as the inner fringe of the Asteroid Belt.
Of course, with all that CO2 in the atmosphere, there wouldn't
be very many sunny days!