Greenhouse effect: Other planets
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Earth isn’t the only planet in the Solar System with an atmosphere. Finding out about the composition of atmospheres on the other rocky planets, and how these atmospheres affect temperature, can help scientists to understand the greenhouse effect on Earth.
Mars is the fourth planet from the Sun and is both smaller and colder than Earth. Its atmosphere is 95% carbon dioxide, which is a common greenhouse gas. Despite this, Mars’s greenhouse effect is actually very weak. This is because the Martian atmosphere is 100 times less dense than the atmosphere on Earth – too thin to absorb much of the outgoing heat. Mars is also further from the Sun than Earth, so it receives less solar energy. Combined with the planet’s weak greenhouse effect, this leaves Mars very cold, with a surface temperature of about –60 °C.
Venus is the second planet from the Sun. It has a very thick atmosphere composed of 96% carbon dioxide (CO2) and is shrouded in thick clouds made of sulphuric acid. The combination of a dense atmosphere, high CO2 concentrations and perpetual cloud cover results in an extremely strong greenhouse effect that keeps the planet’s surface temperature at about 460 °C. Scientists think Venus used to be much cooler, but a series of positive feedbacks released huge amounts of CO2, heating Venus up to its present blistering temperature. Venus was vulnerable to this ‘runaway greenhouse effect’ partly because it’s so close to the Sun – fortunately, the same won’t happen on Earth.
Mercury is surrounded by an extremely thin atmosphere which is completely unlike those of Earth, Venus and Mars, the other rocky planets. The gases surrounding Mercury are actually an ‘exosphere’, similar to the outermost layer of our Earth’s atmosphere – so sparse that gas molecules can often escape to space. Being the closest planet to the Sun, Mercury receives lots of solar energy during its daytime. But the lack of a substantial atmosphere means Mercury has no greenhouse effect, and therefore nothing to prevent heat escaping from its surface at night. So while Mercury’s daytime temperatures can reach 400 °C, night-time temperatures can drop to –170 °C.
Titan is the largest moon orbiting Saturn, the sixth planet from the Sun. It’s the only moon known to have a substantial atmosphere, composed of 95% nitrogen, 3% methane and traces of other gases. Titan’s atmosphere absorbs some of the infrared heat energy given off by the surface. This gives Titan a greenhouse effect which keeps it about 21 °C warmer than it would otherwise be – similar to the strength of the greenhouse effect on Earth. But Titan also has a large albedo caused by a layer of haze that blocks out some sunlight, cooling the moon by about 9 °C and so counteracting some of Titan’s greenhouse effect. Overall the atmosphere increases Titan’s surface temperature by about 12 °C.
An atmosphere’s greenhouse effect acts to delay the progress of outgoing heat, keeping the surface of the planet or moon warmer than it would otherwise be. But atmospheres can also contain gases or particles that have a cooling effect – preventing some sunlight from reaching the surface. The amount of sunlight reflected by a planet or moon is known as its albedo. Earth has a moderate albedo, reflecting about 30% of sunlight back to space. Saturn’s moon Titan also has a significant albedo owing to a high-altitude layer of haze. Venus’s perpetual cloud cover gives it a very high albedo, preventing more than 70% of sunlight reaching the planet’s surface.
Some scientists call the differences between Venus, Mars and Earth the ‘Goldilocks principle’ – ‘Venus is too hot, Mars is too cold, Earth is just right’. This is partly because Venus is closer to the Sun, while Mars is further away, and partly because of the differing greenhouse effects of the planets’ atmospheres. The atmospheres of Mars and Venus are about 95% carbon dioxide. But Venus’s atmosphere is nearly 100 times denser than Earth’s, with Mars’s 100 times thinner – giving Venus a much stronger greenhouse effect. Earth’s atmosphere is dense enough to trap heat, but – with just 0.038% carbon dioxide – isn’t dominated by greenhouse gases, leaving our planet at a comfortable temperature for life as we know it.
Sometimes the best place to look for answers is where you’d least expect them. Ingo would agree – he tries to find out how Earth’s atmosphere works by studying other planets. ‘I use spacecraft observations to measure atmospheric gas properties at other planets,’ he says. Ingo also uses supercomputers to model extraterrestrial atmospheres. Understanding the behaviour of gases on other planets helps scientists to understand what goes on in the Earth’s atmosphere. ‘The most exciting discoveries I’ve been involved in relate to the atmosphere of Saturn’s largest moon, Titan, a body resembling Earth in many ways, but yet so different.’