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The inhospitable neighbours in our Solar System

Posted in Exploration, Science, Space, Technology on Tuesday, 30 August 2011

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This edited article about the Solar System originally appeared in Look and Learn issue number 1049 published on 17 April 1982.

Halley's comet, picture, image, illustration

Halley’s Comet by Harry Green

The Earth’s closest planetary neighbours in the Solar System are Venus, which is a little closer to the Sun than we are, and Mars, a little farther out. Astronomers define the average distance of the Earth from the Sun as one astronomical unit, or 1 AU. So in terms of these units the distance of Venus from the Sun averages out over its orbit as 0.72 AU, and the distance of Mars averages out at 1.52 AU.

Mars is almost exactly twice as far from the Sun as Venus is, and Earth lies between the two, but those differences are sufficient to make Venus and Mars lifeless deserts today, while the Earth has become a haven for life. Had the Sun been just a little hotter or cooler, or had the orbits of the planets been slightly different, conditions on those planets could have been very different indeed.

In terms of size, Venus is the nearest thing to a twin planet of the Earth in the Solar System. It has 95 per cent of the radius of the Earth, 88 per cent of the volume and 82 per cent as much mass. If Venus had formed from the disc of material around the young Sun at the distance of the Earth’s orbit, it would have ended up just like the Earth. Instead, it has a super-dense atmosphere of carbon dioxide gas, and a surface baking at a temperature of over 500 degrees C.

Because Venus is closer to the Sun, there was very little water present in the material it formed from; the water was driven out by the Sun’s heat to the distance of the Earth’s orbit and beyond. So when an atmosphere began to form around Venus by outgassing from volcanoes – in much the same way that the Earth’s atmosphere formed – there was very little water vapour and no chance for oceans to form.

The extra heat which Venus receives by being a little closer to the Sun helped to ensure that any traces of water vapour stayed in the atmosphere as vapour, and did not fall as rain. So all of the carbon dioxide which is released by the volcanoes remained in the atmosphere. None could dissolve in oceans and no carbonates were laid down as sedimentary rocks, because without water there was no sedimentation.

Measurements taken by space probes have revealed the fact that the atmosphere of Venus contains almost exactly as much carbon dioxide as there is locked up in the rocks of the Earth.

The result is an atmosphere so thick that the pressure at the surface of Venus is 90 times the atmospheric pressure at the surface of the Earth. And that thick carbon dioxide atmosphere traps heat by the “greenhouse effect”.

Energy from the Sun can get through the atmosphere and warm the Venusian surface, but this reflects infra-red heat energy back out towards space. Carbon dioxide absorbs infrared energy, so the heat is trapped and the surface of the planet becomes much hotter than it would be if the infra-red energy could escape, rather as a greenhouse retains heat.

Probably because of this strong heating, the winds on Venus blow with super-hurricane force, at 360 kilometres per hour. The clouds in the atmosphere sweep right around the planet in only four days, although the planet itself takes 243 days to turn once on its axis – a “day” on Venus is about eight Earth months long! And those clouds seem to be composed not of water vapour, but of sulphuric acid. Altogether, Venus is a very inhospitable place.

Mars is also inhospitable, but only just outside the range of conditions we find on Earth. With only half the radius of our planet, and one-tenth of its mass, the gravity of Mars was unable to hold on to the atmosphere that volcanic activity wrapped around it in the early days of the Solar System, while any water that was released quickly froze. A year on Mars lasts almost twice as long as our year, but the day is 24 and five-eighths hours, almost the same as ours.

The thin atmosphere provides no greenhouse effect warmth, so even on the Martian equator the temperature at noon never creeps above 10 degrees C, while at the poles in winter temperatures plummet to -120 degrees C, cold enough to freeze carbon dioxide, which falls like snow forming white polar caps like the ice-caps of the Earth.

There is no liquid water on Mars today. But pictures taken by space probes show features which may be dried-up river beds, and plains which seem to have been swept by flood water in the past. This would mean that Mars only just missed being a home for life.

Astronomers explain the geological traces left by running water on Mars by assuming that a great deal of water is frozen beneath the visible Martian surface. If it could be warmed just a little, at least in the equatorial regions, the water would flow to form rivers and oceans. One way to make Mars warmer would be, somehow, to make the atmosphere thicker, so that the greenhouse effect could get to work. One theory is that millions of years ago, something happened to vapourise the carbon dioxide now locked up in the Martian polar caps. That could just have tilted the balance for a short-lived Martian “spring”‘ which thawed out the planet. But as the atmosphere leaked away into space, the planet inevitably froze again before life could appear on it.

Our planet Earth seems to sit in the middle of the life zone around our Sun. Venus is just too close to the Sun, Mars is just too far away and too small. Many astronomers say that this illustrates how fortunate we are, and how rare water planets are.

But look at this another way. If the Sun had been a little bit cooler, Venus could also have been a water planet. If the Sun had been a little warmer, or if Mars had been big enough to hold on to its atmosphere and let the greenhouse effect get to work, Mars could have been a water planet. In both cases conditions could still have been just right for life on Earth! It seems that really we are unlucky in having just one water planet in our Solar System.

Throughout the Galaxy there are stars like the Sun; there must be rocky, terrestrial planets like the Earth, with zones of life-promoting conditions big enough to stretch across the orbits of one or two planets. The perfect conditions for life on Earth, and the near misses of Venus and Mars, indicate that there may well be millions of Earth-like planets.

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