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Life in the Solar System: Mercury & Venus

In the previous lecture, the existence of life in extreme terrestrial environments was discussed. This lecture looks at the possibility of life existing (past, present or future) elsewhere in the Solar System, beginning with Mercury and Venus.

  1. Mercury
    1. Background
    2. Environment
    3. Mercury: Prospects for Life
  2. Venus
    1. Background
    2. Environment
    3. Prospects for Life

Mercury

Background

Mercury, the smallest of the terrestrial planets, orbits at an average distance of 0.39 AU from the Sun, with an orbital period of 88 days and a rotation period of 59 days. Its diameter is about one-third of the Earth's, and its mass about 5% of the Earth's. The high density of Mercury comes from the fact that it has a large iron core, which occupies about 75% of the planet's volume. This core is responsible for the generation of Mercury's magnetic field.

Mariner 10 image of Mercury

Mariner 10 image of Mercury

Most of our knowledge regarding Mercury comes from 3 encounters by the Mariner 10 mission in 1974-1975. Images taken by the spacecraft revealed a barren, heavily-cratered surface.

Environment

Mercury is a very hostile environment: temperatures range from 400C during the daytime to -170C at night. It has a very thin atmosphere of oxygen, sodium and helium, but the low gravity means that this atmosphere is continually evaporating and being replenished by incoming material from the solar wind. The lack of a thick atmosphere accounts for the heavily-cratered surface of the planet.

Surprisingly, radar observations of Mercury taken in 1991 showed the presence of highly-reflective regions at the planet's north pole, with the signature of ice at low temperatures. Many scientists now believe that ice can survive in steep-sided craters on Mercury's poles, where it is shielded from direct sunlight and therefore escapes being evaporated. It is thought that the ice may have arrived there from cometary impacts, or from the outgassing of water vapour from the planet's interior.

Radar observations of Mercury

Radar observations of Mercury

It is hoped that NASA's MESSENGER mission will resolve the question of Mercury's polar ice, and also answer other puzzles concerning the planet. The spacecraft is due for launch in spring 2004, and will commence orbit around Mercury in 2009, where it will spend a year collecting data. Amongst other instruments, it will be carrying a gamma-ray and neutron spectrometer (GRNS), which can be used to establish once and for all whether the reflective polar regions are indeed made of ice.

The MESSENGER spacecraft

The MESSENGER spacecraft

Mercury: Prospects for Life

Even though ice may exist on Mercury, the prospects for life are poor. Any ice which reached high enough temperatures to thaw would instantly evaporate and be lost into space. Furthermore, the proximity of Mercury to the Sun means that the levels of UV radiation are probably too high for life to survive there. To date, there is no evidence for life on the planet.

Venus

Background

With the exception of Earth, Venus is the largest of the terrestrial planets, with a diameter of 95% of Earth's. It is similar to Earth in a number ways, with a mass 80% of Earth's, and a slightly lower density. However, its orbit is significantly closer to the Sun, with a mean radius of 0.72 Au. Furthermore, Venus's rotation is very slow, with a period of 243 days, longer than the Venusian year of 225 days!. The slow rotation is probably the reason why Venus does not possess a significant global magnetic field.

Venus

Venus

Environment

Unlike Mercury, Venus has a thick atmosphere, which accounts for the planet's high albedo of 0.57. The first close-up study of Venus was undertaken by the Mariner 2 spacecraft, which revealed high surface temperatures (in excess of 400C), and very little water in the planet's atmosphere.

The Mariner 2 spacecraft

The Mariner 2 spacecraft

More recent studies of Venus have revealed that the atmosphere is primarily comprised of carbon dioxide, with smaller amounts of sulphur dioxide, sulphuric acid and hydrogen sulphide. As discussed in [link:diploma-3|Lecture 3], the presence of these gasses in Venus's atmosphere led to a runaway greenhouse effect, where incoming radiation from the Sun was trapped, causing the continual rise in the planet's temperature.

The source of Venus's atmosphere was outgassing from volcanic activity, which accounts for the high levels of sulphur. The Magellan spacecraft, which orbited Venus in 1990 and measured the plant's terrain, found evidence that this volcanic activity was still occurring in the planet's recent past (10 million years ago), and may be continuing today.

Computer-generated image of Maat Mons, one of Venus's volcanoes

Computer-generated image of Maat Mons, one of Venus's volcanoes

Prospects for Life

Like Mercury, the prospects for life on Venus appear to be poor. There is very little water on the planet, since most of it has been lost through photodissociation (see [link:diploma-3|Lecture 3]). What water does remain has no chance of being liquid on the surface, since the temperatures there are far too high.

However, recent evidence has caused some scientists to rethink their views of life on Venus. The presence of sulphur dioxide and hydrogen sulphide together in the atmosphere is puzzling, since these gasses quickly react with one another and are destroyed. Although volcanic activity can replenish the gasses, the fact that their concentrations are highest far above ground level (around 50km) suggests something else might be going on.

Temperatures within Venus's atmosphere

Temperatures within Venus's atmosphere

Pressures within Venus's atmosphere

Pressures within Venus's atmosphere

Interestingly, at around 50km above the Venusian surface, the temperature ranges between 30C and 80C (see figure above), allowing water to condense into droplets. Furthermore, the pressure is similar to Earth's sea-level pressure. It is conceivable that acidophiles may exist in these droplets, accounting for the unusual concentrations of sulphur dioxide and hydrogen sulphide.

Ultraviolet image of Venus

Ultraviolet image of Venus

Support for the existence of such lifeforms comes from ultraviolet images of Venus, which show dark patches and bands. If living organisms in the Venusian clouds were synthesizing food using a form of anoxygenic photosynthesis, they would absorb UV radiation and lead to the dark patches observed.

To resolve whether there is indeed life on Venus, further missions to the planet are needed. As part of its trip to Mercury, a fly-by of Venus is scheduled for the MESSENGER mission (see above). More significantly, the European Space Agency (ESA) has now approved the Venus Express mission, after some uncertainty.

The Venus Express spacecraft

The Venus Express spacecraft

Venus Express will leave for Venus in December 2005, arriving at the planet the following summer. It will spend 2 Venusian days (around 500 Earth days!) orbiting the planet, performing an analysis of the atmosphere and surface.


Updated 2009-10-13 12:29:49