Life in the Solar System

Goals• Summary of the key requirements for life• Life on Mercury, Venus, or the Moon• Life on the outer worlds• Astrobiological space missions

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Three key requirements for life

At last, we are ready to expand our vision of life beyond the Earth. In our studies we have found three key requirements for life

1. Source of molecules (easily accomplished)– The biologically important atoms C, H, O, N are universal;– Amino acids are also available in the environment;– Amino acids are easily manufactured (Miller Experiment).

2. Source of energy (a little more difficult)– Solar radiation (drops with inverse square law);– Chemical reaction rates drop by a factor of 2 for each 10ºC drop;– Hydrothermal vents;– Other exotic energy sources.

3. Liquid water (a key limitation)– Water transports nutrients in, and toxic wastes out;– Water expands when it freezes;– Water is liquid over a very wide range of temperatures.

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Polar and nonpolar moleculesSince liquids like water seem to be so important, let us look more closely at them.

The attractive forces between the molecules in a compound tend to keep that compound in liquid form. Water (H2O) and ethane (C2H6) are both liquid over very large temperature ranges.

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Freezing Boiling ΔT Water (H2O) 0ºC 100ºC 100ºC

Ammonia (NH3) -78ºC -33ºC 45ºC

Methane (CH4) -182ºC -164ºC 18ºC

Ethane (C2H6) -183ºC -89ºC 94ºC

(Temperatures given for 1 atm pressure)

Polar and nonpolar molecules

Symmetric molecules are called nonpolar; asymmetric molecules are polar.

Liquids made of polar molecules can dissolve solids made of polar molecules—water can dissolve salt.

Similarly, nonpolar liquids can dissolve nonpolar solids—gasoline can dissolve wax or pine resin.

But polar and nonpolar compounds do not dissolve each other. This is why oil and water do not mix.

Lipid (fat) molecules in water tend to form membranes.

Since lipids are nonpolar, water does not dissolve such membranes. This means that RNA-enclosing cells can spontaneously form if water is available.

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Let’s look for life

With our set of tools that tell us where life might be, let us quickly review our entire solar system, and see where life may or may not be likely.

The search for places where life can occur is essentially the same as looking for places where water (or some other compound) is liquid.

Mercury – Small, low gravity lost its atmosphere;

– No volcanism no atmosphere enhancement;

– 58.6 day rotation, 87.9 day orbit extreme temperatures(425ºC daytime, -175ºC night);

– Liquid water surely prohibited, but ices probable in deep polar craters?

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Let’s look for life

The Moon– In a part of the solar system called the

“habitable zone”;– Small, low gravity lost its atmosphere;– Impact origin drove away volatiles;– Liquid water surely prohibited;– Ices detected in deep polar craters;– In general, a dry world.

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Let’s look for life

Venus– Nearly identical to Earth in mass, size;– Atmosphere: 90 atm, CO2;– Greenhouse effect: 470ºC;– With no liquid water and little (if any tectonics), no CO2 cycle;– Perhaps habitable in the past when the sun was dimmer?– Perhaps remnant microbes in the atmosphere?

Mars– There is too much to talk about…let’s wait until the next chapter.

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Let’s look for life

Jupiter and Saturn– Organic compounds abound;– There are many sources of energy (electrical/magnetic fields);– No solid surfaces, but potential habitats exist in the atmosphere;– The atmosphere is too cold at high elevations, and too hot at low

elevations, but there are temperate regions between:

Cloud tops Temperate 10% into Jupiter125 K, 1 atm 400 K, 18 atm 2000 K, 500,000 atm

…but…

There is extreme atmospheric turbulence that moves gas vertically.

How could anything survive the ride?

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Let’s look for life

Uranus and Neptune– Colder still than Jupiter and Saturn (~60 K);– More of that turbulence;– No apparent valuable energy source;

So, it seems unlikely that life could occur in the atmospheres of these planets.

…but…

Because of the high internal pressures in these water-rich worlds, their interiors are composed largely of liquid water cores at extreme pressures.

Could anything survive such conditions?

How could we ever know?

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Let’s look for life

Asteroids and comets– Organic compounds are present;– Gravity is low, atmospheric losses are high!– Fairly safe to rule these out.

Large satellites of the outer worlds– There is too much to talk about—let’s

save them for another chapter!

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Astrobiological space missionsFlyby

– The craft flies by the destination;– Relatively easily accomplished;– Voyager, Pioneer 10 & 11.

Orbiter– The craft enters into an orbit around the object;– More difficult to achieve;– Galileo (to Jupiter).

Lander– A (usually) soft landing is intended;– Considerable deceleration must be achieved;– Phoenix, Mars rovers.

Sample & return– Samples are obtained and returned to Earth;– Extremely difficult;– Often financially draining, but politically popular;– e.g. Apollo, Luna

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