Are we alone?

Could there be life on the other rocky planets?

Venus
Mars

In this part, we look at how the planets and their moons match up to these conditions, which will help us to see where it might be worth looking for life elsewhere in the solar system.

Mercury

(Copyright Calvin J Hamilton)

Mercury is the planet closest to the Sun. If you could visit Mercury, you would see that the Sun looks twice as big there as it does from Earth. It travels round the Sun faster than any other planet; because it so close. It also is the second smallest of the planets - under half the width of the Earth. This means it is too small to have any moons.

On Mercury:

• it is very unlikely there could be liquid water
• there is not enough gravity to keep hold of an atmosphere
• its closeness to the Sun also prevents it holding on to an atmosphere

so the possibility of life existing there is very small.

Images of Mercury’s surface produced with radar show very reflective areas. These shiny circles may be patches of ice in the bottoms of craters at the north and south poles. This may seem very surprising, but despite the intense sunlight falling on Mercury, the raised crater rims keep them in shadow at all times. Without the heat of the Sun on the crater bottoms, they are very much colder than the rest of the surface of Mercury.

The length of one Mercury 'day' - the time it takes to rotate on its own axis - is 59 Earth days. Its 'year' - the time it takes to orbit the Sun - is 88 Earth days. So a Mercury day lasts roughly two-thirds of its year! Because it rotates so slowly relative to its year, temperature variations on Mercury are the most extreme in the solar system - from –200C to 425C. One side of the planet is scorched while the other is frozen!

Mercury has only been visited by one spacecraft, Mariner 10, which flew past three times in 1973/4, and only 45% of the surface has been mapped. It is too close to the sun for the Hubble Space Telescope to look at it.

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Venus

(Copyright Calvin J Hamilton)

So let’s go a little further out from the Sun to our next planet: Venus. Venus is very similar to the Earth in size (it is only very slightly smaller), so it is sometimes called the Earth’s sister planet. It is big enough to have a moon, but does not in fact have one. Indeed, size is about the only thing in which Venus really resembles the Earth.

It does have an atmosphere, like the Earth, but it is nothing like ours - it would be deadly for humans. Venus is covered in thick clouds, which form a much, much thicker atmosphere - it would feel like you were drowning if you breathed it. It is also full of poisonous chemicals and clouds of acids.

Also, despite being further away from the Sun than Mercury, the surface of Venus is actually hotter than that of Mercury - in fact, is the hottest planet in the solar system. At its surface, it is hot enough to melt metals such as lead. This a hot temperature is caused by what we call the ‘greenhouse effect’.

The diagram on the left shows what happens. Heat and light arrive from the Sun, and are then radiated back from the planet's surface. However, this heat cannot escape back into space, because of the thick clouds surrounding Venus. The heat radiation is trapped on the surface.

One Venus 'day' is equal to 243 Earth days, which means it has the slowest rotational period in the solar system. Its 'year' is equal to 225 Earth days, so 1 Venus day is longer than 1 Venus year!

(Copyright NASA public domain)

The Russians had a series of Venera missions, some of which were successful at landing on the surface of the planet, which the image on the right shows. But, remember that this is an incredibly hostile atmosphere.
This is one of the best direct pictures we have of the surface of Venus. It was taken by the Russian lander (Venera 13) that touched down on the Venusian surface in 1982. But, any probe needs to withstand very high temperatures and pressures. This one only survived on the surface for 2 hours, 7 minutes, long enough to obtain 14 images!

Part of the spacecraft is visible at the bottom of the image, including a camera lens cover. Above it, flat rock slabs and soil are visible. It looks just like a hot, stony desert, but lacking both sand and dust. Despite the thick cloud, there is enough light to produce shadows of rocks - in fact, the landscape seen here is as bright as a cloudy day here on Earth.

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Mars

(Credit: MSSS, JPL, NASA
Source http://antwrp.gsfc.nasa.gov/apod/ap030422.html)

What about Mars - the next planet out from the Earth, and often known as the ‘red planet’. Mars is a small, rocky planet - it's about half the size of Earth, although it is bigger than our Moon. Mars has a 'day' that is about 39 minutes longer than Earth's familiar 24 hour day; its 'year' is 686 Earth days.

(Credit/copyright: NASA copyright free policy ,Mars Exploration Rover Mission, JPL, NASA)

Mars has lots of huge geological features - enormous canyons, deserts, heavily cratered plains… and all on scales much larger than any seen on Earth. We know what the surface of Mars is like because several spacecraft have visited it. Rather disappointingly, it just looks like a barren desert with rocks strewn between sand dunes.

Mars is red because it’s rusty! The rocks on the surface contain iron which has been oxidised. When found on Earth, this type of rock is produced by volcanoes. The sky really is pink too, as in the photo on the left. The wind whips up the rusty-coloured dust from the surface to form pink clouds of water ice condensed on reddish dust particles suspended in the atmosphere.

(Copyright Calvin J Hamilton)

Mars is a planet that scientists think is worth investigating for signs of life, as you will know from the news during the last year. It has an atmosphere, although the air is so thin that we could not breathe it; and it would be poisonous to us (carbon dioxide). Its temperature is much colder, but is the nearest we get to the Earth’s in the solar system - the temperature range is -127C to 17C. There is evidence for past volcanic activity, with several volcanoes on the left of the image above, including the one ringed, which is called Olympus Mons (right). This is the biggest mountain in the solar system - it is 550m across, and at 25 km high, is nearly three times as high as Mt Everest (8,850m).

But one of the primary reasons why people are excited about the possibility of life existing on Mars is that there is evidence for water. Mars has ice caps at both poles (see the North pole at the top of the picture of Mars above) which mostly consist of solid carbon dioxide or‘dry ice’. However, they contain water as well. There is also some water vapour in the atmosphere, and you can see hints of some swirling white clouds on the photo.

(Images either NASA public domain or copyright Calvin J Hamilton)

But apart from the ice caps, Mars has surface features, such as dry river beds, that suggest it used to have liquid water - that at one point, water flowed on the surface of Mars. Other features are deep channels presumably cut by flowing liquid water, and large rocks that look as if they have been moved by floodwater. The collage of photos above shows a few such features. Compare these to the channels and patterns you see on a wet muddy beach when the tide goes out. Any water existing now is likely to be under the surface of the planet as ice/frost in the soil, but these features suggest at some time in the past there was very clearly water on the surface.

So the fact that Mars might have had water on it, and that it is the nearest planet to us, making it the easiest to reach - even then it takes at least 6 months for a spacecraft to get there - means that it has been a prime target for exploration. Over the period Dec 2003 - Jan 2004, humans sent five spacecraft to Mars, including the European Space Agency's Mars Express orbiter carrying the Beagle 2 lander, and NASA's two Mars Exploration Rovers. Primary among the scientific goals of such mission is that of searching for and characterising a wide range of rocks and soils that hold clues to past water activity on Mars. The spacecraft are targeted to sites on opposite sides of Mars that appear to have been affected by liquid water in the past.

(Drawing credit: Maas Digital, Mars Exploration Rover Mission Team, JPL, NASA
Source: http://antwrp.gsfc.nasa.gov/apod/ap021126.html)

This is just a painting of a rover, but it shows you what they looks like when they rolled down out of the lander. The rovers performed on-site scientific investigations over the course of their 90-day mission. They took a very close look at both the rocks around the lander and at the soil through a microscope.

(Copyright NASA public domain)

This is it actually in action, photographed from the rover. This rover arm has scientific instruments, including spectrometers that can do a chemical analysis of the composition of the soil and rocks, and a microscope to take images. Previous missions doing similar experiments have shown that the soil is sterile - there has been no organic matter found in it; suggesting any life is going to be hard to find!

Geological signs of Martian water found by the rover include the discovery that the outcrop is rich in sulphate salt deposits arranged in layers, and a mineral called hematite (which is important when combined with sulphates). These are sediments that require liquid water for their formation. The presence of one mineral in particular is typically left behind only when a sea evaporates. This examination of rocks by the landers suggest that they might once have been water-drenched.

(Images copyright Kees Veenenbos
Source: http://home-1.worldonline.nl/~veenen/terragen/mars/mars.html)

So we have evidence that at both landing sites of the Mars rovers, the surface was once drenched with water (left-hand image). There is other evidence from the dry river beds and huge canyons elsewhere that this was true for most of the planet. These oceans must have been deep and long-lasting to have provided all the evidence found. However, the water gradually evaporated or froze leaving high concentrations of salt behind, as in the right-hand picture above.

It is clear that in the past Mars was a lot wetter - and warmer - than it is now. The warmth was probably supplied by the earlier volcanic activity, providing heat from inside the planet. Such volcanic activity may have also have supplied gases for an atmosphere (since evaporated into space). Water does not mean life definitely existed, but it is a key ingredient that makes life possible, and so it looks like Mars had the right conditions to support life sometime in its history. However this does not mean that life actually developed. In any case, we should be clear that what we mean by life is not people or aliens strolling around. The kind of life that might have existed is likely to be microscopic bacteria - bugs that are invisible to the naked eye.

And the question is whether - if life did develop – it could be maintaining a tenuous toehold in the soil of Mars even now… To find out, we would need to collect samples of soil and rock to return to earth for detailed analysis.