Why we look for water

In 2005, the Huygens lander separated from the Cassini spacecraft and soft-landed on the surface of Titan, Saturn’s largest moon.

Titan is unique in the Solar System as being the only moon with a thick atmosphere of nitrogen, methane and other hydrocarbons.

The video transmitted back by Huygens as it descended through Titan's atmosphere is amazing. Just search for the Huygens Titan Landing video. There are several versions available on the web.

First, we see just a thick brown hydrocarbon fog. Then, as we emerge from the bottom of the cloud layer, we see rocks and drainage channels. Finally, the lander settles on what is obviously a dry stream bed, surrounded by rounded rocks and pebbles.

We have lots of seasonal streams on Earth, which are dry for part of the year. However, since the average temperature on Titan is about -180 Celsius, the liquid that flows in that stream bed cannot be water.

On Titan, water will be permanently frozen. The liquid filling the lakes and streams is a mixture of liquid hydrocarbons, which evaporate into the atmosphere, fall as rain and then run back to the lakes and seas through streams and rivers.

This sounds a bit like the water cycle on Earth, where water evaporates from the sea, falls on the land as rain or snow, and then flows back to the sea again.

Without this cycle, the land would be lifeless and the sea a lot less fertile. With liquid hydrocarbons taking the place of water, could we expect to find exotic living things on Titan?

It is certainly possible, but Titan is nowhere near the top of the list of places in the Solar System we think we might find living things. The reason is, we are mainly interested in the presence of water. It has unique properties that hydrocarbons do not.

When we remove grease, paint or varnish with a hydrocarbon solvent, we end up with a mixture of the stuff we removed and the solvent, nothing much more than that.

 Of course, water is useless for dissolving paint and grease, but when we put, say, some Epsom salt (magnesium sulphate) in it, something really interesting happens. The result is not just a mixture of magnesium sulphate and water; the Epsom salt breaks up into magnesium ions and sulphate ions.

Now, if we add another chemical, say washing soda (sodium carbonate), it too breaks up into ions, so now we have a mixture of magnesium, sodium, sulphate and carbonate ions moving around in the water, forming different combinations.

One combination, magnesium carbonate, is insoluble, and falls to the bottom as a white powder.

The other combination, sodium sulphate, remains dissolved. Life here on Earth is driven by chemistry, and water provides an excellent environment for that to happen.

That is why we are looking hard for liquid water in the Solar System and on planets orbiting other stars. Places like Jupiter's moon Europa and Saturn's moon Enceladus. Underneath icy shells are deep, dark oceans of liquid water, heated below due to volcanism driven by tidal forces.

A lot of effort is going into finding a way to get a clean, sterile, probe into those oceans. We know how to land a spacecraft on the surface of those moons, but then there is a layer of ice a few kilometres thick to get through. That’s the hard part.

We think the basic ingredients for life arrive from space when a planet forms. We also know that these ingredients can, with a bit of judiciously applied energy, form amino acids, the building blocks of proteins and life as we know it.

However, the step from amino acids to proteins and then to life is a big one. On the other hand, we know that took place here and probably elsewhere. Getting rid of that “probably” is what we want to do.

  • Venus shines brightly in the west after sunset.
  • Jupiter, Mars and Saturn lie low in the southeast before dawn
  • Mercury lies very low in the dawn glow.
  • The moon will reach first quarter on the 1st. 


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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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