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Monday 7 October 2013

Breaking out of the Habitable Zone

In the search for extraterrestrial life, water is highly regarded as a key ingredient for life to arise. Its polar structure allows it to form "hydrogen bonds," which are crucial for the formation of large organic carbon structures like DNA. Water is so important that scientists are constantly looking for planets in the "habitable zone" around their stars, a land flowing with milk and honey—er, I mean, liquid water. A habitable zone is determined by the properties of the star—with very bright, hot stars, it would be further out; and with dim, cool stars, liquid water could exist much closer in.

Photograph of Jupiter
The instantly recognisable brown bands on Jupiter are of unknown composition. Image credit: tonynetone (Flickr)
However, liquid water can exist in some out-of-the-way places. Several of Jupiter's moons experience such strong gravitational interactions with Jupiter and each other that they are heated up by tidal flexing, making them warm enough to sustain water in its liquid form, despite being five times further away from Sun than the Earth is!

One strong candidate for a watery moon is Europa. It has a frozen ice outer shell, and it's speculated that underneath might be that liquid gold - water. Scientists are still unsure just how much internal heating Europa receives, so it's still not fully known how thick the ice is. Tidal flexing keeps it dynamic and changing, and scientists have observed that the surface is lined with cracks, possibly formed from the ice chaotically pulling apart and smushing back together like giant icebergs in a mosh-pit. This is some of the evidence suggesting there might be a liquid Europan ocean underneath its icy crust.

Giant icebergs in a mosh-pit

We know that disequilibrium and dynamic change are characteristic of global cycles, which could potentially sustain an interacting biosphere. Planets and moons with lots going on are instinctively considered more "alive" than those that are unchanging rocks. So with the active energy source supplied by tidal forcing, and the possibility of a watery abundance, can we expect to find mermaids swimming beneath Europa's ice shell?

Europa: a cracked ball of ice - but is there liquid water underneath? Image taken by the NASA Galileo spacecraft in 1996

Once again, one has to think outside of the box when considering possibilities for Europan life. What might be lethal to some forms of life on Earth might be vital to something that might live on Europa. For example, Jupiter has a strong magnetosphere, which blasts harsh radiation towards its moons. Humans would never be able to survive there. But perhaps some forms of life could have adapted this radiation to their advantage. It's speculated that the radiation splits the water molecules on Europa's surface, allowing the hydrogen to float off into space, and the oxygen to sink into the ocean, mixing down to feed underwater life protected from the external radiation.

Life continues to surprise us, even on Earth. Scientists have discovered an abundance of organisms living at the bottom of our oceans near boiling hot hydrothermal vents, and it is possible that the internal tectonics on Europa cause hydrothermal vents to exist there, too. Maybe we can find analogous life thriving there as well.

But, maybe not. Our definition of life is really quite nebulous, and we only have one example of a planet to base it off of. Perhaps alien life will be so strange that we don't recognize it, or perhaps it is simply not there. But we'll never know if we don't look.

We'll never know if we don't look.

Artist's impression of the JUICE spacecraft near Jupiter.
Image credit: ESA/AOES
The next mission to Europa is called JUICE: JUpiter ICy moon Explorer. It's run by the European Space Agency, and is scheduled to conduct flybys of Europa in 2030. Some groups have also pushed to send a lander to sample surface geology and perhaps melt down through the ice to explore the ocean beneath, but nothing concrete is planned yet.

Though all we can do for the moment is speculate about what might exist there, if life can thrive inside a planet, then we have many more options of where to look. Europa is definitely expanding our ideas of where a habitable zone can exist.

This article is a part of our World Space Week series 2013 series of planetary science articles, and was written by Lori Dajose - a Planetary Science and Philosophy student at the California Institute of Technology (CalTech). Lori would love to find alien life one day, and is an avid fan of coffee.

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