The Null Hypothesis: When Do We Declare a Barren World?

This is a guest post by Euan Monaghan, a post-doctoral researcher in the Department of Physical Sciences at The Open University, where he studies the habitability of the subsurface of Mars. You can find him on Twitter


Astrobiology is the search for life elsewhere in the universe. When this search is focussed on a specific world, there’s a chance—quite a good chance it would seem—that this search will turn out to be fruitless; that there will be no life to be found except the terrestrial life we bring along with us in the process. But can we ever say for sure?

This piece is focussed on Mars, but the idea applies to all worlds targeted for astrobiological exploration. The particular habitats on Europa, Titan or Kepler-62e might be different to those found on Mars, but the question is the same everywhere: does this world host life?

Scientific progress has made the martians of our imagination progressively smaller and more insignificant. No longer the grand canal builders of old—no longer even considered to be multi-cellular—the optimistic amongst us imagine microbes in briny pockets kilometres beneath a hostile surface; their presence deep underground given away by a subtle disequilibrium in the gases of Mars’ tenuous atmosphere. If the martians are there, they’re in hiding.

As we gain a greater understanding of the geologic and climatic history of Mars, a subterranean biosphere doesn’t seem so unreasonable. While Mars was likely warm and wet long before the Earth was, it is also smaller and so cooled faster. It couldn’t hold onto a thick, warming atmosphere for long and so its surface water was gradually lost, both out into space and down into the planet’s interior, to be fixed within the structure of minerals, frozen as permafrost or trapped in groundwater aquifers beneath layers of ice. And as Mars cooled and the water descended, so did the planet’s habitable zone, until it was hidden from view.

Recurring slope lineae in Coprates Chasma may be due to active seeps of water; a clue to a possible subsurface biosphere? (Credit: NASA/JPL/University of Arizona, HiRise)

The habitability of any extra-terrestrial environment is estimated through the study of life adapted to extreme conditions on the Earth. This ‘envelope of life’, with its upper and lower boundaries of temperature, pressure, salt tolerance and so on, is expanding all the time. The relatively recent discovery of our own deep subsurface biosphere, as well as its remarkable diversity and extent, has broadened our concept of what we consider to be a habitable environment. It is with this ever-more subtle knowledge of our own world that we turn back to the planets in our search for life.

The next logical step in that search, for Mars at least, is a detailed study of its atmosphere. In early 2016 the European Space Agency will launch a mission to do just that: the ExoMars Trace Gas Orbiter (TGO) will perform a more comprehensive inventory of the martian atmosphere and the respective abundances of its gases than ever before. It is hoped that the results of this study will provide an insight into active processes occurring deep underground. But then again there is the very real possibility that the TGO will arrive in orbit and find no signs of life, however tentative. The null hypothesis—Mars is a barren world—would still stand. Should we then give up on our search, or do we commit time and resources to a strategy of ever more sophisticated astrobiological exploration, all the while striving to prevent accidental contamination by terrestrial life?

The inevitable moments when we decide to re-focus our search for life beyond the Earth should not be considered moments of pessimism. The universe has too much potential.