The last refuges for life on the far-future Earth
Billions of years from now, life on Earth will be extinguished when the dying Sun scorches the surface of our planet. New research has aimed to determine what the last life forms on Earth will be, and what kind of abodes they will cling to before the Earth becomes sterilized.
A habitable niche in an inhospitable world
As the Earth’s oceans evaporate, the few remaining pools of water could provide a last refuge for some microbes. The present average depth of the oceans is four kilometers, but this extends to eleven kilometers in the Mariana Trench, which is the deepest known ocean trench. Trenches carved in the sea bed could be among the last places to harbor liquid water, with the looming walls offering some source of shade from the unforgiving Sun. However, this potential haven is not quite as inviting as it may first seem. Air moving into the trench will become compressed as it sinks lower, and this pressure will greatly increase the air temperature above the water.
“By the time we get to the point where there's a trench with a small pool of water at the bottom, a large mass of ocean water would have evaporated, so surface temperatures on the planet would be rapidly increasing,” explains Jack O’Malley-James of the University of St. Andrews, and lead author of the work. “Therefore water at the bottom of a trench wouldn't remain cool enough for long enough to make a good refuge for life.”
Another potential haven for the last microbial life on Earth could be in underground caves. Microbes have been found living in caves on the present-day Earth without any need for sunlight. Most caves in the far-future Earth would not be suitable for life, as temperatures increase with depth. However caves that have large chambers below a narrow entrance might be colder, as the dense cold air is sucked in, but lighter warmer air is barricaded out.
Such caves are formed from collapsed lava tubes, and the cold air in the caves will cause in-falling snow to compact into ice during the winter, as well as freeze any incoming water. When the outside temperature climbs again, the cold air is still trapped within the cave, along with the ice. However, the ice will melt eventually as heat is conducted through the walls of the cave, so it must be continually replaced and therefore some source of water would still be necessary on the far-future Earth for such a cave to retain its cool climate.
At the other end of the scale, temperatures will decrease by around 6.5 degrees Celsius per kilometer above the Earth’s surface. This is because the surface of the Earth re-radiates heat that has been received from the Sun, thus heating the lower atmosphere. The lower temperatures at high altitude would encourage microbial life on the far-future Earth to reach for the skies and seek refuge in the last remaining lakes in the mountains in an attempt to escape the heat. However, as tectonic plates cease to crash into each other, there will no longer be a force to drive mountains upwards. Instead, the mountains will succumb to weathering and eventually there will be fewer regions of high altitude on the planet.
The remaining high-altitude regions would likely be comprised of volcanoes, as convection of molten rock in the mantle of the Earth will still occur even after the cessation of plate movement. The lack of plate tectonics will allow these “hot spot” volcanoes to reach heights that are currently impossible to achieve today.
“Sites around active volcanoes on Earth today host life, so living near an active volcano shouldn't be a challenge for extremophilic microorganisms,” says O’Malley-James. “It's likely that volcanic activity would decline as the planet cools, but it may not stop completely during the time period in which planet is still habitable.”
Biosignatures of a dying planet
Studying what life will be like on Earth at the end of the habitable era helps scientists to narrow down what kind of biosignatures might exist on Earth-like exoplanets orbiting aging stars near the end of their main sequence. So what kind of biosignatures would the last life on Earth exhibit?
Thermohalophiles, such as those found at volcanoes in the Atacama desert, use carbon monoxide to obtain energy. The by-products of their metabolic processes include carbon dioxide, hydrogen, and ethanol. Carbon dioxide could be seen as an indicator of life, considering that the carbon dioxide inherent to the planet would have been severely reduced million of years previously. Carbon dioxide itself is not a biosignature and its presence, such as on Mars, does not indicate that life exists on a planet. However, biologically produced carbon dioxide would cause a disequilibrium of the CO2 in the atmosphere that could reveal the presence of microbial life. Similarly, the biological production of hydrogen by the thermohalophiles could create an excess of hydrogen in the atmosphere, which could be used as an indicator of life. However, all of these biosignatures would likely be weak, as biological productivity would be severely diminished in a dying world.
“It looks like they would be similar to the biosignatures for early-type microbial biospheres, but the strength of the various atmospheric signatures would be much lower for the late-type microbial biospheres,” explains O'Malley-James. “So it may be possible to distinguish between early and late microbial biospheres purely by looking at the strength of the various biosignature gases in the atmospheric spectra of Earth-like planets.”
Future work will seek to refine what these biosignatures could be, and ultimately search for the telltale signs of a dying habitable planet among the Earth-like planets that have been discovered so far.
The paper has been published in the International Journal of Astrobiology and the preprint can be found here: http://arxiv.org/abs/1210.5721