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.
Life could also exist in subsurface environments other than ice caves.
Microbial life today has been found at depths of 3.3 miles (5.3 km)
below the Earth's surface. The increase of temperature with depth is around 86 degrees Fahrenheit (48 degrees Celsius) per mile (1.6 km);
however, the exact increase depends on the type of rock. Such a subsurface refuge could be one of the last to contain life on Earth.
At the other end of the scale, temperatures will decrease by around 18.9 degrees Fahrenheit (10.5 degrees Celsius) per mile 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," said 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."
Isolated pools from the remnants of the ocean will have high salt concentrations, meaning that bacterial life would have to withstand high saline as well as high temperatures.
Such microbes are called thermohalophiles, and they exist today in such conditions around hydrothermal vents. Microbes on the far-future Earth would also have to contend with being bombarded with high doses of ultraviolet radiation, as the ozone layer would have been stripped away when the oxygen in the atmosphere diminished. (Wipe Out: History's Most Mysterious Extinctions)
Biosignatures of a dying planet
Studying what life will be like on Earth at the end of the habitable era helps scientists 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 Chile's Atacama Desert, use carbon monoxide to obtain energy, and 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 by 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. (7 Potentially Habitable Alien Planets)
Microbes can adapt to extreme conditions, such as the harsh conditions that existed on the early Earth. The first life to appear on Earth, as far back as 3.8 billion years ago, was unicellular life. Similarly, microbes will be the sole occupants of the Earth during its final days as a habitable planet.
Microbial biospheres would exhibit biosignatures that are very dissimilar to what is present on the current Earth, but whether late-type biospheres would appear similar to early-type biospheres is another question.
"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," said 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."
ANALYSIS: Extreme Life: Surviving on Fumes : Discovery News
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. The preprint can be found here.
This story was provided by Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program.
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