Earth may teem with life, but it might not be the best cradle for it. Ocean dynamics are crucial to life here, and a new study says that a slightly different world would allow ocean life to be even more widespread and healthy. This insight might help us find such worlds and search for signs of life there.
On Earth, life in the ocean faces a tension between the availability of sunlight and nutrients. Most organisms are concentrated fairly near the water’s surface, where exposure to sunlight means that they can harvest the sun’s energy for photosynthesis. But important minerals for life like phosphorus tend to sink to the sea floor. Living things therefore depend on these chemicals being stirred up and buoyed to the surface by a process called upwelling.
Advertisement
“Photosynthetic life must live at the surface where there is light, but gravity is always going to act to accumulate nutrients at the bottom of the ocean,” says Stephanie Olson at the University of Chicago. “If you look at life in Earth’s oceans today, it is overwhelmingly concentrated in areas of upwelling for that reason.”
Upwelling occurs primarily because the wind pushes around the surface water – deeper water then flows upwards to fill the gaps. Olson and her colleagues have simulated a series of worlds that are slightly different to Earth to figure out how various planetary characteristics could affect upwelling and other facets of ocean circulation, and how that might affect life.
The group found that upwelling was maximised, raising up most nutrients, on a planet not quite like our own. “For many of these parameters that you could vary to maximise upwelling and nutrient cycling in an ocean, Earth is not the sweet spot – life on other planets could be even more productive than it is here,” says Jennifer Macalady at Pennsylvania State University. “It would look greener and slimier and more seaweedy.”
To make this lush world, the team found that the most sea-life-friendly planet would be slightly larger than Earth, with continents and a salty ocean like ours to maximise upwelling as wind blows off the coast. It should also be rotating slower than Earth and have a spin that doesn’t quite align with its orbit so it has strong seasons, changing the way the seas circulate throughout the year.
Because the wind is so important to upwelling, an important factor is atmospheric thickness: the thicker the atmosphere, the more air particles the wind carries, allowing it to push the water with more force. An ideal planet for life at the sea’s surface, then, would have a thick atmosphere and high surface pressure.
This would allow photosynthetic life to thrive at sea. And the more photosynthetic sea life there is on a planet, the easier that life will be to detect, Olson says. That is because this sort of life pumps oxygen into the atmosphere. An oxygen-rich atmosphere is the first hint that many astrobiologists would look for to indicate that there might be life on a distant world.
Learn more about Virgin Galactic: Explore the past and future of space travel on a żěè¶ĚĘÓƵ Discovery Tour
Detectable life
This doesn’t account for life at the bottom of a sea or on land, but those are likely to have signatures that are harder to detect from afar, says Olson. “Life as a general phenomenon might not care about upwelling, but remotely detectable life definitely does.”
Most of the properties that Olson and her colleagues simulated will be detectable with new telescopes that are planned for the next decade, says Macalady. Finding planets that have thick atmospheres and slow spins could help us identify the best planets to search for signs of life.
“This kind of thinking might eventually help us distinguish between planets that are merely habitable, which used to be an exciting thing but is pretty everyday now, and those that are habitable and have remotely detectable life,” says Macalady. We will probably still have to keep working on the technology to detect that life, but at least we’ll know where to look.
Reference:
