“Carbon dioxide alone is not enough,” Kite tells Inverse. “And so that's been a problem for years: What's the extra warming agent?”
That “extra warming agent” is a key to understanding the potential for life on Mars. There's not a good reason to think that it should have had liquid water. It only receives 44 percent of the sunlight of Earth. It’s cold and inhospitable today — and should, by all measures, always have been. But once, Mars had flowing rivers and pooling lakes. For a window of time, it had all the right ingredients for life.
Kite, assistant professor of geophysical science at the University of Chicago, and his colleagues dissected the history of early Mars, using data from the Mars Reconnaissance Orbiter to perform some forensics on the Red Planet. As it ends up, their answers were in the clouds, literally. Ancient clouds on Mars trapped in enough heat to keep water on the ground stable, and thus create all the right opportunities for life.
Their study, published Monday in Proceedings of the National Academy of Sciences, reconstructs Mars’ early history, and suggests that its clouds warmed the planet enough for it to hold patches of water on its surface. And with those patches of water came the potential for ancient life.
WHAT’S NEW — Mars has always been a puzzling place. There’s abundant evidence that it once had water, but where it went — and how the planet was warm enough in the first place to have it — have always vexed researchers.
Although, Mars' atmosphere is 96% carbon dioxide, and carbon dioxide is a known climate regulating greenhouse gas (meaning that it absorbs and then emits radiation, trapping heat in a planet’s atmosphere), that alone wasn’t sufficient to explain how it once had water.
Kite turned to the clouds.
Clouds can either cool or warm a planet. In Earth’s atmosphere, low clouds tend to cool the surface while high clouds tend to warm it. On Mars, the same sort of effect is in play, but the planet also has water-ice clouds for most of the year.
These clouds are in the equatorial region between about 6 to 19 miles above the surface, and they absorb infrared light emitted from the ground during daytime. Water ice clouds in the current atmosphere provide a minor warming effect of less than 1 Kelvin per year.
But during its early history, when Mars’ atmosphere was more substantial, water ice clouds could have provided significant greenhouse warming.
Kite’s team created a global climate model of Mars, simulating the greenhouse effect produced by Mars’ clouds which could have warmed the planet to temperatures capable of supporting liquid surface water.
Through their model, the team found that the greenhouse effect would have resulted in warm enough temperatures for areas of surface water features on Mars that are spaced further out from each other rather than entire oceans.
“Our models produced a warm, arid climate, but only if the spatial distribution of surface water is quite patchy,” Kite says.
HERE’S THE BACKGROUND — Mounting evidence suggests Mars was once a wet, warm, and possibly habitable planet. Atmospheric loss caused rapid climate change and water loss that left the planet cold and with a thin atmosphere just 1 percent the pressure of Earth’s
Tanya Harrison, a planetary scientist and director of science strategy for Planet Labs who was not involved in the study, says that scientists look for pieces of data to help them reconstruct a more complete picture of Mars.
“It really all ties back to answering that question about whether or not Mars ever had life, so we really want to understand how habitable Mars was back when we think it was warmer and wetter and more conducive to supporting life, at least as we know it,” Harrison tells Inverse.
The first detailed images of Mars obtained by spacecraft in the 1960s showed a dry desert with no signs of water. But as observations continued on the Red Planet in the ensuing decades, hints of past rivers and lakes were revealed.
“That means Mars at least had to be warm enough compared to today that you could have liquid water running rampant across the surface,” Harrison says. “That doesn't necessarily mean that it was, you know, a tropical oasis or anything like that.”
WHY IT MATTERS — Anywhere there’s water on Earth, some form of life has managed to survive. Given that Mars had large bodies of surface water in the past, it had a lot of the right conditions for life billions of years ago.
But in order to question whether or not Mars ever had life, scientists need to understand the planet’s history and past habitability.
Besides answering the question of habitability, Mars is also the most similar planet to Earth out of the Solar System. Therefore, revisiting Mars’ history allows us to peer back in time into the history of our own planet.
“We think that Mars and Earth were very similar early on in terms of temperature and evolution of the planets,” Harrison says.
WHAT’S NEXT — NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) mission arrived at Mars in 2014, tasked with reconstructing the loss of atmosphere that stripped Mars of most of its surface water. But in order to build a more holistic picture, NASA has turned to rover data like that of Curiosity or Opportunity.
But in recent months, the agency has a new tool in its arsenal. On February 18, NASA’s Perseverance rover landed on Mars to begin its mission of searching for clues of ancient life, improving on the instruments on-board the Curiosity rover.
Perseverance will explore Jezero Crater, a dried-up, ancient lake that may have once housed microbial life billions of years ago.
Exploring Jezero Crater will provide scientists with more clues since that environment used to house water billions of years ago.
“We care about the river delta for at least two reasons,” Kite says. “The first is that they tend to concentrate organic matters that are a good place to look for biomarkers and drill for samples that have a higher probability of evidence of past life. And the second reason is they're just telling us that the climate was a lot warmer and wetter in the past.”
“So that's the basic problem: how do you explain the climate being warmer, wetter?”
That answer may come soon.
Abstract: Despite receiving just 30% of the Earth’s present-day insolation, Mars had water lakes and rivers early in the planet’s history, due to an unknown warming mechanism. A possible explanation for the >102 -y-long lake-forming climates is warming by water ice clouds. However, this suggested cloud greenhouse explanation has proved difficult to replicate and has been argued to require unrealistically optically thick clouds at high altitudes. Here, we use a global climate model (GCM) to show that a cloud greenhouse can warm a Mars-like planet to global average annual-mean temperature (T) ∼265 K, which is warm enough for low-latitude lakes, and stay warm for centuries or longer, but only if the planet has spatially patchy surface water sources. Warm, stable climates involve surface ice (and low clouds) only at locations much colder than the average surface temperature. At locations horizontally distant from these surface cold traps, clouds are found only at high altitudes, which maximizes warming. Radiatively significant clouds persist because ice particles sublimate as they fall, moistening the subcloud layer so that modest updrafts can sustain relatively large amounts of cloud. The resulting climates are arid (area-averaged surface relative humidity ∼25%). In a warm, arid climate, lakes could be fed by groundwater upwelling, or by melting of ice following a cold-to-warm transition. Our results are consistent with the warm and arid climate favored by interpretation of geologic data, and support the cloud greenhouse hypothesis.