The debate over whether Mars used to be warmer and wetter or colder and wetter earlier in its history has been a long and contentious one. Now, a new study suggests it may be the latter, that Mars was indeed wetter, as overwhelming evidence has already shown, but that it was still a rather cold and icy climate overall.
There is broad agreement that Mars used to be much wetter than it is now, but as to how warm or cold it was, not so much.
The study, conducted by researcher Robin Wordsworth of the Harvard Paulson School of Engineering and Applied Sciences and his colleagues, used a 3-dimensional atmospheric circulation model to compare a water cycle on Mars under different scenarios 3 to 4 billion years ago (the late Noachian and early Hesperian periods). The first scenario was that of Mars as warm and wet with an average global temperature of 50 degrees Fahrenheit (10 degrees Celsius) and the second as cold and icy with an average global temperature of minus 54 degrees Fahrenheit (48 degrees Celsius).
According to their results, the colder scenario turned out to be more likely, based on what is currently known about history of the Sun and the tilt of Mars’ axis 3 to 4 billion years ago. Currently, Mars only receives 43 percent of the solar energy that Earth does, and during this time on early Mars, it is thought that the Sun was 25 percent dimmer than it is today. The colder model would seem to be a good fit for that, and can also explain various ancient water-related features seen on the surface of the planet.
The results were just accepted in the AGU’s Journal of Geophysical Research – Planets.
In this colder scenario, there would have been a thicker atmosphere of mainly carbon dioxide, but the overall climate would still have been quite cold. The extreme tilt of the Martian axis at the time would have pointed the planet’s poles at the Sun and driven polar ice to the equator, where water drainage and erosion features are still seen today. Also, highland regions at the equator would get colder and northern low-lying regions would get warmer; this is the so-called “icy highlands effect,” which results in the peaks of mountains on Earth being snow-covered.
The climate models had a more difficult time showing early Mars as warm and wet, however, even with the thicker atmosphere, clouds, and dust taken into account. This might not seem too surprising, with Mars being smaller and farther from the Sun than Earth.
Another problem is why there are many water-eroded valleys near the equator, when there shouldn’t have been, under the warm/wet model. One such valley, Margaritifer Sinus, would have received less rain than some other areas due to the Tharsis bulge causing drier air to flow over that region, yet shows evidence of large amounts of water having flowed there.
According to Wordsworth, “Even then, however, the warm/wet early Mars does not explain the patchwork of Martian water erosion features and valley networks observed on the planet today, and why these features tend to be concentrated near the planet’s equator.”
Wordsworth acknowledges that the cold model isn’t perfect. It allows for frozen water to be located near drainage features still seen today, but liquid water would still be necessary to create them. Meteor impacts and volcanic eruptions could be what melted the ice, in this scenario.
“I’m still trying to keep an open mind about this,” he said. “There is lots of work to be done.”
Proving that a cold climate on early Mars led to the features seen on the planet today is a “big question”, said Bethany Ehlmann, a planetary scientist at California Institute of Technology and NASA’s Jet Propulsion Laboratory in Pasadena, Calif. She was not directly involved in the new study.
“We know from rover-and orbiter-based data that there were lakes on ancient Mars,” she said. “Key questions are: how long did they persist? Were they episodic or persistent? And does the feeder valley network demand rain or is snow and ice melt sufficient?”
“The 3-D climate modeling used in the new study begins to address these questions with a new level of sophistication by investigating how specific locations might have accumulated rain or snow,” she added.
Indeed, the Curiosity rover has found strong, in-situ, evidence that a large freshwater lake used to exist inside Gale crater where it is exploring, as well as fast-flowing streams emptying into that lake. What’s not known yet is how long it lasted or how warm or cold it was. The Opportunity and Spirit rovers have also found evidence for ancient playa lakes and groundwater, although they were more salty and acidic than the lake in Gale crater. Spirit also found evidence for ancient hot springs; while Mars may have been cold on the surface, it could still have been quite toasty below ground, providing a potentially habitable environment.
There is also growing evidence for current liquid water brines as well as widespread buried glaciers on Mars. Much of Mars’ past water seems to now be buried as ice, while more was gradually lost to space over time as the atmosphere thinned. As to the amount of that water, other evidence has pointed to the existence of a now long-gone ocean in the northern hemisphere. How that might be reconciled with this new study isn’t clear, although other studies have suggested it might have been a colder, icy ocean rather than a warmer one. It may have been more like polar oceans on Earth, dotted with icebergs.
Whichever scenario is correct, or perhaps something in-between, it appears that the debate about conditions on early Mars will continue for some time to come.