Change is the one constant in our world– moving in ways tiny and enormous, constructive and destructive.
We’re living now in a time when a rampaging pandemic circles the globe and when the climate is changing in so many worrisome and potentially devastating ways.
With these ominous changes as a backdrop, it is perhaps useful to spend a moment with change as it happens in a natural world without humans. And just how complete that change can be:
For years now, planetary scientists have debated whether Mars once had a large ocean across its northern hemisphere.
There certainly isn’t one now — the north of Mars is parched, frigid and largely featureless. The hemisphere was largely covered over in a later epoch by a deep bed of lava, hiding signs of its past.
Because our sun sent out significantly less warmth at the time of early Mars (4.2-3.5 billion years ago,) climate modelers have long struggled to come up with an explanation for how the planet — on average, 137 million miles further out than Earth — could have been anything but profoundly colder than today. And if that world was so unrelentingly frigid, how could there be a surface ocean of liquid water?
But discoveries in the 21st century have strongly supported the long-ago presence of water on a Mars in the form of river valleys, lakes and a water cycle to feed them. The work done by the Curiosity rover and Mars-orbiting satellites has made this abundantly clear.
An ocean in the northern lowlands is one proposal made to explain how the water cycle was fed.
And now, In a new paper in Journal of Geophysical Research: Planets, scientists from Japan and the United States have presented modelling and analysis describing how and why Mars had to have a large ocean early in its history to produce the geological landscape that is being found.
Lead author Ramses Ramirez, a planetary scientist with the Earth-Life Science Institute in Tokyo, said it was not possible to determine how long the ocean persisted, but their team concluded that it had to be present in that early period around 4 billion to 3.5 billion years ago. That is roughly when what are now known to be river valleys were cut in the planet’s southern highlands.
Only a northern ocean of some size on early Mars, he said, would provide sufficient water on the planet — via a water cycle — to produce all the fossil rivers, lakes and deltas that have been found. The alternate theory that seasonal melt water created the rivers and lakes simply cannot provide enough H2O.
“We show that without a relatively large northern ocean and corresponding hydrologic cycle, no cold-and-icy early Mars scenario can explain the valley networks and other observed surface erosion,” Ramirez wrote in an email.
“The northern ocean had to come in before the valleys could be incised. Without an ocean, there could be no water source, and no valleys.”
As described by Ramirez, who authored the paper with Robert Craddock of the Smithsonian Air & Space Museum and Tomohiro Usui of the Japanese space agency, the precipitation that originally filled the northern ocean could have come in much the same way that it did for the Earth.
This means that a condensing steam atmosphere early on would have brought in the initial water, and perhaps some more could have been supplied via successive impacts early on.
Subsurface groundwater could also have contributed to the formation of some of the early water features. But that too, Ramirez said, requires an aquifer to be charged by precipitation.
Some scientists argue that the riverbeds and deltas seen now could have been formed when the planet was not too different in terms of climate from today — arid and cold.
As measured by the Viking landers in the 1970s, surface temperatures today range from 1 degree F to -160 F. Mars scientists have since found that can get well above freezing at the equator in summer, but the average surface temp for the planet is -81. So under this scenario, river carving would have been the result of seasonal melts, briny water or localized heat caused by geological events.
The Ramirez et al paper takes the view that the planet once had to be significantly warmer to be wet enough to leave behind the features now being found. And to produce those conditions, something in the atmosphere clearly had to be supplying a kind of greenhouse effect.
So how did the planet warm sufficiently to allow for an ocean of water?
The authors propose that the rise of the Tharsis volcanoes, as well as the enormous Olympus Mons shield volcano, took place at around the time that Mars was getting wetter and warmer — rather than millions of years earlier, as is often described.
The volcanoes would spit out hydrogen and carbon dioxide which, when the molecules collide at high atmospheric pressure, produce a thick greenhouse atmosphere. (There is not nearly enough carbonate now on the surface of Mars to propose an earlier substantially carbon dioxide greenhouse.)
The result of the hydrogen-carbon dioxide greenhouse, they write, was hardly a tropical Mars. Rather, it was a planet that had temperatures slightly above freezing, which was enough to produce an ocean over about 35 percent of the planet. The lakes and riverbeds detected on Mars, he said, are consistent with that kind of semi-arid environment.
But for their model to work, they also needed modifications to the often cited narrative of volcanism and mountain-building on Mars.
The previous models presupposing a cold and icy Mars had also assumed that Mars’ biggest mountains were already fully-formed before this river valley period. If the mountains had already risen, then they would have caught snow and ice at high elevations — produced melt water in warmer seasons – and created an super-arid rain shadow in precisely the areas where the river valleys were cut.
An earlier rise for this Tharsis range is a motivator for many present cold and icy scenarios, Ramirez said.
“The problem is that such scenarios do not agree with the geologic evidence. The most up-to-date geologic mapping shows that Tharsis was much flatter during valley network formation. Indeed the entire complex was forming during and after this time, suggesting that Tharsis volcanism may have been an important component to the warm temperatures required during this period.”
So if the appearance of the Tharsis occurred early in the history of Mars, then a melt water source for the river valleys is all that’s available. But if the mountains came later, there would be no rain shadow over the river valley area and a water cycle could develop.
And when water was flowing, why would it head north?
That the water would have pooled in the northern lowlands makes sense because the region — some 40 percent of the surface of Mars — is as much as 5 kilometers (3.5 miles) below the southern highlands, creating what is called the “dichotomy.”
As with most everything regarding the northern ocean, there is rigorous debate about how the lowlands were formed, with a huge impact in the very early history of Mars being a leading expIanation. But however it was formed, the so called Borealis Basin is the largest flat piece of real estate in our solar system.
How long might a Northern Ocean have lasted?
“We do not give an estimate for how long the ocean could have existed there, but we give an estimate for how long it takes to form the observed surface valleys. Based on the level of observed surface erosion, we estimate that it would have taken no longer than 10 million years, and possibly considerably less than that.”
So the Northern Ocean might also have existed for a relatively short time, geologically speaking. An additional big question is whether an ocean would have been present for a long enough time to help jump start potential life on Mars, but that is a complete unknown.)
The relatively short lifetime of a northern ocean is consistent with evidence that the atmosphere of Mars began to be stripped away rather early, and as a result the ocean would have frozen or disappeared into space.
Those unconvinced by the science of the Northern Ocean point to the absence of thick glaciers left behind after the oceans froze at the end of the warm period. Ramirez counters that the proposed ocean would only cover that 35 percent of the planet, compared with 70 percent of the surface of the Earth covered by water. So the hydrologic cycle would have produced a warm and semi-arid climate, rather than the broad array of climates on Earth, including tropical. There are no thick left-behind glaciers on Mars, and Ramirez says that is consistent with the model they present.
This will certainly not be the final word on the Northern Ocean theory. Unanswered questions remain and the theory involves processes at work and conditions that existed so long ago.
But with the likelihood growing that Mars did once have a substantial ocean, it is quite remarkable — and a testament to the power of change — that it would have disappeared with hardly a trace. Or without a trace identifiable with certainty from those, on average, 137 million miles away.