This NASA image of Mars at sunset taken by the Spirit  rover, evokes the conditions on early Mars when the planet received only 70 percent of the of the solar energy that it does now.  (NASA/JPL/Texas A&M/Cornell)

When our sun was young, it was significantly less luminous and sent out significantly less warming energy than it does now.  Scientists estimate that 4 million years ago, when the sun and our solar system were 500 million years old, the energy that the sun produced and dispersed was about 75 percent of what it is today.

The paradox arises because during this time of the faint young sun Earth had liquid water on its surface and — as has been conclusively proven in recent years — so did Mars, which is 61 million miles further into space.  However difficult it is to explain the faint young sun problem as it relates to early Earth, it is far more difficult to explain for far more frigid Mars.

Yet many have tried.  And because the data is both limited and innately puzzling, the subject has been vigorously debated from a variety of different perspectives.  In 2018, the journal Nature Geoscience published an editorial on the state of that dispute titled “Mars at War.”

There are numerous point of (strenuous) disagreement, with the main ones involving whether early Mars was significantly more wet and warm than previously inferred, or whether it was essentially cold and arid with only brief interludes of warming.  The differences in interpretation also require different models for how the warming occurred.

Was there a greenhouse warming  effect produced by heat-retaining molecules in the atmosphere?  Was long-term volcanic activity the cause? Or perhaps meteor strikes?  Or heat from the interior of the planet?

All of these explanations are plausible and all may have played a role.  But that begs the question that has so energized Mars scientists since Mars orbiters and the Curiosity rover conclusively proved that surface water created early rivers and valley networks, lakes and perhaps an ocean.  To solve the “faint young sun” paradox as it played out on Mars,  a climate driver (or drivers) that produces significant amounts of heat is required.

Could the necessary warming be the result of radioactive elements in the Martian crust and mantle that decay and give off impressive amounts of heat when they do?

A team led by Lujendra Ojha, an assistant professor at Rutgers University, proposes in Science Advances that may well be the answer, or at least part of the answer.

A vertically exaggerated and false-color perspective of a large, water-carved channel on Mars called Dao Vallis. Whether channels like these on Mars were carved by surface water or groundwater is highly debated. The channel is ~40 km wide, ~2.5 km deep, and more than 500 km in length.
Credit: ESA/DLR/FU Berlin. 3D rendered and colored by Lujendra Ojha.

Ojha that even if a greenhouse effect is introduced into models of the early Martian atmosphere, it would still be “a difficult struggle” to get temperatures above the freezing point of water.

“The way you can get large amounts of liquid water on early Mars is through basal melting at the bottom of the ice sheets,” he told me.  “Heat from below the sheets would melt the ice and create salty lake water.  And after you have those concentrations of water, you get rivers and river networks under the glaciers carved by subsurface melt water.  That happens all the time on Earth under glaciers in places like Antarctica and Greenland.”

Lujendra Ojha is an assistant professor of planetary sciences at Rutgers University. As an undergraduate, he was the first to identify “recurring slope linae,” thin streams of briney water or grain that slide down sunny hillsides in the equatorial regions of Mars. (Rutgers University.)

In other words, all those fossil rivers seen on Mars did not necessarily form via precipitation and a water cycle, but instead were sometimes the remains of rivers flowing deep under ice.  Over eons the ice then dissipated and left the river channels exposed and dry.

To reach the conclusion that heat from the interior could have played a signiificant role in creating flowing water,  Ojha and his colleagues first modeled the thickness of ice deposits on the Martian southern highlands.  Based on estimates of the amount of water in the atmosphere at the time, and found that the ice sheets averaged about 1.2 miles .

After also estimating the planet’s mean annual surface temperature and surface heat flow (the movement of heat from Mars’ interior to its surface) four billion years ago, the researchers assessed whether the heat flow would have been strong enough to melt the base of the planet’s ice sheets.  They determined that heat flow from the mantle (not just the crust) would have indeed been sufficient to melt the ice and demonstrated that this amount of heat flow would have been present.

The model, Ojha said, is grounded in data showing existing concentrations radioactive thorium, potassium and decay products of uranium on Mars.  He also pointed to the presence of minerals and clays on Mars that can only be formed in presence of hot water, such as that produced by geothermal forces.

The presence of the radioactive material, he said, was confirmed by readings from the gamma-ray spectrometer on the Mars orbiting Odyssey satellite.  Two of the minerals only produced in hydrothermal settings are prehnite and serpentine, and the hydrothermal origins of many Martian clays has been reported using data from the Curiosity rover.

This view that radiogenic heating played a major role in forming subglacial rivers and lakes on early Mars has been discussed for years, and was proposed by Steven Squyres of Cornell University, who was also the principal investigator of the Opportunity and Spirit rovers on Mars.  What this new paper does, Ojha said, is to model the amounts of ice and heat needed to create deep subsurface waterways, and finds that the math works to support the theory.

Many rivers and 400 lakes exist below the ice sheets of Antarctica.  Lake Vostok is the largest lake and is the 16th largest lake by surface area on Earth.  The authors of the Science Advances paper argue that early Mars could have had similar liquid water systems beneath its ice sheets. (NASA/JPL)

A corollary to this view of early Mars is that the most likely place for life to potentially emerge and evolve is beneath the ice sheets, where liquid water and nutrients would be plentiful and microbes would be protected from the harmful radiation that hits the planet’s surface.

A release accompanying the article reports that “the authors also concluded that whatever the Martian climate may have been like at the time, the planet’s deep subsurface region was likely the most habitable. They suggest that life may have lingered deep beneath the Martian surface, sustained by below-ground water, even as the planet lost its magnetic field, its atmosphere thinned, its global temperatures dropped, and Mars’ surface became uninhabitable.”

This conclusion is in line with evolving research that has found microbes and DNA in deep subsurface water and ice, including in the ice abutting the liquid water of Lake Vostok — 13,000 feet below the Antarctic surface.

This composite, false-color image of Mount Sharp inside Gale crater on Mars shows geologists a changing planetary environment, that we now know included megafloods.  Independent of the megafloods, scientists have established that Gale crater was also home to numerous lakes. On Mars, the sky is not blue, but the image was made to resemble Earth so that scientists could distinguish stratification layers. (NASA/JPL-Caltech)

Another recent advance in understand Mars and its ancient water history was a report that Gale Crater — where the Curiosity rover has explored since 2012 – experienced a huge megaflood some four billion years ago. Researcher Alberto G. Fairén of Cornell University and the Centro de Astrobiología in Madrid and colleagues published in November the peer-reviewed findings in Scientific Reports.

These scientists said that a meteorite impact and the heat it produced likely created the megafloods by melting ice on the Martian surface. The finding is a bit of a surprise, since the telltale geologic signs of a megaflood were first seen by Curiosity scientists.

“We identified megafloods for the first time using detailed sedimentological data observed by the rover Curiosity,” Fairén said in a release. “Deposits left behind by megafloods had not been previously identified with orbiter data,”

Scientists found giant wave-shaped features in the sedimentary layers of Gale Crater. The features – called megaripples  or antidunes – are about 30 feet tall and spaced about 450 feet apart. Co-author Ezat Heydari,  a professor of physics at Jackson State University in Mississippi, said they are evidence of megafloods on the bottom of the crater and that their appearance is strikingly similar to features created on Earth by giant ice melts about 2 million years ago.

The meteorite impact not only melted the ice but also released carbon dioxide and methane, which had been frozen and trapped below the surface, the authors explained in the release. This release created warmer and wetter conditions for a short period of time. The resulting condensation in the atmosphere formed water vapor clouds similar to those on Earth and from those clouds came torrential rains over much of the planet.

In the case of Gale Crater, the rain water combined with other water washing down from Mount Sharp, the large mountain in the middle of the crater. All that water produced huge flash floods which formed the ripple features that can still be seen today.

So the story of water on Mars continues and gets ever more interesting.

Last year, for instance, European researchers reported that they had discovered the first evidence of a huge groundwater system that once existed below the surface of Mars. Another study in Nature Geoscience from 2019 reported that at higher elevations, Gale Crater’s Mount Sharp once had a number of briny ponds.  This was surprising because further down Mount Sharp were signs of the long-ago presence of large freshwater lakes.