As 2014 comes to a close, Astrobiology Magazine is counting down our ‘Top 10’ stories from the past year. Number 7 on the list are new discoveries concerning the scale and persistence of liquid water on the surface of ancient Mars. Scientists have long theorized that liquid water was once stable on Mars, but NASA’s team of robotic Mars explorers has provided stunning evidence for the extent of Mars’ ancient ocean and the length of time in which it covered large areas of the planet.

The first story, New Evidence For Ancient Ocean on Mars, was originally published on February 15, 2014.

The second story, NASA’s Curiosity Rover Finds Clues to How Water Helped Shape Martian Landscape, was originally published on December 9, 2014.

New Evidence For Ancient Ocean on Mars

By: Johnny Bontemps

Did a vast ocean once cover Mars’ northern plains?

The idea has been hotly debated among scientists for the past 20 years, ever since Viking Orbiter images revealed possible ancient shorelines near the pole. Later findings even suggested that the primordial ocean–dubbed Oceanus Borealis–could have covered a third of the planet.

But even if the evidence has mounted steadily, fostering our hopes of finding signs of past life on the Red Planet, the case for an ancient Martian ocean remains unsettled.

Now a new study by Lorena Moscardelli, a geologist at the University of Texas, Austin, puts forward yet another line of evidence.

Today, large fields of boulder-size rocks blanket parts of Mars’ northern plains. By pointing to analogue geological features on our Earth, Moscardelli suggests that the boulders were delivered to their current locations by catastrophic underwater landslides–bolstering evidence for an ancient Martian ocean.

The boulders were spotted by the HiRISE camera on the Mars Reconnaissance Orbiter a while ago. So Moscardelli is not reporting their presence as something new, but rather a new interpretation of the processes behind their origin. The paper was published this month in a journal of the Geological Society of America.

Terrestrial Analogy

In the past, geoscientists thought of ocean sediments as mostly fine-grained, floating in the water column and settling like a slow “rain” on the sea floor, Moscardelli explained. But we now know it’s not the only possible scenario.

“We know that ‘submarine landslides’ can transport big boulders–sometimes as big as a house–for hundreds of kilometers into the deep-water of the Earth oceans,” she said. “Imagine a huge landslide affecting the entire state of Texas, but happening in the ocean.”

In her new study, Moscardelli documents several sites where these events have occurred on Earth, such as the Pennsylvanian Jackfork Group of south-central Arkansas; the outcrops of the Guandacol Formation in the Pangazo Basin, Argentina; or in the Santos Basin, offshore Brazil.

She even shows that these underwater events can affect huge areas, as with a massive landslide that covered thousands of square kilometers in the Barents Sea, north of Russia, about a million years ago.

Some scientists have suggested that the boulders of Mars’s northern plain could be the product of meteorite impacts. But to Moscardelli, that’s not a fitting theory.

“That’s possible for some of the boulders, especially those found close to craters,” she says. “But how do you explain boulder fields that can cover thousands of square kilometers without any impact craters around? The submarine hypothesis provides a feasible alternative.”

The Case for a Martian Ocean

In the 1980s, Viking spacecraft images revealed two possible ancient shorelines near the pole, much like those found in Earth’s coastal regions. But further observations showed the coastlines varied in elevation, undulating like a wave, and thus casting much doubt on the Martian ocean hypothesis. However, later studies eventually showed that the deformation could be simply explained by the movement of Mars’ spin axis.

What’s more, the northern plains of Mars–also called the northern lowlands–lie at a lower elevation than the southern hemisphere, much like ocean basins found on Earth.

In addition to the boulders of the northern plains, Moscardelli had previously documented other geological features which can form underwater on Earth, including teardrop-shaped islands and polygon-shaped areas.

“There are many hypothesis out there and we still need to learn a whole lot before we can be confident about which one is right or wrong,” she said. “I have an informed opinion based on my technical observations, but I am cautious and humble about it because I could be wrong! That said, I think my case is a strong one.”

Some of the evidence for terrestrial analogues came from 3-D seismic surveys, a tool traditionally used by the oil and gas industry. So she hopes her approach will encourage more inter-disciplinary research.

“It is amazing to see how little the planetary science and the marine geoscience communities interacts,” she said. “If anything, I hope my contributions can help improve that kind of cross-pollination and cooperation.”

NASA’s Curiosity Rover Finds Clues to How Water Helped Shape Martian Landscape

Observations by NASA’s Curiosity Rover indicate Mars’ Mount Sharp was built by sediments deposited in a large lake bed over tens of millions of years.

This interpretation of Curiosity’s finds in Gale Crater suggests ancient Mars maintained a climate that could have produced long-lasting lakes at many locations on the Red Planet.

“If our hypothesis for Mount Sharp holds up, it challenges the notion that warm and wet conditions were transient, local, or only underground on Mars,” said Ashwin Vasavada, Curiosity deputy project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “A more radical explanation is that Mars’ ancient, thicker atmosphere raised temperatures above freezing globally, but so far we don’t know how the atmosphere did that.”

Why this layered mountain sits in a crater has been a challenging question for researchers. Mount Sharp stands about 3 miles (5 kilometers) tall, its lower flanks exposing hundreds of rock layers. The rock layers – alternating between lake, river and wind deposits — bear witness to the repeated filling and evaporation of a Martian lake much larger and longer-lasting than any previously examined close-up.

“We are making headway in solving the mystery of Mount Sharp,” said Curiosity Project Scientist John Grotzinger of the California Institute of Technology in Pasadena. “Where there’s now a mountain, there may have once been a series of lakes.”

Curiosity currently is investigating the lowest sedimentary layers of Mount Sharp, a section of rock 500 feet (150 meters) high, dubbed the Murray formation. Rivers carried sand and silt to the lake, depositing the sediments at the mouth of the river to form deltas similar to those found at river mouths on Earth. This cycle occurred over and over again.

“The great thing about a lake that occurs repeatedly, over and over, is that each time it comes back it is another experiment to tell you how the environment works,” Grotzinger said. “As Curiosity climbs higher on Mount Sharp, we will have a series of experiments to show patterns in how the atmosphere and the water and the sediments interact. We may see how the chemistry changed in the lakes over time. This is a hypothesis supported by what we have observed so far, providing a framework for testing in the coming year.”

After the crater filled to a height of at least a few hundred yards, or meters, and the sediments hardened into rock, the accumulated layers of sediment were sculpted over time into a mountainous shape by wind erosion that carved away the material between the crater perimeter and what is now the edge of the mountain.

On the 5-mile (8-kilometer) journey from Curiosity’s 2012 landing site to its current work site at the base of Mount Sharp, the rover uncovered clues about the changing shape of the crater floor during the era of lakes.

“We found sedimentary rocks suggestive of small, ancient deltas stacked on top of one another,” said Curiosity science team member Sanjeev Gupta of Imperial College in London. “Curiosity crossed a boundary from an environment dominated by rivers to an environment dominated by lakes.”

Despite earlier evidence from several Mars missions that pointed to wet environments on ancient Mars, modeling of the ancient climate has yet to identify the conditions that could have produced long periods warm enough for stable water on the surface.

NASA’s Mars Science Laboratory Project uses Curiosity to assess ancient, potentially habitable environments and the significant changes the Martian environment has experienced over millions of years. This project is one element of NASA’s ongoing Mars research and preparation for a human mission to the planet in the 2030s.

“Knowledge we’re gaining about Mars’ environmental evolution by deciphering how Mount Sharp formed will also help guide plans for future missions to seek signs of Martian life,” said Michael Meyer, lead scientist for NASA’s Mars Exploration Program at the agency’s headquarters in Washington.