The evidence of water on Mars continues to grow. And because water is so important for life as we know it, this bodes well for our efforts both to send humans farther from home and to search for signs of extraterrestrial life.

In late 2019, for example, NASA released a "treasure map" of water ice embedded in the Martian surface, illustrating not only the planet's abundance of frozen water, but also how much of it lies just 2.5 centimeters (1 inch) deep at high and mid-latitudes. Published in the journal Geophysical Research Letters, this could be a critical resource in planning future missions to Mars with humans onboard.

Liquid water can't last very long in the thin Martian air, instead quickly evaporating when exposed to the atmosphere, NASA explains. Scientists have found evidence of frozen water deeper underground at the planet's mid-latitudes, but this new image maps out shallower — and thus more accessible — water ice. Rather than trying to move large amounts of water from Earth, any human missions to Mars will likely need to harvest this kind of ice for drinking water and other purposes.

"You wouldn't need a backhoe to dig up this ice. You could use a shovel," says the study's lead author, Sylvain Piqueux of NASA's Jet Propulsion Laboratory, in a statement. "We're continuing to collect data on buried ice on Mars, zeroing in on the best places for astronauts to land."

Those astronauts will want to avoid zones on this map colored black, which represent areas where a landing spacecraft would sink into fine dust. There are lots of places on Mars where scientists would like to visit, NASA points out, but not many would be practical landing sites for astronauts. The northern mid-latitudes include some popular options, thanks to more sunlight, warmer temperatures and lower elevations, which offer more atmosphere to slow down a spacecraft before it lands.

One of the most intriguing targets lies in a region called Arcadia Planitia, according to NASA, and this new map suggests it is a good candidate, with lots of blue and purple that indicates water ice less than 30 centimeters (1 foot) below the surface.

Underground lakes

The landing site for the Mars 2020 rover is expected to be chosen by the end of 2018. (Photo: NASA)

Earlier in 2019, researchers from the European Space Agency (ESA) and the Mars Express project announced they had not only found historical evidence of water flowing in craters around Mars' northern hemisphere, but also that a system of ancient, interconnected lakes lurks underground.

The team studied 24 craters with floors about 4 kilometers (2.5 miles) below Martian "sea level." The floors have features indicating water once flowed through them, including channels on the crater walls, valleys, deltas and ridged terraces, all of which could have only been formed by the presence of water. These findings line up with the previous discovery of an ancient Martian ocean, they added.

"We think that this ocean may have connected to a system of underground lakes that spread across the entire planet," said study co-author Gian Gabriele Ori, director of the Università D’Annunzio’s International Research School of Planetary Sciences, Italy. "These lakes would have existed around 3.5 billion years ago, so may have been contemporaries of a Martian ocean."

"Findings like this are hugely important; they help us to identify the regions of Mars that are the most promising for finding signs of past life," said Dmitri Titov, ESA’s Mars Express project scientist.

One area researchers think might hold evidence of life is the southern ice cap.

Polar ice caps

NASA/ESA Hubble Space Telescope snapped this portrait of Mars, which shows the southern polar ice cap at the bottom. NASA/ESA, J. Bell (Cornell U.

In 2018, the Italian Space Agency announced evidence of liquid water underneath Mars' southern polar ice cap. Using the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument (MARSIS) aboard the ESA's Mars Express spacecraft, radar detected a subglacial lake roughly 20 km (12.5 miles) wide and 1.6 km (1 mile) below the surface.

MARSIS used 29 radar profiles to send radio pulses to measure the planet's surface reflection from May 2012 to December 2015. The pulses detected brightness under the ice caps, and researchers were able to determine the presence of water. They said other theories for the brightness — such as a carbon dioxide ice layer above or below the ice cap, or water ice with a very low temperature — aren't possible because they wouldn't cause a reflection as strong as liquid water would.

Other experts, however, were not immediately able to confirm MARSIS' findings.

"We don't see the same reflector with SHARAD [Shallow Radar sounder onboard the Mars Reconnaissance Orbiter], not even when we recently summed together [thousands] of observations to create CATSCAN-like 3-D views of both polar caps," Nathaniel Putzig, Mars Reconnaissance Orbiter SHARAD deputy team leader and senior scientist at the Planetary Science Institute, told CNN. "We're hoping to carry out that same imaging process with the MARSIS data next. I'm excited to see how the 3-D imaging will clarify the view of this detection and whether we will find similar ones elsewhere beneath the polar caps."

Liquid water or flowing sand?

These dark streaks, called recurring slope lineae (RSL), flow downhill at several locations on the surface of Mars. Initially thought to be signs of liquid water, they may also be caused by 'granular flows' of sand and dust, research suggests. NASA/JPL/University of Arizona

In 2015, NASA announced evidence of liquid, flowing seasonal water on the red planet, although further research later cast doubt on that interpretation, suggesting what seemed like evidence of flowing water might actually be caused by "granular flows" — i.e., sand or dust. NASA acknowledged this in a statement, although it noted the clues behind these dueling conclusions "remain puzzling."

The clues in question are mysterious features known as "recurring slope linea," or RSL. Dark streaks seemingly flow down steep slopes at several locations on the Martian surface, appearing and disappearing over time in a way that hints at seasonal flows of liquid water on the surface. "These are dark streaks that form in late spring, grow in the summer and disappear by fall," Michael Meyer of NASA's Mars Exploration Program said in 2015.

This image shows RSL on the walls of Garni crater, where the dark streaks are up to a few hundred meters in length. NASA/JPL/University of Arizona

The news was based on research published in Nature Geoscience, which showed how scientists were able to study RSL on the planet's surface. These streaks had been previously seen in photos, but because the streaks are only about 5 meters (16 feet) across, researchers couldn't get a good enough look to determine what was causing them. Eventually, however, they found a way to analyze data from the Mars Reconnaissance Orbiter by extracting data from the pictures on a per pixel level. This let scientists study smaller details on the red planet's surface, and those details provided the new information.

Evidence of water would mean many things, Mary Beth Wilhelm of NASA's Ames Research Center said at the time, not the least of which is the possibility of microbial life. Of course, water on Mars could also be a big boost for human exploration of the planet, providing a vital resource for visiting astronauts or for long-term colonists.

In 2017, however, another study in Nature Geoscience concluded these RSL were more likely caused by granular flows of dry material, not liquid water. "We've thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand," said co-author Colin Dundas of the U.S. Geological Survey's Astrogeology Science Center in a statement about the research. "This new understanding of RSL supports other evidence that shows that Mars today is very dry."

That doesn't mean we can't still learn a lot about Mars by studying RSL, though. And even if they are just sand, the red planet remains a tantalizing place to search for signs of water, both past and present, as well as any hidden hints of life.