Scientists have found water on Mars.

You have probably heard that one before.

It seems that every so often, the discovery of water on Mars is announced again.

Despite this, flowing, liquid water on Mars has never been found. So what gives?

This discrepancy is caused by two things, said Alfred McEwen, principal investigator for the High Resolution Imaging Science Experiment (HiRISE) onboard NASA’s Mars Reconnaissance Orbiter.

McEwen and his team at the University of Arizona’s Lunar and Planetary Lab have been imaging the surface of Mars in great detail for 11 years.

The first reason is sociopolitical, McEwen said. It reflects NASA’s interest in promoting Mars missions, because frankly, people dig Mars, and the news media like to write about it.

The other reason is scientific. “There’s a lot to the story of water on Mars, in time and space and so forth, so that it’s not just one story. It really is multiple stories,” McEwen said.

“There’s frozen water, there’s atmospheric water and then there’s liquid water,” McEwen said. “Of course, liquid gets people most excited for the possibilities of life.”

But, for life to thrive, you have to have all the right conditions in a given environment, which includes more than just water. “The thing is, on Mars, we find pieces of these types of environments,” said Alexis Rodriguez, senior scientist at the Tucson-based Planetary Science Institute.

Timeline of discoveries

When scientists knew less about the Red Planet, they conjectured more.

Ever since Galileo began looking at astronomical objects through telescopes, people have been able to observe the seasonal growth and decay of the Martian polar caps. Until recently, scientists assumed they were analogous to the ones on Earth.

In 1877, Italian Giovanni Schiaparelli observed lined features on Mars which he called canali, meaning channels. Percival Lowell, observing from Flagstaff in 1894, believed they were man-made (or more accurately “alien-made”) canals and proposed they were the remnants of an ancient civilization.

Those ideas inspired science fiction, such as H.G. Well’s “War of the Worlds,” the first of many sci-fi tales about Martians invading Earth.

It wasn’t until the first flybys of Mars by the Mariner missions in the 1960s that scientists got a good look at the surface and found no evidence of aliens. While there were ancient imprints of dry riverbeds, there were no artificial canals.

The atmosphere was also very thin, suggesting that the seasonal caps are carbon dioxide ice, said Shane Byrne, associate professor at the University of Arizona’s Lunar and Planetary Laboratory.

In the 1970s, scientists discovered the perennial caps are composed of water ice in the north and carbon dioxide in the south and that they both sit atop layered ice deposits containing dust.

It wasn’t until 2008 when the radar instrument Sharad (SHAllow RADar) on the Mars Reconnaissance Orbiter observed the poles that they were confirmed to be mostly water ice. Liquid water on Mars, past or present, was still a possibility.

It has been NASA’s motto to “follow the water” because — if we follow Earth-based logic — where there’s water, there should also be life.

In the mid-’70s, the Viking 1 and 2 orbiters and landers captured images of the surface and ran soil samples for signs of microbial life. The results were inconclusive, according to NASA.

More American robots were sent in starting in the mid-’90s through the early 2000s with missions to search for habitable conditions past or present. This included the Mars Global Surveyor orbiter, the Pathfinder lander and Sojourner rover, the 2001 Mars Odyssey orbiter, the Spirit and Opportunity rovers, the Mars Reconnaissance Orbiter (on which HiRISE is installed), and the Phoenix Mars lander — a mission headed by the University of Arizona.

The latest rover, Curiosity, landed in 2012 and is still exploring. The latest orbiter, MAVEN, continues to study the Martian atmosphere. The next mission, a lander named InSight, is planned for 2018 and will collect seismic data to reveal more about the interior of Mars.

All missions inform scientists about the conditions on Mars in the past or present and whether such conditions were suitable for water and — by extension — life.

A warm, wet past?
Not so fast

Evidence is mounting that water was once abundant on Mars, according to Dan Berman, research scientist at the Planetary Science Institute.

Many features on Mars look as if they were eroded by water — dry tributaries, river valleys, riverbeds, lake sediments, shorelines that appear to have been wracked by tsunamis, and a canyon as long as the United States.

A much thicker Martian atmosphere containing carbon dioxide — a greenhouse gas — could have existed early in its history, adding enough pressure and trapping enough heat for liquid water to flow on the surface. Today, the atmosphere on Mars is one-one hundredth as thin as Earth’s.

There is a hitch in these models of a warm, wet, ancient Mars. Some scientists think that Mars’ gravity was not strong enough to hold a thick-enough atmosphere for water to exist billions of years ago.

Others argue that the newborn sun would have been too dim and too cool to sustain liquid water on the surface.

A possible explanation is that liquid water existed beneath a layer of surface ice, according to Byrne. The problem with this hypothesis is that it’s hard to explain how to renew the sources of flowing water without water returning to the atmosphere.

“There’s definitely been water on Mars,” McEwen said. “There’s no mystery, but there’s lots of debate still about when and where and how.”

“You have to always keep an open mind and remember where ideas came from,” Berman said. “We’re continuously accumulating more data and exploring new ideas.”

The state of the liquid

Scientists know that Mars would have lost a lot of water as the protective atmosphere was slowly stripped away, according to McEwen. What’s left today is the ice on the poles and subsurface ice in the mid-latitudes.

Present-day liquid water remains elusive.

Features called Recurring Slope Lineae are the most promising evidence scientists have for present-day water on Mars.

These dark streaks seem to flow downhill. Repeated observations by HiRISE show them growing in summer and shrinking in winter.

Lineae were discovered in a HiRISE analysis in 2010, but it wasn’t until 2015, when CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) data showed that these dark streaks contained hydrated salts, that scientists were sure that water played some sort of role in their formation.

There are some problems, though.

NASA announced in the 2015 news conference that it had found evidence for flowing water on Mars. McEwen, one of the authors of the paper on which that claim was based, said he was not consulted before NASA’s announcement and wasn’t comfortable with the term “flowing.”

Instead these might simply be streaks of damp dirt. The hydrated salts are not proof that they were solely formed by water, Berman said.

“For the last few years, people have been tying themselves in knots trying to explain how to get liquid water to run down slopes on modern-day Mars,” Byrne said. Some have proposed that RSLs are created by dry processes.

“There are still lots of problems with that alternate idea as well, so I would say that the jury is still out.”

Distant cousins
or a separate creation

Mars is not the only place NASA and planetary scientists think they can find water in our solar system.

Europa and Enceladus are moons of Jupiter and Saturn, respectively, where scientists have discovered evidence for liquid oceans beneath layers of rock-hard ice. Both release their contents through water volcanoes, making them prime targets in the search for life.

Titan, another moon of Saturn with methane oceans on the surface and methane clouds in the atmosphere, is also believed to have subsurface oceans.

Finding life on these moons would be different from finding life on Mars, McEwen explained.

Mars and Earth have been exchanging meteorites for billions of years. Life could have originated on either planet and infected the other. If life were found on Mars, it could provide clues to the origins of life on Earth.

The moons of the outer solar system are too distant for a similar exchange to have likely occurred. If life emerged on Enceladus or Europa, “that’s more likely to be a separate creation, so that’s very interesting for a totally different reason,” McEwen said.

McEwen is deputy principal investigator for the Europa Imaging System — a pair of cameras to be launched in the 2020s onboard NASA’s Europa Clipper to map the surface of the Jovian moon.

Mikayla Mace is a graduate student in the University of Arizona School of Journalism and an apprentice at the Arizona Daily Star.