Water on Mars : A Brief History

RSL animated gif. (Image: NASA/JPL-Caltech/Univ. of Arizona)

Today, our newest MSc at PVL examines a large and well-known problem in planetary science: the water inventory of Mars and how we achieved the state of knowledge we now posses. Grappling with such big picture issues is as important for trainees as is the fine details of their own research.

By Brittney Cooper

The internet was recently abuzz with the latest results from the Mars Advanced Radar for Sub-Surface and Ionosphere Sounding (MARSIS) instrument, as it returned evidence for a sub-surface lake near Mars’ south pole. Of course, that’s not how a lot of us saw it communicated in the news and on social media. It can be incredibly hard to distill intricate and niche scientific findings for the public’s palate, and often you see media outlets striving less to find that balance in favour of simply slapping on a sensationalist title and making sweeping assumptions.

A misleading headline that once again reared its ugly head in many publications was the age-old classic “Water Discovered on Mars!”. The important distinction with this newest discovery is that the water is “liquid”, and while making that distinction may seem like a small oversight, it makes a big difference when considering the geologic, atmospheric, and astro-biological ramifications. Furthermore, water has been known to exist on Mars in both gaseous and solid states  since the 1970s, and in 2015 scientists also claimed to have found salty liquid water on Mars’ surface in the form of recurring slope lineae (see photo above).

Our understanding of water on Mars has been a tumultuous journey, and it began notably with Shiaparelli’s Martian canals, which he first observed when Mars was at opposition in 1877. In those times, astronomical observations consisted of hand-drawn sketches by astronomers after hours of sitting and staring through an eyepiece. Multiple other observations by other astronomers followed, but with canals in different positions and new ones added. By the time the Mars opposition of 1894 came around, Percival Lowell and a few other astronomers had adopted the belief that the canals were built and operated by intelligent beings for irrigation, and that theory was a widely believed – until technology caught up, and his doubters had all the evidence they needed. Mars was too cold, and didn’t have a substantive atmosphere, making it difficult for intelligent life (let alone any life) to have evolved, if liquid water wasn’t stable on the surface. Mariner’s fly-by in 1965 finally lay to rest any remaining suspicions with up-close photos depicting a barren and crater-marked surface. What was Lowell seeing? Some say he was looking at the reflection of his eye, but it was probably a combination of that, and the need to perpetuate an exciting possibility.

Our understanding of Mars’ past and present water grew immensely with Mariner 9’s arrival in 1971. While we got back the first images showing Mars to be lifeless and barren, we also got back the first evidence of ancient liquid flows with distinct photographic evidence of canyons and valleys. Mars’ next visitors were the Viking orbiters in 1976, and they continued to provide striking evidence of geomorphological features resulting from liquid water with each orbit. Ancient broken dams, riverbeds and valley networks abounded over the planet, seemingly untouched since their days of activity.

 

Geomorpholofical features formed by ancient water flows on Mars. (Image: Jim Secosky modified NASA Viking image)

When the Viking lander tested samples with it’s mass spectrometer, every sample detected water at present, as well as chemical compounds that were likely residues that remained after the sea water on the surface evaporated in ancient times. What’s more, the Viking 2 lander touched down at a northern latitude on Mars’s surface, and provided the first in situ photographic evidence of ice water in the form of frost on the surface.

 

Frost on Mars’ Surface as seen by Viking 2 lander.

With every mission to Mars that followed, our understanding of water on Mars continued to grow, as observations re-affirmed those initial findings of Mariner 9 and Viking 1 and 2, and provided new discoveries that continued to surprise the scientific community. We began to develop our understanding of the seasonal and diurnal atmospheric processes involving water, including the formation of water ice clouds and the deposition and sublimation of carbon dioxide and water vapor onto the polar caps, as well as the global atmospheric pressure variations that resulted from said sublimation and deposition.

The Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS) provided chemical evidence that parts of Mars have been dry for an extremely long time, while images from MGS provided evidence that elsewhere on the planet, small gullies were evidence of more recent fluvial activity.

The Viking landers and later Mars Pathfinder measured diurnal temperature and pressure cycles of the surface, finding the temperature and pressure of Mars’ surface to be too low for liquid water to exist. Findings did note that water could possibly exist if mixed with certain salts, and imagery showed evidence of rocks that had been positioned and worn down by an ancient river.

In 2003, it was announced that Mars Odyssey had found evidence of great amounts of frozen water covering the entire planet, just below the surface with it’s Gamma Ray Spectrometer (GRS). In 2008, the Phoenix lander confirmed the presence of water ice just below Mars’ surface with a robotic arm that scraped the surface. Phoenix also showed evidence that strongly implied once the sub-surface ice was exposed to the atmosphere, it sublimated into vapour.

 

Images showing the sublimation of water ice exposed to the atmosphere, from the Phoenix mission. (Image: NASA)

The Phoneix mission was also the first mission to actively photograph and monitor the morphologies and physical properties of Martian water ice clouds from the surface with a camera and a lidar system. In doing so, it discovered virga (also known as fall streaks) – the first evidence for solid precipitation of water ice on Mars, revealing that Mars meteorology was perhaps much more important and dynamic than previously thought.

Clouds and Virga visible in lidar data from the Phoenix mission. (Image: J. Whiteway and the Phoenix LIDAR team/York University & NASA)

Following Phoenix, we entered a new era of rover missions with great longevity that began with the Mars Exploration Rovers (MER, also known as Spirit and Opportunity) reaching Mars in 2004, and continued with Mars Science Laboratory (MSL, also known as Curiosity) in 2012. These missions allowed (and still allow) us to continually observe the mineralology, geomorphology, atmospheric parameters, and water ice clouds over the span of multiple Martian years by continuously gathering data. 

 

Processed animated gif of clouds observed by the Phoenix lander.
(Image: J.E.Moores /University of Arizona/Texas A&M/NASA).

The constant accumulation of data allows us to understand the seasonal cycles of the atmosphere, which can be compared with orbital data, and used to investigate variations in the surface geology up-close, along a path chosen by geologists. This also led to the direct in-situ observation of recurring slope lineae (RSL) near Curiosity in 2015, which are hypothesized to be the first evidence of a present flowing liquid water brine on Mars’ surface (though this watery origin is now disputed by other groups). These RSL were also observed with Mars Reconnaissance Orbiter’s instrument Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) which detected a perchlorate salt that cloud allow water to exist in a liquid form in the regions where the RSL are present.

            To conclude this brief overview, I will say that our understanding of Mars as a habitable, and hospitable world is greatly dependent on the existence and presence of water, and more importantly liquid water. We have been trying to understand the long history of water on Mars and with each mission, and with each new discovery we come one step closer to unearthing the secrets of Mars’ past. It is important to have a healthy skepticism when reading sensationalist headlines, but also see scientific evidence for what it is – another piece of the puzzle.