Originally published November 26, 2015
Are we alone in the universe? Humanity has been searching for evidence of life elsewhere in the cosmos for hundreds of years, and recently, the key to life on Earth, liquid water, was found on our neighboring planet Mars. In September of 2015, the National Aeronautics and Space Administration (NASA) confirmed evidence of liquid, flowing water on the planet Mars. The lead author of the new report, Lujendra Ojha, noted the presence of hydrated minerals on some of the hills of Mars, which appear to flow and change direction over time. Given the growing public interest in the Red Planet, in part evidenced by the success of modern space dramas like The Martian, we wanted to highlight the importance of NASA’s new discovery in the context of previous water-related discoveries on Mars. While few people expect to find little green men, the discovery of even single-celled organisms on the Red Planet would finally answer one of humanity’s greatest questions.
Scientists have speculated about the possibility of water on Mars since the early days of telescopic observation, when William Herschel, the famous German astronomer, recorded his observations of the Red Planet in the 1780s, noting striking similarities between Mars and Earth. Like other scientists at the time, Herschel believed that Mars’s polar ice caps were evidence of water, but he also believed that large, dark spots on Mars’s surface were oceans and clouds. Herschel went so far as to postulate that the differences in surface topography in relation to seasonality were caused by inhabitants on Mars who were growing vegetation.
Herschel’s popularization of the theory of Martian residents likely influenced one of the greatest Mars sensationalists, Percival Lowell. Taken in by a popular scientific idea of the 1890s, Lowell constructed the Lowell Observatory in Flagstaff, Arizona to help observe and make detailed drawings of the planet’s surface, believing that the “inhabitants” of Mars were building visible canals on the Martian surface to direct water flow. His ideas were rejected by later astronomers, though the Lowell Observatory was ultimately used by Clyde Tombaugh to discover the dwarf-planet Pluto and Lowell himself was able to generate tremendous public enthusiasm to hunt for life on the Red Planet.
Despite ongoing public and scientific interest, water-related discoveries on Mars did not truly surmount until modern analytical techniques for astronomical observation were developed. In the 1930s, astronomers began to observe Mars using the techniques of spectroscopy, which works by splitting light into different wavelengths and analyzing those wavelengths separately. Early experiments by Walter Adams and Theodore Dunham showed effectively no water vapor or oxygen in the Martian atmosphere; later, more refined experiments determined the amount of oxygen to be around one percent that of Earth and even water vapor was detected in small quantities, though not until 1963. Eventually, and after years of contention, a number of observations determined that Mars has two polar water-ice caps – one that has a perennial dry-ice coating and one that acquires a dry-ice coating during the Martian winter.
Modern spacecraft-based Mars exploration began with the Mariner 4 flyby in 1965. The spacecraft’s images and measurements showed a very thin Martian atmosphere and a pox-marked surface, indicative of many asteroid collisions and little to no geologic activity. Such a lack of geologic activity suggested a lack of flowing water on the Martian surface and a thin atmosphere implied that any liquid water on the surface would either quickly boil or freeze. These observations led many to question the possibility of any significant water on the surface, and the chance of finding life on Mars seemed unlikely to much of the scientific community.
The view of Mars as a “dead” planet hampered future exploration; however, nearly a decade after the Mariner 4 mission, Mariner 9 was the next probe to reveal significant information of past water on Mars in 1971. This discovery rekindled scientists’ hope to find water and life on Mars. While previous spacecraft had only conducted flybys, Mariner 9 was the first to enter the orbit of another planet and remain there for its entire mission, which proved to be a much more desirable method of planetary observation. Mariner 9 was able to reveal not only the presence of riverbeds and canyons, but also weather fronts, fog, and other past and present indicators of liquid water on Mars.
The success of the Mariner program, and Mariner 9 specifically, influenced the design of the following Viking missions to Mars. Between 1976 and 1982, the probes Viking 1 and Viking 2 provided a wealth of information about Mars and were the first successful rovers to land on the Martian surface. Chemical analysis of the soil by the rovers indicated the possible presence of organic materials and water in the surface, though it was noted that the presence of strong UV light and perchlorate in the soil would make it extremely difficult for life to exist in the Martian top-soil. Combined data from the Viking orbiters and rovers showed a wealth of information for water-based erosion on the Martian surface. Strong evidence was found for past river valleys, natural dams, streams, rainfall, and even mud caused by heating of select locations by meteor strikes or volcanism.
The Viking program was retired on November 13, 1982, and the information garnered by it has been in use to this day. Since then, a variety of data about water on Mars have been collected by multiple probes and rovers. In the late 90s, the Mars Global Surveyor discovered evidence of past lava flows, implying geologic activity and warming. In 1997, Pathfinder found evidence of wet soil. Between 2002 and 2008, Mars Odyssey, Phoenix, and Mars Express each found evidence of past water distribution, while in 2004, Opportunity found evidence of past oceans and coastlines. Some of the latest, most influential evidence for water on Mars comes from the Mars Reconnaissance Orbiter (MRO). The September 2015 announcement by NASA came in light of flowing, hydrated minerals detected by MRO.
Using similar spectroscopic techniques as Walter Adams and Theodore Dunham had nearly 80 years prior, MRO was able to detect these hydrated minerals on “recurring slope lineae” when local temperatures were above -10 degrees Celsius. While flowing water was previously thought to be impossible given current conditions on the Martian surface, the extreme saltiness of these hydrated minerals could allow for liquid water to exist, much like how salt spread on roads can help keep water liquid below 0 degrees Celsius. Of course, the big question on everyone’s minds is now “Can we find evidence of life on these slopes?” to which the answer is “Maybe”. The hydrated minerals are not present on the surface year round, and evidence for ground water beneath the slopes is still unclear. Additionally, the extreme saltiness of these minerals and the possible water around them is toxic to all but the most resistant forms of known life. Future expeditions will attempt to characterize more recurring slope lineae as well as other potential sources of current water on Mars, and the Curiosity rover is currently searching for fossilized bacteria on the Martian surface when it’s not too busy taking selfies.
Faced with the current worldwide situation of economic instability and crisis, the value of Mars-research and space exploration in general are often undercut, such as with the Obama administration’s 20% budget reduction of NASA’s planetary-sciences division – but the importance of Mars-research and space exploration cannot be overstated. From an economic standpoint, the discovery of water on Mars would make colonization and resource-extraction efforts profoundly more plausible and could be a long-term solution to humanity’s population explosion and resource-mismanagement. Not only is Mars-exploration economically enticing, but humanity has been fascinated by the Red Planet since ancient times, and the prospect of life on Mars feeds our curiosity about the universe and could help tell us whether or not Earth is the only planet capable of harboring life. From William Herschel with his telescope, to NASA’s Jet Propulsion Lab, we have spent hundreds of years studying Mars and launched over 50 missions to investigate the planet. The discovery of water on Mars is not simply bringing closure to decades of past speculation, but is the beginning of new possibilities and a chance to challenge our current solitude in the universe.