Updated: May 6, 2021
Today we see Mars as a cold and dry planet. The average temperature at the surface is close to -50°C and the atmosphere is too thin to create an atmospheric pressure high enough to support liquid water stability on the surface. But, was this always how Mars used to be?
Was Early Mars filled with surface water? An artist's rendition (Image Credit: Astrobiology.com)
The most prevalent idea among researchers is that Mars was once filled with abundant quantities of liquid water. There are many pieces of evidence to support this hypothesis, such as geomorphic features like valleys, outflow channels, lakes, etc. , and the origin of all of these features could be explained to be carved by flowing liquid water. Drawing parallels, researchers have claimed for decades that water was abundant in the Martian past. Even to an untrained eye, that might seem true. The presence of a plethora of hydrated minerals further supports this idea.
For hosting huge quantities of liquid water on any surface, water stability conditions must be met. This means that the planet must be adequately warm and must have had a relatively greater atmospheric pressure at the surface to hold liquid water - something which can be met if the planet had a thicker atmosphere. If that were true, where did all the thick atmosphere disappear?
To explain the loss of atmosphere, researchers have suggested that the atmosphere got sequentially stripped off by solar winds and radiation, owing to the weakening of the planet's magnetism. (A process informally referred to as 'Spluttering')
However, what cannot be ignored here is the fact that there are a few loopholes to this idea. For instance, the early Solar System was considerably fainter during the early period of its formation as the Sun, too was in its initial stages (referring to the first billion years of the solar system's history). Also referred to as The Faint Young Sun Paradox, due to the faintness of the young Sun during the early periods of the Solar System's history, the stability of liquid water on Mars was likely impossible. So, on one hand, we have valleys and other geomorphic features which were likely carved by abundant quantities of liquid water (in the early Noachian period of Martian history), while on the other hand, the stability of liquid water could not be explained due to the faint young Sun paradox. How were the valleys in Mars formed if there was no liquid surface water?
A recent article published in Nature Geoscience journal , lead-authored by Dr. Anna Grau Galofre (affiliated to SESE- Arizona State University and University of British Columbia), has provided pieces of evidence to solve this paradox, arguing that the valleys on Mars, could have been carved by moving ice glaciers as opposed to by flowing surface water. An earth-based analog at the Canadian Arctic, called the Devon Island, has multiple valley networks that are nearly identical to Martian valleys. In fact, previous studies led by planetary scientists, such as Dr. Pascal Lee, (of SETI Institute and Mars Institute) have laid the foundations for this idea by studying Devon Island back in 1998. Dr. Lee has been a major proponent of the early cold Mars hypothesis (refer to articles , ,  ). Climate experts, such as Dr. Robin Wordsworth (Harvard University), has also argued that the colder scenario is a more likely scenario. Mars gets ~43% of the solar energy of Earth. During the early history of the Solar System, Sun was likely 25% dimmer than what it is today, corroborating the idea that early Mars was cold and icy.. 
So, how does it matter to us to know whether the early Mars was dry or wet? One of the many ideas it effects is regarding past habitability. If Mars had surface water and was sufficiently warm during the early history of the planet, chances are that it would also have had conditions conducive to life forms. Search for alien life, extant or extinct, has been a major goal of almost every planetary mission, ranging from the times of Viking missions, and currently, Mars 2020 rover (Perseverance) is on its way to Mars to search for fossil records of any past life form. Most of NASA's rover and orbiter missions were launched under the tandem, 'Follow the water', which would definitely change if water had never actually been stable on the surface. Had Mars been a cold and dry planet throughout its history, the entire planning for planetary missions and analog study on Earth, to search for any potential life activities could dramatically change. For instance, life forms could also be found in cold Arctic sheets of ice. However, the planning for major astrobiological targets might considerably change owing to a major change in our understanding of Mars. Of course, it goes without saying, more research is needed to understand how the planets evolved.
 Galofre, Anna Grau, A. Mark Jellinek, and Gordon R. Osinski. "Valley formation on early Mars by subglacial and fluvial erosion." Nature Geoscience (2020): 1-6.
 Irion, Robert. "Geologists See Mars in the Canadian Arctic." (1998): 210-211.
 Lee, P., and J. W. Rice, Jr. 1999. Small valley networks on Mars: The glacial meltwater channel networks of Devon Island, Nunavut Territory, Arctic Canada, as possible analogs. 5th Mars Conference, Jul 1999.
 Lee, P. C. P. McKay 2003. Mars: Always Cold, Sometimes Wet? 34th Lunar Planet. Sci. Conf., Mar, 2003. [#2127]. http://www.lpi.usra.edu/meetings/lpsc2003/pdf/2127.pdf  https://www.eurekalert.org/pub_releases/2015-06/agu-nsf061515.php