In the history of Mars studies, opinions have swung from ‘I can see seas and oceans’ to ‘not a single drop of water there’ to ‘I can see an ocean!’ again.
This picture shows Mars as it could be seen in the 19th century.
The white polar caps on Mars closely resembled those on Earth. Black spots were declared by some to be oceans, while others said they were jungles. On the whole, the little red world seemed pretty similar to ours, which secured the Martians a place in our earthly fiction for a century and a half ahead.
Methods of astronomic observation had somewhat improved by the mid 20th century, and photography began to be used. Fewer and fewer scientists could make out any canals on Mars, and the Red Planet seemed more and more to be a dead or dying desert world. Even approximate calculations of temperature on Mars yielded staggering results of about -100 C, which people found impossible to believe back then.
All Martian seas and forests were done with once and for all when Mariner-4 broadcast its first photographs from Mars’s orbit, followed by imagery from Mariner-9 and the Soviet Mars-5 orbiter.
It became obvious that Mars can boast neither artificial canals, nor any traces of vegetation, nor large pools of open water. Even its polar caps were found to be covered with frozen carbon dioxide, or dry ice.
Instead, scientists discovered dry river beds. That find gave hope that at least there was water in the past, but that brought about the question of where it is now. Nonetheless, Mars being dry did not stop scientists from formulating a reliable hypothesis that seasonal variations of Mars’s polar caps depend on the freezing and thawing of the carbon dioxide they contain, whereas those scraps of the caps that do not change from year to year on the poles are made of water ice.
You can read about this theory in the Big Soviet Encyclopaedia published as early as 1968.
The initial investigation of Martian topsoil yielded negative results. NASA’s Viking 1 and 2 landed in the northern middle latitudes of Mars. They were tasked with not only looking for microorganisms in the soil, but also with finding out if there was water in the soil at all. The findings were pretty disappointing, with the gas chromatograph detecting no more than 1% of moisture in the topsoil layer.
The investigation was continued in the 1990s. The Mars Global Surveyor orbiter imaged even more evidence of Mars’s watery past, for example, this classic river delta inside Eberswald crater.
There used to be a lake inside the crater, fed by a river. Soil particles, drawn by the current, ended up on the lakebed, forming multilayered sediments. These crater and delta are being considered as a possible future rover destination.
Mars Pathfinder landed on Mars in 1997 carrying the precursor of all NASA rovers, Sojourner. It was sent to investigate an area which closely resembled a wide riverbed from orbit, similar to what the Amazon would look like if it were dry.
To the disappointment of many, there were very few signs if any of past water to be found. The plain in question was strewn with volcanic rocks and covered with sand. All Soujourner could do was establish that these rocks were once exposed to water, but only to a very small extent.
After the initial disappointment a full-scale hunting campaign for Martian water was launched. NASA launched the Mars Odyssey orbiter in 2001, followed by ESA’s Mars Express orbiter. Both satellites were equipped with various instruments to look for water, including several tools contributed by Russia.
Mars Odyssey had the GRS gamma ray detector and the HEND neutron detector on board (the latter provided by the Institute of Space Research of the Russian Academy of Sciences). These devices captured cosmic gamma rays as well as neutrons reflected off the Martian surface. Hydrogen atoms contained in the topmost soil layers detain neutrons and release gamma photons when bombarded by cosmic particles which allowed scientists to calculate approximate amounts of hydrogen in the soil. As hydrogen cannot stay in the soil in its gaseous form, it can only be found there in its bonded form, most likely in water molecules.
Thanks to these two instruments, the first water distribution maps of Mars were created.
So, things were not so bad after all. There was indeed water on Mars. The data from either of these instruments corroborated that from the other one. But scientists still yearned to see and ‘touch’ this water, that is, they needed to analyse it in situ.
The OMEGA spectrometer on board Mars Express managed to separate water ice from the dry ice found on the poles. It discovered that the top layer was made of frozen CO2 (pink in the image) and the blue of H2O ice (blue) in deep crevasses.
In 2005 Mars Express unfolded its MARSIS radar antenna, which was capable of scanning the Martian crust up to the depth of several kilometers. Thanks to this radar a sectional scan was obtained for the Martian polar caps, similar to ultrasound scans of a patient’s body made by doctors.
This scanning helped establish that there was a 1.7 km thick layer of water ice in the north polar cap, whereas in the south polar cap it reached the depth of 4 km. As for the dry CO2 ice, it reached the depth of only 8 meters on the south pole. Moreover, data was obtained by the radar in some other areas which can be interpreted as potential bodies of liquid water. In other words, a vague hope still remains that there might be an under-ice lake similar to Lake Vostok in the Antarctic. For those still hoping to find life on Mars this is the best place to look. That said, there is still no proof yet of any kind of life in Lake Vostok, so we simply don’t know if life can survive in its possible Martian analogue.
Meanwhile, in 2004, twin rovers Spirit and Opportunity landed on Mars. They were sent to investigage areas once heavily exposed to water.
Opportunity landed in Meridiani Planum, in an area with a high hematite concentration. Hematite is a form of iron ore which is formed in groundwater aquifers or at shallow lake bottoms.
Spirit was sent to investigate Gusev crater, where scientists hoped to find evidence of a past river, a lake or even a sea bay.
Ironically, Opportunity had accomplished nearly all of its goals the moment it landed on Mars. The rover found hematite, layered sediments, salt crystals – all of them evidence of a body of water present there in the past. Working on the surface, it managed to confirm what was quite obvious – that Mars used to have abundant water and that the physical conditions on the planet’s surface could keep it in the liquid state. In other words, Mars used to be a warmer place and had a denser atmosphere.
It was a challenge for the rover’s scientific team to find the next goal for it, so they had to drive it around the plains quite a bit first. It was eventually tasked with finding clay, which can be deposited only in fresh water. The rover had previously found evidence only of briny or acid water pools the conditions in which weren’t favourable for life.
Later on, the rover did manage to stumble on some clay deposits. As of today, it has covered the marathon distance of 42 km and keeps on rolling. Its mission is still far from complete.
Spirit had lived a short but eventful life. It didn’t find any evidence of past water on the surface at first. Having travelled another 5 km it stumbled on remnants of ancient geysers, aided in this find by its faulty wheel. By the end of its career, Spirit found more evidence of a dried pool when it came across a lump of salt in a pit. And it was the salt deposit that actually killed it.
In the next publication you will learn how much water still remains on Mars, where to look for it apart from the polar regions and where seasonal flows come from.