Today, Mars is a cold, rocky wasteland. However, 3 billion years ago, the conditions on the planet were much more hospitable to life. There was an atmosphere, glaciers from which rivers originated, muddy lakes quite suitable for living organisms, and an ocean stretching to the north. However, over time, all of this disappeared.
What modern Mars looks like
Thanks to the successful missions of spacecraft, scientists know that today’s Mars is a lifeless, cold desert with no water and an atmosphere that is very thin and consists almost entirely of carbon dioxide. The existence of nearly all terrestrial organisms on it is impossible. Only some extremely resistant algae could theoretically survive there. And it’s not just the lack of water and oxygen, but also the solar radiation, which is not retained by either the fragile atmosphere or the magnetic field, which the planet does not have.

However, this was not always the case. Studies by the same automatic vehicles and reconstructions made by scientists on Earth have helped to find out that Mars once had an atmosphere and hydrosphere quite comparable to those on Earth. Let’s try to figure out why it lost them.
The Noachian period on Mars
As a planet, Mars was formed at the same time as the Earth. And the mechanism of its formation was the same. Large pieces of undifferentiated material from the protoplanetary disk, called planetesimals, collided. The energy of the impacts was converted into heat. Therefore, it is believed that at the beginning of its history – about 4.6 billion years ago – Mars was molten.
It was during this period that its primary atmosphere was formed. It was about a thousand times denser than today’s and consisted mainly of hydrogen, which is an even more effective greenhouse gas than carbon monoxide or water vapor. Therefore, despite the decrease in the temperature of the Mars surface, its gas shell continued to retain a large amount of heat.
From the point of view of areology (analogous to geology for Mars), scientists divide the history of the Red Planet into three major periods or eras. The first of them, the Noachian, began immediately after the formation of a solid surface. It got its name from a formation known as the “Noachis Terra” – an ancient hilly plain about 4.1 billion years old.

The structures that appeared during this period cover almost 40% of the planet’s surface. Relatively recent studies have shown that some of them could have been formed even before the Noachian period. Be that as it may, the climate on Mars was quite specific back then.
First of all, there was still a lot of “debris” in space left over from the birth of the solar system. Meteorite impacts occurred 500 times more often than now. Craters with a diameter of more than 100 km were formed almost every million years.
These impacts triggered volcanic activity. Lava flows flooded the Martian surface. However, the pressure was high, and the temperature had already dropped, so the water was able to condense and it began to rain. That’s why the ancient craters that cover the landscapes of the Noachian period are heavily eroded by water.
There were no permanent rivers or large bodies of water on the surface of Mars at that time. But scientists have found many small networks of streams, very similar to those that form in Earth’s deserts after rains.
There could likely have been large puddles on Mars at that time, and hydrothermal springs could have formed in volcanic regions. The Martian conditions of that time may not seem very favorable for living organisms, but life was emerging on Earth at that time and in similar conditions.

However, no such ancient traces of organic matter have been found in samples examined by probes on the Red Planet itself. But it is known for certain that the Martian soil of that era was very similar to the Earth’s. It did not yet have a characteristic red hue and consisted mainly of silicates and aluminates. Through grinding, they formed clays that still lie on the surface. Their study can bring interesting discoveries.
The Hesperian period
About 3.7 billion years ago, the climate on Mars began to change. On the one hand, the planet’s atmosphere became different. Hydrogen and water vapor fell to the surface as rain or simply dissipated into the surrounding space. Their “escape” was facilitated by the low gravity, which was unable to hold the molecules that radiation provided with additional energy. In addition, the magnetic field was weakening and could not effectively stop high-energy particles.
The Hesperian period came when the gas shell of Mars already consisted of carbon dioxide with oxygen impurities. Its density is estimated differently, but it is quite possible that it was in no way inferior to the Earth’s. On our planet, the Archean Eon was just underway, and the atmosphere was no better.
On Earth, this did not prevent life from evolving, going from the simplest molecules capable of self-replication to fairly complex bacteria and archaea. What happened on Mars at this time is a great mystery. Numerous finds of organic matter date back to the Hesperian period.

The Martian climate at this time became drier and cooler, and instead of a dense network of short streams, real rivers hundreds of kilometers long formed. They mostly flowed from the south, where Mars has highlands, to the north with a more low-lying landscape.
Scientists do not have a definite opinion about the source of these rivers. Some believe that it was the rains that continued to fall in some areas. Others talk about glaciers on the surface or permafrost in the ground.
The water flows formed numerous lakes, where a lot of silt was deposited. One such reservoir was located in the Jezero crater. It appeared in the previous, Noachian period, but was filled with water only in the Hesperian. Now the sediments in it are being carefully studied by the Perseverance rover. Later, the collected clay samples are planned to be delivered to Earth. Almost all Martian rivers carry their water to the ocean in the northern part of the planet. Throughout the Hesperian period, it was constantly changing its banks, dividing into separate basins.

Source: NASA/JPL-Caltech/MSSS/JHU-APL
All this was superimposed by volcanic activity that intensified in the Hesperian period. It enriched the planet’s atmosphere with sulfur oxides. At the same time, aluminate and silicate rocks began to be replaced by sulfate rocks. Relatively speaking, something similar to gypsum was formed instead of clay.
In general, the Red Planet remained an extremely inhospitable place during this period. A person could live there for only a few minutes. However, even such conditions were better for life than Mars offers today. At least, they were acceptable for single-celled organisms.
The Amazonian period
Scientists have not yet decided when the Hesperian period ended on the Red Planet, but they assume that it happened between 3.2 billion and 2 billion years ago. If bacteria existed on Mars then, the situation was generally similar to the Earth.

But then the paths of the two planets began to diverge more and more. Organisms evolved on Earth that produced molecular oxygen in the course of their life, which gradually became more and more abundant in the atmosphere and hydrosphere, and eventually, the so-called Great Oxidation Event took place. Some living organisms became extinct, while others adapted and received a new powerful source of energy – oxidation. The conditions were ripe for the emergence of multicellular organisms.
On Mars, the Amazonian period began and is still going on. The planet was gradually cooling and losing its atmosphere. Due to the low pressure, the water evaporated from the surface. The ocean in the northern trench disappeared.
The ice began to melt intensively, causing short-term floods, but eventually, most of the water disappeared from the surface of the Red Planet. How long this process lasted is one of the main mysteries of Mars that researchers are trying to find out. The actual change from relatively humid Hesperian conditions to modern Amazonian conditions could have occurred as early as 2.5 billion or 2 billion years ago, meaning that the transition period could have lasted hundreds of millions of Earth years.
However, with the disappearance of the main ice reserves on Mars, the history of water on its surface may not have ended. Scientists have recently discovered an interesting formation – a disorderly jumble of table mountains and flat-topped hills called the Hydraotes Chaos. Approximately 3.4 billion years ago, during the Hesperian period, there was a powerful water flow in that region, and for a long time, scientists believed that it was this flow that formed the local terrain.

However, new evidence suggests that the last time water could flow there was about 1.1 billion years ago. It was at this time that volcanic activity began under the Hydraotes Chaos, which melted the ice beneath the surface, forming reservoirs of water. Further heating led to the emergence of geothermal springs and a system of muddy lakes on the surface. From there, the water flowed for some time to the long-vanished ocean in the north. It is quite possible that life could have existed in those conditions.
Of course, 1.1 billion years ago is also a very long time ago. There were no multicellular organisms on Earth then. However, likely, this is not the last such event on the Red Planet, as volcanic activity has continued there for the last billion years. Recently, scientists have found that in some places eruptions occurred less than a million years ago. This could have triggered the melting of the ice and created conditions suitable for life.
According to current scientific understanding, throughout the Amazonian period, there are cyclical changes in the inclination of Mars’ axis of rotation to the plane of its orbit, as well as the shape of the orbit itself. They cause processes similar to the Earth’s Milankovitch cycles. In our country, they lead to the onset of ice ages, while on the neighboring planet, on the contrary, they can make the climate warmer and more humid. So it is quite likely that the last time water and life were there was only a few million years ago. Moreover, this condition may return in the future.
This article was published in Universe Space Tech magazine #1 (190) 2024. You can buy this issue in the electronic version in our store.