The climatic differences between Mars and Earth, considered “sister planets” due to some basic similarities, such as rotation and axis, size and rock composition, have long intrigued scientists. Today, these distinctions are what drive most scientific research on the red planet.
One such research instrument, NASA’s Curiosity rover, currently in Gale Crater on the Martian equator, has provided some previously unseen details about how the ancient climate of Mars went from being potentially suitable for life, including liquid water available on its surface, to a desert hostile to life developing under terrestrial conditions.
In a study published in the journal Proceedings of the National Academy of Sciences (PNAS), researchers used Curiosity’s instruments to produce an isotopic composition of the crater’s carbonates, materials rich in carbon. Atoms of the same element with different numbers of neutrons, the isotopes reveal information about the origin, physical processes and environmental conditions of the formation of these materials.
Isotopic analysis of the history of climate on Mars
The choice of Martian carbon isotopes was not by chance. A fundamental element for life, carbon is an important marker when it comes to climate records. These minerals can retain signatures of their original formation contexts, including the temperature and acidity of the water, and the chemical composition of the water and atmosphere.
The research follows the following principles: as the water evaporated, light versions of carbon and oxygen were more likely to escape into the atmosphere, leaving behind heavier versions that ended up incorporated into the carbonate rocks on the surface.
After reading the paper, the article proposes two possible processes for the formation of carbonates on Gale. The first points to a scenario in which the carbonates are formed by a series of wet-dry cycles within the Martian crater. In the second, the carbonates form in very salty water under cold ice-forming (cryogenic) conditions within the crater.
Two Proposals for the Climate of Mars
In a statement, co-author Jennifer Stern, a planetary geochemist at NASA's Goddard Space Flight Center in Maryland, USA, explains that each formation mechanism represents a different climate regime. These, in turn, can influence the habitability of the planet over time.
Thus, the wet-dry cycle suggests alternating periods between conditions more favorable and less favorable to life. During the wet phases, there may have been liquid water available on the surface or in underground layers, which is essential for the existence of chemical and biological processes that sustain life.
The cryogenic conditions, predominant in the middle latitudes of Mars, indicate extremely cold temperatures, with most of the water confined in the form of ice. In addition, this water is highly saline, which makes the environment inhospitable to the formation of life as we know it.
Comparing the isotopes of Mars with those of Earth
Martian carbonates have significantly higher values of carbon and oxygen isotopes than those found on Earth. According to the authors, these are the highest values ever recorded for any known Martian material, indicating that unique environmental or geological processes influenced the formation of these minerals on the planet.
For the paper’s first author, David Burtt of NASA Goddard, these exceptional isotopic values indicate intense evaporation, two to three times greater than any observed on Earth. This intensity explains the enrichment in heavy isotopes and their preservation, showing that processes capable of creating lighter isotopes had less impact or intensity on Mars.
Although wet-dry and cold-salty climate scenarios for the ancient red planet have previously been proposed based on modeling and minerals, this study is the first to provide direct isotopic evidence. Analysis of rock samples confirms previously suggested climate models.