Scientists’ ideas about life on Mars have changed dramatically in less than 100 years of studying it. The Red Planet was first considered simply harsh for life, like the Sahara on a summer afternoon, and then it became clear that biological life known to us could not have existed on it for at least the last few billion years. Before this, there could have been life on Mars, but how it died was revealed by new discoveries from Curiosity.
What “watery” Mars might have looked like in ancient times. Image source: NASA
Long-term observations of Mars and remote sensing of its geology and soils suggest that approximately 4 billion years ago this planet had a vast shallow ocean, lakes, rivers and streams. But then the climate changed dramatically and irrevocably. What happened on the planet and what its climate became was shown by the latest findings of the NASA Curiosity rover in the area of the 154 km wide Gale crater impact crater. This crater was formed as a result of a meteorite impact 3.5–3.8 billion years ago.
Studies of rock samples from the crater floor by the rover’s onboard instruments (Sample Analysis at Mars and Tunable Laser Spectrometer) show that there was water in the crater and, therefore, minerals characteristic of a humid environment, such as clays, sulfates and carbonates, arose there. From the point of view of assessing climate change, carbonates formed from carbon and oxygen are considered the most valuable. Light isotopes of atoms quickly evaporate into the atmosphere, while heavy ones remain. By the ratio of one to the other, one can judge the climate, including temperature, water acidity, as well as the composition of water and the atmosphere.
«Isotopic readings from these carbonates indicate extreme amounts of evaporation, suggesting that these carbonates likely formed in a climate that could only temporarily support liquid water, said NASA’s David Burtt. “Our samples do not suggest [the existence of] an ancient environment with life (biosphere) on the surface of Mars, although this does not exclude the possibility of the existence of a subsurface or surface biosphere that began and ended before the formation of these carbonates.”
The carbonate states indicate that habitable Mars died in two processes simultaneously or separately. Firstly, periodic intense “outbreaks” of moisture evaporation began to occur on the planet. Secondly, the water began to freeze, and together with evaporation, this led to an exorbitant increase in its salinity. In such an environment, nothing known to us could survive, not even bacteria. There remains hope for the search for life (at least microbial) under the surface of Mars at depth, but it is unlikely that Earth science will be capable of this in the next 10–15 years.