Water is essential for the emergence and maintenance of life as we know it. The chances of finding life-friendly conditions on Venus are therefore zero. On the other hand, the situation is different on Mars.
With a temperature of almost 500 degrees Celsius, the surface of Venus is hostile to life – at least to life as we know it from Earth. But there is a zone of life-friendly temperatures in the planet’s high atmosphere. Could this region be populated by microorganisms? An international research team has now answered this question with a clear no. According to the scientists in the scientific journal “Nature Astronomy,” there is too little water available for life processes. Mars offers better prospects. Lakes on the red planet offer not only sporadic, but over a longer period of time uninterrupted life-friendly conditions, as a second team also reported in Nature Astronomy.
Only recently, the alleged discovery of the gas phosphane in the atmosphere of Venus had caused renewed discussions about possible life on the planet. “However, studies on this have so far overlooked the role of water activity in making an environment conducive to life,” explain John Hallsworth of Queen’s University Belfast and his colleagues. Biologists use water activity to describe the proportion of unbound water, i.e. water available for life processes, with a value between 0 and 1. Life is only possible if water activity is greater than 0.585 – an important threshold value also for the shelf life of food. Because if the water activity is less than 0.585, no microbes that are hazardous to health can exist there.
Based on a chemical analysis of the conditions in the well-tempered zone of Venus’ atmosphere, Hallsworth and his team show that the water activity in this region must have a value lower than 0.004. The biological availability of water is reduced to this low value in particular by the droplets of sulfuric acid present there, the scientists say. This rules out the existence of microbes there, they said.
Mars just under the limit
The situation is different on Mars: For its thin atmosphere, Hallsworth and his colleagues get a water activity of at most 0.537, just below the limit of life-friendliness. Since the climatic conditions on the red planet may have changed considerably in the course of its history, the atmosphere may well have been more life-friendly in earlier ages.
This is also supported by the results of a study by Elisabeth Losa-Adams of the University of Vigo in Spain and her colleagues. The team analyzed X-ray measurements of sediment deposits in Gale Crater taken by the U.S. rover Curiosity. Curiosity has been traveling in Gale Crater since 2012, where there was a lake in the early days of Mars. “Until now, however, we didn’t know whether the sediments were formed by a slow depositional process or by selective, brief flooding,” Losa-Adams and her colleagues explain.
With the help of X-ray measurements, the researchers were able to examine the crystalline structure of the sediments and draw conclusions about the depositional process. Their finding: The sediments must have been deposited over a long period of time in a calm body of water. The lake in the Gale crater was therefore not just a sporadically existing body of water, but offered life-friendly conditions for a longer period of time – presumably up to ten million years. Also the temperatures must have been pleasant at this epoch, because neither did the water freeze to ice, nor did it evaporate too fast. So, according to the researchers, life could well have developed in the water-rich environment of young Mars.