Why is terraforming Mars even considered

The lake must be extremely salty

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The researchers do not provide direct evidence of water. After all, nobody was there to drill into the lake below the surface; a robot that could do that would first have to be developed and then flown there. However, they do provide a strong indication with the help of the indirect measurements. The method of investigation is known from Earth. "With the help of radar waves, water below the Antarctic ice sheet was detected and analyzed," writes the glaciologist Anja Diez from the Norwegian Polar Institute in Tromsø in an article accompanying the study (Science, 2018). "The radar waves reflected from a water surface are stronger than those from rock or sediment," she explains. They can therefore be seen relatively brightly in the radar gram that depicts the recordings - exactly as Orosei and his team have described.

Solar system

What is a planet

In the literal sense of the word, a planet is a celestial bodies buzzing around. To be considered a planet according to the definition of the International Astronomical Union (IAU), it must meet three criteria:

1) It moves in an orbit around the sun.

2) Its mass is large enough to keep it in hydrostatic equilibrium. In other words: It is approximately spherical.

3) It is the dominant object of its orbit, which means that over time it has gotten everything else out of the way through its gravitational field.

The solar system

"My (Mercury) father (Venus) explains (earth) to me (Mars) every (Jupiter) Saturday (Saturn) our (Uranus) nine (Neptune) planets (Pluto)" - this is a memento for the planets of our solar system. The Donkey bridge lists them in the correct order, starting with Mercury, the closest to the Sun.

But be careful: it has been in effect since 2006 Pluto officially only as a dwarf planetbecause it does not meet the third planetary criterion. A controversial decision that is being discussed further in professional circles. Sometimes there is a new candidate for this: Planet X, also called Planet 9, which is hidden outside the orbit of Neptune. However, its existence has not yet been proven. For the time being, the following slogan applies to our solar system: "My (Mercury) father (Venus) explains (earth) to me (Mars) every (Jupiter) Saturday (Saturn) our (Uranus) night sky (Neptune)."

Astronomers believed they had a good understanding of how the planets in our solar system were formed. But now there are hundreds of distant star systems and thus thousands Exoplanets known that question the previous model.

Theoretically, there may be other explanations for the abnormalities, for example a layer of carbon ice. "But the team excluded them all," writes Diez. The work of Orosei leaves a liquid layer of water or soaked sediment as the only explanation.

"The authors have provided convincing evidence," says Stephen Clifford, who first considered the existence of such a lake 30 years ago. "But it's a discovery that the rest of the radar community should scrutinize to rule out alternatives." A mission originally planned for 2022 could have done this, but it is currently unlikely to take place.

Alina Schadwinkel

Editorial manager Spektrum.de, former knowledge editor at ZEIT ONLINE

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How did Clifford come up with the idea that there could be water under the pole? "Because there is a large network of subglacial lakes and canals under the Antarctic and Greenland ice sheets on Earth, it seemed logical to assume similar conditions on Mars," says Clifford. At the end of the 1980s, the surface temperature of the planet was known and there was a good idea of ​​the thickness and thermal conductivity of the ice, he explains. What was unknown, however, was whether the planet had enough internal heat to raise the temperature at the bottom of the ice to the melting point. Or even salts that support this. "Sodium chloride, for example, calcium chloride or even better suitable salts such as magnesium perchlorate - the latter was only discovered in the soil of Mars in 2008," says Clifford (AGU Fall Meeting Abstracts: Fisher et al., 2008).

For everyone involved, one thing is certain: The lake that has now been discovered must be extremely salty, because temperatures of up to minus 70 degrees Celsius prevail at a depth of one and a half kilometers under the ice and conventional water would therefore be frozen (Journal of Geophysical Research: Fisher et al., 2010). Here the earth's extreme zones help again. Such extremely salty waters are known from the Antarctic dry valleys, for example. While oceans have a salinity of 32 to 37 PSU (Practical Saliniy Units), it is 200 in Antarctica. A super salt lake is therefore not a theory born out of necessity.

What does the find say about life on Mars? "Maybe nothing," says Clifford - "if the salt content is actually as high as the study authors described." According to current knowledge, there are no organisms that could survive in solutions saturated with perchlorates at around minus 70 degrees Celsius. "But it is possible that the water under the polar cap is not liquid because of salts, but because of geothermal processes." The combination of higher temperature and lower salt content is well within the tolerance limit of some terrestrial organisms.

"The conditions are not exactly life-friendly," says study author Orosei. There is a lot of salt, but neither oxygen nor light. But on earth the unicellular organism did not prevent it from existing. In a lake under the Antarctic ice, for example, organisms survived for 35 million years without contact with the outside world. Why should it be any different on Mars? Numerous measurements from recent years indicate that there was once life there. Most recently, Mars rover Curiosity found numerous building blocks for it in three billion year old rock (Science: Eigenbrode et al., 2018).

So it can be said: According to the latest findings, liquid water on Mars is more than likely. But also: Direct evidence is pending. Likewise the evidence of life - whether these days or millions of years ago. But those who still suspect extraterrestrial organisms on Mars now have one more argument.