New research indicates that some methanogens, Archaea which breath hydrogen and carbon dioxide while excreting methane, can survive the temperature extremes currently found on the surface of Mars. I find this to be a very interesting piece of data.
Various bacteria have been shown to survive the extreme conditions of vacuum, temperature, and radiation found in space. This second piece of interesting data indicates that bacteria can potentially be transferred from Earth to Mars, where they could survive.
Meteorites are routinely found here on Earth, including a rare few which contain gases matching those found in the atmosphere of Mars. This third piece of interesting data indicates the potential transfer of rocky material from Earth to Mars.
Together, these data indicate that living things could be found on Mars, if they had the opportunity to be transferred there at some time.
This leads us to another piece of data, referred to as the late heavy bombardment. Approximately 4 billion years ago, large rocky fragments were crashing about the inner solar system. The crater-scarred surface of the moon is the most obvious piece of evidence to this time period. Impact craters on Earth and Mars which would have been produced at the same time have been worn away by the erosional processes of the thicker atmospheres and oceans both planets held in the times soon after the bombardment ended. Some of the impacts would have been large enough to sterilize the surface of either planet, evaporating nascent oceans and scattering rocky bits all throughout the solar system. Living cells which had formed on Earth would have been carried into local space on those ejected fragments, only to come raining down later to reseed the cooling planetary surface.
The early environments of Earth and Mars were very much the same. Primordial reducing atmospheres with water oceans would have been found on both. Those rocky seeds which returned life to the sterilized surface of Earth would have also carried life to Mars. The same process would have carried Martian life to Earth, of course, but we have so little evidence from that time period that we really can't say which direction the transfer went.
A few billion years after the late heavy bombardment concluded, we evolved and started asking questions.
The core of Mars had cooled and lost its magnetic field, leading to the Martian atmosphere being stripped away by the persistent solar wind. Without a thick atmosphere, Mars cooled and dried out. However, even now there is evidence for liquid water on Mars. There is also evidence for methane release into the atmosphere of Mars.
Given how persistent life has been found to be on our planet, living in every place it could possibly find a way to live, I strongly suspect there is life on Mars today. That life would be the last remnants of a formerly thriving biosphere, much like our own, which was descended from the simple forms of life scattered through the solar system during the late heavy bombardment.
Hopefully, we will get the opportunity to go and look while I'm still here to know about it.
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