Looking for life on the Red Planet
The search for Martians
Jezero crater is the 28-km diameter remnant of an ancient impact on Mars.
Until maybe three billion years ago it contained a large lake that had a major river system flowing into it, forming a large delta.
This crater is now being closely examined using the Perseverance Rover for signs of ancient life. Why would this location be such a good site to search for ancient life on the Red Planet?
It would be very hard to find a three-billion-year-old crater anywhere on Earth.
Plate tectonics is continually recycling the Earth's crust. The only really ancient rocks remaining today occur on the Canadian Shield, in the Canadian Arctic, and in parts of Australia.
If Mars ever had any plate tectonics and recycling of its surface, it ended long ago. That is why there are so many ancient craters and other landforms visible on Mars today.
Three or so billion years ago Mars was a watery, warm world, just like Earth. There were rivers, lakes and possibly seas, lying beneath a thick atmosphere. There may have been ancient living things on Mars, as there were on Earth at the time. The creatures swimming in the Earth's seas were single-celled, and tiny, but there were lots of them. The situation on Mars was probably the same. Life on Earth became much more diverse and complex some 500 million years ago, during the Cambrian Period. Mars never reached this point.
Mars is smaller than the Earth, and its core cooled faster. When it solidified, any tectonic recycling of its surface stopped. From there on the only things changing the surface were erosion and meteoric impacts. More importantly, when the core solidified the flows of molten core material ceased too, which shut down the dynamo processes generating the Red Planet's magnetic field.
As the magnetic field decayed, it stopped shielding the planet from the solar wind. The atmosphere was scrubbed away, the greenhouse effect keeping the planet warm ended, and the planet became a frozen, almost airless desert. There would never be the Martian equivalent of the Cambrian explosion of life. The Earth still has a strong magnetic field, which is keeping the solar wind away from the top of the atmosphere.
If we were looking for life on Mars, where would be a good place to look? There may be things eking out a tough life there now, but as is the case on our world, there should be ancient rocks containing the remains of the creatures living in the rivers lakes and seas that existed around three billion years ago.
On Mars, where should we look?
In seas and lakes, there is a continual rain of particles and other things falling and accumulating on the bottom. Included in this rain are the remains of living things: shells, bones, carapaces and other hard parts, and more rarely, soft parts. These get buried by further sediments, and over millions of years, the sediment layers become layers of rock. So we should look at the rocks formed from these sediments.
Perseverance has sent back images of suitable sedimentary rocks. In addition, creatures leaving their remains in a river would have some of those remains, especially those of tiny life forms, carried downstream by the current.
As a river flows into a lake, the flow slows and the river dumps this material forming a delta. Therefore the deltaic materials could contain a concentration of animal and plant remains. So Perseverance is going to focus a lot of its attention on the rocks derived from deltaic sediments. These sediments will also tell us the history of the river and the lake, including the increasing number of droughts as the Red Plant dried up and froze. Maybe this space mission will tell us about the sad story of Martian life, and maybe detect some tough descendants eking out a life someone beneath the surface.
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• After sunset, Jupiter and Saturn will be low in the southwest and Mercury will be low in the predawn sky.
• The Moon will be full on Oct. 20.
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