They do though:
"The WATSON (Wide Angle Topographic Sensor for Operations and eNgineering) is a reflight of the MAHLI (MArs Hand Lens Imager) that is a part of the Curiosity rover (Edgett et al., 2012). WATSON obtains full-color images from microscopic scales (∼13 μm/pixel) to infinity and is used for initial textural analysis of rock and regolith targets, as well as to assess potential proximity science targets and the safety of robotic arm activities (Edgett et al., 2012). The ACI (Autofocus Contextual Imager) is a fixed field, 10.1 μm/pixel resolution grayscale imager used to obtain best-focus and colocate laser spots with surface feature analyzed during SHERLOC spectroscopic investigations (Bhartia et al., 2021)."
From: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022EA00...
See also:
https://en.wikipedia.org/wiki/Perseverance_(rover)#Instrumen...
https://science.nasa.gov/mission/mars-2020-perseverance/scie...
10-13 μm per pixel is nowhere near good enough when a typical bacterium is 0.5 - 5.0 μm in size!
I remember the discussions around the mission plan for both Opportunity and Curiosity where NASA kept making "mumbly" noises about why they can't ship decent optics with these things.
Anything that would definitely eliminate (not just "potentially find") the presence of either life or water is never included. It's always omitted, for "reasons".
Water and life must forever remain possible things for the funding to keep flowing.
Water isn't an abstract possibility on Mars. It's a reality. They've found minerals that only form in water, they've found ice, they've observed erosion. We don't understand the hydrology of Mars but it isn't some kind of conspiracy. It's a laborious process, which they continue to chug away at.
Looking for life isn't the primary mission of Mars rovers. They're remote controlled geologists. The search for life really has nothing to do with funding for Mars missions. No one expects to find it.
https://en.wikipedia.org/wiki/Thrombolite
https://en.wikipedia.org/wiki/Stromatolite
Technically, a one-foot diameter dog's vomit slime mold is a single cell.
A good enough microscope can easily tell the difference between life and non-life, especially in the presence of water. If it moves on its own, it is almost certainly alive!
Certain kinds of chromatographs can conclusively determine that no complex chemicals are present, the kind essential to life. I.e.: if only simple metal oxides and the like are present, then you have only a rock.
NASA (and also the Soviet Union and ESA) have repeatedly designed Mars sample return missions, but have not done them for budgetary reasons; it would be tremendously difficult and expensive.
Here's the current one: https://en.wikipedia.org/wiki/NASA-ESA_Mars_Sample_Return - however, given that it was hitting funding problems even _before_ ol' minihands gutted NASA funding, it seems destined to become yet another NASA/ESA canceled program (there's a bit of a history of ambitious NASA/ESA collaborations which die when one side or the other pulls the budgetary plug; JWST was likely lucky to escape this fate, say).
This puts it in a particularly weird place, as the earth return section is already built and due to launch on an Ariane 6 in two years (it will then proceed, slowly, to Mars using an ion drive, and await the lander and Mars launcher, which will presumably never arrive because budgets).
If that's so, I can produce a sample of material from the center of the Amazon rain forest that will conclusively prove to you that Earth is also lifeless.
They are still arguing over this one three decades later: https://en.wikipedia.org/wiki/Allan_Hills_84001
https://www.science.org/doi/10.1126/science.abg7905
" Inorganic precipitation processes are capable of producing a wide range of morphological outputs. This range includes shapes with both crystallographic and non-crystallographic symmetry elements. Among the latter, morphologies that mimic primitive living organisms are easily obtained under different physico-chemical conditions including those that are geochemically plausible. The application of this information to the problem of deciphering primitive life on the early Earth and Mars is discussed. It is concluded that morphology cannot be used unambiguously as a tool for primitive life detection. "
https://www.spiedigitallibrary.org/conference-proceedings-of...
you should go work for spacex and show them how to do the sample return. they've thought about it for at least a decade now and haven't yet. so you can go there and show them how since it's so easy. you'll be millionaire real quick i promise.
But no traces of life were found ever.
If there is life on Mars, it is hidden underground in vulcanic active areas and alike and no mission we can do today, could conclude with certainty that there is no life on Mars. But we have been looking real hard.
But in german and no idea if it was ever translated, but I assume similar books exist in english.