The possibility of finding extraterrestrial life excites many people and is a popular topic. Instead of searching for life, many scientist focus their search for biosignatures which can range of things from chemicals, physical structures. Several possible biosignatures have been found on Mars but have not led to the conclusion that life once existed there.
The Vikings Mission had three specially designed tests for life of Mars that focused on the metabolic aspect in the definition of life. The test results were seemingly positive but were negated by a mass spectrometer and gas chromotograph which did not find organic compounds. As a result, scientist concluded that biosignatures were not found so there was no life on the surface of the planet.
One of the more convincing evidence of a Martian biosignature are microbial mats that were photographed by the Curiosity rover. The mats are also common in certain places on Earth and remnants of microbial ecosystems that typically grow in shallow waters. Scientists who spent their careers studying microbial mats could not find significant distinguishing features between the Martian one and terrestrial ones, yet were extremely cautious to use this as proof of Martian life. A sample return mission is required for more analysis on the possible microbial mats to know more, and such a mission is unlikely to happen anytime soon.
While there have been several indications that biosignatures how been found on Mars, granted some are more convincing than others, no conclusions have been made. This leads to the question, what would be considered sufficient proof of extraterrestrial life? A certain level of skepticism is healthy, especially in science, but will there ever been evidence strong enough to for a census and will that be possible if the life has gone extinct and there are only remnants?
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I love the angle you take on this topic. Questioning not the validity of particular evidence, but the standards by which we would consider evidence sufficient. The hesitance in classification of something as life on other planets and I think ties directly into the lack of direct observation. Humans tend to be very sure in their judgement of life when observing it first hand.
As far as negative aspects of this post, you could talk a little more about why this evidence was discounted and perhaps better ways to collect and analyze evidence.
Overall, good job.