"No indigenous Red Planet organisms have ever been discovered. But it is possible that life on Mars — if it ever existed — may have made its way to Earth at some point, many scientists say." [emphases mine]
The "likely" term is chiseled away by the more realistic terms of possibilities. It is interesting that the author chose the term "many" to describe scientists. Why? Perhaps, to give the possibility more weight than it deserves and lend credence to the use of the term "likely."
After reading the article, I wonder if the author should have stated "Some chemicals that life may require may have come from Mars." Or maybe the author thinks, despite the lack of evidence for life on Mars, the conditions on Mars may have been more conducive to life and maybe it started there, survived a necessarily violent upheaval in order to fly it off Mars toward Earth, survived radiation and vacuum and atmospheric reentry and a crash landing to seed life on Earth. Maybe.
If you read this for class, you might ask the students to come up with questions like: Why is life "likely" to come from Mars? How many is "many?" What does it mean that scientists acknowledge the possibility of something? Why do some scientists believe that life must have these chemicals to begin life? Are they saying that life on Earth is unlikely to begin without rocks from Mars? Is there evidence?