It is a claim for which justification is provided and that, if true, establishes the truth of the argument's main conclusion. It is a claim for which some justification is provided and that is required in order to establish the argument's main conclusion. It is a claim that provides some support for the argument's conclusion but that neither ensures the truth of that conclusion nor is required in order to establish that conclusion. It is a claim for which no justification is provided and that, if true, ensures the truth of the argument's main conclusion.
Astronomer: Earth was bombarded repeatedly by comets and asteroids early in its history. This bombardment probably sterilized the surface and prevented life from originating during this early period in Earth' s geological history. Meanwhile, Mars escaped severe bombardment, and so there could have been microbial life on Mars prior to there being such life on Earth. Because many meteorites originating from Mars have landed on Earth, life on Earth may have started when living microbes were carried here from Mars on a meteorite.
Which one of the following most accurately describes the role played in the astronomer's argument by the statement that there could have been microbial life on Mars prior to there being such life on Earth?