139. Read the following passage, then answer the question below.

Of all the planets, Mars has attracted the most robotic visitors, largely because it is the most Earth-like world within reach. Today it is cold, dry, and bathed in radiation, but orbiters and rovers have revealed channels, deltas, and minerals that form only in the presence of liquid water. The question that now drives exploration has shifted from whether Mars was once wet to whether it was ever alive.

Answering it is difficult because of distance. Radio signals take between several minutes and more than twenty minutes to travel from Earth to Mars, depending on the planets' positions. A rover cannot be steered in real time like a toy car. Engineers instead send a day's plan, and the vehicle carries out the instructions, using onboard cameras to avoid hazards. Increasingly, rovers choose their own routes and even select rocks to study. This autonomy is not a convenience but a necessity: without it, a mission would cover only a few meters between each exchange of messages.

Another difficulty is that the most informative tests are impossible to perform on Mars. A rover carries only a small laboratory, limited by weight and power. For that reason, NASA's Perseverance rover collects rock cores and seals them in tubes, to be retrieved by a later mission and analyzed in laboratories on Earth. The plan is expensive and technically daunting, since it requires launching a rocket from the surface of another planet, something not yet attempted.

Some scientists question whether such effort is justified when human missions might do the work far more quickly. A geologist on site could examine in an afternoon what a rover needs weeks to study. Yet astronauts would bring microbes of their own, and the risk of contaminating the very sites that might hold native life complicates the argument for sending people first.

Why does the author mention the microbes carried by astronauts?