Final answer:
The phrase 'PAN-PAN' is used in urgent situations that are not immediately life-threatening, differing from a 'MAYDAY' distress signal. If on the moon, communication should be done with visual signals such as flashing a light, since there is no atmosphere for sound to travel.
Step-by-step explanation:
The phrase "PAN-PAN" in a radio message indicates that there is an urgency on board a vessel, aircraft, or other vehicles, but that at the moment, there is no immediate danger to anyone's life or to the vessel itself. This is different from a "MAYDAY" call which is used in a distress situation where there is a grave and imminent threat requiring immediate assistance.
In a hypothetical scenario where you're on the moon, the best way to communicate a broken radio to your friend would not be through yelling due to the lack of atmosphere for sound to travel, but rather through flashing a light or using some form of visual communication that does not rely on sound waves.