The Martian Officer is a Hardmode, post-Golemenemy that spawns during the Martian Madnessevent. It will always initially spawn with a protective Bubble Shield, which provides the Officer significantly added defense and hit points, as well as immunity to lava and knockback, until it is depleted. After the Bubble Shield is destroyed, the Officer can now actually be harmed. If the Martian Officer is not killed within three seconds after the Bubble Shield is depleted, the Bubble Shield will re-activate with full strength. The Officer's health will not be restored.
The Bubble Shield is one of the few "enemies" that does not have an Expert Mode buff or boost.
The Bestiary entry for the Martian Officer: "Lacking offensive weaponry, officers in the Martian forces favor defensive measures to focus on relaying tactics to the soldiers."