Oops, estimation fail, thanks! Attempt to double spend "closer" - on both "smaller" and "cooler". A sun-hot Moon gets you only a second Sun. A lunar-mantle-hot Moon gets you merely an extra dim red dwarf "sun". The "100x" brighter was wrong. More like 1000x dimmer?

The ChatGPT looks surprisingly ok. It struggled where textbooks and such are less than wonderful. So it raises the irrelevant Sun's core, and says "The Sun's energy output is vastly greater due to nuclear fusion reactions, emitting a massive amount of radiation across the electromagnetic spectrum. The Moon, lacking such reactions, emits no comparable energy.", perhaps reflecting the textbook "Sun shines because fusion" vs "Sun glows because hot, and hot things glow (with color and intensity proportional to temperature^4), and fusion is why it's still hot billions of years after forming". Though in a way it's true - "The Moon lacking such reactions, and being small, has cooled off and greatly dimmed since its last big formation impact". :)

The "Brown dwarfs typically range between about 1,000 and 2,500 degrees Celsius at their surface, far hotter than the Moon's mantle." seems inconsistent with [1].

Thanks for the check!

[1] graph: https://media.springernature.com/lw685/springer-static/image... source: Fig 2 on page 9 of https://hal.science/hal-02863845/document [PDF]

Although curiously, most processes theoretically capable of exposing large amounts of lunar mantle would likely generate a decent fraction of that heat just on their own merit.