There's a hidden message on this family portrait that an Apollo astronaut left on the moon

On April 20, 1972, Apollo 16 astronaut Charles Duke took his first steps on the moon. He was 36 at the time and is the youngest human in history to ever walk on the lunar surface.

But that's not the only achievement of Duke's that lives on in American history.

(NASA John W. Young )
Astronaut Charles M. Duke Jr., Lunar Module pilot of the Apollo 16 mission, is photographed collecting lunar samples at Station no. 1 during the first Apollo 16 extravehicular activity at the Descartes landing site.

While he was on the moon, he snapped this family portrait of him, his two sons, and his wife, which remains on the moon to this day.

(Courtesy of Charles Duke)
"I'd always planned to leave it on the moon," Duke told Business Insider. "So when I dropped it, it was just to show the kids that I really did leave it on the moon."

The photo has since been featured in numerous popular photo books and is a great example of the "human side of space exploration," Duke said.

When Duke was training to be an Apollo astronaut, he spent most of his time in Florida. But his family was stationed in Houston. As a result, the children didn't get to see much of their father during that time.

"So, just to get the kids excited about what dad was going to do, I said 'Would y'all like to go to the moon with me?'" Duke said. "We can take a picture of the family and so the whole family can go to the moon."

More than 43 years have passed since Duke walked on the moon. And while the footprints that he made in the lunar soil are relatively unchanged, Duke suspects the photo is not in very good shape at this point.

"After 43 years, the temperature of the moon every month goes up to 400 degrees [Fahrenheit] in our landing area and at night it drops almost absolute zero," Duke said. "Shrink wrap doesn't turn out too well in those temperatures. It looked OK when I dropped it, but I never looked at it again and I would imagine it's all faded out by now."

Unfortunately, there is no way to determine just how faded the photo is because it's too small for lunar satellites to spot.

Regardless, the photos "was very meaningful for the family," Duke said. In the end, that's all that matters, right?