That counters his administration's policy that NASA should focus on returning to the Moon for a landing in 2024.
The confidence of Trump's assurance that the moon is somehow "part" of Mars is remarkable.
Nasa has not publicly responded to Trump's latest theory.
According to journalist Matthew Gertz, Trump's "of which the moon is part" business comes from his attempt to live-tweet what he saw on Fox News, whereon a NASA guy said that a mission to the moon will help them ultimately get to Mars.
However, Dr. Gu said he took issue with such an explanation, slamming Trump's words as "confusing and incoherent beyond measure" and saying the president "speaks gibberish".
"They should be focused on the much bigger things we are going, including Mars (of which the Moon is a part), Defense and Science!"
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The NASA website also provided details on the space agency's plans for making the moon a jumping off point for future missions to Mars and a place to test equipment and technology for other forays out into the solar system.
The tweet is at odds with a directive Trump signed in December 2017 to send USA astronauts back to the moon for the first time since 1972.
It's possible he meant that going to the moon would be a kind of staging mission for going to Mars, but still.
Last month, the president said he wanted to include an additional $1.6 billion in his budget so the United States could "return to Space in a BIG WAY!"
"Moon is part of Mars is going to lead to moon truthers and Qanon people joining forces", reporter Yashar Ali jokingly predicted.
When asked about the tweet, a NASA media relations specialist pointed CNET to a response tweet by NASA administrator Jim Bridenstine.
It would be challenging for NASA to talk about humans on Mars without first talking about the moon.
There are probably some frantic emails flying between NASA headquarters and the White House right now.