It's that last bit some scientists are a little squeamish about.
Space agencies all over this world have meticulously protected other planets from Earth's germs for decades. NASA even has an entire department dedicated to the task, aptly named the Office of Planetary Protection.
If we do find alien life, NASA wants to make sure it's actually alien, and not some Earthling (likely bacteria) that hitched a ride on one of its spacecraft. That's why you always see people building spacecraft wearing white plastic suits in cleanrooms making sure the spacecraft stays immaculate.
An illustration of the Interplanetary Transport System - the spaceship SpaceX plans to take humans to Mars in - refueling in orbit.
"There's really nothing on the surface of Mars, I think," the SpaceX CEO said during a press call Tuesday evening. "There may be subterranean, chemotrophic bacteria. I would expect they're pretty hardy, and there's not much that we could do to kick them out even if we wanted to."
As Emily Lakdawalla wrote for The Planetary Society, the organization Bill Nye heads that has proposed humans orbit Mars first and think about landing later, Earthlings are notoriously germy.
"If we keep our filthy meatbag bodies in space and tele-operate sterile robots on the surface, we'll avoid irreversible contamination of Mars - and obfuscation of the answer to the question of whether we're alone in the solar system - for a little while longer," Lakdawalla wrote in a 2015 blog post. "Maybe just long enough for robots to taste Martian water or discover Martian life."
What a human Mars mission could look like
On the flip side, alien Martian life could be just as dangerous to humans as we could be to it, which is another argument scientists make for keeping missions sterile. NASA has been planning a human mission to Mars, and the agency has of course weighed the risks of contaminating astronauts, or the planet they might land on.
Agency scientists who studied the issue in 2005 concluded: "All operations of an initial human mission to Mars should include isolation of humans from any direct contact with materials from Mars for planetary protection purposes."
That means a NASA trip likely wouldn't look like the one in the 2015 blockbuster, "The Martian," with Mark Watney growing potatoes, suitless inside an oxygen-filled habitat, on the planet's surface.
The SpaceX colony would probably look a lot different, too, and be much more permanent. Musk spoke about the distant possibility of growing plants on the Red Planet if they change the atmosphere, or even creating oceans where water used to flow millions of years ago.
He wouldn't be the one to make the decision to change Mars so it looks more like Earth in a process called terraforming, though.
"Terraforming would take us a long period of time, and I think, would ultimately be the decision of the people on Mars. We have to get there in the first place," Musk said. "The larger point is creating a self-sustained civilization on Mars to provide insurance for life as a whole - life as we know it."
'Out there among the stars'
SpaceX/YouTube
Elon Musk presenting SpaceX's human missions to Mars at the 67th International Astronautical Congress in Guadalajara, Mexico.
Musk kept repeating this phrase, "life as we know it," which seems to basically mean "not aliens." And that is the difference between Musk's ideas and those of scientists who want to keep space free of Earth's scourge - he's prioritizing "life as we know it."
"I think, really, the planet we should be considering for protection is Earth," he said. "We can bring life as we know it, and breathe life into Mars where it doesn't exist today, and ensure that if there is some kind of cataclysmic event on Earth, that life as we know it continues to exist."
Musk's defense for sending humans to colonize Mars may not convince some scientists, but it sure is poetic:
"I think a future where we are a space-faring civilization and out there among the stars is infinitely more exciting and inspiring than one where we are not."