Oh, this is a fun one. I can think of all kinds of things that would be pretty awesome to say to share that moment in words with the rest of humanity. Here are a few:
"We are now a multi-planetary species!"
"With the hopes and dreams of our species for a greater role in the cosmos, I now step forward onto Mars and we now step forward to the future."
"We now step foot onto Mars, the Red Planet, the fourth large rock from the Sun, and we embrace all of human curiosity and passion and the drive that has brought us from an ancient and primitive life on Earth to our future as a species embarking forward into the vastness of space. May time recall these moments as but the earliest steps forward to a greater future for us all"
Thought this might be appropriate:
"should have worn darker shoes"
"I thought Ray Bradbury would meet me here?"
"Space for all!"
"we're here to stay as humankind"
Humankind is indeed more accurate than mankind
"That was an even bigger leap for mankind"
Where are we of to next? Or... where's the water ice? Imagine being the first person to consume something from another planet!
"I don't think we're in Kansas anymore Toto"
Aww..I love this one !! :D However, right now I'm way too excited to think of anything sensible. I guess the first person who will land on Mars will "exactly" know what to say as he/she will only have the total experience of joy,pain, happiness, success and will-power to prepare and finish such a difficult expedition. I believe it should be kept as natural and spontaneous as possible even though plannings are usually involved. Like for instance according to Wikipedia before the moon expedition it was already decided from before that if landing occur successfully Neil Armstrong will be the first man to land on moon as Neil had less ego.
Words and sentences can be anything (as long as it makes sense) but it's the EMOTIONS, little flaws,grammatical errors behind the words that captivate us and makes history more REAL in our history books. Then again discussions are always fun and the first person on Mars might get some tips from us and then may add something on it's own. But too bad for me, right now I'm speechless about this ! :/