Not a green laser.
The green color of the laser depicted above is for illustrative purposes
A chemical camera sitting atop Curiosity, the Mars rover, has spotted signs that the Red Planet may have once had oxygen in its atmosphere, fuelling further speculation that it was once Earth-like. Today, Mars is a barren wasteland. Its surface is dry and caked in rust-coloured iron oxide dust particles – a stark difference …
Probably Mars had actual oceans of water, but due to the low gravity a lot of water molecules were getting high enough to be disassociated by UV from the sun. The result is hydrogen and oxygen gas. The hydrogen gradually escapes to space, and the oxygen bonds with whatever's available (iron mostly).
This is happening here too, but the rate is far lower, so we still have oceans. As the sun ages it will get brighter, raising more water vapor up to altitudes where the UV is strong. At some point the rate of water loss will skyrocket and the oceans will dry up just like Mars.
So yes, there was atmospheric oxygen on Mars, while its water was going away.
Not only that, but the only reason there *is* more than trace atmospheric oxygen on Earth is life. Biotic processes are what separate the Oxygen from the rocks (Iron and Calcium, predominantly). Otherwise, Earth would look rather a lot like Mars, with much of our oxygen tied up in the rocks.
Of course, it's spectacularly unlikely that Mars (or Earth) formed with all the Iron being iron oxide. Oxygen is a very common element, but to reach the all-pervasive nature of iron-oxide on Mars there would have had to have been atmospheric oxygen on Mars at some point...and for quite a while too.
"Mars had an Oxygen atmosphere" isn't exactly news. Proof that it didn't would be novel. Alternately, proof that there was once enough Nitrogen on Mars to serve as a buffer gas would be interesting. If there was that much at one point, where'd it all go? Mars should be able to hold on to Nitrogen...I think? Hmm...now I have to go look that up...
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