Meanwhile, on Mars...

The rover Curiosity is busy chomping on sand and spitting out reports on its contents and lack thereof. There's chlorinated compounds, and an unusual ratio of deuterium to hydrogen in water samples.

Deuterium, it turns out, is a significant indicator of the initial formation of the universe during the period immediately after the Big Bang, and the ratio of deuterium to hydrogen (protium) hasn't changed much since that Big Bang. There's an interesting statement in Wikipedia's page on Deuterium, which reads as follows:-

"In fact, the discovery of deuterium/protium ratios in a number of comets very similar to the mean ratio in Earth's oceans (156 atoms of deuterium per million hydrogens) has led to theories that much of the Earth's ocean water has a cometary origin.
Deuterium/protium ratios thus continue to be an active topic of research in both astronomy and climatology."

Now then - let's turn that around and look at it this way:-  It would be more logical to assume that a planet's oceans had become comets following some cataclysmic event such as the collision of that planet with another similar body from space, rather than to assume that somehow, magically, a bunch of comets were attracted to a planet where these dirty snowballs then melted to form its oceans. 

Older texts report that Mars about three and a half billion years ago was struck by a meteoroid of between 100 and 200 Kms in diameter which punctured its crust causing a major tectonic event. That 'major tectonic event' probably involved an interior explosion inside the core of Mars which would then cause immediate if momentary expansion of the planet's core. That in turn might have thrown off its skin of hard surfaces, or most of those, including its oceans. This debris then very probably drifted off in space, forming what's now called the Asteroid Belt and our periodic Comets, samples from which have indicated sodium content, suggestive of salt water origins.
I think my theory makes more sense than some kind of reversed process involving mysterious comets forming oceans on planets in some unexplained manner.

But getting back to Mars - today's Question Everything is: How can they tell whether the elements found on Mars are those which are native to its own origins, or whether those have arrived later, such as via that meteoroid which collided with it long ago?