We tend to assume that the Earth is a more-or-less closed system... but that assumption has recently been proven wrong.

Today I learned that in its early history, the Earth's oceans contained significantly more water than they do today; hydrogen from split water molecules has escaped into space.

Actually, this is pretty fascinating.

Although water covers 70 percent of the Earth's surface, water is actually a rare substance that represents just 0.05 percent of the Earth's total mass.

Water has nevertheless played a crucial role in the emergence of life on Earth. Without water, the Earth would in all likelihood be a dead planet.

The amount of water on the planet has not always been the same, however. A research group at the Natural History Museum of Denmark has discovered this by measuring how hydrogen isotope ratios in the oceans have changed over time.

"The water that covered the Earth at the dawn of time contained more of the lighter hydrogen isotope than the heavier hydrogen isotope, known as deuterium, than it does today,” says Emily Pope, a post doc, who has played a central role in the study.

“By examining how the ratio of these isotopes has changed, we have been able to determine that over the course of around four billion years, the Earth's oceans have lost about a quarter of their original mass."

And the reason the hydrogen left our atmosphere is probably because it got flushed away by the solar wind.

Solar wind is a plasma (a soup of charged particles) emitted by the sun that washes the whole solar system. Thanks Earth active core, our atmosphere is protected by a magnetic shield that stops charged particles, pole auroras are the remaining of this epic battle, they are caused by charged particles colliding into the atmosphere.

Mars is less lucky, it is too small, its core cooled fast and it's magnetic field disappeared long time ago.

So its atmosphere got washed away by the solar wind, and probably it's surface liquid water too.

However, it's  still possible that some liquid water remains under the surface, and that's why we're sending robots to track it. Because liquid water is a clue on the search for life as we know it.

from NASA

space wind blew it over to earth...

Solar wind is fascinating concept. 

Thanks for the explanation, Oce. I'm glad we're sending robots to look deeper.

More on Mars' former giant ocean:

http://pandawhale.com/post/59619/mars-was-once-home-to-a-giant-ocean