

Mars: The Planet That Lost an Ocean’s Worth of Water - dnetesn
http://www.eso.org/public/news/eso1509/

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dredmorbius
Loss of water (and/or dissociated hydrogen) on rocky planets is an interesting
area of study. A few weeks back I ran across a reference that the Earth is
thought to have lost a quarter of its water to space over the past 4.5 billion
years:

"The Earth has lost a quarter of its water" [http://sciencenordic.com/earth-
has-lost-quarter-its-water](http://sciencenordic.com/earth-has-lost-quarter-
its-water)

The determination was made by a similar H2O / D2O ratio analysis.

Given that the Earth has seven seas, that would be 1.75 oceans worth of water
lost. The process is moderated by the presence of oxygen in the atmosphere
(which recombines with dissociated hydrogen), so, curiously, not only is water
necessary for life, but life may well help retain water on planets.

~~~
warbastard
"...so, curiously, not only is water necessary for life, but life may well
help retain water on planets."

An interesting thought. So perhaps old planets that still have water are more
likely to contain life. We can detect water in some forms on exoplanets
already [1] so soon we may be able to predict how common life is in the
galaxy?

[1] [http://www.nasa.gov/press/2014/september/nasa-telescopes-
fin...](http://www.nasa.gov/press/2014/september/nasa-telescopes-find-clear-
skies-and-water-vapor-on-exoplanet/)

~~~
dredmorbius
There are a few chemical signatures that are likely to indicate life. Oxygen
would be one (though there are cases in which it can exist by other means).

Spectroscopy (how signatures are detected) is more _qualitative_ than
_quantitative_ , so it's _very_ difficult to get a good sense of _how much_ of
something exists, as I understand. Though it's _tremendously_ sensitive and
can detect atoms or molecules across light years, and for plants, the range is
~30-300 or so from hazy recollection.

Water's almost certainly a weaker signal of life than oxygen, but it's a good
sign all the same. And a strong water signal from an older but small (e.g.,
Earth-sized) rocky world would probably show that life does, or at least did,
exist.

