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Comment by dredmorbius | Hacker News Reader

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

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.

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"...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...

At least, those planets of a small enough size to lose water. Jovians probably don't count.

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.

Wouldn't it be more correct to say "the presence of oxygen in the atmosphere helps retain water on planets"?

We don't really know how well presence of oxygen correlates with presence of life yet, do we? :)

Well, sort of. It takes a lot of free oxygen to overcome all the various reducing agents usually found on planets (iron, hydrogen, aluminum, lithium, silicon, etc). The only process we know of capable of generating a continuous long-term flow of oxygen is (aerobic) life. The presence of an oxidizing atmosphere (rather than the standard reducing atmosphere) is thus considered an indicator of life.

Happened that way of Earth, anyway. See http://en.wikipedia.org/wiki/Great_Oxygenation_Event

While it's possible, I suppose, that there could be places with such naturally abundant oxygen that there could be free oxygen left over to create a static oxidizing atmosphere, even in spite of geologic churn, astronomy and cosmology says it's unlikely.

Also it might help answer the question of where did the water from Earth (and Mars) came from.

I had a late night, so I'm a little slow this morning. Can you explain the connection? I read the linked article and couldn't see how that explains where planetary water comes from.

I'm speculating, but it might help, or at least give some clues.

If we can establish the history of water on Mars then compare it with Earth. Or composition, presence of elements, etc. Do they share a common source? Common processes?

It is speculated that some water came through comets, for example

See http://en.wikipedia.org/wiki/Origin_of_water_on_Earth

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