burnt water

I was reading Cec’s answer about why water doesn’t burn. My question is–if water is a major by-product of burning, how come more planets don’t have water. I must of been asleep in science class for this one.

And the column in question is, Water contains hydrogen and oxygen. Why doesn’t it burn?"

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Well, water can undergo photolysis (by UV light) to hydrogen and oxygen again. The hydrogen is light, and therefore fast-moving, enough to escape from a Terrestrial-type planet; the oxygen, although it won’t escape, will react with unoxidized materials in the regolith to form (unsurprisingly) oxides. This is generally considered to be the cause of the loss of the water both outgassed from Venus and Mars, and brought to them by comets (Mercury may have been hot enough and small enough that the unphotolyzed water could escape; I actually don’t feel like doing the calculations).

The Earth, OTOH, evolved both photosynthetic life and a stable ozone layer (the latter being more important; the former may, but is not necessarily, a prerequisite for the second). Thus, it kept most, if not all, of its secondary water.

The outer giants (Jupiter through Neptune), kept a lot of primary water: the stuff existing in the original solar nebula (the inner planets probably had all of this swept away, pre-formation, by the increasing solar radiation and solar wind). Both temperatures and irradiation at the orbit of Jupiter are low enough that such minor bodies as its moons Europa, Ganymede, and Callisto still have large amounts of water (Io, OTOH, has been heated to such an extent by tidal action that it has pretty much lost its water).

And of course, a great many planets do have water, to some degree. If I recall correctly, it’s the second-most common compound in the Universe (after methane), and it’s been found in at least trace amounts on all of the planets and comets and most of the moons in the Solar System. It’s very seldom found in liquid form (pretty much just here, sporadically on Mars, and probably under Europa’s crust), but that’s just because of temperature: Most places that have it are frozen.

 On most planets it's known by another name: Ice.

Speaking of planets and water, I have always wondered why the claim is often made that “water” must have been flowing once upon a time on some planets such as Mars.

The geological formation that prompts this claim is usually an obvious “flowing” mark on the land that looks exactly like Earth’s rivers and tributaries as seen from the air. I don’t doubt that <something> was flowing, but how can we assume it was H2O? Wouldn’t any liquid of similar viscosity would produce the same pattern under the influence of gravity? And many other chemicals are known to exist on Solar System planets.

So is the “water was flowing” claim based on sound science or wild speculation?

As a rule, chemicals aren’t liquid over a big temperature range. Given a reasonably likely temperature range for Mars in the past and given a reasonably likely chemical profile (e.g., no seas of pure mercury) water’s about the only candidate that makes sense.

Alcohols of various sorts have a similar range and cosmically speaking there is an awful lot of alcohols in space. Not only in comets, but also dominant in interstellar space.

I’m not saying alcohol ran free on Mars, just you can’t say water is the only possible candidate.

Similarly, other types of hydrocarbons are possible and may even be produced by life forms living in the crusts of planets (see Thomas Gold on this)

Alcohols are common compared to most chemicals, and they are indeed found in nebulae and such, but they’re still not anywhere near as common as water. The only reason that you don’t read about astronomers detecting water in nebulae is that everyone’s known for years that they have water.

There’s also the matter that we know that Mars has water now (mostly in permafrost and the polar caps), and that same water was probably around back then, too, when it would have been liquid.

The presence of water now does not imply the presence of water then.

Comet debris are continually adding water to planets in significant quantities. There is probably even water-ice on the moon from this source.

My point still stands. Water is the most likely candidate for creating erosion features on Mars, but other liquids, or liquid like substances, shouldn’t be ruled out. Especially if the erosion occurred at an early period in Mars’ history.

Liquid candidates include water, alcohol, and low boiling point hydrocarbons.

Liquid-like candidates include pyroclastic flows from volcanic or meteoric origin.

 Said liquids need to meet two properties:

 1) They need to be sufficiently abundant that large enough quantities would flow to cause erosion.

 2) They need to be a liquid at the temperature of the planet.

 For planets anything near Earth's temperature, these requirements leave exactly one suitable substance: H20.
 Sure, there could be alcohols on Mars. In sufficient quantity to make up a river, though???

 As for hydrocarbons, what's going to be a liquid at Mars temperatures?

*Originally posted by Loren Pechtel *

Who says Mars was the same temperature as today ?

It is quite probable Mars had a substantial atmosphere and was considerable warmer way back then.

I don’t suppose you remember the great ice catastrophe on earth when the entire surface of the earth was covered in ice miles deep? That lasted for a pretty long time until volcanism managed to create enough greenhouse gases to melt most of the ice and recreate the oceans.

Hooray for greenhouse gases!

(And for the picky types, Im not referring to any current ice ages, I’m talking about the great grand-daddy one early in earth’s history)

I now know why I have a degree in English. :slight_smile:

*Originally posted by jezzaOZ *

 What simple hydrocarbon (ie, that could occur naturally in large quantities) is liquid at anything from Mars' current temperature to Earth's?

One more candidate recently proposed. CO[sub]2[/sub] flows apparently form similar patterns.

BTW, although water is fairly common, I believe free oxygen is not (apart from the aforementioned minor amounts due to water or something else oxygen has reacted with gaining enough energy from UV to split up)? In looking for life in the universe (at least one type of potential life) couldn’t we be looking for a strong signature of oxygen once the Terrestrial Planet Finder is up in orbit?

Is this something extremely obvious that they are planning to do, or is there some bit of non-living chemistry that also cranks out large amounts of oxygen?

In fact, that’s TPF’s primary mission. They’re actually going to be looking for lines of ozone, not O2 (ozone lines are easier to detect), but the reasoning is the same: Nothing other than life produces significant amounts of O2, and nothing produces significant amounts of ozone without plenty of O2 as a starting point. Ozone, then, implies life.

 Slight problem--to get CO2 flow, you need a *LOT* of atmospheric pressure--something Mars isn't likely to have had. At pressures we consider normal, CO2 has no liquid state. That's why it's called *DRY* ice--it goes straight from a solid to a vapor with no liquid stage in between.

 As for oxygen detection--if a spectrascope detects appreciable free oxygen on a planetary body, it's a clear sign of life.

 However, until our telescopes are a *LOT* better, it's rather useless information. Point a spectrascope at a likely candidate and all you'll get is the light from the star. Free oxygen there is *NOT* an indication of life, merely an indication of it's extreme temperature (enough to tear apart any compound that oxygen might have made.)

OK, looking for free oxygen is a good indication of life BUT early life on Earth (and probably in other places) did not have this requirement. In fact as you need life forms to get abundant free O2, this is a bit obvious.
Just because here the high energy benefits of the aerobic life led to the current well known higher life-forms, it does not mean that such evolution could not occur where a “reducing” environment prevailed. Note that O2 is dangerous at high concentrations, and can even be toxic at low concentrations. Aerobic organisms need sophisticated protection mechanisms to counteract this, so the anaerobic life may have some benefits!