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InAwe
2008-Sep-02, 01:51 PM
I've been a visitor to this forum for a few weeks, and have found it to be both informative and fun. I was very surprised to learn of the apparent discovery of perchlorate in Martian soil, and even more surprised that it has generated so little discussion here.

Based on my limited understanding of the history of the Earth, the initial planetary conditions were characterized by a chemically reducing environment. It was only after the innovation of photosynthesis (and considerable time) that a surplus of oxygen was produced to yield the currently moderately oxidizing environment.

That makes me wonder what process could produce something so strongly oxidizing as perchlorate on Mars. I am not aware of any other body in the solar system that is known to have a non-reducing condition, but that could be simply my own ignorance. I'd be grateful for thoughts from those with more knowledge on the subject.

geonuc
2008-Sep-02, 02:22 PM
I believe there has been discussion of perchlorate in the Phoenix on Mars thread.

slang
2008-Sep-02, 02:40 PM
Phoenix on Mars (http://www.bautforum.com/space-exploration/74917-phoenix-mars.html) thread in the Space Exploration forum.

InAwe
2008-Sep-02, 04:14 PM
Thanks for the lead. I'd missed those posts.
I believe there has been discussion of perchlorate in the Phoenix on Mars thread.
Perhaps I'm just not seeing it, but I could not find any discussion of the origin of the perchlorate. The closest I found was a link to another article with the assertion, "In nature perchlorates form photochemically in the atmosphere, and then settle randomly on a planetary surface." I have found papers reporting preliminary evidence for the photochemical formation of perchlorate on Earth from chlorine/oxygen precursors, but it seems a stretch to extrapolate to Mars, and it raises the question of where the precursors would have come from.

InAwe
2008-Sep-02, 04:20 PM
I'm providing links in a separate post because I assume there will be a hold placed on a post with links.

Article referenced in the Phoenix on Mars thread:
http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=2849

Example of report of photochemical formation of perchlorate:
http://phys4.harvard.edu/~wilson/arsenic/conferences/ACS_abstracts_MARCH_2006/envr_38

geonuc
2008-Sep-02, 05:35 PM
Perhaps you should jump in the Phoenix thread and lead the perchlorate discussion. :)

01101001
2008-Sep-02, 06:21 PM
Perhaps you should jump in the Phoenix thread and lead the perchlorate discussion. :)

I'd be fine with a thread for Mars chemistry, like this (but maybe better moved to Space Exploration), separate from Phoenix activity news, if chemistry is destined to be discussed deeply.

But, I'd add the speculation as to the source of the perchlorate would be most fruitful if preceded by strong evidence that the perchlorate actually exists. I haven't seen that reported in the informal news, yet, and certainly not published in the literature.

Jim
2008-Sep-02, 07:09 PM
Let's try it in Space Exploration.

Swift
2008-Sep-02, 07:58 PM
This blog (http://martianchronicles.wordpress.com/2008/08/07/phoenix-update-pondering-perchlorates/) offers some thoughts on how perchlorates may have formed on Mars, assuming they formed there (not some rocket contaminant).


Alternatively, the perchlorates may have been formed on Mars. In the lab, perchlorates can be created by evaporating the right kind of acid. However, perchlorate salts also occur naturally in Earth in extremely arid environments, like the Atacama desert in Chile. One study has shown that these salts can form by exposing typical chloride salts (think NaCl) to sunlight or ultraviolet light for long periods of time (months). This is a pretty appealing case for Mars, since certain salts that often form with chloride salts have been detected all over the north polar region (ref: my thesis!), and the surface of Mars receives a ton of UV through the thin atmosphere.

InAwe
2008-Sep-02, 10:05 PM
. . . the speculation as to the source of the perchlorate would be most fruitful if preceded by strong evidence that the perchlorate actually exists. I haven't seen that reported in the informal news, yet, and certainly not published in the literature.
Excellent point! Perhaps I'm being hasty.
But, the reason I am asking has a more basic question at its root. Based on my understanding of the atmospheres on Venus, Mars, and the early Earth, I would not have expected the environment on Mars to be compatible with the spontaneous formation of an oxidized species such as perchlorate. If it is, how did those conditions arise? (On this planet, living organisms were required, I think - not to produce perchlorate, but to produce an atmosphere that is sufficiently oxidative to allow perchlorate to form.)

In short, my initial response is one of utter amazement that something as reactive as perchlorate could be present on Mars. The fact that the overwhelming assumption was not that it must be a contaminant is what caused me to wonder if there is some aspect of its formation that could be compatible with a highly reducing atmosphere and my question was aimed at learning what that is.

JonClarke
2008-Sep-02, 10:32 PM
G'day!

Several things to remember.

First of all on Earth, and apparently on Mars too, perchlorate is is a surficial process, formed by UV action of surface minerals. Its probably constantly being broken down and renewed.

Second, Mars does not have a reducing atmosphere, it has a near neutral one, dominated by oxidised (and weakly odixising) CO2. The presence of a soild oxidant, formed by photochemical reactions, is consistent with it.

If Mars had a reducing atmosphere it would have been very early in its history, with the probable presence methane. Under such circumstances perchlorate is unlikely to form because the methane would form photochemical smog through UV action and prevent the UV reaching the surface. If it did form it would probably be consumed through reaction with atmospheric and surface organics.

Lastly, the amount of perchlorate present in proably small, I would be surprised if it is over the 100s of ppm range, and may be less.

Jon

InAwe
2008-Sep-03, 07:15 PM
Jon,
Thank you for your informative response. I assumed my surprise was due in large part to my own ignorance.

Do you have a guess as to what type of substance might supply the oxygen? Is it CO2? And what consumes the electrons?

Do you know if there is any experimental evidence of such a reaction occurring under Martian conditions?

01101001
2008-Sep-03, 07:33 PM
Atmosphere of Mars (Wikipedia (http://en.wikipedia.org/wiki/Atmosphere_of_Mars)):


Oxygen 0.13%

What does it need to be?

JonClarke
2008-Sep-03, 10:50 PM
Jon,
Thank you for your informative response. I assumed my surprise was due in large part to my own ignorance.

Do you have a guess as to what type of substance might supply the oxygen? Is it CO2? And what consumes the electrons?

Do you know if there is any experimental evidence of such a reaction occurring under Martian conditions?

G'day

I don't know of any studies of how perchlorate forms on Mars. It is hard enough understanding how it forms here on Earth!

There are many potential oxygen sources for such reactions, I suspect. There is oxygen in sulphates silicates, water and CO2 ice. Also the atmosphere contains traces of O2 as 01101001 pointed out.

Jon

Ivan Viehoff
2008-Sep-04, 03:37 PM
I thought it was well known that the surface of Mars is very oxidised. All that Fe2O3 making the place red.

The Labelled Release experiment carried by the Viking landers, in which the soil had some nutrient solution added to try and get any "life" to metabolise and give off gases, resulted in the surprising vigorous release of CO2. http://en.wikipedia.org/wiki/Viking_biological_experiments The simplest inorganic explanation for this is that the soil contained a vigorous oxidising agent. So when I heard they had found perchlorate, I wasn't in the least surprised. But I'm simplifying, I think it is more controversial than that.

01101001
2008-Sep-04, 05:34 PM
The simplest inorganic explanation for this is that the soil contained a vigorous oxidising agent. So when I heard they had found perchlorate, I wasn't in the least surprised. But I'm simplifying, I think it is more controversial than that.

NASA Phoenix Mission: Audio Briefing, August 5 (http://www.nasa.gov/mission_pages/phoenix/multimedia/audiobriefing-20080805.html)


Eric Hand:
Hi. Yeah. I was hoping you could put this in the context of previous probes. I think someone else asked this question. But, maybe you could describe how this perchlorate finding is different from what Viking found.

They didn't find this. There's been a lot of debate over what the oxidizer was that Viking found. A lot of folks thought it was hydrogen peroxide. Does this suggest that perchlorate is the main oxidizer on Mars. How does this relate to what Viking found?

Richard Quinn:
This is Richard Quinn speaking. You're absolutely right about the statement of Viking. The leading hypothesis for what was seen in the Viking biology experiments was the presence of small amounts of peroxide or superoxide. For those who are not familiar with it, it was the light detection experiment.

What they saw was, when they wetted the soil, they saw a small amount of oxygen released. They saw some decomposition of some of the organic nutrients that they brought with them. Perchlorate is very different. Perchlorate is typically not very reactive in solution.

It would not typically give off oxygen when wetted. So the perchlorate is not likely, in my opinion, it is not responsible for the Viking biology experiment. It's not the same compound. But, I do think that it all does point to a very interesting photochemistry that I mentioned earlier.

The hypothesis with Viking is that the small amounts of peroxide were also produced in the atmosphere photochemically and then deposited down on the surface. So again, yeah, I think that there will be some very interesting ties in the future to these types of compounds, how they interact with water and how they move through soils and what the implications are there.

JonClarke
2008-Sep-06, 07:34 AM
It is quite possible, indeed probable that there are several different oxidising agents present.

It is also likely that the abundance and proportions of these vary across the surface of Mars.

Jon