We'll soon know what kinds of crops we can grow on Mars! Bonus, the fruits will be already freeze-dried!

Handy list of preferred pH's of common garden plants.

-Mike

That's really a difficult question to answer, considering the actual Martian data that we have to work with.

'Don't know, it'll be interesting to find out'. Tick.

Considering the fact that there is water ice and some salt, basic minerals, I think we can make a rough assumption about the pH value of martian soil.
However the verdict is still open.

My money is on fairly acidic.

Oh man, my camp is dead last! The curse of the far side continues...

Brian

The results indicate that, as a group.... we don't have a clue!

The robotic arm has just delivered the first sample of soil to Wet Chemistry Laboratory.
Click to view attachment

The universe's first, interplanetary Pot Noodle?

Poll = Fail

The results from the first sample are a pH between 8 and 9

Doug

News: WCL analysis indicates pH of "8 or 9." Very surprising!

--Emily

Someone spilled the baking soda....that explains all the bright patches Phoenix is seeing - Stu, you have your answer!

Nope. Not baking soda. That would hit a pH of 7 (neutral). More like sodium carbonate.

Looks like we'll be sipping soothing mint tea on Mars...

-Mike

(P.S. my guess was at the other end of the pH scale)

Not surprising. The pH is a rather quirky parameter that is also temperature dependent (although that may not be entirely relevant here). The WCL takes a sample of the regolith/soil and introduces it to water at an un-martian temperature. I get many hits to "phoenix pH WCL" and need to sort through them.

My first thought is that the high pH is from hydroxyl ions, but we'll need to see the entire chemistry to figure out what is happening.

Here is a quick-and-dirty explanation of pH _vs_ temperature:

http://www.lenntech.com/Correlation-betwee...Temperature.htm

which, again, may not be entirely relevant here.

--Bill

High -OH does not have to be present in the original sample.
A simplified view :
High pH means that the salts involved have a "weak acid" part, thus anions that form strong acids are probably low concentration in the sample:

Cl-, Br-, I-, SO4--, NO3-, ClO3-, ClO4-

As Mike said, CO3-- (carbonates) or any other weak acid would be a good candidate.

Remember this is a real sample, with many salts probably present so common ions effects might also be present.

Let's wait and hear what the Phoenix team's martian experts will say !

Poll now closed, because that would be like...cheating


Doug

Baking soda has a pH of 8 or 9.

I stand adjusted.

I made a basic error.

<ducks>

Lol.

May not be relevant here, but silica (SiO2) itself can be considered a weak acid, so that just freshly pulverized basalt (probably a major component of martian sand) will yield a mildly basic solution in contact with liquid water. You don't need a basic "salt" as such - any basaltic silicate mineral, olivine or pyroxene or plagioclase feldspar, will do.

-- HDP Don

Doesn't matter much, since a pH of 8-9 was not one of the choices anyway

Airbag

DOI: 10.1306/74D70E75-2B21-11D7-8648000102C1865D
Dissolved Products of Artificially Pulverized Silicate Minerals and Rocks: Part II
W. D. Keller, A. L. Reesman
Journal of Sedimentary Research
Volume 33 (1963)

ABSTRACT

Indeed, slightly basic pH (8.2 to 9.2) is reported by these authors for most silicate rocks.
More recent literature anyone ?

PS. It seems we were all biased by the "past acidic conditions" and the tons of sulfates at Meridiani :-)

I was biased by the prospect of acidic windblown dust or salts making up the top layer of the surface.

I guess that didn't pan out.

<ducks and runs for cover>

Except those of us who can barely remember from school which side of 7 is basic and which side is acidic, nevermind guessing what the conditions on Mars might be!

It may be worth stressing that at the briefing they pointed out that conditions may vary considerably with depth or with location on Mars. Today's result is likely just one data point rather than "the answer" to what the pH is on Mars.

Apparently we're all going to be eating asparagus on Mars.

Stinky urine, anyone?

Edit: that article has some ... interesting units. A cubic metre of soil? Blimey!

"The 1 cubic meter (35 cubic feet) of soil was taken from about 1 inch below the surface of Mars and had a pH, or alkaline, level of 8 or 9. "

Oh boy. Did they take the cartoon of Phoenix burying itself seriously ?

Doug

Emily's blog is, as ever, far more informative - and suggests it was a cubic centimetre of soil sampled.

Oh well, Reuters was only out by six orders of magnitude. ;-)

The free-style mixing of standard units and metric ought to be blacklisted in Mars exploration PR.

Some people -- you give 'em an inch, and they'll take a hectare.

I agree. Non-metric has to go. It just acts as a fog.

I mean, Fahrenheit - for goodness sake spare us . . .

but save the pint! (no impact on science).

Well, save the Imperial pint. The US pint has got to go.

Brian

That article left out half his sentence though...

"Preliminary results showed the soil had a pH between 8 and 9, researchers said. A pH less than 7 means the solution is acidic, while a pH over 7 means it is alkaline. Phoenix also detected the presence of magnesium, sodium, potassium and chloride in the mixture.

"It's very typical of the soil here on Earth minus the organics," Kounaves said during a teleconference from Tucson, Ariz.

On Earth, asparagus, green beans and turnips could be planted in such an environment and chemical-loving bacteria would thrive there, he said.

Planetary scientist David Paige of the University of California, Los Angeles, said it is too early to tell whether the minerals found in the soil could support life. Paige, who had no role in the mission, said the find was not surprising because rocks weather over time and bits of minerals mix with the soil."

http://news.yahoo.com/s/ap/20080626/ap_on_...3uBRCTV7nL0kPUI

What does a pH of 8-9 do to the "missing carbonates" paradox? If there was liquid water on the surface of the Mars at some point of time and CO2 in the atmosphere, then pH 8-9 (in contrast to an acidic pH most of us expected) is ideal for precipitation of carbonates (e.g. limestone). Where are all those carbonates?

From the article:
"When told the pH levels, one colleague 'jumped up and down as if he had the winning lottery ticket,' mission soil analysis specialist Michael Hecht told a telephone news conference."

Evidently he was in the UMSF poll.

Careful! It's an American mission. If they get antsy it'll more likely be the metric measures which will get scrapped, in which case you guys are going to have to learn to live with feet and miles as well as inches.


Mixing measures of length with measures of area in your metaphors serves only to confuse rather than to clarify.

======
Stephen

Note for admin - the top quote in post 34 there is mis-attributed, not sure how that can happen.

Strange - somebody must have got there units mixed up - I've fixed it - James

Back on the pH, I have a question for the chemists. On Earth clays in soil act as pH buffers. Is this only because of their associated organics, or would organic-free martian clays be expected to do the same?

Just to clarify things I wrote earlier in this thread: the fact that the pH of the first soil sample was 8-9, i.e. slightly basic, does not mean that the main salts present are carbonates, phosphates or whatever other "weak acid" anions. The first results from the team, also mentioned in the nice sum up in Emily's blog, show they found Na+, Ca+, K+, and Cl-, and are currently measuring the levels of sulfates, SO4--
Combinations of the above ions form salts where both ions are very soluble in water, and thus, do not alter the original pH of the water brought from earth, which was 7, although they might be the main salts in the soil concentration-wise. Basic salts could be just a very small proportion, or even absent if martian basaltic silicate minerals in sand have a slight alkaline pH, as mentioned by D. Burt.

Buffers can be either inorganic and organic.

So an inorganic salt could be a buffer (example: sodium bicarbonate solution).

Hydrate complexes formed by cations (K+, Mg++, Ca++, etc.) dissolved in water also have their own pKa's (and pKb's) and could also act as buffers.

-Mike

Hmm. What does this tell us (if anything at all) about the existence of superoxides at the site? Straining my memory of ancient high-school chemistry here, I would expect such compounds to be aggressive electron donors & therefore acidic.

(Okay, now tell me I got that backwards. Plus, I know it's a subsurface sample, but you'd expect this stuff to get circulated down at least a couple of cm due to wind action over time.)

Yup. The pKa of hydroperoxy anion (HOO-) is 4.88 (acidic like vinegar). So at any pH above 5, the predominant form will be the superoxide anion (O2--)

At pH 9, the ratio of superoxide anion to hydroperoxy anion will be about 10,000 : 1.

(Source: Wikipedia/superoxide + others to confirm)

-Mike

As you feared, you actually have it a little reversed. An oxidizing agent like oxygen gas, peroxide ion, or superoxide ion wants to gain electrons (be an electron acceptor) - all want to turn into the oxide ion, O2-. A reducing agent like iron metal wants to lose electrons (be an electron donor), to either the ferrous ion Fe2+ or the ferric ion, Fe3+. Combine them and you get either FeO (wustite) or Fe203 (hematite). Oxidizing and reducing are clearly relative terms, because an ion like Fe2+ can either be oxidized to Fe3+ or reduced to Fe metal.

Regarding acids, by the electronic (Lewis) definition of acids and bases, an acid is electron pair acceptor, generally with a large charge to radius ratio (the ultimate acid is the tiny naked proton H+), whereas a base is an electron pair donor (e.g., the oxide ion, O2-, which has a pair of electrons to donate). Combine them and you get neutral H2O or water. In aqueous solution, these two ions are unstable - the proton combines with a water molecule to form H3O+ (hydronium) and the oxide ion takes on a proton to form OH- (hydroxide). By common convention (acidic and basic, like reducing and oxidizing, being relative terms in a continuum), an aqueous solution with an excess of hydronium is called acidic and one with an excess of hydroxide is called basic.

As you correctly recalled, there is some overlap between the definitions of oxidized and acidic, and between reduced and basic, inasmuch as both definitions involve electrons and ionic charge. This is accounted for in the so-called Usavovich definition, only rarely used by geochemists, which does not differentiate between oxidizing agents and acids, or reducing agents and bases. A familiar example of its importance is to compare the acid strength of reduced hydrogen sulfide H2S (very weak) with that of oxidized sulfuric acid H2SO4 (very strong). That is, if it's oxidized, its generally more acidic, as you correctly you pointed out.

Relevance to Mars? That's a longer story, and this post is already too long.

-- HDP Don

No one else has dared to tackle this, one of the biggest unanswered questions of Mars, so I'll foolishly try. One common hypothesis is that the ancient martian atmosphere was very different from today's, owing to a very high content of volcanic sulfur dioxide, itself caused by extremely active volcanism in the past. By this hypothesis, the acidification of surface waters by the atmosphere rendered deposition of carbonate rocks impossible. This would imply that the rate of absorption of acid species from the atmosphere always greatly exceeded the rate of neutralization of those acid species by the pulverized basic rocks of the martian crust. One chemical problem with this hypothesis is that seasonal or permanent ice cover on Mars would completely cut surface waters off from contact with the acidic atmosphere, allowing neutralization by reaction with rocks, at least seasonally. Another one is that surface waters on Earth (I know, terrestrial analogs are dangerous for Mars) are only rarely acidic, despite all the acids (carbonic, sulfuric, and nitric) constantly being tossed into the atmosphere by our civilization, not to mention our extremely active volcanism (compared to Mars). The implication is that surface rocks (on Earth at least) generally buffer the pH of surface waters to neutral or basic, even without an ice cover, and with active volcanism, coal-burning power plants, smelters, and so on.

Another common hypothesis, a far simpler one, is that, owing to low pressures and low temperatures, liquid water on Mars was always somewhat transient. That is, if present, it never stuck around long enough to reach chemical equilibrium with either surface rocks or the atmosphere, and thus to precipitate appreciable quantities of carbonates (other than the minor quantities found along fractures in some meteorites from Mars). Offhand, I'm not aware of any chemical problems associated with this hypothesis, but you may be. Also, keep in mind that the two hypotheses need not be mutually exclusive (that is, you could logically have a sulfur dioxide-rich ancient atmosphere, or any other ancient atmosphere, and transient or no liquid water at the same time).

Anyone else have any thoughts?

-- HDP Don

pH is a quirky parameter and we may be putting too much stock in this value. I do wish that the media would stop proclaiming "conditions favorable for life" when the conditions are simply "not hostile to life". I'll be wanting to see an anion-cation balance and Piper-Stiff diagrams (et al) when the full chemistry comes in. It's been centuries since I've done one manually, I'll need to unearth my textbooks...

--Bill

It's not really the media's fault. The Phoenix panel came out of the gate with a story from Hecht about how he called a friend working in Antarctica and told her that they now knew what they needed to know about how to grow stuff in Martian soil, because they knew what "nutrients" were there. And then they seemed confused when the reporters asked, okay, what kind of life are you talking about? Viruses? Bacteria? Multicellular stuff? Plants? And they weren't prepared to answer. Finally they said the pH was compatible with asparagus, turnips, and green beans, and there was nothing incompatible about the soil. But they'd started talking about life, so the reporters were asking, what about the trace elements? What about organics? And of course they couldn't answer those questions, because they couldn't do those tests.

--Emily

I really wish the panelists were a bit more conservative and better prepared in this last press conference.
After all, we know that the popular media will exaggerate whatever they hear to infinite proportions.
The press here today is full of comments about how easily "you could grow asparagus on Mars that is as tasty as in our own garden".

<sigh>, in a manner of speaking, this "Mars soil is yummy" comment... snowballed?

--Bill

Well....in one sense the press has picked up on a key finding, if you look at it from the eventually-there-will-be-colonists-on-Mars in a "Farmer in the Sky" point of view: the results of the basic soil of this one analysis help bring Mars down to Earth. I can imagine what kinds of crops might be grown there someday.

The Big Three, NPK (nitrogen, phosphorous, and potassium), can be adjusted by large amounts of fertilizer, and trace micronutrients can also be easily added (I use Greensand) to soil. Soil microorganisms and plants can be coaxed to live as long as there aren't any wierd yukkies (funkybad salts like arsenic) or other reservoirs of difficult reactive species.

Soil pH, while adjustable, can be a pain to deal with. Ask anyone from Texas trying to grow acid-loving rhododendrons in alkaline caliche. You can do it, but you will be adding soil amendments forever. It is a losing battle, as the background soil will continually be trying to buffer out the tiny acid patch you've installed to grow your rhodys.

So when the full results of the soil analysis are complete, (and assuming no funkybad stuff) it will be possible to picture what bags of soil amendments and other materials would be required to mix into martian soil to make it suitable for agriculture.

I have no idea why, but I find that concept both fascinating and comforting.

-Mike

Mike, you consistently amaze me with your ability to coin great band names..."The Funkybad Salts" would definitely kick gluteus maximi!

Thanks to Don also for the illumination. From what I gather, a pH reading alone is not enough to extrapolate detailed chemical properties and compositional details on Mars, even when compared to mass spec data. It helps, of course, but it also seems too easy to apply terrestrial analogues too freely, 'cause that's what we know. MSR is a must.

Mike... so do I.... "The Farmer in the Sky" has just gotten a tiny bit closer to reality .....
To me this is like being in a hard science fiction story... and so much more to come.

Craig

It's still looking better. The last "key finding" in '76 suggested a horridly reactive superoxide mix, and you can add the "salty" sulfates at Meridiani. I'd love to put a chunk of ice into the WCL.

At home, I have to "make" soil for my flower and wildflower gardens. Except for paltry 6" of organic "A" horizon, most of my soil is a clay soil derived from the weathering of Pennsylvanian sandtones and shales, so I have to add lime, organics, sand and nutrients to the mix.

--Bill