The following is a post recovered from the NewMars forum thread on a Minimally Terraformed Martian Atmosphere:


You know, I've been wondering about this nitrogen thing for a while now. I'm not so sure atmospheric nitrogen is a prerequisite for a stable ecosystem. It would seem to me that once there is "fixed" nitrogen in a biological system (e.g. ammonia, nitrates, nitrites) there can exist a nitrogen cycle entirely decoupled from the atmosphere. See the EPA image below:

external image nitrogen_cycle_EPA.jpg

Through plant and animal decomposition and assimilation, the nitrogen makes a complete cycle and you have a happy ecosystem. The only rub is that "denitrifying" bacteria, who live in anoxic conditions such as the soil and stagnant water, like to use nitrate instead of oxygen as their metabolic electron acceptor. This turns the fixed nitrogen into atmospheric N2, which is useless to all life except nitrogen fixing bacteria, who help return it to a biologically accessible form. There are also abiotic ways nitrogen is fixed though, such as lightning and volcanic activity.

In some sense the question comes down to this: Are we setting up our terraformed ecosystem using gaseous N2 or in a fixed form like nitrates? Either will work, though if we use gaseous N2 we need to make sure we have >5 mbar. In addition, it will take a while for fixing bacteria to build up enough fixed nitrogen from this for an ecosystem to operate. If we initially introduce nitrates, however, higher plants will be able to immediately use it as a nutritive resource, and, through them, animals will also have access to nitrogen. The only requirement is that we make sure that there are >5 mbar worth of nitrates so that when denitrifying bacteria start bleeding nitrogen off to the atmosphere there will be enough pressure for nitrogen fixing bacteria to reabsorb it.

There is a second reason nitrates are attractive: they probably already exist on Mars. Many people claim that this is speculation since we haven't observed any nitrates on Mars, but this is false. The truth is that: 1) nitrates are extremely hard to detect using spectrographic techniques, and 2) there is every reason to believe that nitrates would only exist at in substantial quantities at depths of ~1 meter. An excellent paper by (Sutter et al. 2007) demonstrates this by using soil from the Atacama Desert, an oft used terrestrial Martian analog. The Atacama is a hyperarid (50 times less water than Califronia's Death Valley), with heavy saline deposits, nearby volcanos, and has thin air and high UV due to its elevation. It also boasts the world's most incredible nitrate deposits: in many regions >7% of of the soil is nitrate by weight (Prellwitz et al. 2006). However, as (Sutter et al. 2007) show, even in the areas with the most concentrated nitrate deposits we cannot discern nitrates via spectrometry until they occupy ~1% of the soil by weight, and that doesn't happen until a depth of ~1 meter has been reached. Below this depth the nitrate fraction can quickly explode up to 30% of the soil's weight.

Ok, so the Atacama has huge nitrate deposits; why think that Mars does too? It is thought that Atacama's deposits are due in large part to the volcanism of the surrounding area, which has been estimated to have fixed 2800 megatons of nitrogen from the air (Oyarzun & Oyarzun 2007). Extrapolating this to the global volcanism of Mars using (Geeley & Schneid 1991), we would expect 180 million megatons, or 45 mbar worth of nitrogen to be fixed as nitrates and nitrites. In addition it has been experimentally demonstrated by (Segura et al. 2005) that UV radiation could have fixed almost 2 megatons of nitrogen every year on early Mars. Between the time of Mars formation 4.6 Gyr ago and the beginning of the Late Heavy Bombardment 4.0 Gyr ago, this mechanism could have fixed 250 mbar of nitrogen in the soil before impactors blew off much of Mars' atmosphere. This would also be before the Martian global magnetic field decayed and solar wind erosion began.

But did Mars even have this much nitrogen to start with? By analogy with Earth a planet of Mars' mass could have accumulated 120 mbar of N2 during formation, and by analogy with Venus it might have begun with 560 mbar. Even if Mars began with an extremely pessimistic 1 bar CO2, the ratio of C/N = 20 in comets and terrestrial planet atmospheres means it should have begun with at least 25 mbar N2. And since (Phillips et al. 2001) estimate a magmatic outgasing of 1.5 bar of CO2 during the formation of Tharsis, anywhere from 38 to 250 mbar N2 could have been added to the atmosphere as well, based on possible magma N2 contents from (Segura et al. 2005).

Ok, so Mars had the ability to fix large amounts of nitrogen in the past, as well as a significant inventory from which to fix it. So where is it? Well, nitrates are extremely soluble, meaning that during Mars' wet period(s) it would have quickly dissolved and been transported to the northern plains. As the water froze/evaporated the nitrates would have been deposited primarily in the Utopian basin, which seems to have originally been an impact crater the size of Hellas which filled in with sediment due to its watershed amounting to over 2/3 of the Martian surface (Bandert 2004). Other locations of heavy nitrate deposits might be the Acidalia, Amazonis, Argyre, Chryse, Echus, Hellas, and Isidis plains.

Interestingly, the crustal thickness at these locations is very thin, on the order of that found at Iceland or thinner, suggesting that the geothermal gradient there may be sufficient to support liquid water at relatively shallow depths. According to (Halevy et al. 2007) such sites could experience large carbonate deposit formation after the recession of Mars' acidic seas. I find it intriguing that large deposits of both nitrates and carbonates might be located in the same places, along with the geothermal means to decompose them directly into CO2, N2, and O2. Just drill down to ~650°C, pipe down your carbonates and nitrates with some sand, and you get all those nice atmospheric gases boiling out plus a "slag" of useful building materials like sodium silicate (water glass) and calcium silicate (the prime component of Portland cement). If you don't throw in the sand you can get sodium hydroxide (lye) and calcium oxide (lime), both of which are also very useful. This is all currently within our technological reach, and perhaps could be used to provide raw materials for early research facilities and colonies.

However, even if you could magically use all the geothermal heat flux of Mars (~4 terawatts) to decompose nitrates for 1000 years, you still wouldn't have 5 mbar N2 in the air. One could conceivably use nuclear or solar to decompose nitrates, but why go through all that trouble when you can just spread the nitrates around like fertilizer, which is all it really is. Just let the ecosystem absorb it and redistribute it naturally.


References:

Terrestrial analogs for interpretation of infrared spectra from the Martian surface and subsurface: Sulfate, nitrate, carbonate, and phyllosilicate-bearing Atacama Desert soils
B Sutter, JB Dalton, SA Ewing, R Amundson, and CP McKay, Journal of Geophysical Research, Vol. 112, G04S10

Nitrate Concentrations in Atacama Desert soils and Their Implications for the Antiquity of the Atacama Desert.
J Prellwitz, J Rech, G Michalski, B Buck, MS Howell, and A Brock, 18th World Conference of Soil Science, July 15, 2006
http://a-c-s.confex.com/crops/wc2006/techprogram/P18247.HTM

Massive Volcanism in the Altiplano-Puna Volcanic Plateau and Formation of the Huge Atacama Desert Nitrate Deposits: A Case for Thermal and Electric Fixation of Atmospheric Nitrogen
J Oyarzun & R Oyarzun, International Geology Review, Volume 49, Number 10 / October 2007

Magma Generation on Mars: Amounts, Rates, and Comparisons with Earth, Moon, and Venus
R Geeley & BD Schneid, Science 15 November 1991: Vol. 254. no. 5034, pp. 996-998

Ancient Geodynamics and Global-Scale Hydrology on Mars
RJ Phillips, MT Zuber, SC Solomon, MP Golombek, BM Jakosky, WB Banerdt, DE Smith, RME Williams, BM Hynek, O Aharonson, and SA Hauck II, Science, Vol 291, 30 March 2001

Implications of the Utopia gravity anomaly for the resurfacing of the northern plains of Mars
WB Banerdt, Second Conference on Early Mars (2004)

A Sulfur Dioxide Climate Feedback on Early Mars
I Halevy, MT Zuber, and DP Schrag, Science Vol 318, 1903, 21 December 2007