On Oct 21, 3:29*pm, Robert Clark wrote:
On Oct 18, 9:40*pm, Bolaleman wrote:





On 13 oct, 09:57, Robert Clark wrote:

On Oct 12, 9:48*pm, xxein wrote:

On Oct 12, 9:59*am, Robert Clark wrote:

Large deposits of carbonates had been expected to be wide spread on
Mars because of the extensive water in the distant past and because of
the CO2 atmosphere. Previously however deposits of carbonates had not
been seen from orbit. What was seen from orbit was small amounts of
carbonate in the form of dust sprinkled over the planet at perhaps the
2% amount. *This was confirmed on the ground by the MER rovers which
also saw carbonate only in the form of dust in small amounts.
*However, the Phoenix lander has found carbonates in significant
amounts at its landing site, perhaps in the 6%-8% range. The presence
of the carbonate here might be due to the alkalinity of the soil at
the Phoenix site compared to acidic soil, as indicated by the presence
of sulfates, at the other lander sites.
*A new report however to be presented at the upcoming "Workshop on
Martian Phyllosilicates: Recorders of Aqueous Processes?" will argue
that carbonate best fits the spectra in a deposit in the Nili Fossae
region on Mars:

PHYLLOSILICATES, ZEOLITES, AND CARBONATE NEAR NILI FOSSAE, MARS:
EVIDENCE FOR DISTINCT ENVIRONMENTS OF AQUEOUS ALTERATION. B.L.
Ehlmann1, J.F. Mustard1, G.A. Swayze2, J.J. Wray3, O.S. Barnouin-Jha4,
J.L. Bishop5, D.J. Des Marais6, F. Poulet7, L.H. Roach1, R.E.
Milliken8, R.N. Clark2, S.L. Murchie4, and the MRO CRISM Team. 1Dept.
of Geological Sciences, Brown University, 2U.S. Geological Survey,
Denver, 3Cornell University, 4JHU-Applied Physics Laboratory 5SETI
Institute 6NASA Ames 7IAS, Université Paris-Sud, 8JPL-Caltech
)http://www.lpi.usra.edu/meetings/aqu...8/pdf/7019.pdf

This is to be published in an upcoming article in Science.

Other interesting reports from this conference:

Program
Workshop on Martian Phyllosilicates: Recorders of Aqueous Processes?
October 21–23, 2008http://www.lpi.usra.edu/meetings/aqueous2008/pdf/program.pdf

* * Bob Clark

xxein: *And? *Do you think that would be significantly strange?

*Planetary scientists were convinced there would be wide spread
carbonate deposits found on Mars, limestone and the like, because the
orbital imagery suggests there were flowing rivers, lakes, perhaps
even seas very early in Mars history and because of the carbon dioxide
atmosphere should have created abundant carbonate.
*The lack of carbonate deposits found in the spectra taken from orbit
was somewhat of a embarrassment. So much so that many scientists began
to doubt Mars ever had a warm, wet period. Instead some proposed that
the extensive river channels seen on Mars were actually due to liquid
CO2 flows.
*If this really is a carbonate deposit of this size that would mean
there was indeed large amount of liquid water present at some point.
Also, ancient life is frequently seen in association with carbonates
on Earth. In some cases the carbonate is even made up of ancient
lithified life. It will be important to accurately constrain the
placement period of this deposit.
*If it is indeed carbonate it will also be important to understand the
geology and geochemistry of this region to serve as a guide for other
large deposits.

* * Bob Clark- Ocultar texto de la cita -

- Mostrar texto de la cita -

"If this really is a carbonate deposit of this size that would mean
there was indeed large amount of liquid water present at some point."
Is this conclusion true? Theoretically, calcium carbonate can be
formed just from Calcium oxyde and CO2, without water.

* On Earth such large deposits are always formed in the presence of
water. The addition of the phyllosilicates (clays) in the same
location makes it very likely this deposit formed in the presence of
water as well.

* * Bob Clark- Hide quoted text -

- Show quoted text -


I aggree that on earth calcium carbonate deposits are formed in a
liquid (water)-solid-gas reaction. In a purely solid-gas reaction, the
reaction might have stopped in a certain depth of the calcium oxide
deposits. Here is an article about the diffusion controled reaction of
the reaction of calcium oxide with CO2:

The carbonation rate of 15-20 micro-m, nonporous, calcium oxide
crystals has been studied over a temperature range of 550-1100 C and a
CO2 pressure range of 1-11.7 atm. At temperatures greater than 600 C,
the carbonation rate decreases more rapidly with time than would be
expected from diffusion through a uniform product layer and the
activation energy is initially low but increases with conversion. The
product layer consists of crystalline grains, and these product layer
grains grow by coalescence from less than one m diameter to the
approximate dimension of the particle. The carbonation rate can be
described by a model where CO2 pressure-independent grain boundary
diffusion and diffusion through the carbonate crystals act in
parallel. The relative importance of bulk diffusion through the
product layer crystals increases with time relative to transport
through the grain boundaries and has an effective activation energy of
57 kcal/mol. Diffusion through the crystal boundaries has low
activation energy and low dependence on CO2 pressure. At temperatures
greater than 900 C and times greater than 600 min, bulk diffusion
through the product layer becomes dominant and the rate approaches
first order in CO2 pressure. (see
http://pubs.acs.org/cgi-bin/abstract...ef980266f.html)