BBC: Mars 'not a watery world'

Scoria writes "Contrary to a belief shared among many scientists, new evidence established by NASA's Mars Global Surveyor suggests that the planet may not have once possessed a temperate climate capable of sustaining life. Instead, an absence of carbonate rock deposits, which require the presence of liquid water to be produced, lends credence to those who believe that Mars is perpetually frozen."

Comments

Grrr...

[daeley]

Submitted this story yesterday....

Anyhow, JPL sent out a press release yesterday: New Findings Could Dash Hopes for Past Oceans on Mars

After a decades-long quest, scientists analyzing data from NASA's Mars Global Surveyor spacecraft have at last found critical evidence the spacecraft's infrared spectrometer instrument was built to search for: the presence of water-related carbonate minerals on the surface of Mars.

However, the discovery also potentially contradicts what scientists had hoped to prove: the past existence of large bodies of liquid water on Mars, such as oceans. How this discovery relates to the possibility of ephemeral lakes on Mars is not known at this time.... [continues]

Re:Is this just one group stating their opinion?

[Hittite Creosote]

What the story is, is that carbonates should have formed abundantly if Mars had large seas. The amounts they have found with this mission are tiny.
This means either that Mars didn't have large seas, or that any carbonates that did form were in basins that have since been covered up, and hence weren't detectable by this mission.

Another article here

Re:Is this just one group stating their opinion?

[barakn]

Large oceans of liquid water under an atmosphere with CO2 tend to form large deposits of carbonate minerals. In other words, we should find wide areas of carbonate rock, especially at low elevations. No carbonate rocks were found, only carbonate signatures in soil. What's news is that the Arizona team spent 6 years using the thermal emission spectrometer to look for carbonates, and didn't find thick layers of it. Other (better) articles on the same news release can be found here or here.

Carbonate precipitation

[barakn]

Here on Earth, the precipitation of carbonates is often done biologically. Calcium and magnesium carbonates make good cement for the "skeletons" of a large variety of sea creatures ranging from plankton to molluscs to coral (even whale bones, though they're not important quantitatively.)

Carbonates can also precipitate unassisted. A dramatic example is the somewhat speculative theory known as the snowball Earth. For periods of 10 million years and repeating perhaps up to 4 times starting 3/4 and ending 1/2 a billion years ago, the Earth froze. Large glaciers covered the land and the oceans were capped by a kilometer of ice. Without getting into a heated discussion about how this occurred, the escape sequence is the interesting part. Volacanoes poke their way through the ice and vent CO2 into the atmosphere. Since there was no biological activity, the CO2 kept building up until the greenhouse effect can melt the ice. The newly liquid oceans then absorbed CO2 from the extremely high concentrations in the atmosphere, and then rapid carbonate precipitation commenced, leaving, in some cases, crystal clusters as tall as a person.

doesn't seem to us much

[penguin7of9]

Carbonate deposits would most likely be found in the basins, but those are also the areas most likely to be filled with dust, lava, and ice. On Earth, those low points with their deposits eventually get lifted up into mountain ranges, which is why they get exposed, but I believe on Mars, that doesn't happen very much. So, the result that we don't see a lot of carbonates isn't all that surprising even if there were moderately sized oceans on Mars a long time ago.

Re:doesn't seem to us much

[barakn]

Carbonate precipitation generally occurs in shallow waters (on Earth). Partly this is because the solubility of calcium carbonate increases with depth, so there is a depth in the ocean (known as the Carbonate Compensation Depth, or CCD) where the rate of downfalling Ca carbonate precipitate equals the rate of dissolution of Ca carbonate. Below this depth, little Ca carbonate survives.

CO2 + H2O + CaCO3 => Ca2+ + 2*HCO3-

The reaction will be driven to the left by the removal of CO2. from the water. As your flat Mountain Dew can attest, depressurizing and warming a liquid is a good way to get rid of CO2. Carbonate thus precipitates in warm shallow water. We would expect "bathtub rings" of carbonate around ocean basins. These would be less likely to be covered, but even if they were, erosional products from them would still get mixed into the soil and be visible in from space. Only a small amount of magnesite (magnesium carbonates being virtually impossible to precipitate from water, at least when calcium is around) was found, suggesting problems with the idea of oceans.