Wet, warm Mars Options

[dburt]

In that case would pre-LHB zircon crystals really have survived intact? And it seems that the Acasta formation in Canada is older than at least the later part of the LHB. If the surface was completely remelted I would think that all the radiologic ages would have been reset.

tty - From a talk I heard over a year ago (by Stephen J. Mojzsis, Univ. of Colorado), many pre-LHB zircon crystals did indeed survive on Earth, but they tend to show an age anomaly at 3.8-3.9Ga (i.e., age zoning: only the cores give older ages) that suggests crustal remelting during the LHB. Some terrestrial rocks seem to overlap the approximate time of the LHB, as you mention, but 60 years of intense geochronlogical research has yet to find any older on Earth. Also keep in mind that the overlapping rocks could have been dated by their zircons, and I imagine that an average age for the entire zircon tended to be determined in older work (although geochronology is not my field, I freely admit). This could yield spuriously old ages for the rock as a whole (based on the faulty assumption that the zircon crystallized at the same time as the rest of the rock). Sorry, I'm too busy getting ready for LPSC next week to look up the relevant literature.

-- HDP Don

[dburt]

Well... I completely agree with you (especially with that proviso "largely"), as far as Mars' southern hemisphere is concerned. But north of the dichotomy line (which isn't exactly equatorial), very few remnants of the LHB are seen to survive. That land *has* been heavily resurfaced, somehow, since the LHB.

So -- *half* of Mars fits your description above. Half doesn't...
-the other Doug

Good point Doug, but the "resurfacing" of the northern plains need not have been done by a climate greatly different from today's. All the dust raised by impacts has to end up somewhere, and lower elevations seem logical. Ditto for whatever the wind was blowing around (just look at the dust and dunes nearly filling craters like Victoria), plus what was deposited by the groundwater (brine) breakouts of the outflow channels, plus possible volcanic contributions. All fluids and dense suspensions tend to flow downhill, not just liquid water.

Also keep in mind that this cover clearly is very shallow - MOLA maps reveal the ghosts of underlying craters, and recent radar data has revealed that, under its shallow cover, the northern plains are just as heavily cratered as the exposed highlands. Herb Frey has published many papers and abstracts on this topic. So all you seem to be pointing out is the global dichotomy in elevation itself, not anything revealing about climate or erosion or the LHB.

-- HDP Don

[Juramike]

How long would it take earth's atmosphere to change if all life here disappeared tomorrow?

I'm not sure of the relative removal rates of CO2 due to geological, oceanic, and biological processes. But I'll wager that all these helped make Earth a really nice place to live. (Most of these processes went away during the putative Snowball Earth phase, leading to the buildup of CO2 which eventually caused the Big Melt)

(I'd speculate that if plate tectonics magically stopped during the Big Melt (or any other time early in Earth's history), our atmosphere would eventually resemble Venus'.)
-Mike

OK. So my curiosity got the better of me:

From Wikipedia (carbon dioxide):http://en.wikipedia.org/wiki/Carbon_dioxide
and Wikipedia (carbon cycle, with cool-o diagram showing carbon fluxes in GTC):http://en.wikipedia.org/wiki/Carbon_cycle

So, assuming that sequestering and plate tectonics totally magically shut down on Earth. No oceanic exchange, no sediment sequesteration. How long would it take to build up a sizable CO2 atmosphere?

Output of CO2 from Earth volcanoes is: ca. 200E9 kg year-1
(130-230 million tonnes year-1 x 1000 kg per tonne)

[Makes sense, the sediment sequestering flux is about 0.2 GTC year. Up until man got all industrial and stuff, things were pretty much in balance]

Total mass of the atmosphere is 5.14E18 kg. (Wikipedia again)


5.14E18 kg / (200E9 kg year-1) = 25 Myr to generate an extra atmosphere of CO2 (ignoring stuff like m.w. difference between CO2 and N2)

So after 25 million years you would have a pressure of 2 atm and a 50% CO2 atmosphere.

Venus' atmosphere is 90 x that of Earth. So assuming that volcanoes still keep burping out CO2 at the same rate, it would take 2.2 Gyr to build up a Venus-like 90 atm atmosphere here on Earth. [1% N2, 99% CO2]

"Venus, there but for the grace of tectonic CO2 sequesteration, go I."

(Now won't y'all sleep better tonight knowing a global terrestrial ice age would be reasonably short-lived?)

-Mike

[Aussie]

Good point Doug, but the "resurfacing" of the northern plains need not have been done by a climate greatly different from today's. All the dust raised by impacts has to end up somewhere, and lower elevations seem logical. -- HDP Don

Pathfinder and the Viking landers didn't exactly find sand seas with particle sizes reflecting those that could be with be mobilised in the current Martian vacuum and to continue the Venus comparison, the impact craters on that planet have resisted weathering despite the presence of a thick atmosphere. (Plate tectonics rule). There would seem to be sufficient degradation of southern highland craters to suggest that significant aeolian and indeed liquid water erosion took place after the LHB ended.

If Mars did have a volcanic recharged greenhouse atmosphere that lasted until volcanic activity shut down, which seems most plausible, then would not this fit both the impact proposition and the Cornell warmer wetter theory. Following the LBH there was a period of unknown duration where an environment fitting the Cornell model existed. The findings from the descent into Victoria do seem to substantiate the Cornell proposition. Once the atmosphere dissipated impact again became the dominant influence, but with a sedate tempo.

[dburt]

Aussie - Not wishing to revive ancient arguments, but Pathfinder and Viking found a rocky surface made up of what could reasonably be interpreted as young impact ejecta, and no evidence of water other than the ubiquitous salts in the soil. Evidence of wind erosion abounds, but evidence of liquid water erosion after the LHB seems largely resticted to outflow channels (interpreted to be temporary groundwater breakouts). You can hypothesize or model volcanic recharge of a Martian greenhouse atmosphere, but don't confuse this with evidence. Similarly for the descent into Victoria - I've seen no feature that requires more than minimum moisture, despite the models. And Knauth and I noted that concentrated brines could exist under present Martian surface conditions as long ago as 2000 (preceded by G.W. Brass in 1980). No greenhouse required.

-- HDP Don

[Aussie]

Aussie - Not wishing to revive ancient arguments, but Pathfinder and Viking found a rocky surface made up of what could reasonably be interpreted as young impact ejecta, and no evidence of water other than the ubiquitous salts in the soil.

the "resurfacing" of the northern plains need not have been done by a climate greatly different from today's. All the dust raised by impacts has to end up somewhere, and lower elevations seem logical.
-- HDP Don

The two statements are not really compatible since the resurfacing must have occurred after the LHB. I think I will sit fair and square in the other Doug's corner on this one.

Exactly. In point of fact, a good number of the well-developed river drainage systems observed on Mars developed in the crests between crater rims in the heavily battered southern hemisphere.

It rained on Mars after the LHB had pretty much breathed its last.

-the other Doug

[tty]

5.14E18 kg / (200E9 kg year-1) = 25 Myr to generate an extra atmosphere of CO2 (ignoring stuff like m.w. difference between CO2 and N2)

So after 25 million years you would have a pressure of 2 atm and a 50% CO2 atmosphere.

Venus' atmosphere is 90 x that of Earth. So assuming that volcanoes still keep burping out CO2 at the same rate, it would take 2.2 Gyr to build up a Venus-like 90 atm atmosphere here on Earth. [1% N2, 99% CO2]

"Venus, there but for the grace of tectonic CO2 sequesteration, go I."

(Now won't y'all sleep better tonight knowing a global terrestrial ice age would be reasonably short-lived?)

Actually that mechanism is thought to have ended the very large neoproterozoic ice ages when Earth possibly froze over more or less completely. The large deposits of "cap carbonates" apparently deposited at very high temperatures right on top of the glacial deposits certainly seems to indicate that a lot of CO2 had accumulated in the atmosphere.

However at that time plate tectonics was going at full blast. If plate tectonics somehow stopped as you posited, then volcanism and CO2 emissions would pretty quickly decline (though probably not stop completely) since no new easily-remelted organics-rich seafloor rocks would descend into the mantle.

[Juramike]

Actually that mechanism is thought to have ended the very large neoproterozoic ice ages when Earth possibly froze over more or less completely. The large deposits of "cap carbonates" apparently deposited at very high temperatures right on top of the glacial deposits certainly seems to indicate that a lot of CO2 had accumulated in the atmosphere.

However at that time plate tectonics was going at full blast. If plate tectonics somehow stopped as you posited, then volcanism and CO2 emissions would pretty quickly decline (though probably not stop completely) since no new easily-remelted organics-rich seafloor rocks would descend into the mantle.

Sorry my post wasn't clear: I was whipping out a hypothetical exercise to see what would happen IF plate tectonics magically stopped on Earth AND volcanoes continued to burp. This was just to show the happy effect that plate tectonics has on preventing CO2 buildup.

I never meant to imply that plate tectonics stopped on Earth at any point in our history. (Although individual plate boundaries might lock up a bit on a fairly long time scale)

But what was cool (!) about this back-of-the-envelope calculation is that the time frame matches up nicely with the estimated time (in Wikipedia) it took to get out of the Snowball Earth scenario.

[From Wikipedia (Snowball Earth):
"The carbon dioxide levels necessary to unfreeze the Earth have been estimated as being 350 times what they are today, about 13 percent of the atmosphere.*[12] Since the Earth was almost completely covered with ice, carbon dioxide could not be withdrawn from the atmosphere by the weathering of siliceous rocks. Over 4 to 30 million years, enough CO2 and methane, mainly emitted by volcanoes, would accumulate to finally cause enough greenhouse effect to make surface ice melt in the tropics until a band of ice-free land and water developed;[13] this would be darker than the ice, and thus absorb more energy from the sun — initiating a 'positive feedback.'"]

-Mike

* actually, I think 13% is one of the lower estimates, other models seem to require a higher amount of CO2.

[dvandorn]

Just to re-state a bit... in my initial post, I did say that Mars was incontrovertibly once warm and wet *enough* for water to form sinuous, meandering (and sometimes braided) river channels, and to allow for catastrophic floods. That's all I really said. Not that it was that warm and wet for billions of years -- just that it was warm and wet *enough*, for long enough, to allow the features we see to have formed.

I do find it fascinating that, when confronting this fact, those who are belief-level-certain that Mars has always been cold and dry (just as it is today) seem driven to ask themselves how such conditions could be generated on extremely short time scales, such as hours or days, to allow such things to have happened without challenging their dearly-held beliefs. And postulating that impacts had to have both stripped Mars' atmosphere and, *at the same time*, thickened it for brief periods.

I'm not saying that these are impossible scenarios. I'm just saying that my own reaction to them is they "feel" like they push Occam's Razor quite far in order to account for observed features which cannot be created on the Mars they are just certain must always have existed...



For what it's worth, I completely agree that Mars does indeed seem to have been as cold and dry as it is now since the beginning of the lava piling that resulted in the Tharsis Bulge, since those lavas have never been broken down chemically by abundant liquid water. My own opinion is that Mars' magnetic field died as a result of LHB interactions and that its atmosphere, thinned by the LHB to just above a "line" where liquid water was commonly possible, fairly quickly became depleted by solar wind interactions (within a few million years) to where the only further erosion Mars would see from that point on was aeolian- and impact-related. The very last water erosion was probably the catastrophic flooding, set off when frozen reserves of water (and very cold reserves of liquid brine) were mobilized by the heat of the magma rising into the Tharsis region. So, in my own personal perspective, the lavas which define the end of a Mars wet and warm enough for liquid water to be a major erosional factor were actually responsible for the final era of such aqueous erosion.

-the other Doug

[dburt]

...Not that it was that warm and wet for billions of years -- just that it was warm and wet *enough*, for long enough, to allow the features we see to have formed.

I do find it fascinating that, when confronting this fact, those who are belief-level-certain that Mars has always been cold and dry (just as it is today) seem driven to ask themselves how such conditions could be generated on extremely short time scales, such as hours or days, to allow such things to have happened without challenging their dearly-held beliefs. And postulating that impacts had to have both stripped Mars' atmosphere and, *at the same time*, thickened it for brief periods...

For what it's worth, I completely agree that Mars does indeed seem to have been as cold and dry as it is now since the beginning of the lava piling that resulted in the Tharsis Bulge, since those lavas have never been broken down chemically by abundant liquid water...

-the other Doug

OD - I don't think we disagree about very much, then. From models I've seen, the temporary greenhouse warming following a major impact could last 1000 years or more, "long enough" to allow significant channel erosion following catastrophic downpours (or blizzards), without much chemical weathering. Your stating "hours or days" would be putting words in my mouth. Temporary greenhouse warming would be mainly caused by vaporized steam that would later condense; the atmosphere that was permanently lost "at the same time" was presumably different, mainly CO2. And I've only come around to the "mostly cold and dry" camp very slowly, kicking and screaming as more evidence came in (I initially wanted to study Mars for its clays, which have turned out to be severely time and space restricted - especially surprising for a planet that was supposedly swimming in acids). I don't deny an early volcanic greenhouse effect, I just don't feel that it need be significant, given the overwhelming abruptness and magnitude of the LHB effects (i.e., it was probably a secondary effect, one that declined rapidly with time after the end of the LHB, at the same time that the remaining atmosphere was itself being thinned by solar effects). Enjoyable discussion, although it might save time if we could read each other's minds.

Well, I'm off to LPSC, to blather about this stuff some more, and photograph alligators.

-- HDP Don

[tty]

From models I've seen, the temporary greenhouse warming following a major impact could last 1000 years or more, "long enough" to allow significant channel erosion following catastrophic downpours (or blizzards), without much chemical weathering.

I suppose in those models the residual heat of the crater keeps the hydrological cycle going for a while, because the water lofted by the initial impact would certainly rain (or snow) out within a few years at the very most.
Most of the greenhouse warming would have to be due to water, since CO2 by itself is a rather feeble greenhouse gas.
Also I wonder, is a thousand years really enough to create for example the Eberswalde delta? There are deltas created in about that time frame on Earth (e. g. marginal deltas that evolved in Scandinavia during the Younger Dryas) but they are not nearly as complex morphologically.

[Juramike]

A nice freely available article on the Eberswalde delta:
Wood, L. GSA Bulletin 117 (2006) 557–566. "Quantitative Geormorphology of the Mars Eberswalde Delta". doi: 10.1130/B25822.1

-Mike

[dvandorn]

I will say that, on geological timescales, a thousand years and ten thousand years are within the same order of magnitude. And I would imagine that the timing of the loss of magnetic field has a lot to do with how long such a Greenhouse Mars could persist -- without a magnetic field, volatiles (including water, broken down into hydrogen and oxygen) in the upper atmosphere are sputtered off into space far more quickly. The atmospheric water vapor content, which drives the water cycle, would be steadily eroded when the protective magnetic shield collapsed -- but could have reached a comfortable equilibrium while the field was in place.

In any event, my "gut" feeling, here, is that the landscape would probably look rather similar today had Greenhouse Mars lasted for a thousand years or a quarter of a million.

The way in which this whole speculation relates to the question of biogenesis, of course, is whether or not life could develop during the geological instant in which Greenhouse Mars could have existed, and could extremophile descendants have adapted to the conditions that followed? Single-cell organisms seem to have developed on Earth within a very short timeframe after conditions allowed it -- were the "relatively wet and warm" period(s) on Mars long enough to allow for biogenesis?

That's one reason I have a strong desire to find paleobacteria on Mars -- if life could start there under the rather more extreme duress than it found on Earth, well, the implications are profound...

-the other Doug

[Aussie]

We don't have any clear indicators as to how long the postulated warmer wetter period on Mars lasted. If the Cornell interpretation of Burns Cliff and the Victoria promentaries is correct then it lasted quite a long time. The cratered southern highlands are not really an indication that Mars has always had the environment evident today. Erosion is evident and IMHO weathering of craters would not seem a particularly accurate yardstick by which to assess climatic factors. For example Gosses Bluff is a big (22 km diameter) crater but it is still pretty impressive despite 140 million years of weathering in Earths highly corrosive atmosphere. Wolf Creek is just a tad larger than Victoria and after 300,000 years is in much better shape. The only point I am trying to make is that the greenhouse mars could have lasted considerably longer than 1000 years. Perhaps I am hoping too hard for that to be true and my judgement is affected. But if it lasted a few hundred thousand years or more then the chances of the Other Doug's paleobacteria arising would be much enhanced.

Edit: just noticed this release http://hirise.lpl.arizona.edu/PSP_003077_1530

[imipak]

We don't have any clear indicators as to how long the postulated warmer wetter period on Mars lasted.

On a related note, from an LPSC blog:

To explain liquid water on the past, there has been a lot of work done to see whether a thicker atmosphere of CO2 could provide high enough pressure and temperature to allow stable water. Today Marc Hirschmann gave an interesting talk about whether volcanoes could give off enough gas to do the job. [...much snipping...] Hirschmann estimated that even in the “best case” scenario, volcanism can only provide at atmosphere with 0.1 bars of CO2: only a tenth of Earth’s atmospheric pressure and not nearly enough to make Mars warm.

http://martianchronicles.wordpress.com/