What you say may well be true, but it's also misleading.

let us restate the observations:-

2a) The erosion patterns in e.g. Warrego Valles are almost impossible to produce without liquid water and precipitation (which could be snow, with basal melting trickling down snow-covered gullies)
2b) The catastrophic floods in various areas of Mars are most easily explained by Mars being WARM enough to melt subterranean ice in a once-only local event
1) Given the above, Mars must have been at least locally and transiently warm and wet enough for liquid water.

Now let's look at the rest of the context. ESA have published a very useful and compelling chronology of the role and activity of water on Mars, as seen from planetwide distributions of mineralogy:-

1- A widespread and active (but not necessarily permanent) water system producing clays - Back in the Noachian
2- A limited water system with strong acid brines producing sulphates - Back in the early Hesperian or late Noachian
3- An extended and planet-wide aridity lasting to the present day - some 3.5 billion years. Local exceptions occur associated with volcanic heating and consequent floods, but little if any chemical evidence results.

The simplest way to interpret all this is in a scenario where the stable state of Mars is arid and frigid, at almost all times. Back in the Noachian, major impact events caused atmospheric transients (lasting tens of years to thousands of years) during which the atmosphere was kicked into a metastable state with liquid water activity. Essentially, after the Noachian, no impact was big enough to cause a global ocean and rain. After the Hesperian, there was nothing left. Thisd model was originally proposed in concept form by myself but developed more carefully by Teresa Segura.

Essentially, we're looking at the evolution of Mars *as it accretes*. During THIS time, the system is energetic enough to enable liquid water (Recall that on earth, at the same time, the atmosphere was beleived to be superheated steam, or even silicate vapour! Mars is colder - there is "only" liquid water, but it's as stable on Mars as superheated steam is on Earth, imho...)

This is a fascinating discussion, and it makes me wonder if anyone here could offer their thoughts on the following question.

If we assume for the sake of argument that infrequent large impacts can produce a temporary warm or wet Martian environment, how long would such conditions last? In particular:

- How dense would the resulting temporary atmosphere credibly be? 0.1 bar, or more?
- What would it comprise - CO2 liberated from frozen deposits, or vaporised rock?
- What would be the process by which the Martian environment reverted to its steady state?

Thanks for any insight you may be able to provide,

Simon

My uneducated guess: To produce heating over a long time or large scale an impact would need to trigger vulcanism on a global scale. So I'd think the chixelub impact, where a ten KM body hit mexico leaving a crater 200 odd km across, and maybe triggered vulcanism in the deccan traps on the other side of the earth, as a reasonable hand waving lower limit. As to how long it would last, I'd guess hundred to tens of thousands of years, but not hundred of thousands or millions unless the impact was really huge (just a stab in the dark). I doubt vapourized rock could comprize an atmosphere that would be cool enough for water, so vaporised ices and volcanic gases are the likely candidates. As the vulcanism or residual heat from a really big impact dies down i imagine the surface becomes cooler, gases making up the temporary atmosphere freeze out and atmospheric pressure drops back toward 0.01 bar. And Nick it's good to see you posting here again.

Yeah, I took a break. I lost my wife, got over it, got Cancer, got over it, got married again, still can't get over how good it is...

As for Mars, and impacts, you need a planetary-scale impact that will deposit an impact sheet AT LEAST 1 m thick. That will keep the atmosphere hot for 1 year, and the surface warm for 10 years. A 10m impact sheet is worth about 30 years in the atmosphere and 300 years at the surface, and a 100m sheet is 1000/10,000 years

But when you look at the size of impact require to acheive this, the mind boggles. Isidis and Hellas are the sort of thing we need. I think Teresa Segura failed in her effort to show that impacts on mars are important for dynamic/transient atmospheres because she statted too late in Mars time, AFTER the giant basin-forming impacts. If we go back to this time, and to the time of ultra-giant basins like the entire northern plains, then it becomes a no-brainer. Clearly, at this time, Mars had some liquid water. It probably had a lot of steam, and then a lot of rain... it was probably at this time that the water was active on Mars and did most of its doings. Later, Mars was a lot quieter, but we still see the last echoes of that early activity...

Congratulations on coming through what sounds like a very rough patch Nick, and congratulations on getting married!
Vulcanism on mars would have been a lot more active at the end of the late heavy bombardment, would that not have kept at least some locales warm between, and for some time after, the giant impacts? I'm not sure when the giant basins were formed. Could a more or less continous stream of smaller impacts contribute to keeping the planet warm? I'm afraid I'm throwing ideas around wildly here as I'm not familiar with how the late heavy bombardment/final planetary accretion was structured in time.

Ah, I phrased my original question rather ambiguously. I was wondering about CO2 liberated from carbonate rocks by the impact; I seem to recall a rather extreme terraforming proposal that involved very enthusiastic, er, nuclear engineering to do this.

Sorry, but no. The Deccan eruptions started well before Chicxulub, which is proven by the fact that the iridium layer from the impact is found in one of the intertrappan beds within the Deccan traps. I think something more like the Hellas impact is probably needed.

Thanks for the info tty, I was under the impression it was still being debated.

Hi Nick, I'm also happy to see you posting here again (although I've enjoyed following your occasional Mars-related comments elsewhere). I seem to have ended up agreeing with most of what you've said in the distant past, even about White Mars, except I feel that it's adequate to suppose that the "white" can be some combination of ice and salts (and of course impact-derived steam), rather than some form of carbon dioxide (not that the latter can be excluded). We met in 2001 when Knauth and I presented our low-melting salts idea at an LPI workshop.

Anyway, in response to sjbradshaw, as I understand it, and as you stated above, a major impact will volatilize nearly everything, including silicate rocks (even ephemerally separating, e.g., Si from O), but the silicate fractions will condense relatively rapidly, possibly to glassy spherules, as on the Moon. Carbon dioxide from target rock carbonates was a major vapor component for e.g., Chicxulub on Earth, but it remains to be seen if such target rocks ever occurred on Mars (probably not). The most important impact-derived greenhouse component on Mars is probably simple steam (water vapor) from impacted ice (or brine), together with sulfur dioxide from impacted sulfides and/or sulfate salts (you don't need unusual episodes of volcanism to make it). Condensation of impact-derived sulfur dioxide as ferric acid salts, and/or late oxidation of impact-distributed sulfides, could account for the mine-dump type mineralogy of the surface of Mars, without requiring acid seas, lakes, or groundwaters (which are nearly impossible from a geochemical standpoint).

Nick, I'm not sure that a Hellas-sized impact is actually required, if one accepts the Late Heavy Bombardment (LHB) as having occurred at about 3.9 billion years ago (long after initial accretion at 4.5 billion years). As suggested by marsbug, a concentrated stream of smaller impacts at the height of the LHB might be sufficient to account for the buried clays, drainage networks, and so on. I agree with you that as the LHB was tailing off, and afterwards, impacts were generally too small to yield much of a greenhouse, and, in fact, late impact debris seem to have covered up the clay minerals in most areas of Mars.

-- HDP Don

Thanks Don. I also have come agree with most of what you and Knauth have been saying. With time, there is a convergence of concepts, driven by the accumulation of evidence. Most Mars scientists are coming to terms with a long-term dry and cold Mars, but that doesn't stop volatiles having an important role at some times and places. The trick is to understand when, where, and how the volatiles act.

I'm still learning...

Another one to chew over:
http://www.sciencedaily.com/releases/2008/...80625093242.htm

Thanks. Haven't read more than that news story, but they seem to be comparing the Atacama Desert with Mars, and inferring that preferential chloride leaching from the surface indicates occasional rain or mist, based on the different relative solubilities of chlorides and sulfates. IMHO, possibly the wrong conclusion for the wrong planet - the old problem of using terrestrial processes as analogs for martian processes, rather than depending on basic physics and chemistry. I've been caught there myself e.g., as regards my recent informal discussion with Nick H. over how to make solid ground ice at the Phoenix site (my suggestion involving liquids was okay for solid salts, but probably not for solid ice, because it neglected the relatively high vapor pressure of ice). In other words, my suggestion explained how ice forms on relatively mild Antarctica, but probably not on much colder polar Mars. Nick was kind enough to set me straight, but also was kind enough NOT to rub my nose in the irony of my being victimized by the Rosenthal effect (finding or seeing what you expect), right after having contrasted that with Occam's razor (the simplest interpretation for all of the data).

Similarly, the authors of this study, if that news release is completely accurate, might appear to have neglected that chlorides, in addition to being more soluble, have much greater freezing point depressions than sulfates (more than 50C vs. less than 5C). Therefore, preferential frost leaching of chloride, on which Knauth and I published in 2002 and 2003 (in Icarus and JGR) might seem a more "Mars-friendly" explanation than rain or mist.

A couple of author's quotes from that news release: "Our study suggests that Mars isn't a planet where things have behaved radically different from Earth" and "It seems very logical that a dry, arid planet like Mars with the same bedrock geology as many places on Earth would have some of the same hydrological and geological processes operating that occur in our deserts here on Earth". In this regard, after weeks of 110+ degree (= 43C) days, Phoenix had its first seasonal dust storm last night. Do dust storms make Phoenix, Arizona hydrologically comparable to the Phoenix lander site, Mars? Perhaps not.

-- HDP Don

I thought you'd have something to say here about the unreliability of earth analogues.

In other words, you knew I wouldn't be able to resist.

-- HDP Don

The impact in the thread next door would probably meet the requirements