Via Space.com -- there's lots of coverage today of a new paper by Head, Schon (and others?) in "Geology". The headline finding is a claim to have dated four distinct flows down a gully by crater-counting (a nearby crater produced many secondaries, I think.) I've not been able to find an abstract online, let alone full-text, but there's good detail in, e.g., the SciAm piece .
(Quick edit after a more leisurely search) - found this paper by the same authors from European Planetary Science Congress (2008):
http://www.cosis.net/abstracts/EPSC2008/00...8-A-00310-1.pdf
Full Version: ~1.25 million year old gully flows
The Geology paper is here (access required for full text):
http://geology.gsapubs.org/cgi/content/full/37/3/207
Abstract:
The origin of gullies on Mars is controversial (e.g., catastrophic groundwater release, debris flows, dry granular flows, or meltwater from surface ice and snow) and their ages are difficult to determine due to their small size. We describe a gully depositional fan that contains a unique chronostratigraphic marker (secondary crater clusters) between episodes of gully activity during fan development. This marker can be traced to its source, a 7-km-diameter rayed crater that we have dated as ca. 1.25 Ma. This age links gully activity to the emplacement of dust-ice mantling deposits interpreted to represent recent ice ages on Mars. This association, together with multiple episodes of depositional fan formation, favors an origin for these gullies from top-down melting of snow and ice during multiple favorable spin-axis and orbital variations. This melting mechanism is consistent with the occurrence of gullies in unique steep-sloped, poleward-facing insolation microenvironments that favor the melting of small amounts of surficial snow and ice.
http://geology.gsapubs.org/cgi/content/full/37/3/207
Abstract:
The origin of gullies on Mars is controversial (e.g., catastrophic groundwater release, debris flows, dry granular flows, or meltwater from surface ice and snow) and their ages are difficult to determine due to their small size. We describe a gully depositional fan that contains a unique chronostratigraphic marker (secondary crater clusters) between episodes of gully activity during fan development. This marker can be traced to its source, a 7-km-diameter rayed crater that we have dated as ca. 1.25 Ma. This age links gully activity to the emplacement of dust-ice mantling deposits interpreted to represent recent ice ages on Mars. This association, together with multiple episodes of depositional fan formation, favors an origin for these gullies from top-down melting of snow and ice during multiple favorable spin-axis and orbital variations. This melting mechanism is consistent with the occurrence of gullies in unique steep-sloped, poleward-facing insolation microenvironments that favor the melting of small amounts of surficial snow and ice.
I'd enjoy seeing the rationale by which a given rayed crater was determined to be 1.25 Ma in age. I take any absolute dating, in the absence of sample examination or in situ radiometric dating, to be speculative at best and a WAG at worst.
Also, there is during the current epoch a remnant CO2 polar cap at both poles in mid-summer, which could be more extensive at Mars' current axial tilt, but completely absent during higher-tilt eras. That CO2 would perforce add more gas to the atmosphere, raising pressures which are already just barely below the important triple-point of temperatures and pressures required for liquid water on the surface. (I'm assuming that the winter pole would not grow significantly larger with higher axial tilts, or would at least account for less additional freeze-out than the residual cap's sublimation would account for added atmosphere gas.)
I bet you get liquid water in some locations (and possibly in minuscule quantities) when Mars' axial tilt goes into the 30-plus degree range -- which could be responsible for some of the gullies we see.
I know that a number of models predict wide fluctuations in Martian axial tilt, but I've never seen anything worked up that attempts to determine how frequently the planet sees high tilt angles, and what the approximate timing has been since the last high-tilt epoch... or until the next one.
-the other Doug
Also, there is during the current epoch a remnant CO2 polar cap at both poles in mid-summer, which could be more extensive at Mars' current axial tilt, but completely absent during higher-tilt eras. That CO2 would perforce add more gas to the atmosphere, raising pressures which are already just barely below the important triple-point of temperatures and pressures required for liquid water on the surface. (I'm assuming that the winter pole would not grow significantly larger with higher axial tilts, or would at least account for less additional freeze-out than the residual cap's sublimation would account for added atmosphere gas.)
I bet you get liquid water in some locations (and possibly in minuscule quantities) when Mars' axial tilt goes into the 30-plus degree range -- which could be responsible for some of the gullies we see.
I know that a number of models predict wide fluctuations in Martian axial tilt, but I've never seen anything worked up that attempts to determine how frequently the planet sees high tilt angles, and what the approximate timing has been since the last high-tilt epoch... or until the next one.
-the other Doug
QUOTE (dvandorn @ Mar 5 2009, 04:13 AM) 
That CO2 would perforce add more gas to the atmosphere, raising pressures
Unless Mars water can precipitate the excess of CO2 somehow, keeping it balanced around the triple point. Either we are lucky to see the martian atmosphere at that point, or it is at that point for a reason.
QUOTE (Fran Ontanaya @ Mar 5 2009, 09:50 AM)
Unless Mars water can precipitate the excess of CO2 somehow
Like forming carbonates?
QUOTE (dvandorn @ Mar 5 2009, 03:13 AM)
I bet you get liquid water in some locations (and possibly in minuscule quantities) when Mars' axial tilt goes into the 30-plus degree range -- which could be responsible for some of the gullies we see.
Yes, that's what this paper says; they interpret the gullies as forming by running liquid water as surface snowpack and ice melt.
A kindly soul sent me copies of this paper, AND four others on the topic of gullies. (I'd forgotten about Pelletier, Kolb, McEwen, Kirk (Geology, 2008) which I'd even posted about here..!
) The others are the original Malin/Edgett (Science, 2006) paper describing new bright deposits and interpreting them as present-day water flows, and Head, Marchant, Krevlavsky (PNAS, 2008), "Formation of gullies on Mars: Link to recent climate history and insolation microenvironments implicate surface water flow origin". This lot will take me a week or two to digest properly, so I've only glanced through the paper that, er, this topic's actually about. (My thanks again to the donor, if you're reading this 
) Hopefully it's OK, copyright-wise, to quote a small extract of the paper for the purpose of review. This seems (on a fast skim) to be the crucial passage relating to the dating of the rayed crater:
(There's an older, outer crater, and a younger inner one which is interpreted as the source of the secondaries on the fan deposits.)
QUOTE
...morphological observations of the outer crater suggest that it predates deposition of latitude-dependent mantling deposits that would obscure fresh crater rays. [...]
Therefore, we interpret the inner crater as the source of the rays and secondary craters of interest, and younger than the most recent episode of latitude-dependent mantling deposition at this low latitude. Our morphological observations suggest that the outer crater predates the end of an obliquity-controlled period of latitude-dependent mantle deposition, while the inner crater appears to post-date the most recent period of mantle deposition (Head et al, 2003). To test this proposition quantitatively we performed crater counts on smooth near-rim units of the inner crater. These units north and south of the inner crater both yield crater retention (CRE) ages of ~1.25 Ma (Fig. 3b) based upon isochrons of Hartmann (2005). The use of small craters for age dating is supported by the small crater production rates observed on Mars by Malin et al. (2006) which agree within a factor of three with the isochron system (Hartmann, 2007). Similarly, Hartmann and Quantin-Nataf (2008) showed that counts of small craters could be used effectively to date young rayed craters. Therefore, our crater counts on the smooth rim deposits yield a robust CRE age of formation for the inner crater of ~1.25 Ma and imply that this crater is among the youngest craters of its size on Mars.
Therefore, we interpret the inner crater as the source of the rays and secondary craters of interest, and younger than the most recent episode of latitude-dependent mantling deposition at this low latitude. Our morphological observations suggest that the outer crater predates the end of an obliquity-controlled period of latitude-dependent mantle deposition, while the inner crater appears to post-date the most recent period of mantle deposition (Head et al, 2003). To test this proposition quantitatively we performed crater counts on smooth near-rim units of the inner crater. These units north and south of the inner crater both yield crater retention (CRE) ages of ~1.25 Ma (Fig. 3b) based upon isochrons of Hartmann (2005). The use of small craters for age dating is supported by the small crater production rates observed on Mars by Malin et al. (2006) which agree within a factor of three with the isochron system (Hartmann, 2007). Similarly, Hartmann and Quantin-Nataf (2008) showed that counts of small craters could be used effectively to date young rayed craters. Therefore, our crater counts on the smooth rim deposits yield a robust CRE age of formation for the inner crater of ~1.25 Ma and imply that this crater is among the youngest craters of its size on Mars.
QUOTE (dvandorn @ Mar 4 2009, 10:13 PM)
I know that a number of models predict wide fluctuations in Martian axial tilt, but I've never seen anything worked up that attempts to determine how frequently the planet sees high tilt angles, and what the approximate timing has been since the last high-tilt epoch... or until the next one.
-the other Doug
-the other Doug
Laskar et al. has spent a significant amount of time modeling orbital dynamics for the solar system. His results are well documented and respected. You can download the orbital parameters for Mars here: http://www.imcce.fr/Equipes/ASD/insola/mars/mars.html
The record covers the last 20 Myr very well. Longer solutions are deterministically chaotic (ie, exceedingly sensitive to initial conditions).
Interesting! Although I note that over longer timeframes, he shows five different solution sets based on five different sets of assumptions.
These projections could also be significantly altered if there has been any crustal movement over the projected timeframes (i.e., the Tharsis volcanic pile acting to move the entire crust such that Tharsis' most massive build-up is now sitting over the equator...)
-the other Doug
These projections could also be significantly altered if there has been any crustal movement over the projected timeframes (i.e., the Tharsis volcanic pile acting to move the entire crust such that Tharsis' most massive build-up is now sitting over the equator...)
-the other Doug
Does anyone know which photo was used for illustrating the articles we can see on the Internet?
Thanks in advance.
Thanks in advance.
It's PSP_002293_1450 .
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