[X] My Assistant

[Guest_BruceMoomaw_*]

I commented in another thread on the fact that the new LPSC abstracts show that the Gully Wars are still raging -- everyone still has their own theory on what creates them (although one can hope that this is one puzzle that MRO will solve). But two of the abstracts are especially interesting because they focus on one of the biggest puzzles about the gullies -- the effect that latitude has on which direction their slopes tend to face.

Lanza and Gimore (#2412) and T. Ishii et al (#1646) point out, as others have before them, that the gullies in the 30-40 deg latitude range (that is, nearer the equator) usually have slopes pointing toward the poles -- while those in the 40-50 deg belt, closer to the poles, have their slopes mostly pointing toward the equator. (Gullies facing toward the eastern or western quadant are scarcer, and seem to exist mostly in the middle zone of 35-45 degrees.) This is obviously telling us something important about their causation mechanism -- but what?

Lanza and Gilmore say only that it look as though there's a Three Bears aspect to the causation mechanism, whatever it is -- it doesn't operate on slopes that are either too cold OR too warm. If the gullies form during Mars' low-obliquity periods, then equator-facing slopes in the equatorial region are Too Warm for them, while pole-facing slopes in the polar regions are Too Cold. If, instead, whatever forms the gullies works during Mars' high-obliquity periods -- when a planet's poles, one at a time, actually get much warmer than its equator ever does during low-obliquity periods, because they face the Sun continuously for months at a time -- then the opposite is true. But in either case, pole-facing slopes near the equator and equator-facing slopes near the poles are Just Right.

Ishii, however, goes further, and comes up with an intriguing new idea, based on a 2003 finding by Kreslavsky and Head ( http://porter.geo.brown.edu/planetary/documents/2880.pdf ) regarding on the steepness of Mars' slopes. They too are dependent on latitude, but in a different way. Poleward of 50 degrees, slopes steeper than 20 degrees are virtually nonexistent. In the 30-50 degree belt, there's an odd transitional effect. Poleward-facing steep slopes start to decrease rapidly in number at 30 degrees latitude and virtually disappear at 40 degrees. Equator-facing steep slopes also start to diminish in number at 30 degrees, but more gradually -- they don't disappear until 50 degrees.

Kreslavsky and Head ascribe this to the fact that, during high-obliquity periods, Mars' polar regions actually get so warm that their near-surface permafrost starts to melt, causing steep slopes to slump and slide. Naturally this effect is stronger for pole-facing (and therefore Sun-facing) slopes than it is for equator-facing ones.

Could this same ice-melting cause the gullies? Unfortunately, Figure 2 in Ishii's abstract, and Figure 1 in Lanza and Gilmore's, reveal a serious problem with this idea. Pole-facing slopes actually do fit this theory quite well -- they show gullies in exactly the same 30-50 degree zone in which they're slumping (with the gullies presumably having been totally erased along with the slopes by the much more severe slumping at more polar latitudes). So do equator-facing gullies poleward of 40 degrees. But equator-facing gullies start slumping in large numbers at 30 degrees (although not as large as the pole-facing slopes), but nevertheless have virtually no gullies until you hit 40 degrees.

So Ishii proposes that the gullies are caused, not by warmth like the slumping slopes, but by a separate combination of steep slopes and cold -- which works only during Mars' low-obliquity periods, like its current one. Specifically, he thinks that the gullies are caused by mid-latitude surface patches of frozen CO2 during the winter: "We would propose that gullies may have been formed by avalanches of seasonal CO2 ice. Observations of particles drifted by wind and their high reflectivity for visible light indicate that CO2 ice will be made up of small particles. Seasonal CO2 ice is expected to sublime from the bottom [in the spring] and form a pressurized gas layer between the CO2 ice and the ground. The pressurized gas may not only trigger avalanches but also fluidize CO2 ice particles...Such granular flows of seasonal CO2 ice may dissect local slopes and result in the development of gullies on Mars." In the 30-40 degree zone closer to the equator, such CO2 snow patches form only on the colder pole-facing slopes. The 40-50 degree zone gets cold enough in the winter for CO2 snow to condense on slopes facing BOTH directions -- but in that zone, there are almost no pole-facing steep slopes, so the snowslides only happen on the equator-facing ones. And the east and west-facing gullies would form on slopes in the intermediate latitude zone, as they in fact do.

It's an interesting notion. But if the gullies are created by avalanches resulting from warming CO2, I wonder if they may be avalanches not of frozen CO2 snow, but of surface dust levitated by the gas from frozen CO2 sublimating out of the soil in spring -- or even by large amounts of gaseous CO2 that was adsorbed by the cold soil particles during the winter (as is known to be the case for Mars), and is then released in the spring. That is, they may be created by the same forces that initiate the beginnings of high-latitude dust storms on Mars. (Released CO2 gas, rather than liquid water, may also explain the springtime dune slides that we see occasionally from orbit.)

Ishii's theory may explain something noted by Lanza and Gilmore: the fact that gullies form on slopes at an average of 18 degrees -- "Many of these slopes are well below the angle of repose, which Heldmann and Mellon point out obviates mass-wasting processes as the sole mechanism of the gully formation" as Gwendolyn Bart suggests (abstract 1345). But if dust is levitated and lubricated by a cushion of released CO2 gas, it could slide on much shallower slopes.

Ishii may also explain something else noted by Lanza and Gilmore (and by Heldmann and Mellon before them): as you move toward the equator, gullies form at deeper and deeper points on their slopes. In the case of pole-facing slopes, as you move toward the equator, points farther down on the slopes would be more shadowed from the Sun at the beginning and end of each day, and thus be colder and allow more frozen or soil-adsorbed CO2 to build up there in the winter. I would think, however, that this effect wouldn't apply to equator-facing slopes -- so the next question is: do we not see the same link between latitude and the formation heights of equator-facing gullies? As far as I know, no one has yet examined this.

[jaywee]

Speaking of craters - is there any "systematic" attempt to catalogise asteroids which could impact Mars in near future (like the NEO does for earth)?

[Guest_BruceMoomaw_*]

A few more interesting Mars abstracts from this year's meetings:

(1) MER-B's Mini-TES team ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2021.pdf ) concludes from its deconvolution of the thermal-IT spectra of the Meridiani outcrop rock that it is indeed sulfates mixed with a large amount of amorphous silica and glass and the phyllosilicate nontronite -- confirming that the stuff mixed with the sulfates is indeed the dregs of basalt extensively modified by exposure to sulfuric acid, rather than unaltered basaltic mud being mixed in with the sulfates.

(2) P.E. Hintze says ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2098.pdf ) that much of the disappearance of organics in Mars' surface soil may be attributable to the plasma produced by electrostatic glow discharges: "The high probability for dust interactions during Martian dust storms and dust devils, combined with the cold, dry climate of Mars, most likely results in airborne dust that is highly charged. Such high electrostatic potentials generated during dust storms on Earth are not permitted in the low-pressure CO2 environment on Mars; therefore electrostatic energy released in the form of glow discharges is a highly likely phenomenon. Since glow discharge methods are used for cleaning and sterilizing surfaces throughout industry, the idea that dust in the Martian atmosphere undergoes a cleaning action many times over geologic time scales appears to be a plausible one."

(3) The Mars Express HRSC team ( http://www.cosis.net/abstracts/EGU06/01915/EGU06-J-01915.pdf ) have seen big dust devils racing across the surface at 15-27 meters/sec, "not consistent with previous assumptions of the wind velocity at the martian surface (~5 m/sec)."

(4) The mole planned for inclusion on the Geophysical Experiment Package that is supposed to be left behind on the surface by the ExoMars rover -- to implant a chain of heat-flow sensors 5 meters deep -- is described in http://www.cosis.net/abstracts/EGU06/09633/EGU06-J-09633.pdf . Heaven knows whether it will actually work, but MEPAG considers Martian heat-flow measurements important if they can be done.

(5) The THEMIS team ( http://www.cosis.net/abstracts/COSPAR2006/...006-A-02673.pdf ) reports that "that the Martian crust, while dominated by basalt, contains a remarkable diversity of igneous materials whose range in composition -- from ultra-mafic basalts to granitoids -- rivals that found on the Earth" (although high-silica rocks are rare). They also note the vast variety of strange markings beneath the seasonal CO2 ice cap, "consistent with a translucent, impermeable CO2 ice cap that sublimates from the base, producing gas flow beneath the ice that erodes the channels and jets that erupt sand-sized grains through vents. These processes are unlike any observed on Earth. The vertical stirring of the polar-layered deposits by this process may have significantly altered the sedimentary record, and may complicate the interpretation of the sedimentary record as it relates to climate history."

[The Messenger]

A few more interesting Mars abstracts from this year's meetings:



(2) P.E. Hintze says ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2098.pdf ) that much of the disappearance of organics in Mars' surface soil may be attributable to the plasma produced by electrostatic glow discharges: "The high probability for dust interactions during Martian dust storms and dust devils, combined with the cold, dry climate of Mars, most likely results in airborne dust that is highly charged. Such high electrostatic potentials generated during dust storms on Earth are not permitted in the low-pressure CO2 environment on Mars; therefore electrostatic energy released in the form of glow discharges is a highly likely phenomenon. Since glow discharge methods are used for cleaning and sterilizing surfaces throughout industry, the idea that dust in the Martian atmosphere undergoes a cleaning action many times over geologic time scales appears to be a plausible one."

Glow discharges are easy to create on Earth, using Mars-like atmospheres and dust grains. But to the best of my knowledge, we have not observed similar phenomenon on Mars. (yes?/No?)

(5) The THEMIS team ( http://www.cosis.net/abstracts/COSPAR2006/...006-A-02673.pdf ) reports that "that the Martian crust, while dominated by basalt, contains a remarkable diversity of igneous materials whose range in composition -- from ultra-mafic basalts to granitoids -- rivals that found on the Earth" (although high-silica rocks are rare). They also note the vast variety of strange markings beneath the seasonal CO2 ice cap, "consistent with a translucent, impermeable CO2 ice cap that sublimates from the base, producing gas flow beneath the ice that erodes the channels and jets that erupt sand-sized grains through vents. These processes are unlike any observed on Earth. The vertical stirring of the polar-layered deposits by this process may have significantly altered the sedimentary record, and may complicate the interpretation of the sedimentary record as it relates to climate history."

The vents are very interesting...could there be a similar process in comets? Could 'base' subliming CO2 or H20 open jets that expel water vapor? Why is the CO2 subliming 'from the base' rather than from the top down? Does Mars have moles

[Guest_BruceMoomaw_*]

Glow discharges are easy to create on Earth, using Mars-like atmospheres and dust grains. But to the best of my knowledge, we have not observed similar phenomenon on Mars. (yes?/No?)

The vents are very interesting...could there be a similar process in comets? Could 'base' subliming CO2 or H20 open jets that expel water vapor? Why is the CO2 subliming 'from the base' rather than from the top down? Does Mars have moles

(1) We haven't yet landed any instruments on Mars that could look for glow discharges and other dust-related electrostatic phenomena. But we certainly will in the not-too-distant future -- scientific interest aside, MEPAG's Mars Human Precursor group says that we badly need to land such an experiment package at least once to determine just how much of a peril Martian electrostatic discharges may actually be to unmanned and manned landers.

(2) The current model of comets has always assumed that, after their initial exposure to the Sun, they develop a lag-deposit crust of rocky material which prevents further release of sublimating gas until the temperature beneath the layer vaporizes enough underlying ices (both water and lower-temperature stuff) to burst a hole in the surface crust to allow a vent jet. CO2 is just one of the lower-temperature ices that are contributors to this sort of thing (CO and probably ethane are also high on the list).

(3) As for why CO2 ice layers near the Martian poles sublimate from the bottom up: it's simply because CO2 ice is relatively transparent to sunlight, so it passes through the ice and warms the dirt underneath, boiling the ice there. There have been a lot of papers written on this subject over the last few years; the underlying gas pressure buildup produces all sorts of weird-looking patterns beneath the CO2 ice that have been photographed by MGS and its successors ("spiders", "fried eggs" "Dalmatian spots", and the Lord knows what else).

[dvandorn]

...

(4) The mole planned for inclusion on the Geophysical Experiment Package that is supposed to be left behind on the surface by the ExoMars rover -- to implant a chain of heat-flow sensors 5 meters deep -- is described in http://www.cosis.net/abstracts/EGU06/09633/EGU06-J-09633.pdf . Heaven knows whether it will actually work, but MEPAG considers Martian heat-flow measurements important if they can be done.

I sure hope this (or something like it) works. I wholeheartedly agree with MEPAG -- I think heat-flow measurements are very important. With some idea of the current heat flow of Mars, we can at least start making informed speculations on subjects that, right now, we can only make wild arm-waving gestures at...

-the other Doug

[RNeuhaus]

It is curious that neither Lanza, Gilmore and Ishii authors about gullies haven't mentioned that the gullies might have caused by a Marsquake. It happens very often that the landslides are caused by from any small Earthquake. Is there any Marsquake?

However, the gullies are most localized on the rim than any other sides and also most frequenent on the equator and poles slopes. That has lead me to think that these are not related to Marsquake. These most gullies are generally localized that any quake can influence that. Mars must have no so much quakes as Earth since it has no active volcan, no active and extensive plate tectonic but is isolated in small zones around where there are most magnetic field.

Rodolfo

[chris]

As I understand it, there is only indirect evidence for marsquakes (see here). I don't think a seismometer has been landed.

Chris

[djellison]

Landed with the Viking landers, but iirc, V1's never worked, and V2's didn't detect anything worthy of note.

Doug

[dilo]

As I understand it, there is only indirect evidence for marsquakes (see here). I don't think a seismometer has been landed.

About pit chains, could be the same mechanism causing "minicraters" at Meridiani on a smaller scale!?

[Bob Shaw]

Whether or not Marsquakes cause gullies, gully formation events certainly cause Marsquakes, so let's all look forward to getting some seismometers up there!

Bob Shaw

[RNeuhaus]

Landed with the Viking landers, but iirc, V1's never worked, and V2's didn't detect anything worthy of note.

It is certain that Viking had seismometer but their data were not reliable since its results has noise caused by the wind vibrations on the spacecraft. A good seismometer, must be cemented on a large rock.

Rodolfo

[Guest_BruceMoomaw_*]

Viking 2's seismometer detected numerous events -- but in every case but one, its wind sensor was operating at the same time and determined that the event was due to a wind gust. In one case, the wind sensor was off at the time, and so we'l never know about that one. So it's open to question how much better Viking 1's seismometer would have done even had it functioned -- the whole experiment was just too optimistic about the seismicity level of Mars (on which it did manage to set an upper limit).

[edstrick]

The "candidate" VL-2 seismic event was recorded at night during a period with a vanishingly low (or something like that) occurrence of wind gusts. The data was taken at the intermediate data rate so they don't have the real seismic or whatever waveform, just amplitude with time and a crude mean frequency (number of zero crossings of the signal) for each amplitude measurement, I think.

Without checking the semi-inaccessible "stacks" of reports and stuff, I think it had a signature similar to what would be observed for an earthlike intraplate quake, magnitide maybe 4.5, at a distance of 250 km. One signal during the total instrument "quiet time" observations implied an upper limit to seismic activity similar to typical intraplate <whatever that is> activity on Earth.

We'll never know if that signal was seismic or not.

[Guest_BruceMoomaw_*]

"Intraplate" is simply the seismic level in the middle of a crustal plate, which of course is a lot less than the level on the edges of plates where they're grating past and/or butting into each other.