Erebus crater

[JonClarke]

The December issue of <i>Geology</i> (Volume 34, Issue 12, pages 1985-1088) has a very nice paper on the sedimentary structures observed by Opportunity at Erebus crater:<br /><br />Sedimentary textures formed by aqueous processes, Erebus crater, Meridiani Planum, Mars.<br /><br />J. Grotzinger, J. Bell III, K. Herkenhoff, J. Johnson, A. Knoll, E. McCartney, S. McLennan, J. Metz, J. Moore, S. Squyres, R. Sullivan, O. Ahronson, R. Arvidson, B. Joliff, M. Golombek, K. Lewis, T. Parker, J. Soderblom.<br /><br />New observations at Erebus crater (Olympia outcrop) by the Mars Exploration Rover Opportunity between sols 671 and 735 (a sol is a martian day) indicate that a diverse suite of primary and penecontemporaneous sedimentary structures is preserved in sulfate-rich bedrock. Centimeter-scale trough (festoon) cross-lamination is abundant, and is better expressed and thicker than previously described examples. Postdepositional shrinkage cracks in the same outcrop are interpreted to have formed in response to desiccation. Considered collectively, this suite of sedimentary structures provides strong support for the involvement of liquid water during accumulation of sedimentary rocks at Meridiani Planum. <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[3488]

Hi Jon,<br /><br />Thanks for that.<br /><br />Does this still suggest that Meridiani Planum was wet, than dry in alternate spells, or was it just one catastrophic episode that Meridiani was wet. <br /><br />Does some research suggest that Mars has been cold & dry for most of its existance with only a short, but intense period of warm & wet conditions, due to geological activity keeping Mars warm before it froze & lost most of its atmosphere?<br /><br />The recent debate & evidence of water flows founfd on MGS images, suggest fossil water leaking out or seeping through?<br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

 

[JonClarke]

Hi Andrew <br /><br />The paper paper reports on small outcrops in Erebus crater, some km from Eagle crater and shows excellent examples of subaqueous ripples and also prismatic dessication cracks that are indistinguishable from those found on Earth. The first indicates that there were sustained gentle subaqueous water flow, the second that there were repated episodes of wetting and drying. I had spotted to current ripples, which are lovely examples of the structure, but I did miss the dessication cracks. But I find their examples convincing.<br /><br />What this means is that the story of salt lakes, high water tables, and episodic gentle water flow that was inferred from Endurance and Eagle craters is holding up, with some variations. The haematite concretions and the evaporite boulds were missing, as were the steep cross beds indicating aeolian deposits.<br /><br />The Meridiani deposits are Late Noachian to Early Hesperian from crater counts. This is the era that the OMEGA team of Mars Express have called Theikian, the era dominated by sulphate deposition. At this time Mars was probably losing its atmosphere and hydrosphere, but was still able to sustain open water bodies. <br /><br />Jon<br /> <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[JonClarke]

The conclusions of the paper state:<br /><br />"Rocks exposed at the Olympia outcrop, Erebus crater, show a distinct set of primary stratification geometries and shrinkage cracks that are consistent with deposition and subsequent modification under aqueous conditions. Trough cross-lamination is distributed on a regional basis, at least across the ~4 km traverse examined by Opportunity, and this suggests that overland aqueous flows were widespread. Prism cracks provide strong evidence for iterative shrinkage and expansion of sediments during desiccation. This may have occurred whenever the capillary fringe of the groundwater table rose to intersect the surface. Subsequent drying would have induced cracking, as commonly happens in playa lake or sabkha-type environments. Intermittent wetting and transient flooding of interdune depressions during groundwater migration provide a simple model that is consistent with observations from all three craters (and several intervening outcrops) examined to date."<br /><br />Cheers<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[3488]

Thanks Jon for explaining that clearly. I understand it fully now. The picture is generally what I had thought.<br /><br />Fascinating stuff.<br /><br />Andrew Brown.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

 

[bobw]

Hi Jon<br /><br />Is a prismatic dessication crack something like the picture below with the voids filled in by a crystal? <div class="Discussion_UserSignature"> </div>

 

[JonClarke]

Nice photo!<br /><br />A prismatic dessication crack would be a stacked series of sedimentary layers where the cracks penetrate through multiple layers, forming a prism, rather than a thin polygon. The cracks themselves can be filled with otherlying sediment or crystalline material.<br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[JonClarke]

Depends on who you talk to <img src="/images/icons/smile.gif" /><br /><br />The old age for the Late Noachian to Early Hesperian is 4.4-3.7, with the boundary at 3.8 Ga. The young age is 3.85-3.1, with boundary at 3.5 Ga.<br /><br />But these are all based on crater counting, which is a very blunt dating instrument and is based on lunar and terrestrial cratering rates only. <br /><br />Jon <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>