Could Impact Sedimentation Solve the Mars Climate Dilemma? D
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11th Planetary Crater Consortium 2020 (LPI Contrib. No. 2251) 2027.pdf COULD IMPACT SEDIMENTATION SOLVE THE MARS CLIMATE DILEMMA? D. M. Burt1, 1School of Earth and Space Exploration, Arizona State University, P.O. Box 1404, Tempe, AZ 85287-1404. Introduction: The early climate dilemma involves The landing sites for the three rovers were, in all making a scientifically acceptable model to account for cases, chosen based on orbital indications of possible conventional interpretations of orbital observations. past water activity. Surprisingly, the ground observa- These observations include ancient valley networks, tions made by the rovers themselves do not actually re- flat-topped basins, exposed sequences of layered sedi- quire sediment deposition by liquid water [3][4], and ment, and spectra unique to hydrous clay minerals. In many features (e.g., primitive basaltic sediment com- addition, older craters are observed to have been partly positions, persistent acidic salts and olivine, abundant erased (smoothed out) in a process called “terrain soft- amorphous materials and immature clays, nearly ubiq- ening.” uitous low-angle cross-bedding, low sediment bulk In almost all cases, by analogy with Earth, the val- densities, high sediment friability, relatively high Ni ley networks are assumed to have been carved by content, planar scouring at unconformities, evidence of streams, the level basins to have been filled by sedi- original dip, and abundant impact spherules of various ments deposited by lakes or oceans, the sequences of types), can actually be interpreted as good evidence layered sediment to have been deposited by water (or, against water deposition, despite claims to the contrary if coarsely cross-bedded and sandy, by wind), and the (see discussion below). clay minerals and terrain softening to have been pro- Features that might be unique to aqueous deposits duced by aqueous weathering. All of these assump- appear to be completely lacking. These could include tions require that early Mars was comparatively warm actual fissile shale beds, channel-confined conglomer- and wet (within the stability field of liquid water), ei- ates, preserved small channels themselves and scal- ther continuously or episodically for relatively long pe- loped scours such as flute marks, strong lateral facies riods. changes related to streams or lakes (such as commonly Warm and wet interpretations are problematic, in- seen at point bars in streams), post-depositional dewa- asmuch as Mars is and always has been a tiny planet tering textures such as sediment deformation and load further from the Sun than Earth, plus the early Sun casts, oscillation ripple marks made in shallow water, may well have been fainter than the modern Sun (faint discrete evaporitic salt layers, and finally, actual mud young Sun paradox). Therefore, making a scientifically cracks (polygonal cracks in dried mud that are clearly reasonable model of how early Mars could have been filled with the overlying sediment, not joints filled both warm and wet has been a problem, especially as with later diagenetic sulfate-filled mineral veinlets). computer modeling has become more sophisticated The putative “desiccation cracks” at Gale [5] would with time [1]. The continuously warm and wet inter- appear to be vein-filled joints, by this criterion. Note pretation requires an unreasonably dense CO2-rich at- that polygonal joint systems can form in many rock mosphere or one containing major unconventional types, by a variety of geologic processes, including greenhouse components. An episodically warm and simple cooling. wet climate has been suggested to result from, e.g., ma- All three sites contain aqueous minerals (clays and jor impact episodes [2], but such episodes are modeled hydrated sulfates) interpreted to have formed after dep- to have been brief. osition of the layered sediments (that is, diagenetically, Ground Observations: As outlined above, the in most cases) but there is no direct evidence, even at warm and wet climate interpretation largely resulted Gale Crater, that water actually deposited the sedi- from orbital observations. After the two Mars Explora- ments themselves. Instead, it altered them after burial, tion Rovers MER A (Spirit) and MER B (Opportunity) forming mainly primitive smectite clays, veinlets of landed on Mars in 2004, with a main NASA-assigned Ca-sulfates, and inside Gale Crater only, typical con- goal of “follow the water” their ground observations in cretions and nodules of highly variable size and shape Gusev Crater and at Meridiani Planum were largely in- [6]. These typical concretions are distinctive from the terpreted in light of this objective (except that this size- and shape-limited hematitic spherules (assumed proved partly impossible at Gusev, where volcanism to be accretionary lapilli) forming a lag deposit at Mer- was invoked). Interpretations based on observations idiani Planum. These presumed impact spherules in made by the Mars Science Laboratory (Curiosity) after blast beds were mistaken for concretions by the MSL 2012 in Gale Crater clearly reflect the same objective, team [3], although they are not cut by bedding or flat- rephrased as “habitability.” tened along bedding planes. 11th Planetary Crater Consortium 2020 (LPI Contrib. No. 2251) 2027.pdf All of the Martian sedimentary deposits appear to surfaces), it also should be the dominant sedimentation have undergone variable descending surficial alteration process on Mars (along with localized dune fields, by acid frost or dew over the billions of years since transient dust deposits, and volcanic ash beds). Yet no deposition. This alteration has formed distinctive acid ancient impact-derived sediments (other than boulders salts such as jarosite that could not persist in the pres- and megabreccias) have yet been interpreted on Mars, ence of liquid water in contact with basalt. either from orbit or on the ground. Virtually every Multiple declarations of aqueous deposition at the sedimentary rock examined by rovers has been attribut- rover sites are clearly interpretations, probably based ed to flowing or standing water or wind (or volcanism on prior expectations (so-called confirmation bias). in the case of the cross-bedded Home Plate sediments Impact Deposits (Blast Beds): Major impacts, as in Gusev Crater). the most energetic events ever to affect the Martian This lack of recognition is surprising because su- surface, were abrupt releases of energy, or explosions. perposition demands that younger blast beds be depos- Volcanic, less energetic explosions presumably also af- ited on top of unexposed older sediments (which might fected the surface of Mars, but volcanism appears to well include water-deposited beds), and orbiters and have become increasingly localized during the Noa- rovers are restricted to observing rocks deposited on chian, when impact cratering also was subsiding. On a the Martian surface. So where are the impact beds? planet with abundant volatiles and an atmosphere (un- Climate Implications: As noted above, if most of like Earth’s Moon), both types of explosions should the sedimentary layers on Noachian Mars were depos- have generated ground-hugging turbulent density cur- ited by liquid water, as currently assumed, then this rents, also known as pyroclastic density currents or places difficult-to-model restrictions (“warm and wet”) PDC’s (if volcanic in origin) and also called base surg- on the temperature range of Martian climates at the es (if relatively dilute and rapidly flowing), as first de- time, presumably over extended periods (as has been scribed for nuclear explosions on Earth. claimed for, e.g., the putative lake in Gale Cra- The most interesting feature of the blast beds de- ter)[7][8]. If most of the sedimentary layers on Mars posited by such density currents is that they superfi- were instead deposited by impact cratering, then few cially resemble layered sediments deposited by water such restrictions are present, and Mars could have been and wind [3]. For example, they display abundant continuously cold and icy except perhaps for very cross bedding, most commonly at low angles, they can short periods when Noachian drainage systems were form dunes and even antidunes, they can scour (erode) eroded. However, note that the ability of rocky or san- the substrate, upon cooling and drying they can devel- dy Martian density currents to erode channels at their op polygonal jointing that resembles patterns of mud base is currently unknown, but such erosion cannot be cracks in shale, and depending on distance from their excluded, especially in the walls of craters. In any source they can vary in grain size from cobbles to dust, case, a predominantly cold and icy Noachian Martian as their sorting (and rounding) increases with distance. surface does not exclude widespread diagenetic altera- Distinctive features of impact-related blast beds tion of impact-deposited sediments by subsurface (compared to wind and especially water deposits) groundwater or hydrothermal fluids, nor does it ex- might include relatively poor sorting, tendency to clude descending surficial alteration by acid frost or drape topography (original dip), low bulk density, mist caused by volcanism or impacts into a sulfur-bear- highly friable nature, cementation by soluble salts, lack ing target. of confinement to channels, breccias that are not con- Acknowledgments: I am grateful to my former fined to channels, lack of primary clays formed by colleagues and co-authors M.F. Sheridan, K.H. Woh- weathering, primitive, homogeneous compositions, ut- letz, and especially L.P. Knauth for instruction and ter lack of shales, abundant glassy amorphous material, ideas that were used in arriving at the above unconven- impact spherules of various types, presence of meteor- tional interpretations. ite fragments, lack of evaporite beds, presence of per- References: [1] Head J.W. et al. (2020) LPS LI, sistent acid salts, and lack of dewatering textures Abstract #2067. [2] Segura T. L. et al. (2002) Science caused by postdepositional compaction. Such features 298, 1977. [3] Knauth L.P. et al. (2005) Nature, 438, are present in all of the sedimentary deposits studied 1123.