Knobby Terrains at the Sources of the Navua-Hadriacus Drainage Systems on Mars: What Are the Knobs? H

Lunar and Planetary Science XLVIII (2017) 1763.pdf

KNOBBY TERRAINS AT THE SOURCES OF THE NAVUA-HADRIACUS DRAINAGE SYSTEMS ON MARS: WHAT ARE THE KNOBS? H. I. Hargitai1 and V. Gulick2, 1NASA Ames Research Center / NPP (MS 239-20 Moffett Field, CA 94035, [email protected]), 2NASA Ames Research Center/ SETI Institute (MS 239-20 Moffett Field, CA 94035, [email protected]).

Introduction: In the East Hellas Region, we have channels on adjacent terrain and rarely have a ridge-like mapped the source areas of three major channel-and- extension. valley systems that are associated with knobby terrain. Knobby terrain is largely concentrated within the upper From the west to the east, Navua “A” (Hargitai et al. segments of Navua Valles (Fig. 2) valleys and channels submitted) originates from the valleys on the rim of a and the drainage of Hadriacus Mons. They are located crater at 84°E 29°S. Navua “B” emerges in the plains on both the inner and outer slopes of the southern, un- and its knobby channel segment terminates in a basin dissected walls of crater at 84°E 29°S, whose northern that is interpreted as a paleolake with a large field of inner walls are dissected, from where a branch of Navua knobby terrain. There similar knobby areas on the flanks A originates. of Hadriacus Mons and knobby patches within the un- channeled uppermost valleys of Hadriacus Mons. We have investigated these terrains to find a plausible ex- planation of their origin and association with the channels. Global survey: We have mapped channel-associated knobs globally on Mars (Fig. 1), using Google Mars’ CTX coverage with THEMIS Day IR fillers, surveying all valleys in the database of [1] and all outflow channels and their surroundings, to find any zonal or regional concentration of these features.We have identified 81 patches of knobby terrains. The largest concentrations are located North of Argyre in the Nereidum Montes, and South of Chryse in Ares, Tiu and Shalbatana Valles, in additon to others scattered in the highlands. The concentrations of knobs in the Argyre and Hellas regions Fig. 2. Knobby terrain within a Navua Valles channel and on are both between 25 and 40° south, but other valleys in its overbank area. (CTX) These knobs are 50-100 m in diam- the same latitude band do not have knobs. In the Eastern eter. Hellas region, they also occur in channels East and Knobby terrain is located in numerous areas along North of Hadriaca Patera, in Ausonia Montes that we the rim, on the floors of parallel valleys, or terraces on have not yet mapped in detail (Fig 3a). the crater wall, in un-channeled valleys connected to channels, in patches in un-channeled valley bottoms, on the uppermost parts on the flanks of Hadriacus Mons, on streamlined islands, in valley heads, uphill beyond valley heads, and on the inside of channeled valleys up- stream from depositional reaches (Fig. 3). Some knobs are even located on interior channels and in small depressions over the channel banks. How- ever, there are no channels associated with the uppermost knobby valleys or terraces on the crater rim and Hadriacus Mons. In the upper reaches of Navua B, Fig 1. Global distribution of channel-associated knobby ter- knobby terrain is limited to terraces, which suggests that rains on Mars (yellow dots). Blue lines are valleys and chan- they formed before the last fluvial episode. nels from [9] Hynek (2010). Background: MOLA DTM. Fractured mounds, 150 m wide, protrude from Description of the knobby terrain: Individual knobs smooth, cracked deposits within the channel of Navua are 50-150 m wide, 100-500 m apart, may be cracked or A and smooth 150 m wide subdued mounds on another flat topped, without central pits (at CTX resolution), similar deposit, which suggests that knobs formed ear- cone to dome shaped or irregular, and circular to ridge- lier and were flooded or covered by the accumulating like with a bright crest-line. Similar high-albedo crest- material or alternatively formed in association with in- lines may also occur in various features beyond the channel deposits. Lunar and Planetary Science XLVIII (2017) 1763.pdf

Hellas occur between -700 and -2600 m altitudes. This suggests that conditions both above and below this band were not favorable for their formation or the occurrence of its material of origin was limited to a particular geo- graphic zone. In a pingo model, above these altitudes, sufficient hydraulic pressures for ground water was not available for pingo formation, whereas below these elevations, the paleo-climate was too warm for even dis- continuous permafrost conditions at the time of knob formation. However, in Ausonia Montes, these putative pingos formed up to 2 km in height, although this site may have had an isolated ground-water table. The knobs described here are much smaller than the pitted knobs in Dao Vallis, which are interpreted as con- structional volcanic domes [9], or those on Pavonis Mons interpreted as sublimation till [6] or those within the caldera of Hadriaca Patera that were interpreted as erosional remnants of a pyroclastic deposit or lava flow [7]. Since knobby terrain in East Hellas is present on

Fig 3. a) Map of knobby channels in the East Hellas Region. both the floors of incised channels and on unchanneled Yellow dots show knobby channels. b) Map of the source areas valleys above the downcut channels, the formation of of the Navua Valles and the channels on the flanks of Hadria- the knobby terrain was closely associated with channel cus Mons. Purple areas are knobby terrains. activity and likely postdated the main channel for- The only group of collapsed knobs occurs in one of the mation. Possible interpretations for the knobs are: till Hadriacus flank valleys, at the southernmost occurrence blocks, uncollapsed cryogenic mounds (pingos), con- of knobby terrain on these slopes (Fig. 4). structional spring mounds or subsurface indurated forms. However, any proposed knob formation model that must explain the following characteristics: 1) their prefererence for, but not exclusive occurrence, on valley floors and in other depressions; 2) concentrations at only higher elevations; 3) their association with Hadri- acus Mons and the high elevation Crater at 84°E 29°S source area of Navua A; 4) their association with sinu- ous ridges, linear forms and deposits in the Navua A valleys; and 5) their occurrence in the Navua B putative paleolake. The sites of our global survey may help elu- cidate common conditions of their formation.

Acknowledgements: This research was supported by a senior postdoctoral research fellowship awarded to H. Hargitai by the NASA Postdoctoral Program (NPP) at Ames Research Center, administered Fig. 4. Collapsed-degraded knobs within a narrow val- by ORAU and USRA through a contract with NASA. V. Gulick is ley on the flank of Hadriacus Mons. (CTX supported by MRO HiRISE CoI funds and also by SETI Institute’s P19_008518_1472_XN_32S269W) NAI CoI funds. References: [1] Hynek BM, Beach M, Hoke MRT (2010). J. Ge- ophys. Res., 115, E09008, doi:10.1029/2009JE003548. [2] Soare RJ. Interpretation: Knobby terrain comprised of sub-km Conway SJ. Dohm,JM., El-Maarry MR (2014). Earth and Planetary Science Letters 398, 25–36. [3] Allen CC, Oehler DZ (2008). Astro- sized knobs in locations not assocated with channels biology 8(6): 1093-1112. doi:10.1089/ast.2008.0239. [4] Chan MA, elsewhere on Mars were interpreted as open system pin- Ormö J, Murchie S, Okubo CH, Komatsu G, Wray JJ, McGuire,P., gos (in Argyre, [1]), hydrothermal or spring mounds (in McGovern JA (2010). Icarus 205(1), 138–153. [5] Oehler DZ, Allen Vernal crater, [2]), knobs related to fluid movement CC (2012) SEPM Special Publication 102. [6] Sheridan MF (1970). Geological Society of America Bulletin, 81, 851–867. [7] Shean DE, through porous, granular material (Candor Chasma, [3]) Head JW, Marchant DR (2005). Journal of Geophysical Research, or mud volcanoes (Acidalia Planitia, [4]). They are also 110, E05001, doi: 10.1029/2004JE002360. [8] Crown DA, Greeley simiar to domical fumarolic mounds, which have been R (1993). J. Geophys. Res. 98(E2), 3431–3451, doi: identified on ignimbite layers in the Bishop Tuff in Cal- 10.1029/92JE02804. [9] Korteniemi J, Kukonnen S, (2013). EPSC ifornia [5]. These knobs in the mapped region in East Abstracts 8, EPSC2013-779.