45th Lunar and Planetary Science Conference (2014) 1917.pdf

ESKERS IN IRELAND, ANALOGS FOR MARS LANDFORMS. X.M. Pellicer1 and M. Bourke2, 1Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland ([email protected]), 2Department of Ge- ography, Trinity College, Dublin 2, Ireland.

Introduction: High resolution images of the Mar- in stationary or retreating ice sheets, and are long, nar- tian surface has allowed the identification of a number row, sinuous ridges. They are commonly composed of of ridge-like features inferred as putative ridges in and gravels with rounded boulders and cobbles. Argyre Planitia [1, 2, 3], Dorsa Argentea [4], Isidis Clasts are usually much larger overall than in other Planitia [5, 6], Rahway Vallis [7], Utopia Planitia [8], glaciofluvial deposits. The gravels are usually bedded, Chryse Planitia [9],Noachis Terra and north of Deuter- the beds often slump towards the flank of the esker, onilus Mensae [10]. indicating collapse as the confining ice walls. There are The name Esker is derived from the Irish word 4 types of in Ireland [12]: “eiscir” meaning ridge or elevation separating two (i) Continuous subglacial tunnel fill represents plains or depressed surfaces. Eskers cover several hun- within tunnels underneath or within an ice dred kilometres of the Irish landscape and are one of body originally used as water escape conduits. When the dominant landforms in the Irish Midlands. (Fig. 1). there is a blockage of the tunnel perhaps because of ice Several well exposed stratigraphic sections and the collapse, large boulders become trapped within the tun- body of existing knowledge due to extensive research nel. Sediment carried by the water in the tunnel accu- carried out on these features [e.g. 11, 12, 13, 14] make mulates at the front of the blockage and a ridge starts the Irish eskers an excellent analog for the esker-like forming, pro-grading upstream. Sediments can range ridges identified in Mars. from well-sorted to very poorly-sorted depending on the hydraulic conditions at the time of deposition. Clas- sically, such deposits are, for the most part, very coarse and may in extreme cases contain boulders up to 5 me- tres in diameter. (ii) Continuous fluvial ice-channel fill is deposited in channels cut into the ice on top of the or down to the substrate and are subsequently infilled by sediment. Thus the sediment is either deposited on ice and subsequently lowered to the substrate by ice melt, or deposited directly on the substrate between ice walls. This type, can form components of eskers that are largely tunnel fill deposits. They also occur on higher ground that emerged through the as it is downwasted. (iii) Long beads - subglacial tunnel fill are seg- mented ridges or beads representing sequential deposi- tion near or at the ice margin as the ice sheet retreats. Long beads are defined by the ratio of their length to their width, usually in the range of 5:1 to 10:1. They are often distinct landforms and are of a uniform size along their length. They are usually offset and may exist as multiple parallel ridges. (iv) Short beads are deposited within a lake. These Fig 1: Spatial distribution of esker ridges in Ireland have been interpreted as sequential deposition of ice- (black line). Circles indicate the three major esker contact subaqueous outwash fans below the water sur- complexes from left to right: Roscommon-Mayo, face or up to the water surface level. Having reached Midlands and Meath esker complexes. surface level, these become equivalent to deltas.

Analogue Potential: Ground Penetrating Radar (GPR) The Irish Eskers: Irish eskers are 0.1 – 80 km surveys carried out on the Irish eskers and their associ- long, 20 – 500 m wide and 4 – 50 m high. These were ated sediments (e.g. subaqueous fans, glaciodeltaic laid down by glacial meltwater in tunnels and sediments) permitted the identification of paleocurrents 45th Lunar and Planetary Science Conference (2014) 1917.pdf

based on cross-bedding dip and direction. These data mits the cataloguing of ‘pristine’ morphologies and assisted in developing models of deglaciation for the morphometries. The shape and sedimentary composi- Irish ice domes [14]. The four categories of esker in tion coupled with the excellent preservation of esker Ireland described above have unique attributes that ridge morphology is strongly conditioned by their rela- distinguish them from eachother and can be used to tive position to the ice mass and the topographic setting indicate formative environmental conditions. [12]. Esker-like ridges identified on Mars are similarly As Irish eskers have significant morphological simi- located in regions of low relief and depressions [1, 4, larities with those identified on Mars (Fig. 2) they pro- 5, 7, 8]. Thus, showing similar topographic conditions vide an opportunity for an insightful morphological and to those in Ireland. These depressions may have poten- morphometric analysis to determine potential formative tially held water bodies, which would have helped to environments on Mars. the preservation of these ridges and are of interest to discovering Mars potential habitability environments. There are several reports of eskers trending up-slope on Mars [e.g., 3, 6, 10], similar to those in Ireland [14]. A throughout investigation of the Irish eskers as Mars analogs using remote sensing methods and GPR will contribute to the understanding of ridge-like fea- tures identified on the surface of Mars.

References: [1] Banks, M. E., et al. (2009) JGR, 114 E09003. [2] Coleman, N.M. (2011) LPS XLII, Ab- stract #1906. [3] Bernhardt H. et al. (2013) Planetary and Space Science, 85, 261–278. [4] Head III J. W. and Pratt S. (2001) JGR, 106, E6, 12275-12299. [5]. [6] Guidat et al, (this volume) LPS XLV. Erkeling, G. (2013) LPS XLIV, Abstract #1919. [7] Ramsdale, J .D. (2013) EGU General Assembly, 15, 13782. [8] Ivanov, M.A. (2013) LPS XLIV, Abstract #1127. [9] Martinez- Alonso S. (2011) Icarus 212, 597–621. [10] Hobley, D.E.J., et al (2014). Journal of Geophysical Research: Planets, 2013JE004396. [11] Synge, F. M. (1950) Pro- ceedings of the Royal Irish Academy 53, 99-110. [12] Warren, W. P. and Ashley, G. M. (1994) J. of Sedi- mentary Research. A64, 433-449. [13] Delaney, C. (2001) North West geography, 1 (2), 23–33. [14] Pel- Fig. 2: (Top) image of ridge-like features in Argyre licer X. M. et al. (2012) Journal of Quaternary Sci- Planitia. (bottom) image of Horseleap esker located ence, 27 (12), 807–818. in the Irish Midlands.

Putative Martian eskers are 2-300 km long, 50- 3000 m wide and 10-150 m high. The Irish eskers are similar in scale and present dimensions within these ranges. Eskers in Ireland are composed of and gravel with cobbles and boulders [11, 12]. Mars esker-like ridges observed on high resolution imagery also show presence of large boulders [3]. Similar to Mars, eskers in Ireland are generally lo- cated in low relief areas and are often associated with glaciolacustrine deposits. Some Midland sites in Ireland are hypothesised to have formed in association with a during deposition. The presence of large glacial lakes during ice withdrawal aided to the out- standing preservation of these features [14]. This per-