Earliest accumulation history of the north polar layered deposits, Mars from SHARAD

Highlights

• SHARAD profiles reveal non-uniform accumulation of water ice in lowermost NPLD.

• The two oldest deposits are limited in extent and confined to two separate areas.

• Lowermost NPLD growth was interrupted once by a retreat event.

• Onset of katabatic winds may have preceded and controlled early NPLD accumulation.

• Topography likely caused local variations in water ice deposition and erosion rates.

Abstract

The approximately 2 km thick north polar layered deposits (NPLD) are often considered to contain the most complete and detailed stratigraphic records of recent climate of Mars. Exposures of the dense layering within troughs and scarps allowed detailed reconstructions of the latest accumulation history of these water ice deposits, but we lack knowledge of their initial emplacement. The Shallow Radar (SHARAD) onboard Mars Reconnaissance Orbiter (MRO) penetrates the NPLD to their base and detects their internal layering, overcoming the limitation of scarce and scattered visible outcrops of the lowermost sequences.

In this study, we map reflectors in SHARAD data that result from discrete stratigraphic horizons in order to delineate the three-dimensional stratigraphy of the lowermost ∼500 m NPLD sequence and reconstruct their accumulation history. We confirm the large-scale lateral continuity and thickness uniformity of the deposits previously detected within the lowermost NPLD. However, stratigraphic complexity—in the form of pinch-outs and significant thickness variations—arises when we examine single radar units. We find evidence of an initially limited geographic stability of water ice within two deposits that are centered at the North Pole and present-day Gemina Lingula. A period of lateral ice sheet growth followed, interrupted only once by a retreat episode. Lower net accumulation is observed on pre-existing slopes, suggesting a reduction of water ice stability due to increased solar radiation incidence and/or transport by katabatic winds. Lateral transport of water ice by wind is also suggested by thickness undulations toward the top of the sequence, resembling cyclic steps.

Water ice accumulation models based on orbital forcing predict a sequence of deposition and retreat events that is generally compatible with our reconstructed accumulation history. Therefore, we interpret the stratigraphic complexity that we observe as regional and, possibly global, climate change induced by orbital forcing. We also find that at least two units are completely buried within the NPLD and do not outcrop, and that NPLD deposition in some places was contemporaneous with deposition of the stratigraphically underlying cavi unit in other places. Both of these findings show that radar reflector mapping is a necessary complement to any stratigraphic reconstruction based on visible exposures.

Introduction

The north polar layered deposits (NPLD) are the largest accumulation of water ice in the northern hemisphere of Mars, and the second largest on the planet. The NPLD make up the upper part of Planum Boreum and lie over the Vastitas Borealis interior unit (Hbv_i) (Tanaka et al., 2008, Tanaka and Fortezzo, 2012) and the basal unit (BU), which is divided into the Rupēs unit and Cavi unit (Brothers et al., 2015, Byrne and Murray, 2002, Edgett et al., 2003, Fishbaugh and Head, 2005, Tanaka et al., 2008, Tanaka and Fortezzo, 2012) (Fig. 1). They consist of a dome-shaped deposit of water ice with up to 10% dust (Grima et al., 2009). Since their discovery (Soderblom et al., 1973), they are thought to hold a valuable record of recent climate change within their stratigraphy (Cutts, 1973), yet little is known about their initial accumulation history and significant uncertainties remain on their estimated age.

To first order, layering has been found to be laterally continuous and characterized by relatively uniform thicknesses throughout the NPLD by visual correlation of layer groups and single “marker beds” (Malin and Edgett, 2001), layer sequences (Fishbaugh and Hvidberg, 2006), spectrally detrended albedo matching (Milkovich and Head, 2005), layer topographic protrusion (Becerra et al., 2016) and radar mapping (Holt et al., 2010, Phillips et al., 2008, Putzig et al., 2009). Attempts have been made to constrain NPLD age by means of crater counting (Tanaka et al., 2008) and correlation of the apparently periodic layering with accumulation models based on orbital forcing assumptions (Fishbaugh and Hvidberg, 2006, Hvidberg et al., 2012, Phillips et al., 2008, Putzig et al., 2009). Crater counting results are statistically weak and only constrain NPLD age to the Late Amazonian (Tanaka et al., 2008), and continuous resurfacing processes are capable of altering this record over timescales of 10²–10³ yr (Banks et al., 2010, Galla et al., 2008, Landis et al., 2016), therefore potentially biasing deposit age estimates to younger ages.

The correlation of fine scale stratigraphy exposed in troughs based on very high-resolution imagery allowed the construction of detailed accumulation models based on different orbital forcing parameters, in turn constraining the modeled age of the NPLD with higher precision (Hvidberg et al., 2012). However, the intrinsic stratigraphic complexity of the NPLD, mainly due to the presence of non-periodic brightness signals (Milkovich and Head, 2005), depositional and erosional hiatuses (Holt et al., 2010, Milkovich and Head, 2005, Phillips et al., 2008, Putzig et al., 2009, Tanaka et al., 2008) and presence of modern dust mantles (Fishbaugh et al., 2010), leads to non-unique solutions (e.g. Putzig et al., 2009) and may undermine the reliability of such accumulation models (Christian et al., 2013, Fishbaugh et al., 2010). In addition, because no detailed stratigraphic models of the lowermost NPLD are available, the accumulation models do not take into account the presence of possible hiatuses in the lowermost ∼500 m of ice deposits. Thus, the first depositional events of the NPLD and their timing are still poorly constrained and only inferred from models extrapolating back in time from the overlying stratigraphic record.

The layering visible in optical images results from a combination of factors including different fractions of ice and entrained dust (Cutts, 1973, Cutts and Lewis, 1982, Thomas et al., 1992) in addition to surface morphology, which controls the local amount of surface dust and frost retention, both of which affect albedo (Fishbaugh et al., 2010). The distinction and correlation of layers across the NPLD at small scales within optical imagery is challenging to impossible in the lowermost sequences where outcrops are scarce.

Radar reflectors, instead, can be easily followed for hundreds of km (Phillips et al., 2008, Seu et al., 2007) and are inferred to originate from dielectric contrasts due to changing ratios of dust and water ice (Nunes and Phillips, 2006). Lalich and Holt (2016) showed that reflector properties in the NPLD are consistent with ∼2–10 m thick, laterally extensive layers prominent at outcrops known as “marker beds” (Fishbaugh et al., 2010, Malin and Edgett, 2001).

SHARAD radargrams show a repeating stratigraphic pattern in the NPLD, generally consisting of four packets with many radar reflectors separated by three inter-packet regions with few or no reflections (Phillips et al., 2008). Putzig et al. (2009) further divide the lowermost packet into two different units: a 200–300 m packet (“Unit C” ) of quasi-parallel reflectors which overlies both the VBi unit and the BU, and an overlying wedge (“Unit D” ) of quasi-parallel reflectors up to 300 m thick confined in the Gemina Lingula region; both units are bound on top by an angular unconformity with “Unit E” .

The NPLD can also be subdivided into different units using an adapted sequence stratigraphy approach. Holt et al. (2010) mapped three main sequences bound by two angular unconformities. The lowermost unconformity delineates the proto-Chasma Boreale and a now-buried chasma. The lowermost sequence, named “radar unit PLD1” corresponds to units C and D mapped by Putzig et al. (2009).

The aim of this study is to use radar stratigraphy from SHARAD to reconstruct the first stages of water ice accumulation and climatic changes of the Martian NPLD. Rather than mapping unconformities separating sequences or packets of reflectors (Holt et al., 2010, Putzig et al., 2009) we delineate the stratigraphy of the first ∼500 m thick sequence of the NPLD (PLD1 in Holt et al., 2010) in three dimensions at the scale of single radar reflectors. This allows us to determine net accumulation and retreat phases via strata geometries and qualitatively compare those to available NPLD accumulation models based on orbital forcing (Greve et al., 2010, Levrard et al., 2007).