Overspilling small craters on a dry Mars: Insights from breach erosion modeling

https://doi.org/10.1016/j.epsl.2020.116671Get rights and content

Highlights

  • Small exit breach craters record wet events on Hesperian/Amazonian Mars.

  • We use a fixed channel width 0-D breach erosion model to predict valley incision in one overspill event.

  • Pollywogs either formed during a single, groundwater-driven crater overspill event, or smaller repeated climate-driven draining events.

Abstract

Understanding when, where, and how frequently liquid water was stable on Mars since the Late Noachian/Early Hesperian (3.2-3.9 Ga) is important for understanding the evolution of Mars' climate and hydrology. Some relatively young features on Mars require multiple wetting events to form, whereas others are consistent with single wetting events. Small and rare exit breach craters or “pollywogs” are craters between 0.5 and 15 km in diameter with valleys leading away from the lowest point on their rims but no visible inlet valleys. These craters must have been filled with water to the point of overspill to form the observed valleys. The two possible water sources are precipitation and groundwater. In this paper we use measurements from Digital Elevation Models (DEMs) of 18 pollywog craters (21 outlet valleys) and a fixed channel width 0-D breach erosion model to determine whether pollywog exit breach valleys are consistent with a single crater overspill event, or if their formation requires multiple overspill events. Our model, which we compare to a selection of dam breaching events on Earth, predicts runaway erosion for two pollywog exit breaches. No runaway erosion is observed. We discuss potential explanations for this mismatch between the data and our model. We show that the majority of pollywog craters on Mars are consistent with formation during a single crater overspill event, incorporating a work around for the long-standing problem of unknown grainsize into our approach. Three pollywog craters require either multiple events or sustained water supply to drive erosion. We discuss potential source mechanisms for crater-filling water and conclude that pollywogs either formed in a single erosion event, driven by groundwater discharge, or through many small erosion events, driven by draining of small meltwater lakes formed on crater-filling bodies of ice.

Introduction

Surface liquid water on Mars requires conditions very different from the present day; either temperatures >273 K resulting from different climate conditions, or transport of water out of the subsurface for example by groundwater outflows (Rodriguez et al., 2015). If the water source is known, morphological features (e.g. fluvial channels, lake deposits) can be used to understand either the evolution of the Martian climate and atmosphere, or the evolution of the hydrosphere. Wet events on Mars since the Valley Network forming period (<3.2-3.9 Ga) (Hartmann, 2005) are recorded by the formation of features such as alluvial fans (Kite et al., 2019; Hauber et al., 2013), and small exit breach craters or “pollywogs” (Wilson et al., 2016). These features indicate a trend from an earlier, wetter climate with intense fluvial erosion (Matsubara et al., 2013), to arid conditions punctuated by occasional surface liquid water (Kite et al., 2019). Pollywogs have outlet valleys extending from well-preserved crater rims, but no inlet valleys (Wilson et al., 2016) e.g. Fig. 1a. The outlet valleys formed by overspill of water from within the crater. Fresh pollywog rims indicate that pollywog outlet incision post-dates the intense Late Noachian and Early Hesperian fluvial activity (Wilson et al., 2016). Pollywog formation requires a water source that left crater rims intact, but was intense enough to supply >107 m3 of water under dry climate conditions. Two possible sources of this water are groundwater (e.g. from a pressurized aquifer at depth), and precipitation and/or melting (Fig. 2). If erosion during a single pollywog overspill event can be constrained, then the observed erosion from pollywog overspill events is an important constraint on the number, duration, and intensity of mid-latitude surface liquid water events on Mars since the Late Noachian/Early Hesperian (3.9-3.2 Ga) (Hartmann, 2005).

Low-latitude alluvial fans require multiple episodes of wetting (Hauber et al., 2013; Kite et al., 2019). Groundwater modeling has shown that outflow channel formation may be more consistent with multiple, smaller-magnitude groundwater discharge events than a single flood (Andrews-Hanna and Phillips, 2007). However, other features are consistent with a single wet event, for example impact-driven melting and precipitation in Mojave and Hale craters (Goddard et al., 2014). Do pollywogs belong to the first group of features (Gale, alluvial fans) that record signatures of global climate conditions during the Hesperian? Or do they result from localized groundwater or impact-induced conditions?

Global climate and local conditions are plausible explanations for pollywogs. Accumulating sufficient surface liquid water to overspill a pollywog through precipitation and snowmelt is challenging in a climate that is consistent with low erosion rates (Golombek et al., 2006) and valley distributions and morphologies consistent with reduced precipitation rates relative to the Noachian (>3.6-3.7 Ga) (Harrison and Grimm, 2005). Models of Martian climate under different orbital conditions can produce ice accumulation in midlatitudes (Mischna et al., 2013), and some melting (Kite et al., 2013). This makes ice-filled craters with melt ponds on their surfaces a plausible watersource for pollywog outlet valley erosion (e.g. Fig. 2). Orbital changes provide a potential mechanism for pacing surface liquid water activity (Jakosky et al., 1995). Large impacts (Steakley et al., 2019; Turbet et al., 2020) may also be able to induce short-lived climates enabling surface liquid water. Groundwater discharge from pressurized aquifers is an alternative mechanism for supplying liquid water to the surface during the Hesperian (Andrews-Hanna and Phillips, 2007; Rodriguez et al., 2015), e.g. in Xanthe Terra (Coleman, 2005), and some large (D>30 km) crater lake breach valleys (Warner et al., 2010).

The expected valley floor erosion from a single pollywog overspill event compared with observed erosion depths is an important constraint on Mars' climate and hydrology since the Late Noachian/Early Hesperian (Fig. 1a) because it can be used to infer the properties of the pollywog-overspilling water source. In this paper, we determine whether pollywog exit breach valleys are consistent with a single overspill event, or if their formation requires multiple overspill events. We use a new fixed channel width 0-D model (Section 2.1) and pollywog measurements from HiRISE Digital Elevation Models (DEMs) (Section 3.1) to predict the erosion that can occur in a single crater overspill event. The model is ground-truthed using terrestrial dam breach flood data (Section 3.2). We compare model predictions and observed breaches (Section 4), and consider the climate and hydrological conditions under which all studied pollywogs could be consistent with a single overspill event (Section 5).

Section snippets

Methods

In Section 2.1 we describe our new model for breach erosion. In Section 3.1 we describe our pollywog measurements, their implications, and how they are used in the breach erosion model. In Section 3.2 we test the model against terrestrial exit breach erosion events.

Results

We apply our model to Mars pollywogs (Section 3.1) and terrestrial breach erosion events compiled from the literature (Section 3.2).

Runaway erosion: predicted but not observed

Runaway erosion is observed for many terrestrial moraine/landslide dam breaching events (i.e. erosion to the base of the dam), whether the dam is a landslide deposit, terminal moraine, or crater rim. Runaway erosion is not observed for any pollywog valleys. Pollywog exit breaches do not erode deeply relative to crater floor depth (Fig. 1). By contrast, megaflood-formed valleys in Xanthe Terra erode hundreds of meters of crust (Coleman, 2005), representing 80 to 100% of the available relief

Are all pollywogs consistent with a single overspill event?

The transition between breaches consistent with a single overspill event and breaches that require multiple breaching events or continuous supply of water (termed “multiple event” breaches) depends on how fast the lake can drain relative to the size of the lake. This can be quantified using the wv/S7/6 ratio of an outlet valley, and D2/L of the pollywog crater and outlet valley. The power of 7/6 is inherited from Eqs. (6) & (8). We define the transition as the value of wv/S7/6 that allows for

Conclusions

Pollywogs are relatively young Martian craters with outlet valleys puncturing fresh crater rims, but no visible inlet valleys. Our new fixed channel width, 0-D model of breach erosion predicts runaway erosion for 2 of 21 measured pollywog crater exit breaches, but evidence for runaway erosion (defined here as erosion through the entire crater rim thickness) is not observed for any pollywogs. The mismatch between our model predictions and the measured pollywog valleys cannot be explained by

CRediT authorship contribution statement

A.O. Warren: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Visualization, Writing – original draft. S. Holo: Methodology, Resources, Software, Writing – review & editing. E.S. Kite: Conceptualization, Funding acquisition, Supervision, Writing – review & editing. S.A. Wilson: Resources, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank Caleb Fassett for pointing out low latitude pollywogs, and the HiWish program for targetting requested pollywogs. We would also like to thank Tim Goudge and an anonymous reviewer for their insightful comments and suggestions which helped clarify the manuscript. We thank Shavonne Morin at the University of Arizona for producing the DEM in Fig. 1. All stereopairs are publicly available on the HiRISE website. Our DEMs are publicly available at https://uchicago.box.com/v/pollywog-DEMs2020.

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