Buried Ice and Sand Caps at the North Pole of Mars: Revealing a Record of Climate Change in the Cavi Unit With SHARAD

Abstract

The cavi unit at the north pole of Mars is a deposit of aeolian sand and water ice underlying the Late Amazonian north polar layered deposits. Its strata of Middle to Late Amazonian age record wind patterns and past climate. The Mars Reconnaissance Orbiter Shallow Radar (SHARAD) reveals extensive internal and basal layering within the cavi unit, allowing us to determine its general structure and relative permittivity. Assuming a basalt composition for the sand (ε′ = 8.8), results indicate that cavi contains an average ice fraction between 62% in Olympia Planum and 88% in its northern reaches beneath the north polar layered deposits and thus represents one of the largest water reservoirs on the planet. Internal reflectors indicate vertical variability in composition, likely in the form of alternating ice and sand layers. The ice layers may be remnants of former polar caps and thus represent a unique record of climate cycles predating the north polar layered deposits.

Flexure of the Lithosphere Beneath the North Polar Cap of Mars: Implications for Ice Composition and Heat Flow

“…The inferred compositions suggest that there must be at least 10 vol% of solid CO 2 (0.15 to 0.17 × 10 6 km 3 , 59 to 67 mbar) within the polar cap if the dust content is larger than 6 vol%, as suggested by the radar estimates of Nerozzi and Holt (2019), the rest being water ice. Increasing the dielectric constant of water ice (from 3 to 3.15, e.g., Grima et al, 2009) or that of the dust (from 6 to 8.8, e.g., Nerozzi & Holt, 2019), or adding more dust to the polar cap, would require an even larger solid CO 2 content.…”

“…• Supporting Information S1 of dust (Herkenhoff et al, 2007;Nerozzi & Holt, 2019). Both MARSIS and SHARAD observed a general lack of downward deflection below the north polar cap with uncertainties of about 200 m everywhere (Selvans et al, 2010) and 100 m across Gemina Lingula (Phillips et al, 2008) (Figure 1).…”

“…The study of Phillips et al (2008) used the global topography as a load when predicting the deflection of the lithosphere, and the presence of a long-wavelength signal arising from the broad Tharsis volcanic province could bias the lithospheric deflection beneath the polar cap. Lastly, we note that constraints on the composition of the polar cap were obtained by assuming that its base is flat and inverting for the dielectric constant using Mars Orbiter Laser Altimeter (MOLA) elevation and radar data (e.g., Grima et al, 2009;Nerozzi & Holt, 2019). If the base were deflected downward, this would require a lower bulk dielectric constant of the polar cap and modify the inferred compositions.…”

Compositional Constraints on the North Polar Cap of Mars from Gravity and Topography

“…Previously, radar investigations found little attenuation of the radar signal by the BU suggesting that it may only contain a few percent contaminants mixed with water ice (Selvans et al, 2010). Further work by Lauro et al (2012) and Nerozzi and Holt (2019) have both found evidence for significantly more contaminants in some parts of the BU. Nerozzi and Holt (2019) used SHARAD data to reveal substantial spatial variations in the bulk dielectric constant of the cavi unit in PB and Olympia Planum.…”

Depletion of Heat Producing Elements in the Martian Mantle

“…We would like to thank Dr. Steven Hauck II for his assistance with the computation. The radar data used in this work are a published database that may be obtained from Nerozzi and Holt () (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GL082114). No other data are generated as part of this work.…”