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

Ojha, L.; Nerozzi, S.; Lewis, K. W.

doi:10.1029/2019gl082294

Cited by 17 publications

(15 citation statements)

References 48 publications

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“…For a heat flow of 20 mW m −2 , the dielectric constant is found to range from 2.40 to 3.15, the elastic thickness is found to be independent of heat flow with a value of at least 330 km, and the density is found to range from 920 to 1,520 kg m −3 . The obtained range of densities is in good agreement with gravity studies at the north (Ojha, Nerozzi, & Lewis, 2019) and south (Wieczorek, 2008;Zuber et al, 2007) poles where the bulk density of both caps was found to be about 1,200 ± 150 kg m −3 .…”

Section: Resultssupporting

confidence: 88%

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

Broquet

Wieczorek

2020

Geophysical Research Letters

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The geodynamical response of the lithosphere under stresses imposed by the geologically young north polar cap is one of the few clues we have to constrain both the polar cap composition and the present‐day thermal state of Mars. Here we combine radar data with a flexural loading model to self‐consistently estimate the density ( ρ) and real dielectric constant ( ε′) of the polar cap, and the elastic thickness of the lithosphere underneath ( Te). Our results show that ρ ranges from 920 to 1,520 kg m −3, ε′ is 2.75 (+0.40, −0.35), and Te is between 330 and 450 km. We determine a polar cap volume that is up to 30% larger than current estimates that all neglect lithospheric flexure. Our inferred compositions suggest that, for dust content larger than about 6 vol%, 10 vol% CO 2 ice is mixed within the polar deposits, which has important implications for the climate evolution of Mars.

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Section: Resultssupporting

confidence: 88%

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

Broquet

Wieczorek

2020

Geophysical Research Letters

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“…Our findings reconcile and confirm the previous findings of Selvans et al (2010), who argued that the entire BU is very rich in ice, and Lauro et al (2012), who measured a relative permittivity at the top of cavi compatible with a basaltic sand sheet with 41–59% porosity occupied by water ice. Analysis and inversion of gravity signals over Planum Boreum also indicate that water ice makes up more than 50% of the BU by volume (Ojha et al, 2019). Our results on the relative permittivity of cavi indicate abundant excess ice, making this unit one of the largest water ice reservoirs of the planet after the polar layered deposits.…”

Section: Discussionmentioning

confidence: 99%

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

Nerozzi

Holt

2019

Geophysical Research Letters

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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.

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“…The lithosphere was found to be thick and cold at the north pole, with an elastic thickness of 330–450 km and a surface heat flow of 11–16 mW

m^{- 2}

(see also Phillips et al., 2008). Whereas the long‐wavelength (

>

400 km) gravity field does not correlate with the surface topography at the north pole (in part because of a poor resolution of the gravity models at high northern latitudes, see also Ojha et al., 2019), the south polar cap shows a clear gravitational signature. It is thus possible to use the gravity field as an additional constraint on the properties of the south polar cap and underlying lithosphere (Ding et al., 2019; Wieczorek, 2008; Zuber et al., 2007).…”

Section: Introductionmentioning

confidence: 99%

The Composition of the South Polar Cap of Mars Derived From Orbital Data

Broquet

Wieczorek

2021

JGR Planets

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The flexure of the lithosphere under stresses imposed by the geologically young south polar cap is one of the few clues we have regarding the south polar cap composition and the present‐day thermal state of Mars. Here, we combine radar, gravity, and topography data with a flexural loading model to estimate the bulk density (ρ) and average real dielectric constant (ε′) of the south polar cap, and the elastic thickness of the lithosphere (Te). Given the uncertainties of the data, our results constrain ρ to be 1,100–1,300 kg m−3 (best fit of 1,220 kg m−3), ε′ to be 2.5–3.4 (best fit of 3.3), and Te to be greater than 150 km (best fit of 360 km). Based on these results, the maximum lithospheric flexure is 770 m, and the polar cap volume could be up to 26% larger than previous estimates that did not account for lithospheric flexure. Our inferred compositions imply that the dust concentration would be at least 9 vol% if the CO2 ice content were negligible, and that the CO2 ice concentration would be more than the known 1 vol% CO2 if the dust concentration were less than 9 vol%. The 1‐σ lower limit on Te implies a surface heat flow that is less than 23.5 mW m−2. This lower limit is significantly less than the range of acceptable values at the north pole (330–450 km, heat flow of 11–16 mW m−2), and helps satisfy global thermal evolution simulations that predict hemispheric differences in surface heat flow.

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

Cited by 17 publications

References 48 publications

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

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

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

The Composition of the South Polar Cap of Mars Derived From Orbital Data

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