The Complex Exhumation History of Jezero Crater Floor Unit and Its Implication for Mars Sample Return

Abstract

During the first year of NASA's Mars 2020 mission, Perseverance rover has investigated the dark crater floor unit of Jezero crater and four samples of this unit have been collected. The focus of this paper is to assess the potential of these samples to calibrate the crater‐based Martian chronology. We first review the previous estimation of crater‐based model age of this unit. Then, we investigate the impact crater density distribution across the floor unit. It reveals that the crater density is heterogeneous from areas which have been exposed to the bombardment during the last 3 Ga to areas very recently exposed to bombardment. It suggests a complex history of exposure to impact cratering. We also display evidence of several remnants of deposits on the top of the dark floor unit across Jezero below which the dark floor unit may have been buried. We propose the following scenario of burying/exhumation: the dark floor unit would have been initially buried below a unit that was a few tens of meters thick. This unit then gradually eroded away due to Aeolian processes from the northeast to the west, resulting in uneven exposure to impact bombardment over 3 Ga. A cratering model reproducing this scenario confirms the feasibility of this hypothesis. Due to the complexity of its exposure history, the Jezero dark crater floor unit will require additional detailed analysis to understand how the Mars 2020 mission samples of the crater floor can be used to inform the Martian cratering chronology.

Diverse Lava Flow Morphologies in the Stratigraphy of the Jezero Crater Floor

“…Along the Artuby ridge, the Artuby and Rochette members range up to 2–2.5 and 2 m thick, respectively. While we can infer the thickness of both members from RIMFAX data (B. Horgan et al., 2023) without seeing the lower portion of Artuby due to regolith cover, these are likely underestimates of the true thickness of the members because the upper portion of Rochette may have been eroded (Quantin‐Nataf et al., 2021, 2023). Likewise, the Artuby member may have been eroded before emplacement of Rochette, and may include erosional surfaces.…”

Samples Collected From the Floor of Jezero Crater With the Mars 2020 Perseverance Rover

“…Because the highest crater densities on the Jezero Crater floor, that is, the mapped Cf‐Fr unit (cf. Stack et al., 2020), occur farthest from the delta (near Hartwell Crater; Quantin‐Nataf et al., 2021), it is likely that this surface experienced the least burial and subsequent exhumation since crystallization. Thus, assuming the rocks exposed near Hartwell Crater are equivalent to upper Máaz, use of the crater density and size distribution observed near Hartwell with the crystallization age of Máaz would provide quantitative constraints on two of the key assumptions in the Mars cratering chronology, specifically the ratios of the bolide fluxes and crater diameters, respectively, between Mars and the Moon.…”

“…However, such tests require knowledge of how long a particular surface was exposed to crater‐forming impact events; this will not generally be equal to the time since an igneous rock crystallized on a planetary surface that has experienced active geomorphic processes. Indeed, the spatial distribution of crater densities observed at the Jezero crater floor clearly indicates that the igneous rocks have also experienced a complex exhumation history, both spatially and temporally, with lowest densities observed near the delta, highest to the NE of the landing site (Quantin‐Nataf et al., 2021). Any tests or calibrations of the crater chronology function will require knowledge of when, and at what rate, this post‐crystallization exhumation occurred, and more generally, how a rock crystallization age can be related to the duration of crater accumulation at a particular surface.…”

Overview and Results From the Mars 2020 Perseverance Rover's First Science Campaign on the Jezero Crater Floor

“…The Naa'táanii member transitions (Figures 3d and 3f) into the higher-standing, gray-toned, massive, blocky meter-scale boulders of the Ch'ał member, which form hummocky landscapes and is the hardest and least altered member from the crater floor (Figure 20). Due to its comparatively more resistant nature, the Ch'ał member is thought to be related to the crater-retaining unit on which crater count ages have been derived (Farley et al, 2022;Goudge et al, 2015;Quantin-Nataf et al, 2021;Schon et al, 2012;Shahrzad et al, 2019). The distinction between the Naa'táanii and Ch'ał members is based largely on outcrop morphology not composition (Wiens et al, 2022).…”

“…A main objective of the crater floor campaign was to collect samples from the crater‐retaining part of the Máaz formation to enable calculations of absolute age for the Máaz formation that could then be related to crater‐count ages for the unit (Section 3.1). Though this objective was accomplished, it is likely that any crater‐count derived ages may reflect the exhumation age of Máaz, rather than the formation age of the unit (Quantin‐Nataf et al., 2021). Future absolute ages derived for the Ch'ał samples will still be useful to reconstruct the timeline of events in Jezero crater, but the potential barrier to calibrating the absolute ages with the Mars crater chronology models is an inherent challenge.…”