Jezero crater is a ~45‐km impact crater on the margin of Isidis basin on Mars. Jezero is the landing site for NASA's Mars 2020 rover mission. The crater hosts a paleolake, and fluvio‐lacustrine ...deposits formed in this lake remain accessible to exploration. A dark‐toned deposit on the crater floor overlies light‐toned carbonate‐bearing deposits and has been interpreted as a lava flow. We determined the average thickness of this deposit at the margins to be ~13.0 ± 0.8 m. We analyzed the statistics of impact craters superposed on this deposit and estimated its model age as 2.6 ± 0.5 Ga in the Hartmann system, placing it most likely in the Early Amazonian. The error estimate here includes an estimate of the uncertainty associated with the crater counts. Acquisition, caching, and eventual return of a sample from this unit could provide an important calibration point for Mars crater chronology.
Plain Language Summary
Jezero crater is a large impact crater on Mars with a diameter of about 45 km. In the Martian deep past this crater hosted a long‐lived lake. On 19 November NASA announced that Jezero crater will be the landing site for the coming Mars 2020 mission. On top of sediments on the central crater floor is a distinctive deposit of dark rocks that possibly originated as a lava flow after the lake had dried out. We studied the statistics of small impact craters on this dark deposit. Such impact crater statistic is widely used as a tool for dating Martian terrains. We report a derived “model age” of 2.6 ± 0.2 Ga for these rocks. One central goal of the Mars 2020 mission is to select and store a cache of rock samples that will be returned to Earth by a later mission for study in terrestrial laboratories. If this dark deposit is indeed lava, a sample could be dated in terrestrial laboratories. Together with our crater statistics reported here, this could provide a crucial tie‐point for recalibration of the crater count dating system for Mars, which now relies on extrapolation from samples collected on the Moon.
Key Points
Based on craters 177‐500 m we estimated the model age of the dark‐toned floor unit as 2.6 ± 0.2 Ga in the Hartmann system
With 25% as an estimate of the uncertainty associated with identifying and measuring diameters, we find a model age range of 2.6 ± 0.5 Ga
The average thickness of the unit at the margins is determined to be ~13.0 ± 0.8 m
The Panoramic Camera (Pancam) on the Mars Exploration Rover mission has acquired in excess of 20,000 images of the Pancam calibration targets on the rovers. Analysis of this data set allows estimates ...of the rate of deposition and removal of aeolian dust on both rovers. During the first 150–170 sols there was gradual dust accumulation on the rovers but no evidence for dust removal. After that time there is ample evidence for both dust removal and dust deposition on both rover decks. We analyze data from early in both rover missions using a diffusive reflectance mixing model. Assuming a dust settling rate proportional to the atmospheric optical depth, we derive spectra of optically thick layers of airfall dust that are consistent with spectra from dusty regions on the Martian surface. Airfall dust reflectance at the Opportunity site appears greater than at the Spirit site, consistent with other observations. We estimate the optical depth of dust deposited on the Spirit calibration target by sol 150 to be 0.44 ± 0.13. For Opportunity the value was 0.39 ± 0.12. Assuming 80% pore space, we estimate that the dust layer grew at a rate of one grain diameter per ∼100 sols on the Spirit calibration target. On Opportunity the rate was one grain diameter per ∼125 sols. These numbers are consistent with dust deposition rates observed by Mars Pathfinder taking into account the lower atmospheric dust optical depth during the Mars Pathfinder mission.
The Panoramic Cameras on NASA's Mars Exploration Rovers have each returned more than 17,000 images of their calibration targets. In order to make optimal use of this data set for reflectance ...calibration, a correction must be made for the presence of air fall dust. Here we present an improved dust correction procedure based on a two‐layer scattering model, and we present a dust reflectance spectrum derived from long‐term trends in the data set. The dust on the calibration targets appears brighter than dusty areas of the Martian surface. We derive detailed histories of dust deposition and removal revealing two distinct environments: At the Spirit landing site, half the year is dominated by dust deposition, the other half by dust removal, usually in brief, sharp events. At the Opportunity landing site the Martian year has a semiannual dust cycle with dust removal happening gradually throughout two removal seasons each year. The highest observed optical depth of settled dust on the calibration target is 1.5 on Spirit and 1.1 on Opportunity (at 601 nm). We derive a general prediction for dust deposition rates of 0.004 ± 0.001 in units of surface optical depth deposited per sol (Martian solar day) per unit atmospheric optical depth. We expect this procedure to lead to improved reflectance‐calibration of the Panoramic Camera data set. In addition, it is easily adapted to similar data sets from other missions in order to deliver improved reflectance calibration as well as data on dust reflectance properties and deposition and removal history.
Key Points
We present an improved method for dust‐correcting calibration target images
The maximum deposited optical depth is 1.5 for Spirit and 1.1 for Opportunity
The two MER landing sites exhibit very different dust histories
The ubiquitous atmospheric dust on Mars is well mixed by periodic global dust storms, and such dust carries information about the environment in which it once formed and hence about the history of ...water on Mars. The Mars Exploration Rovers have permanent magnets to collect atmospheric dust for investigation by instruments on the rovers. Here we report results from Mössbauer spectroscopy and X-ray fluorescence of dust particles captured from the martian atmosphere by the magnets. The dust on the magnets contains magnetite and olivine; this indicates a basaltic origin of the dust and shows that magnetite, not maghemite, is the mineral mainly responsible for the magnetic properties of the dust. Furthermore, the dust on the magnets contains some ferric oxides, probably including nanocrystalline phases, so some alteration or oxidation of the basaltic dust seems to have occurred. The presence of olivine indicates that liquid water did not play a dominant role in the processes that formed the atmospheric dust.
The Rock Abrasion Tool (RAT) on board the Mars Exploration Rovers (MER) is a grinding tool designed to remove dust coatings and/or weathering rinds from rocks and expose fresh rock material. Four ...magnets of different strengths that are built into the structure of the RAT have been attracting substantial amounts of magnetic material during RAT activities from rocks throughout both rover missions. The RAT magnet experiment as performed on Spirit demonstrates the presence of a strongly ferrimagnetic phase in Gusev crater rocks, which based on Mössbauer and visible/near‐infrared reflectance spectra is interpreted as magnetite. The amount of abraded rock material adhering to the magnets varied strongly during the mission and is correlated in a consistent way to the amount of magnetite inferred from Mössbauer spectra for the corresponding rock. The RAT magnet experiment as performed on Opportunity also indicates the presence of a strongly ferrimagnetic phase in outcrops, such as magnetite or an altered version of magnetite. However, the evidence is weaker than in the case of Spirit. According to data from the α particle X‐ray spectrometer (APXS) and the Mössbauer spectrometer (MB), the Eagle crater outcrops should not contain magnetite and their magnetization should not exceed 0.03 A m2 kg−1. However, this assertion seems to be in contradiction with the results of the RAT magnet experiment. The evidence for a strongly ferrimagnetic phase at low abundance in the Meridiani outcrops is discussed.
•Pancam estimates the Lambert albedo at Gusev crater and Meridiani Planum, Mars.•Albedo varies on small spatial/temporal scales due to localized wind events.•Albedo measurements from Pancam, MOC, CTX ...and HiRISE agree to within 15%.
The Mars Exploration Rovers (MER) Spirit and Opportunity have systematically used their Panoramic Camera (Pancam) instruments to estimate the Lambert albedo of the surface across their traverses in Gusev crater and Meridiani Planum. The 360˚ “albedo pan” observations acquired with Pancam's broadband (739 ± 338 nm) L1 filter allow for quantitative estimates of the overall surface albedo and measurements of individual surface features. As of November 2016, over nearly six Mars years of the MER mission, Spirit acquired 20 albedo pans (over 7,730 m of traverse distance) and Opportunity acquired 117 albedo pans (over 42,368 m of traverse distance). For Spirit, this comprises the rover's complete dataset. The ranges of Pancam-derived albedos at Gusev crater (0.14–0.24) and at Meridiani Planum (0.11–0.22, with one anomalously high measurement of 0.27 during the July 2007 global dust storm) are consistent with large-scale albedos of the sites as previously determined by the Viking Orbiter Infrared Thermal Mapper (IRTM), Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES), MGS Mars Orbiter Camera (MOC), Mars Odyssey Thermal Emission Imaging System (THEMIS), Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) and MRO Mars Color Imager (MARCI) instruments. Through comparisons with atmospheric opacity measurements, temporal changes in Pancam albedo values provide insights into interactions between the Martian surface and atmosphere. Pancam observations are also used to “ground truth” measurements from orbit and validate radiometric calibrations, and we present comparisons across the full rover traverses to MOC, CTX, and MRO High Resolution Imaging Science Experiment (HiRISE) data. Albedo averages from the same regions observed by Pancam and all three orbital instruments generally agree to within ± 15%. The few instances found where cross-instrument comparisons exceed the estimated instrument calibration uncertainties can be attributed to atmospheric effects and/or differences in viewing geometries.
On February 18, 2021 NASA's Perseverance rover landed in Jezero crater, located at the northwestern edge of the Isidis basin on Mars. The uppermost surface of the present‐day crater floor is ...dominated by a distinct geologic assemblage previously referred to as the dark‐toned floor. It consists of a smooth, dark‐toned unit overlying and variably covering light‐toned, roughly eroded deposits showing evidence of discrete layers. In this study, we investigated the stratigraphic relations between materials that comprise this assemblage, the main western delta deposit, as well as isolated mesas located east of the main delta body that potentially represent delta remnants. A more detailed classification and differentiation of crater floor units in Jezero and determination of their relative ages is vital for the understanding of the geologic evolution of the crater system, and determination of the potential timeline and environments of habitability. We have investigated unit contacts using topographic profiles and DEMs as well as the distribution of small craters and fractures on the youngest portions of the crater floor. Our results indicate that at least some of the deltaic deposition in Jezero postdates emplacement of the uppermost surface of the crater floor assemblage. The inferred age of the floor assemblage can therefore help to constrain the timing of the Jezero fluviolacustrine system, wherein at least some lake activity postdates the age of the uppermost crater floor. We present hypotheses that can be tested by Perseverance and can be used to advance our knowledge of the geologic evolution of the area.
Plain Language Summary
On February 18, 2021 NASA's Perseverance rover landed in Jezero crater on Mars. In the past, the crater was filled with water, forming a lake, and in the western part of the crater an ancient delta is preserved. Part of the present‐day crater floor has been interpreted to represent a lava flow that was deposited after the lake dried out, meaning that the floor unit would be younger than the western delta. In order to understand how the Jezero crater lake has developed over time, including the potential timeline and environments of habitability, it is necessary to work out the relations between the geologic units in Jezero crater. In this work, we have analyzed orbital images of Jezero crater and reveal how the crater floor and delta deposits relate to each other in time. Our results show that at least some of the deltaic deposits in Jezero overlie the youngest crater floor unit(s). It is therefore possible to learn broadly when fluvial activity in the crater has been effective from the age of the crater floor. Our work presents hypotheses that can be tested by Perseverance to advance our knowledge of how the area has evolved geologically over time.
Key Points
We have studied stratigraphic relations between geologic units in Jezero crater for determination of relative age relations in the crater
Topographic profiles and digital elevation models indicate that the western delta is on top of the youngest crater floor unit(s)
We thus place constraints on the timeline of fluvial‐lacustrine activity in Jezero crater
PDF
