On Feb. 18, 2021, NASA’s Mars 2020 Perseverance rover will hurtle through the martian atmosphere at nearly 1,000 miles per hour toward its landing site at Jezero crater. Once Perseverance reaches its destination on the surface of Mars, attention will turn to the intriguing deposits within and around this ancient crater that might answer one of the most fundamental and enduring questions about the solar system: Was there ever life beyond Earth?
The Mars 2020 Perseverance rover is NASA’s next flagship mission to Mars, and the first step of a multimission campaign to return samples from Mars back to Earth. The mission has four science objectives:
- Characterize the geology and habitability of the rover’s landing site
- Seek signs of ancient life in rocks and soils analyzed by the rover
- Collect a cache of scientifically compelling, returnable samples
- Prepare for future human exploration of Mars
The science instruments onboard Perseverance offer a mix of new and updated science capabilities compared to previous Mars rover payloads and will enable, for the first time, spatially resolved analysis of geochemistry, mineralogy and organics at the mm and sub-mm scales to support the in-situ search for possible biosignatures on Mars. The rover’s newly-designed sampling system allows rock abrasion, dust removal, coring and the collection of regolith samples. The rover carries 43 sample tubes, including several tubes that will act as witness “blanks” throughout the mission, with the anticipated goal of collecting about 30 science samples during the surface mission.
The Perseverance rover is the most complex rover ever sent to Mars and has been designed to accomplish an ambitious set of science and sampling objectives during its surface mission in support of Mars sample return. Thus, the selection of a landing site that would maximize the scientific potential of both in situ and future returned sample science investigations – and would likely be the focus of Mars exploration for a decade or more to come – felt like a particularly high stakes decision.
Landing Site Selection
The landing site for the Perseverance rover was selected in a multiyear process that involved a series of open community workshops at which candidate landing sites were proposed by community advocates. These sites were then evaluated for their engineering safety and feasibility as well as their suitability to address the mission’s science objectives. Going into the fourth and final community workshop in 2018, the list of candidate landing sites had been narrowed to four: Columbia Hills, two landing sites in the Nili Planum region (informally known as Northeast Syrtis), and Jezero crater.
Columbia Hills, the site of the Mars Exploration Rover Spirit mission between 2004-10, was interpreted as an ancient hydrothermal volcanic deposit containing mm-scale digitate opaline silica-bearing structures that can preserve biosignatures. The two Northeast Syrtis sites have been interpreted to expose some of the most ancient crustal rocks present at the surface of Mars. Units of interest in the Northeast Syrtis region included clay- and pyroxene-bearing basement rocks, outcrops of impact megabreccia, an olivine- and carbonate-bearing unit found throughout the region, and a heavily cratered mafic caprock interpreted to be lava flows originating from the nearby Syrtis Major volcanic center. Yet it was Jezero crater – the site of an ancient lake, a well-preserved delta deposit, and diverse mineralogy – that was ultimately selected by NASA as the final landing site for the Perseverance rover.
Geological Features of Jezero Crater
Jezero crater is a 50-kilometer-diameter crater located along the inner rim of the Isidis impact basin, one of the largest and oldest impact basins on the surface of Mars (figure 2). Although the exact age of Jezero crater is unknown, the Isidis basin is thought to have formed at least 3.9 billion years ago, which places an upper limit on the age of Jezero and the deposits within the crater. Many potential crater lake basins that have been identified on Mars, and a number of these sites host preserved delta or lacustrine deposits. Few of these craters, including Jezero crater, have both inlet valleys and an outlet valley. The presence of these channels provides independent and unequivocal confirmation – beyond the preserved intra-crater deposits interpreted to be lacustrine or deltaic – that a lake was once present within the crater. Jezero is host to two deposits that have been interpreted as ancient remnant deltas; an extremely well-preserved deposit emanating from the western interior rim of the crater, and a less well-preserved deposit sourced from the northern rim of the crater (figure 3 at top). A number of layered mesas within the interior of the crater have been interpretated as remnants of once more extensive deltaic or lacustrine deposits that covered the crater floor (figure 4a).
The western Jezero delta contains beautifully preserved exposures of a variety of sedimentary facies and structures visible in 25 centimeters per pixel, high-resolution orbiter images from the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment camera (figure 4). These include a basal sequence of thin, laterally continuous layers that contain Fe/Mg clay minerals and are exposed along the front edge of the delta (figure 4b). This sequence has been interpreted as prodelta deposits. These prodelta deposits are recognized to represent a habitable environment with a high potential for biosignature preservation, and are one of the main exploration and sampling targets for the Perseverance rover.
The thinly layered pro-delta sequence transitions up-section into a series of truncated, curvilinear beds (figure 4c) interpreted to be point bar deposits or, alternatively, subaqueous channel-levee complexes. Analysis of orbiter mineral spectroscopic data shows these deposits to contain carbonate minerals of a likely detrital origin. The truncated, curvilinear deposits are overlain by a series of linear ridges composed of meter-scale blocks (figure 4d) organized into distinct lobes interpreted to be inverted channel deposits of an avulsive fluvial system.
The sequence of facies preserved within the western delta has led to a sequence-level depositional model proposed by Timothy Goudge and co-authors in their 2018 paper, “Stratigraphy and paleohydrology of delta channel deposits, Jezero crater, Mars.” In this model, delta progradation occurred during initial filling of the crater basin. As Jezero continued to fill, the shoreline transgressed, leading to the back-stepping of the avulsive fluvial system represented today by the inverted channel deposits exposed at the top surface of the preserved delta. Transgression continued until the lake breached the crater rim via the outlet valley. As water continued to drain from the crater, the delta may have entered a brief progradational phase. Once ancient lake Jezero drained, the deposit was eroded by wind and fluvial incision to form the deposit that is observed in the crater today. Estimates of the total duration of delta formation vary widely, depending on assumptions of intermittency ranging from 10s of years to upwards of 106-107 years. Longer time-scales would be more favorable for sustained habitability of the Jezero lake system, so constraining the duration of delta deposition is a high priority returned sample science objective for Mars 2020.
The Jezero delta deposits are adjacent to a mafic, crater-retaining unit that forms much of the present-day floor of the crater. Originally interpreted to be a volcanic lava flow, more recent work has favored a sedimentary or volcaniclastic origin. If this unit is volcanic in origin, it would make a prime candidate for absolute age dating and calibration of the cratering chronology used for Mars if the samples are returned to Earth.
Targets of Interest
Additional geologic units of great interest for exploration and sampling by the Perseverance rover are olivine- and carbonate- bearing deposits exposed within the basin and around the inner margin of the crater. These share mineralogic and morphologic similarities with olivine-and carbonate-bearing rocks found outside the crater and throughout the Jezero and Northeast Syrtis regions. Regionally, these rocks have been interpreted as an ultramafic volcaniclastic deposit variably altered to carbonate, and dated to about 3.8 billion years. The olivine-and carbonate-bearing deposits within Jezero may be part of this widespread unit that appears to drape topography throughout the region, although an in situ precipitated component has also been considered for the carbonates around the inner margin of Jezero crater. The presence and concentration of carbonate mineral signatures in deposits along the inner rim of Jezero have raised the possibility that these deposits were precipitates deposited along the shallow margin of ancient lake Jezero. This possibility has led to increased interest in these deposits as one of the prime astrobiological targets for the Perseverance rover.
The final target of interest within Jezero crater is the crater rim itself. Examination of the crater rim will enable investigation of impact processes, the structure of the crust that pre-dated the Jezero impact, and a potential age determination for the Jezero impact if samples are returned to Earth. There may also be impact-generated hydrothermal systems preserved within the crater rim.
At Jezero crater, the Perseverance rover will have the opportunity to explore diverse ancient lake and delta deposits that hosted once habitable environments with high biosignature preservation potential. Mature models for the exploration of lacustrine and deltaic systems, many originating from the oil and gas industry here on Earth, will guide the Perseverance’s exploration and sampling of fluvial, deltaic and lacustrine deposits in search of indigenous martian organics and potential biosignatures. Moreover, Perseverance will have the opportunity to explore and sample potential volcanic rocks in the floor of Jezero crater, carbonate-bearing rocks with high astrobiological potential around the margins of ancient lake Jezero, and habitable near-subsurface environments recorded throughout the rover traverse. Analyses of these rocks will also provide access to a unique paleoclimate record spanning the duration of deposition and diagenesis within Jezero crater.
Landing is Imminent
As the landing date for the Perseverance rover approaches, excitement is building for the rover’s science mission on the surface of Mars in Jezero crater. The diverse set of rocks in and around Jezero enable exploration of many compelling scientific questions – including the potential for ancient life on Mars – that can be answered by a combination of in situ investigations on the surface of Mars and by returned sample science if and when the samples are returned to Earth by future missions.
Acknowledgments
This article was written at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.