On 18 February 2021, NASA’s Perseverance rover is expected to arrive at Jezero impact crater, the site of a former lake on Mars. The High Resolution Stereo Camera on ESA’s Mars Express has provided important context for the landing site and its surrounds.
Mars Express has been imaging the Red Planet in three dimensions and in colour since 2004. The data it collects make it possible to create images of large areas of the martian surface – not only as colour mosaic images, but also as digital terrain models. These provide researchers with important information about the terrain and elevation of the land. Accurate topographical information is critical for ensuring a safe landing. The detailed data from Mars Express has been used to support the selection of numerous landing site candidates, including that of NASA’s Mars 2020 mission that will see Perseverance rover land in Jezero Crater this week.
The images shown here are derived from ‘map sheets’ covering millions of square kilometres, depicting the wider geographical context around the landing site. But the images not only provide a large-scale overview of the geographic location; thanks to their high resolution, they can be greatly enlarged for a closer look at individual details of the landscape in exceptional quality.
Jezero in context
Jezero crater lies exactly on the border between the ancient highland region of Terra Sabaea, where rocks from 4.1–3.7 billion years ago can still be found, and the 3.9 billion year old Isidis impact basin. The Nili Fossae graben system, which roughly traces the curved shape of the rim of Isidis basin, was formed by the impact event as a result of tectonic fractures. The Isidis Planitia (plains) is made up of much younger deposits and formed in the martian ‘middle ages’ some 3.7-3 billion years ago, modified up to the present day. To the southwest of Jezero Crater is the volcanic region of Syrtis Major, where lava flowed around 3.7-3 billion years ago. Thus, the rocks and deposits in and around Jezero Crater span the geological history of Mars.
Jezero mineralogy in depth
Jezero Crater has a diverse mineralogy, which can be used to infer the environmental conditions at the time of the minerals’ formation. Perseverance rover will be studying them directly.
The detailed map of Jezero Crater (top right inset in the images shown here) shows that the crater rim is breached by three valleys that were former rivers. Neretva Vallis and Sava Vallis were inflow channels that have created two deltas on the western and northwestern rim of the crater. Perseverance will examine the larger of the two in the west in more detail.
Pliva Vallis in the east of Jezero was an outflow channel through which water was discharged from the crater. As such, Jezero is known as an ‘open basin lake’, a type of lake that is thought to have once been numerous on Mars. Compared to closed basins (with inflow but no outflow), they are interesting because they hosted freshwater lakes with a stable water level. Lakes in closed basins, on the other hand, were subjected to more frequent periods of drying out, which turned them into salt lakes, thus making them less promising in the search for conditions that are conducive to life.
The large map shows the water catchment area of the two inflow channels, from which material from the surrounding area was transported by the rivers into the crater and deposited in the two deltas. Spacecraft orbiting Mars have used spectrometers to detect a variety of minerals in this catchment area. These are mainly silicates from the olivine and pyroxene groups, both mineral classes that originate from magma within the martian mantle and indicate basaltic volcanic deposits that were not subject to long-term weathering by water. Carbonates identified on the inner rim of Jezero Crater, together with the clay minerals, testify to weathering by water of rock with a volcanic source.
Volcanic minerals, carbonates and clay minerals are observed both in the delta and elsewhere in the crater. Some carbonates (limestones) are thought to have been formed directly in the lake. Such lake carbonates, and especially the clay minerals, indicate freshwater conditions and have the potential to preserve traces of life.
However, other types of minerals, such as sulphates that contain iron oxide, amorphous silicon oxides, and hydroxides, have also been observed; these tend to form in acidic waters that gradually dried up. These minerals indicate that the environmental conditions in Jezero Crater became drier and less conducive to life at a later stage. However, even among these minerals there are some in which biosignatures can be very well preserved.
Studying rocks and minerals with a wide origin – whether formed by volcanic activity, by water, or modified in giant impact events – will provide important insights into the history of Mars.
Preparing for a future sample return mission
Rover missions provide essential ground work, but even more insights can be gleaned by humans working in high-tech laboratories back on Earth. That is why Perseverance will also collect samples from a range of locations and store them on the surface, to be retrieved by future missions. Perseverance marks the first stage of the Mars Sample Return joint venture between ESA and NASA, which foresees a future rover to retrieve the cached samples, and an Earth-return orbiter to bring them ‘home’.