At the time of writing it has been 21 days - or 20 sols - since NASA’s nuclear powered Perseverance rover made a spectacular landing on the Martian surface. A day on Mars is 24 hours and 37 minutes and is referred to as a ‘sol’. The descent and touch down were recorded in remarkable video imagery which, for the first time, showed the full process of descent under parachute, followed by a precision touch down using a rocket powered “sky crane”. This is a ground navigation radar equipped platform carrying the rover which slows to a hover a few feet above the surface before gently lowering the rover on cables to the ground. This complex procedure is necessary because the Martian atmosphere is only 0.6% as dense as Earth’s and consequently lacks the density required for parachutes to slow a descending spacecraft to a safe touch down velocity by themselves.
The landing location is Jezero Crater; a ~45 km diameter crater in the Northern Hemisphere which was once a lake. How do we know this? The North and Western rims of the crater are pierced by two meandering dried up river valleys which descend in altitude towards the crater. A similar channel is observed in the Eastern rim which continues to descend in altitude away from the crater and functioned as an outflow valley. At the mouths of the inflow valleys are fan shaped deposits of carbonates and clay minerals; primarily smectite, which were washed downstream from higher altitude watersheds to the West. The Western fan shaped deposit in particular shows a series of meandering channels which demonstrate that this was once a river delta, spreading out over the floor of the crater. In this remarkable hi-res panorama video from the landing site, comprised of images from Perseverance’s onboard cameras, the enormous mouth of the Western inflow valley is visible at 24 minutes, closely followed by a series of layered escarpments, approximately 1-mile from the rover, which mark the periphery of the delta which is the primary target for the mission. Why? Because smectite clays of the kind found here are very good at preserving organic material on Earth, including fossils, and are known to form only in liquid water. Furthermore, because these deposits were made by water flowing down from a large watershed region to the West, samples of minerals from a large region are conveniently concentrated into a small area which is accessible to the rover.
The broad history of water on Mars is determined by the evolution of the planets atmosphere. During the Noachian epoch, which lasted from approximately 4.1 to 3.7 billion years ago, Mars had a denser atmosphere than today which may likely permitted a hydrological cycle like the one we presently have on Earth. River valleys, lakes, hydrated minerals, and ancient coastlines dating from this era are all visible on the surface of the planet. However, at approximately 4 billion years ago the internal dynamo of the planet shut down and Mars lost its global magnetic field. Over the next few hundred million years this enabled the solar wind - a continuous outflow of plasma from the Sun into interplanetary space - to directly interact with the Martian atmosphere. Coupled with Mars’ weaker gravitational field, and thermal escape, the solar wind gradually stripped away the Martian atmosphere, carrying it off into space. This is process is also currently ongoing at Venus (another unmagnetised planet). However, a recent paper also suggests that much of Mars early water was sequestered in the planets crust.
The loss of atmospheric pressure at the surface of Mars eventually became so low that surface water gradually either evaporated or froze. The greenhouse effect from any CO2 atmosphere largely dissipated and the planet entered its present cold and arid state. The Martian climate of today is thought to have been largely unchanged since the end of the Noachian, albeit with temporary increases in pressure and temperature caused by volcanic activity, meteorite impacts, and changes in the planets inclination. A major unresolved question is how long water was stable on the Martian surface? Was it present for long enough that life could gain a purchase, at least temporarily? Also, was the climate of Noachian Mars warm, wet, and humid, or wet, cold, and icy? The Sun was about 30% dimmer at this time than it is today, but did Mars have sufficient greenhouse gases to offset this and provide a more clement environment suitable for life? Numerous studies have been published arguing for all of these positions. It is hoped that Perseverance may shed some light on these questions.
In the meantime, Perseverance is preparing to begin science operations. On 4th. March the rover performed a first short test drive on the surface and it has begun deploying its various science instruments. One early upcoming milestone is to test-fly Ingenuity, a small helicopter drone, which would be the first time an aerial vehicle has been flown on the Martian surface. The rover has also extended its robotic arm, and its cameras have already returned thousands of images of the landing site. The instruments include microphones which have recorded the sound of wind blowing about the landing site, motors working on the rover, and the snapping sound of a laser which the rover uses to spectroscopically analyse rock targets - an early result of a first target rock obtained yesterday indicates the rock is basalt, or water altered basalt, and is thus of volcanic origin. The floor of Jezero appears to have been partially resurfaced by lava at some point after the formation and drying up of the lake, although no obvious signs of a nearby volcanic vent have been found. Whilst the science objectives of the rover are extremely interesting, I cannot help reflecting when looking at images of the now desolate and wind swept landing site that billions of years before the first humans ever walked the Earth, there were flowing rivers and a lake here with, possibly, a living ecosystem of its own.