The climate of Mars has evolved over time. Early in its history, between 3.7 and 4.1 billion years ago, the climate was warmer and wetter and the atmosphere thicker than it is today. Erosion rates were higher than today, and liquid water flowed on the planet’s surface, carving valley networks, filling lakes, creating deltas, and weathering rocks. This implies runoff and suggests rainfall and/or snowmelt. Oceans may have existed. Over time, the atmosphere thinned, erosion rates declined, water activity ceased, and cooler and drier conditions prevailed. Ice became the dominate form of surface water. Yet the climate continued to evolve, driven now by large variations in Mars’ orbit parameters. Beating in rhythm with these variations, surface ice has been repeatedly mobilized and moved around the planet, glaciers have advanced and retreated, dust storms and polar caps have come and gone, and the atmosphere has collapsed and re-inflated many times. The layered terrains that now characterize both polar regions are telltale signatures of this cyclical behavior and owe their existence to modulations of the seasonal cycles of dust, water, and CO2. Contrary to the early images from the Mariner flybys of the 1960s, Mars is and has been a dynamically active planet whose surface has been partly shaped through its interaction with a changing atmosphere and climate system.
Robert M. Haberle
The planetary boundary layer of Mars is a crucial component of the Martian climate and meteorology, as well as a key driver of the surface-atmosphere exchanges on Mars. As such, it is explored by several landers and orbiters; high-resolution atmospheric modeling is used to interpret the measurements by those spacecrafts. The planetary boundary layer of Mars is particularly influenced by the strong radiative control of the Martian surface and, as a result, features a more extreme version of planetary boundary layer phenomena occurring on Earth. In daytime, the Martian planetary boundary layer is highly turbulent, mixing heat and momentum in the atmosphere up to about 10 kilometers from the surface. Daytime convective turbulence is organized as convective cells and vortices, the latter giving rise to numerous dust devils when dust is lifted and transported in the vortex. The nighttime planetary boundary layer is dominated by stable-layer turbulence, which is much less intense than in the daytime, and slope winds in regions characterized by uneven topography. Clouds and fogs are associated with the planetary boundary layer activity on Mars.
Cosmogenic nuclides are produced by the interaction of energetic elementary particles of galactic cosmic radiation (GCR) and their secondaries with atomic nuclei in extraterrestrial or terrestrial material. In extraterrestrial samples cosmogenic nuclides produced by energetic particles emitted by the Sun (SCR) are also detectable. Cosmogenic nuclides usually are observable only for noble gas isotopes, whose natural abundances in the targets of interest are exceedingly low, with some radioactive isotopes having half-lives mostly in the million-year range, and a few stable nuclides of elements such as Gd and Sm whose abundance is appreciably modified by reactions with low-energy secondary cosmic-ray neutrons. In solid matter, the mean attenuation length of GCR protons is on the order of 50 cm. Therefore, cosmogenic nuclides are a major tool to study the history of small objects in space and of matter near the surfaces of larger parent bodies. A classical application is to measure “exposure ages” of meteorites, that is, the time they spent as a small body in interplanetary space. In some cases, the previous history of the future meteorite in its parent-body regolith can also be constrained. Such information helps to understand delivery mechanisms of meteorites from their parent asteroids (mainly from the main belt) or parent planets, and to constrain the number of ejection events responsible for the meteorites in collections worldwide. Cosmogenic nuclides in lunar samples from known depths of up to ~2 m serve to study the deposition and mixing history of the lunar regolith over hundreds of million years, as well as to calibrate nuclide production models. Present and future sample return missions rely on cosmogenic nuclide measurements as important tools to constrain the sample’s exposure history or loss rates of its parent-body surfaces to space. First measurements of cosmogenic noble gas isotopes on the surface of Mars demonstrate that the exposure and erosional history of planetary bodies can be obtained by in situ analyses. Exposure ages of presolar grains in meteorites provide at present the only quantitative constraint of their presolar history. In some cases, irradiation effects of energetic particles from the early Sun can be detected in early solar system condensates, confirming that the early Sun was likely much more active than later in its history, as expected from observations of young stars. The increasing precision of modern isotope analyses also reveals tiny isotopic anomalies induced by cosmic-ray effects in several elements of interest in cosmochemistry, which need to be recognized and corrected for. Cosmogenic nuclide studies rely on the knowledge of their production rates, which depend on the elemental composition of a sample and its “shielding” during irradiation, that is, its position within an irradiated object, and for meteorites their pre-atmospheric size. The physics of cosmogenic nuclide production is basically well understood and has led to sophisticated production models. They are most successful if a sample’s shielding can be constrained by the analyses of several cosmogenic nuclides with different depth dependencies of their production rates. Cosmogenic nuclides are also an important tool in Earth sciences, although this is not a topic of this article. The foremost example is 14C produced in the atmosphere and incorporated into organic material, which is used for dating. Cosmogenic radionucuclides and noble gases produced in situ in near-surface samples, mostly by secondary cosmic-ray neutrons, are an important tool in quantitative geomorphology and related fields.