Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies (including the Moon, Mercury, the asteroids, comets, and some of the moons of other planets) incur many weathering processes:

• collisions of galactic cosmic rays and solar cosmic rays,
• irradiation, implantation, and sputtering from solar wind particles, and
• bombardment by all sizes of meteorites and micrometeorites. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — Galactic cosmic rays ( GCRs) consist of those Cosmic rays that enter the solar system from the outside For the 1962 Bruce Conner film see Cosmic Ray (film Cosmic rays are energetic particles originating from space that impinge on Irradiation is the process by which an item is exposed to Radiation. Sputtering is a process whereby Atoms are Ejected from a solid target material due to bombardment of the target by energetic Ions It is commonly used for The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. A meteorite is a natural object originating in Outer space that survives an impact with the Earth 's surface A Micrometeoroid (also micrometeorite, micrometeor) is a tiny Meteoroid; a small particle of rock in space usually weighing less than a Gram

Space weathering is important because these processes affect the physical and optical properties of the surface of many planetary bodies. Therefore, it is critical to understand the effects of space weathering in order to properly interpret remotely sensed data.

An illustation of space weathering's different components.

History

Much of our knowledge of the space weathering process comes from studies of the lunar samples returned by the Apollo program, particularly the lunar soils (or regolith). Lunar soil is the fine Regolith found on the surface of the Moon. Regolith ( Greek: "blanket rock" is a layer of loose Heterogeneous material covering solid rock. The constant flux of high energy particles and micrometeorites, along with larger meteorites, act to comminute, melt, sputter and vaporize components of the lunar soil, as well as to garden (or overturn) it. Comminution is one of the four main groups of mechanical processing and describes the movement of the Particle size distribution (grains drops bubbles into a range of finer

The first products of space weathering that were recognized in lunar soils were agglutinates. Agglutinates are created when micrometeorites melt a small amount of material, which incorporates surrounding glass and mineral fragments into a glass-welded aggregate ranging in size from a few micrometers to a few millimeters. Agglutinates appear black to the human eye, largely due to the presence of nanophase iron. Nanophase materials are materials that have grain sizes under 100 Nanometres They have different mechanical and optical properties compared to the large grained materials of Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Agglutinates are very common in lunar soil, accounting for as much as 60 to 70% of mature soils.

TEM image of space weathered rim on a grain of lunar soil 10084

Space weathering also produces surface-correlated products on individual soil grains, such as glass splashes; implanted hydrogen, helium and other rare gases; solar flare tracks; and accreted components, including nanophase iron. It wasn't until the 1990s that improved instruments and techniques allowed for the discovery of very thin (60-200 nm) patinas, or rims, develop on individual lunar soil grains as a result of the redepositing of vapor from nearby micrometeorite impacts and the redeposition of material sputtered from nearby grains. ^[1] These weathering processes have large affects on the spectral properties of lunar soil, particularly in the UV/Vis/NIR wavelengths. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency.

Effects on spectral properties

The spectral effects of space weathering are threefold: as a surface matures it becomes darker (the albedo is reduced), redder (reflectance increases with increasing wavelength), and the depth of its diognostic absorption bands are reduced^[2] These effects are largely due to the presence of nanophase iron in both the agglutinates and in the accreted rims on individual grains. The albedo of an object is the extent to which it diffusely reflects light from the sun An absorption band is a range of Wavelengths (or equivalently frequencies) in the Electromagnetic spectrum which are able to excite a particular The darkening effects of space weathering are readily seen by studying lunar craters. Young, fresh craters have bright ray systems, because they have exposed fresh, unweathered material, but over time those rays disappear as the weathering process darkens the material. A ray system comprises the radial streaks of fine Ejecta thrown out during the formation of an Impact crater.

Space weathering on asteroids

Space weathering is also thought to occur on asteroids,^[3] though the environment is quite different from the Moon. Impacts in the asteroid belt are slower, and therefore create less melt and vapor. Also, fewer solar wind particles reach the asteroid belt. And finally, the higher rate of impactors and lower gravity of the smaller bodies means that there is more overturn and the surface exposure ages should be younger than the lunar surface. The lunar surface (or the surface of the Moon) differs greatly from that of Earth. Therefore, space weathering should occur more slowly and to a lesser degree on the surfaces of asteroids.

However, we do see evidence for asteroidal space weathering. For years there had been a so-called "conundrum" in the planetary science community because, in general, the spectra of asteroids do not match the spectra of our collection of meteorites. Particularly, the spectra of S-type asteroids, the most abundant type, did not match the spectra of the most abundant type of meteorites, ordinary chondrites (OCs). S-type asteroids are of a silicaceous ( stony) composition hence the name The Ordinary chondrites (sometimes called the O chondrites) are a class of stony Chondritic Meteorites They are by far the most numerous group and comprise The asteroid spectra tended to be redder with a steep curvature in the visible wavelengths. However, Binzel et al. ^[4] have identified near Earth asteroids with spectral properties covering the range from S-type to spectra similar to those of OC meteorites, suggesting an ongoing process is occurring that can alter the spectra of OC material to look like S-type asteroids. There is also evidence of regolith alteration from Galileo's flybys of Gaspra and Ida showing spectral differences at fresh craters. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher TemplateInfobox Planet. --> 951 Gaspra (ˈɡæsprə is an S-type Asteroid that orbits very TemplateInfobox Planet. --> 243 Ida (ˈaɪdə eye'-də) is a Main belt Asteroid With time, the spectra of Ida and Gaspra appear to redden and lose spectral contrast. More recent evidence from NEAR Shoemaker's x-ray measurements of Eros indicate an ordinary chondrite composition despite a red-sloped, S-type spectrum, again suggesting that some process has altered the optical properties of the surface. The Near Earth Asteroid Rendezvous - Shoemaker (NEAR Shoemaker renamed after its launch in honor of planetary scientist Eugene M TemplateInfobox Planet. --> 433 Eros (ˈɪərɒs irr'-os) is the first discovered Near-Earth asteroid

Space weathering on Mercury

The environment at Mercury also differs substantially from the Moon. For one thing, it is significantly hotter in the day (diurnal surface temperature ~100 °C for the Moon, ~425 °C on Mercury) and colder at night, which may alter the products of space weathering. In addition, because of its location in the solar system, Mercury is also subjected to a slightly larger flux of micrometeorites that impact at much higher velocities than the Moon. These factors combine to make Mercury much more efficient than the Moon at creating both melt and vapor. Per unit area, impacts on Mercury are expected to produce 13. 5x the melt and 19. 5x the vapor than is produced on the Moon. ^[5] Agglutinitic glass-like deposits and vapor-deposited coatings should be created significantly faster and more efficiently on Mercury than on the Moon.

The UV/Vis spectrum of Mercury, as observed telescopically from Earth, is roughly linear, with a red slope. There are no absorption bands related to Fe-bearing minerals, such as pyroxene. This means that either there is no iron on the surface of Mercury, or else the iron in the Fe-bearing minerals has been weathered to nanophase iron. A weathered surface would then explain the reddened slope. ^[6]

References