Interplanetary Weathering: Surface Erosion in Outer Space

What happens to bare rock and icy surfaces when they lie exposed to the interplanetary space environment for millions of years? This fundamental question is, of course, very difficult to answer, but planetary geologists and field and particle physicists are working together to do so.

Our most valuable resources for studying space weathering are the lunar samples of surface soils and the underlying rocks that the Apollo astronauts scraped from the surface of the Moon. The lunar surface soils differ from their source rocks in at least one important way: they look different. For eons, the Moon's surface has been bombarded by meteoroids, by solar ultraviolet photons, by solar wind and solar flare ions, and by cosmic rays from our galaxy and others. These processes can result in structural, chemical, and physical changes to planetary surfaces, lunar and otherwise, over time (see figure (1)).

Figure 1. This is a false-color picture of an area on the Moon taken by the Galileo spacecraft in 1992. The colors indicate the relative freshness of surface materials. Recent craters are indicated by blue compared to the older surrounding regions, which appear as red. Red materials are thought to be more optically altered by exposure to space weathering processes. (Image processing by the U.S. Geological Survey, Flagstaff, Ariz.)

Many spacecraft and orbital telescope studies (for example, Voyager, Hubble, and the International Ultraviolet Explorer) of the outer solar system have generated interest in a different class of "weathered" surface: the icy surfaces of the moons of the giant planets. Although ice is the "rock" of small outer solar system objects, it is more volatile than silicates, and, hence, icy surfaces are more easily altered. Thus surface material in the outer solar system and on the Moon matures at different rates.

Ice and rock are not the only surfaces affected by weathering in the space environment; even microscopic space dust is vulnerable. Recently, Earth orbiting spacecraft and U2 aircraft have collected interplanetary dust particles (IDPs) from the uppermost layers of Earth's atmosphere. These tiny particles, thought to be asteroidal and cometary in origin, have been extensively weathered by interactions with energetic charged particles since they were released from their parent object.

Scientists Study Weathering of Surfaces in Space

The term "weathering" is borrowed from the geological processes of erosion and degradation caused by air and water. As used here it means surface alteration in outer space. In this environment, ions and micrometeorites take the place of air and water.

We are now beginning to understand space weathering processes and how they affect lunar soils and interplanetary dust. However, problems lie in extrapolating these processes to the asteroids, Mercury, the Galilean Satellites of Jupiter, etc., and predicting their effects on optical properties. A number of issues related to weathering are under investigation by scientists. Work is under way to model lunar surface alteration and quantify its effects, and to study the production and composition of altered materials, transient atmospheres, coronae, and levitated dust layers. Some are studying results from particulate bombardment simulations and ion and photon irradiation while others work toward prediction. Methods are also being developed to help researchers "see" through the weathering and understand the underlying geology. To achieve this ultimate goal, the weathering processes, their effects, and the rates at which they occur must be better understood.

Data from NASA's Near-Earth Asteroid Rendezvous mission (NEAR) is expected to answer some of the questions about space weathering of rocky surfaces. Similarly, the Galileo and Cassini spacecraft results for the icy moons of Jupiter and Saturn will help enormously to resolve space weathering questions about icy surfaces. However, while awaiting new spacecraft data, lunar and IDP data are being studied, and advances are being made in theoretical and experimental simulation of weathering processes.

The Moon

A comprehensive model for lunar surface alteration shows that impacts and solar wind sputtering (the ejection of elements by energetic kicks from incoming particles) on the Moon produces a melt and a vapor from surface soils. On the lunar surface, Fe²⁺ is reduced when heated by impacts and forms submicroscopic iron metal particles in the glass that results from the melted materials. In addition, as the vapor phase recondenses, submicroscopic iron is deposited on surface mineral grains. This submicroscopic iron appears to be responsible for the optical changes that rock exposed on the lunar surface undergoes. The two most dominant of these optical changes are a decreased brightness and an increased redness of the surface with time.

Other research shows that if optical surface changes are properly taken into account, then accurate maps of subsurface lunar "bedrock" are possible. This exciting and important result will undoubtedly interest planetary geologists who work with remote sensing techniques. For example, in light of these findings, one researcher is currently investigating the differences in modification history of the surfaces of Mercury, the Moon, and the asteroids.

Also of interest is the recent remote sensing discovery that the Moon is richer in iron-bearing minerals than the Earth. This finding excludes models for the origin of the Moon, such as fission and coaccretion, that require the Earth and Moon to have the same compositions. Instead, it favors more catastrophic scenarios, such as formation by giant impact or capture of a passing planetesimal.

For the Moon, at least, we have come far in understanding surface alteration effects and are using this understanding to consider what lies beneath the weathering. These advances in remote sensing determinations have greatly aided our studies of the geology and the formation of our Moon.

Mercury

Mercury was traditionally thought to have as much iron in its rocks as the Earth's Moon, but several recent observational studies indicate otherwise. Observing Mercury in the midinfrared from the Kuiper Airborne Observatory, several scientists concluded that Mercury's surface consists of minerals more depleted in oxidized iron than those on the Moon.

However, Mercury is thought to be strongly affected by the impact melting of surface materials. Due to its high temperatures, compositional studies using reflected light are difficult at best on Mercury, and it is possible that they are further complicated by intense space weathering alteration of surface grains. Theoretical studies of space weathering effects at Mercury suggest a scenario in which three physical processes can account for Mercury's apparent lack of oxidized iron minerals at the surface.

First, dayside-nightside temperature contrasts on Mercury could produce very small grains at the optical surface, decreasing absorption band depths if they exist. Temperature contrasts tend to break bigger rocks down into smaller ones due to differences within the rocks in thermal expansion and contraction rates of minerals.

Second, the nearness of the Sun makes for many hot atoms at the surface. Third, when combined with reducing chemistry conditions (that is, the presence of lots of protons stripped of their electrons which tend to "reduce" or "combine with" ions with electrons to share) due to high hydrogen fluxes, these conditions could result in the chemical reduction of iron even if no impact-generated heat is supplied.

The Asteroids

Very recently, Galileo spacecraft images of asteroids 951 Gaspra and 243 Ida (see figure (2)) revealed dramatic evidence that the surfaces of asteroids, too, undergo alteration. These images are the first glimpse of the response of asteroids to space weathering, and constitute the first clear evidence that low-gravity rock surfaces are modified through time. One of the most surprising implications of these images is the rapid rate at which weathering must occur: two morphologically similar young craters appear to show different degrees of optical alteration. This could only be explained by a slight, although as yet unquantified, difference in age! The big question now is: what is altering the surface and how does this process work?

Figure 2. This is a false-color picture of the asteroid 243 Ida taken by the Galileo spacecraft in 1993. Ida is thought to be composed of silicates and is possibly rich in nickel and iron. Young craters appear in blue as compared to the older surroundings, which are red in this image. On Ida, as on the Moon, recent craters appear to be optically different than more mature craters, possibly indicating that space weathering processes are altering the properties of materials exposed at the surface. (Image obtained from the Galileo SSI web page.)

Unfortunately, a straightforward extrapolation of the lunar model of rocky surface alteration cannot explain the asteroid data: spectroscopic data from asteroids are not consistent with lunarlike space weathering effects. This means that a new and different model for asteroid space weathering that is sensitive to asteroid composition must be developed.

In an effort to simulate weathering processes that may be affecting asteroid surfaces, several researchers are experimenting with irradiation effects on minerals. These experiments are aimed at understanding chemical changes due to irradiation and their possible correlation with reflectance changes on the surface. Using photoelectron spectroscopy, they have measured chemical reduction that may occur due to ion bombardment by the solar wind. The degree to which these chemical changes affect optical properties is being examined to ascertain their importance for the spectroscopic appearance of asteroids. For the asteroids, then, it is not yet known what mechanisms are responsible for the surface alteration seen in the Galileo imaging data. Laboratory simulations are under way, and the meteorite record is being scrutinized, but with meteorite samples from so many lithologically different asteroid sources, investigators will be busy for some time to come. The problems of relating asteroid spectra to meteorite spectra are far from settled.

Interplanetary Dust Particles

Irradiation effects on Interplanetary Dust Particles (IDPs) are also being studied. So far we know that ionizing radiation causes major structural and compositional changes in IDPs. In particular, silicate minerals are amorphized, foreign elements are implanted, cations are selectively removed, and specific elements are enriched. These chemical changes may be important components of the optical changes that occur on bare mineral surfaces. In fact, some parts of the IDPs may have been "weathered" by charged particles in the interstellar medium.

Our best understanding of the microphysical effects possible from weathering in the space environment may come from the study of these IDPs. If it turns out that the chemical changes wrought by ionizing radiation significantly affect optical properties, then this mechanism and its rate will have to be included in the equation of competing alteration mechanisms.

Icy Moons

While the effect of charged particle weathering on the optical properties of surfaces that refract light is not fully established, the effects on icy moon surfaces in the outer solar system have been well studied. Although subtle, the effects can be clearly seen in reflectance changes with longitude.

The orbits of icy moons at Saturn and Jupiter are locked to their respective planets and the plasma rotates with the planets. Because of this peculiar geometry, the ion bombardment rate varies with longitude resulting in longitudinal variation in reflectance. The changes produced by ion bombardment compete, as usual, with other weathering processes. The icy moons are thus excellent "laboratories" for studying the dynamics of space weathering.

It is clear that weathering processes in the space environment are important surface alteration mechanisms. Given time, bombardment processes can alter the nature of refractory and icy surfaces. Understanding space weathering mechanisms and their effects is thus a necessary prelude to the interpretation of remote sensing of planetary surfaces.

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