Elsevier

Surface Science

Volume 500, Issues 1–3, 10 March 2002, Pages 838-858
Surface Science

Far-out surface science: radiation-induced surface processes in the solar system

https://doi.org/10.1016/S0039-6028(01)01556-4Get rights and content

Abstract

Interplanetary space is a cosmic laboratory for surface scientists. Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids. Many of these bodies exist in ultrahigh vacuum environments, so that direct particle–surface collisions dominate the interactions. In this article, we discuss the origins of the very tenuous planetary atmospheres observed on a number of bodies, space weathering of the surface of asteroids and comets, and magnetospheric processing of the surfaces of Jupiter's icy satellites. We emphasize non-thermal processes and the important relationships between surface composition and the gas phase species observed. We also discuss what laboratory and computational modeling should be done to support the current and future space missions––e.g. the Genesis mission to recover solar wind particles, the Cassini mission to probe Saturn, the Europa Lander mission to explore the subsurface ocean hypothesis, and the Pluto/Kuiper Express to sample the outer reaches of the solar system.

Introduction

Interplanetary space is a cosmic, ultrahigh vacuum laboratory for surface scientists. The “space” between planetary bodies, and even the rarified atmospheres above a number of large bodies, correspond to much better vacuum than that generally attainable in laboratories. Although interplanetary space conjures up a mental image of a vast, peaceful void of total emptiness, in reality, it is a stormy and sometimes very violent environment permeated by energetic particles and radiation constantly emanating from the Sun. This outpouring from the Sun is called the solar wind, and is an expanding mixture of fully ionized gaseous material; i.e., an electrically neutral plasma containing electrons and ions that carries a magnetic field and constantly streams outward from the inner solar corona. In addition to the solar wind, photons, solar flare ions and cosmic rays bath the solar system, so that most planetary and interplanetary surfaces are, in fact, continuously bombarded by radiation. Mankind is rather fortunate since the magnetic field and atmosphere of Earth shield us surface inhabitants from the harmful effects of this continuous unfriendly assault!

The magnetic field of the Earth creates a magnetosphere that “funnels” most of the charged particles near the poles creating the great northern and southern light shows, i.e., the beautiful optical displays of the aurora borealis and aurora australis, respectively. A little farther from home, the photons from the Sun can cause our Moon to “glow” even during the daytime by stimulating the removal of alkali metal atoms from its surface. As we proceed beyond the outer reaches of the so-called traditional “habitable zone”, we encounter the asteriod belt which is composed of a multitude of “minor” planets or, more simply, planetary fragments. On an unfortunate occasion, an asteriod can plummet towards the Earth (a scenario that Hollywood box offices find attractive and one that has been suggested to explain the extinction of dinosaurs). Asteriods that do not annihilate themselves by slamming into larger planetary objects are space weathered by radiation. The small interstellar ice grains and dust particles, which collectively account for much of the measurable mass of the universe, are also constantly “processed” by radiation. In the outer solar system, the magnetospheres of Saturn and Jupiter trap energetic ions and electrons that irradiate the low temperature ice present in the rings of Saturn and the rocky and icy material of Jupiter's satellites. The four major satellites of Jupiter are Io, Ganymede, Europa and Callisto. These spectacular objects were first observed by Galileo Galilei in 1610 and described in Sidereus Nuncius (Starry Messenger): Magna longeque admirabilia spectacula pandens suspiciendaque proponens unicuique. (Revealing the great and most spectacular display to everyone to contemplate.) The surfaces of these heavenly bodies are significantly altered by radiation bombardment!

In this paper, we focus on bodies in the solar system which have tenuous atmospheres, comparable to ultrahigh vacuum (UHV) in the laboratory (<1×10−8 Pa). These include the planet Mercury, the Moon, the icy satellites (moons) of Jupiter and other satellites, asteroids, etc. In a recent article in Newsweek [1] entitled “Shoot the Moon”, a headline trumpeted “with 66 Moons in the solar system, it's sheer lunacy out there––Moons with water, atmospheres or both are good places to discover how life on earth got started”. For most of these bodies, thermal processes and direct particle–surface collisions dominate the surface interactions and lead to fascinating surface chemistry and physics, including the formation of tenuous, ballistic atmospheres (i.e., UHV atmospheres in which gravity is the dominant force affecting the trajectories of atmospheric atoms and molecules).

There is a wide and complex spectrum of materials whose surfaces are exposed to the space environment. Some of our earliest and most direct information about materials in space has literally fallen from the sky in the form of meteorites. Most are chondrites, iron-rich bodies containing silicates and ppm traces of most of the elements in the periodic table [2]!

The Moon is the only planetary body (besides Earth) for which rock samples have been collected from known locations, thanks to the Apollo Moon-lander missions. The lunar samples are predominately silicates, with SiO2 as the dominant constituent. Other oxide components vary from location to location, and include Al2O3, MgO, FeO, CaO and TiO2, with traces of Na2O and K2O [2]. Lunar Prospector maps of the Moon show the surface distribution of Fe, Ti, Th and K [3]. Optical reflectance studies of Mercury provide evidence for Mg silicates.

The cold planets (Jupiter and beyond) and the icy satellites of Jupiter are largely covered by layers of condensed molecules (e.g., NH3, CH4, H2O, CO2, N2,…) [2]. The icy satellites of Jupiter also contain large amounts of water ice and non-ice regions that may contain hydrated minerals (possibly from a subsurface ocean) and radiation-processed materials, including sulfuric acid.

Thus, the challenge to surface scientists is in the study of oxide surfaces, including complex multi-component minerals, and the surfaces of condensed molecular solids containing dissolved minerals. These are far more complex than the elemental solids and binary oxides typically studied in surface science laboratories!

Section snippets

Solar photons and the solar wind

Solar photon radiation is by far the largest source of radiant energy for surface processing, and most of this energy is in the form of infrared and visible photons. Some of this energy is absorbed by objects in the solar system, and causes heating of the surface regions. In addition to possibly inducing equilibrium chemistry on exposed surfaces, the heating leads to thermal desorption of gases from a number of `airless' bodies (see Section 3.1, below). Here we will primarily describe processes

Thermal processes and desorption from surfaces

The temperature extremes due to weather on Earth, from the coldest night in Antarctica to the hottest day in the Sahara, range from roughly 200 to 330 K. Much greater temperature extremes are experienced in the solar system due to the presence (or absence) of a flux of solar infrared photons, especially on surfaces that are not protected by an atmosphere. Solar heating at “noontime” at Mercury's equator causes surface temperatures of ∼700 K, while the side of Mercury away from the Sun cools to

The origin of the Moon and Mercury glow

It has been shown that space weathering on the Moon involves high velocity and large momentum impact events (e.g., bombardment of the surface by micrometeorites––a form of “space dust” in the solar system). These result in the production of a melt and vaporization. Subsequent re-deposition of material produces “optical” coatings containing submicron sized particles of reduced iron. Photon and charged particle interactions with the lunar surface do not appear to control the lunar optical

Upcoming missions

There are a number of exciting experiments that will be flown on upcoming NASA space missions, and these will present challenging opportunities to the surface science community. We describe a few of these below.

Acknowledgements

T.E. Madey acknowledges fruitful collaborations with Dr. Boris Yakshinskiy and Prof. V.N. Ageev, and partial support from the National Science Foundation under grant no. 0075995, and by NASA Planetary Atmospheres Division. R.E. Johnson thanks W.L. Brown and L.J. Lanzerotti for introducing him to this subject and for many fruitful collaborations, and acknowledges long-term support from NSF and NASA. Thom M. Orlando thanks Prof. Tom McCord of the Hawaii Institute of Geophysics and Planetology,

References (83)

  • A. Bar-Nun et al.

    Ejection of H2O, O2, H2 and H from water ice by 0.5–6 keV H+ and Ne+ ion bombardment

    Surf. Sci

    (1985)
  • R.E. Johnson

    Sodium at Europa

    Icarus

    (2000)
  • J.S. Kargel

    Brine volcanism and the interior structures of asteroids and icy satellites

    Icarus

    (1991)
  • M.K. Pospieszalska et al.

    Magnetospheric ion bombardment profiles of satellites: Europa and Dione

    Icarus

    (1989)
  • D.B. Chrisey et al.

    Ejection of sodium from sodium sulfide by the sputtering of the surface of Io

    Icarus

    (1988)
  • R.C. Wiens et al.

    Sputtering products of sodium sulfate: Implications for Io's surface and for sodium-bearing molecules in the Io torus

    Icarus

    (1997)
  • Shoot the Moon, Newsweek 26 (1999)...
  • K. Lodders et al.

    The Planetary Scientist's Companion

    (1998)
  • Science (special issue on the lunar prospector) 281 (1998)...
  • R.E. Johnson, in: J. Klinger, D. Reidel (Eds.), Ices in the Solar System, 1985, p....
  • R. Lundin

    Erosion by the solar wind

    Science

    (2001)
  • J.F. Cooper, R.E. Johnson, B.H. Mauk, H.B. Garrett, N. Gehrels, Energetic ion and electron irradiation of the icy...
  • T.E. Madey et al.

    Desorption of alkali atoms and ions from oxide surfaces: relevance to origins of Na, K in atmospheres of Mercury and the Moon

    J. Geophys. Res

    (1998)
  • D.P. Woodruff et al.

    Modern Techniques of Surface Science

    (1994)
  • D.A. Williams, E. Herbst, It's a dusty Universe: surface science in space, Surf. Sci. 500 (2002)...
  • T.E. Madey, F.M. Zimmermann, R.A. Bartynski (Eds.), Proc. of Eighth International Workshop on Desorption Induced by...
  • T.E. Madey

    Electron and photon-stimulated desorption: probes of structure and bonding at surfaces

    Science

    (1986)
  • R.C. Elphic et al.

    Lunar surface composition and solar wind-induced secondary ion mass spectrometry

    Geophys. Res. Lett

    (1991)
  • A.E. Potter et al.

    Discovery of sodium in the atmosphere of Mercury

    Science

    (1985)
  • A.E. Potter et al.

    Discovery of sodium and potassium vapor in the atmosphere of the Moon

    Science

    (1988)
  • A.E. Potter et al.

    Coronagraphic observation of the lunar sodium surface

    J. Geophys. Res

    (1998)
  • M.A. McGrath et al.

    Sputtering of sodium on the planet Mercury

    Nature

    (1986)
  • M. Mendillo et al.

    Constraints on the origin of the Moon's atmosphere from observations during lunar eclipse

    Nature

    (1995)
  • B.V. Yakshinskiy et al.

    Evidence for role of photon stimulated desorption as a source of Na in the lunar atmosphere

    Nature

    (1999)
  • D.F. Heath, M.P. Thekaekara, in: O.R. White (Ed.), The Solar Output and its Variation, Colorado Assocation University...
  • S. Verami et al.

    Possible detection of meteor stream effects on the lunar sodium atmosphere

    Planet. Space Sci

    (1998)
  • E.D. Whipple

    Potentials of surface in space

    Rep. Prog. Phys

    (1981)
  • A.A. Sickafoose et al.

    Photoelectric charging of dust particles in vacuum

    Phys. Rev. Lett

    (2000)
  • R.H. Manke

    Plasma and potential at the lunar surface

  • J. Benson

    Direct measurement of the plasma screening length and surface potential near the lunar terminator

    J. Geophys. Res

    (1977)
  • H.A. Zook et al.

    Large scale lunar horizon glow and a high altitude lunar dust exosphere

    Geophys. Res. Lett

    (1991)
  • Cited by (0)

    View full text