Life, love and soil

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Sir

Daniel Markewitz1 has objected to the use of the term ‘soil’ for material recently studied remotely by Mars Sojourner, because he thinks soil should be defined as a medium for life. But there is a very compelling reason why the US National Aeronautics and Space Administration and many scientists choose to talk of soil on the surface of the Moon and Mars. They wish to be understood. Soil, like love and home, is difficult to define, but is, at the same time, a familiar and comforting concept2.

A geological as opposed to an engineering or agricultural definition of soil is material altered in place at the surface of a planetary body by physical, chemical or biological means3. This definition of soil introduces the idea of alteration in place as a distinction between soil and sediment. This important distinction is deliberately blurred by terms such as regolith and gradation, also used in planetary geology4, especially where only remote-sensing data are available. But in martian images, sediment, which is bedded, can be distinguished from soil, which is clayey and salty3.

References

  1. 1

    Markewitz, D. Nature 389, 435 (1997).

  2. 2

    Hole, F. Geoderma 25, 75–112 (1981).

  3. 3

    Retallack, G. J. Soils of the Past (Unwin-Hyman, London, 1990).

  4. 4

    Greeley, R. Planetary Landscapes (Unwin-Hyman, London, 1985).

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