Originally published in:
Encyclopedia of Planetary Sciences, edited by J. H. Shirley and R. W. Fainbridge,
905-907, Chapman and Hall, New York, 1997.
Venus is sometimes characterized as Earth's 'twin' because of its close proximity in solar system location (~ 0.72 AU heliocentric distance compared to 1.0 AU) and its similar size (~ 6053 km radius compared to - 6371 km radius), but other close resemblances are few. Besides the more obvious atmospheric composition and pressure differences, and the related extreme temperatures at the surface described elsewhere in this volume, events in the history and evolution of the interior of Venus have left that planet with practically no intrinsic magnetic field. The consequences for the space environment and atmosphere are numerous, ranging from the presence of an 'induced' magnetotail in the wake, to an ionosphere and upper atmosphere that are constantly being scavenged by the passing solar wind.
Venus, like the other terrestrial planets, was presumably accreted from iron and silicate-bearing planetesimals some 4.5 billion years ago. These new planets are all likely to have differentiated in a similar manner, so that they have the common feature of a molten iron-rich core of about half the planet's radius, covered by a crust of the remaining (mainly silicate) material. Only indirect information is available about these cores, but seismic measurements on the surface of Earth tell us that a solid inner core, with a size depending on the size of the planet and on its thermal history, may also be a common feature. Since no seismic measurements have been obtained on the surface of Venus, we cannot be as certain about its interior; however, the large value of the mean density of ~ 5.25 g cm-3 derived from satellite orbits suggests that Venus contains an Earth-like core. Essentially, all other deductions about the interior of Venus are based on models of Earth-like planets with internal temperatures and pressures adjusted for the slightly different radius and possible compositional differences. One of these models has led to the hypothesis that the core of Venus may be completely solid or 'frozen' today, while others propose that core solidification has not yet commenced or has stopped at some time in the past (e.g. Stevenson, Spohn and Schubert, 1983). In all cases, evidence cited in support of these hypotheses always includes the known weakness of the intrinsic magnetic field.
When Mariner 2 flew by Venus in 1962 at a distance of 6.6 planetary radii (Rv), it did not detect any evidence of an Earth-size magnetosphere. Mariner 5, passing within 1.4 Rv in 1967, detected the signatures in the solar wind of deflection around an 'obstacle' at Venus. The small inferred size of that obstacle placed an upper limit on the magnetic dipole moment of Venus of ~ 10-3 that of Earth. Later Venera 4 made magnetic measurements down to 200 km altitude, still detecting no planetary field but providing data that reduced this estimate by about an order of magnitude. In a 1974 flyby, Mariner 10 merely confirmed the existence of a small, nearly planet- size obstacle. Venera 9 and 10 were put into orbit around Venus in 1975, but did not approach Venus closer than ~ 1500 km. Nevertheless, the data that these spacecraft obtained in the wake of the planet provided the first evidence that an Earth-like magnetotail was absent, and that instead a structure related to the interplanetary magnetic field occupied that region of space. The most definitive measurements of the magnetic moment of Venus were obtained during the Pioneer Venus Orbiter mission in its first years of operation (1979-1981). Repeated low-altitude (~ 150 km) passes by that spacecraft over the antisolar region, coupled with dayside observations to the same altitude, proved the insignificance of a field of internal origin in near-Venus space. The observed fields for the most part could be explained as solar wind interaction-induced features, to be described below. The new upper limit on the dipole moment obtained from the Pioneer Venus Orbiter wake measurements placed the Venus intrinsic magnetic field at ~ 10-5 times that of Earth.
Of course, the weakness of the present measurement does not imply that Venus has always been bereft of an intrinsic field. Theories of the dynamos operating in the liquid cores of the newly accreted terrestrial planets suggest that there was a magnetic moment of Venus of the same order as Earth's for about the first billion years of Venus' life. During that time, thermal convection from the heat left over from accretion drove the dynamo. However, after that energy source diminished, there was apparently no source to replace it. While solid core formation in Earth's interior maintains its dynamo to this day by virtue of the related 'stirring' of the molten core around it, Venus appears to either lack the necessary internal ingredients (chemical or physical) for solid core formation, or to have ceased such processes at an earlier time if they resulted in complete core solidification or arrested core solidification. It is important to note that, contrary to popular belief, dynamo theory does not credit the smallness of the magnetic moment to the slow rotation of Venus (a Venus day of ~ 243 Earth days is almost equal to the length of its year of ~ 224 days, and its sense of rotation is retrograde). It is also notable that Venus would not have maintained any remanent crustal magnetic fields from its proposed early period of dynamo activity because the temperatures in the crust are expected to be above the Curie point (below which such fields could persist in rocky materials).
The 'magnetosphere' of Venus that was detected by spacecraft is now known to be an example of an 'induced' magnetosphere. In an induced magnetosphere, the solar wind interacts directly with the planetary ionosphere. The fields and plasmas that are observed are generally of solar wind or ionospheric origin. There are no belts of trapped radiation such as Earth's Van Allen belts, and there is no 'magnetotail' composed of fields of planetary origin. The basic features of an induced magnetosphere are shown in Figure 1. The ionospheric obstacle to the solar wind is defined by a surface called the ionopause. At the ionopause, pressure balance exists between the solar wind dynamic pressure on the outside and the thermal pressure of the ionospheric ions and electrons on the inside. Outside of the ionopause the solar wind interaction has all of the features characteristic to a planetary magnetosphere. A bow shock forms upstream of the obstacle. An interesting feature of the Venus bow shock is that it appears to have a location that varies with the solar cycle. The 'nose', or subsolar position, of the bow shock at sunspot maximum is near 1.5 Rv, but the terminator location moves in to - 2.1 Rv. Inside of the bow shock the solar wind plasma is deflected around the obstacle in a magnetosheath region, which is sometimes referred to as an ionosheath since the obstacle is an ionosphere. The embedded interplanetary magnetic field is compressed and draped around the obstacle in the magnetosheath region in the usual way.
|Fig. 1. Illustration of the major features of the solar wind interaction with the ionosphere of Venus. The solid dots represent the neutral atmosphere, while the circled plus symbols represent ionized atmosphere. The ionized atmosphere above the ionopause is removed by the solar wind.|
Inside of the ionopause the plasma changes from solar wind-dominated to ionospheric in origin. During the primary Pioneer Venus mission, which occurred at a time of high solar activity when the planetary ionospheres are densest, this boundary between the solar wind and ionosphere proper occurred at an average altitude of about 300 km, flaring to ~ 800 km average altitude near the terminator. The boundary, which moves up and down in response to changing external (solar wind) pressure, was typically thin at a few tens of kilometers, although it increases in thickness as its altitude decreases. The observations indicated that the magnetic fields of interplanetary origin in the magnetosheath generally remain confined above the ionosphere proper, although small-scale (dimensions of a few kilometers) field increases (to ~ 100 nT) of still-unknown origin were observed. These field intrusions appeared to have twisted internal structures and so were dubbed 'flux ropes'. The exception to this behavior occurred on the rare occasions (about 15% of the time) when the solar wind pressure was high enough to drive the pressure-balance boundary to altitudes of ~ 250 km or less (the ionospheric thermal pressure increases as altitude decreases down to about 190 km altitude). At these times it appeared as if the interplanetary magnetic field in the magnetosheath penetrated the ionosphere to at least the spacecraft minimum altitude of ~ 150 km. Its magnitude in the ionosphere can reach -150 nT.
The nightside solar wind interaction features at altitudes below several hundred kilometers also show a dichotomy with solar wind pressure. When the conditions for tile 'unmagnetized' dayside ionospheres prevail, the nightside ionosphere is supplied by planetary plasma flowing across the terminator from the dayside. The observed nightside ionospheric magnetic fields are fluctuating and weak (~ 10 nT) at these times, and do not appear to be twisted like the dayside flux ropes. Near midnight, however, steady, almost vertical magnetic fields of a few tells of nanotesla were observed in conjunction with ionospheric density depletions called 'holes' by their discoverers (Brace, et al., 1982). These features are up to a significant fraction (~ 1/4) of the planetary radius in horizontal scale, and they appear to have a field 'polarity' (e.g. sunward or antisunward) that depends on the interplanetary magnetic field orientation and the associated draped field in the magnetosheath, Their origin and nature remain controversial. The 'holes' disappear when the solar wind pressure is high. The nightside counterpart of the large-scale magnetosheath field penetration into the dayside ionosphere appears to be a large-scale horizontal field of somewhat smaller magnitude (tens of nanotesla) throughout the nightside. Its relationship to the dayside field is still poorly understood. It should be noted that the high solar wind pressure scenario is expected to be common during solar minimum, when the ionospheric pressure is always weaker than at solar maximum.
The high-altitude wake of Venus is permeated with structured magnetic fields that generally point sunward or antisunward and often exhibit a 'double-lobed' structure like an intrinsic planetary magnetotail. However, examination of the polarities of the fields in the lobes shows them to be coupled closely to the interplanetary field and resulting draped magnetosheath field orientations. As shown in Figure 1, this 'induced' magnetotail can be pictured as an extension of the magnetosheath, with the draped interplanetary fields sinking into the ionospheric obstacles' wake. The draping of the field in the induced magnetotail is observed to be enhanced beyond that in the surrounding magnetosheath. This enhancement has been attributed to the 'mass loading' of those interplanetary flux tubes that pass closest to the ionopause and form the magnetotail by virtue of heavy ionospheric ion production on those passing flux tubes. In this sense, Venus can be likened to a comet, which has an induced magnetotail of similar origin.
The authors are supported for work on this subject by NASA grant NAGW 2- 501 through the Pioneer Venus project.
Brace, L. H., Theis, R. F., Mayr, H. G. et al. (1982) Holes in the nightside ionosphere of Venus. J. Geophys. Res., 87, 199.
Hunten, D. M., Colin, L., Donahue, T. M. and Moroz, V. I. (eds) (1993) Venus. Tucson: University of Arizona Press.
Luhmann, J. G. (1986) The solar wind interaction with Venus. Space Sci. Rev., 44, 241.
Russell, C. T. (ed.) (1991) Venus Aeronomy. Space Sci. Rev., 55, London: Kluwer Academic Publishers.
Russell, C. T. (1987) Planetary magnetism in Geomagnetism, Vol. 2 (ed, J. A. Jacobs) London: Academic Press, pp. 457-523.
Stevenson, D. J., Spohn, T. and Schubert, G. (1983) Magnetism and thermal evolution of the terrestrial planets. Icarus, 54, 466.