20 Encyclopedia of the Solar System
FIGURE 9 A suspected cometary interplanetary dust particle.
The IDP is a highly porous, apparently random collection of
submicron silicate grains embedded in a carbonaceous matrix.
The particle is∼ 10 μm across. The voids in the IDP may have
once been filled with cometary ices.
an important comparison with the IDPs collected by high-
flying aircraft. An example of a suspected cometary IDP is
shown in Fig. 9.
Extraterrestrial particulates are also collected on the
Earth in Antarctic ice cores, in melt-ponds in Green-
land, and as millimeter-sized silicate and nickel–iron melt
products in sediments. The IDP component in terrestrial
sediments can be determined by measuring the abundance
of^3 He.^3 He has normal abundances in terrestrial materials
of 10−^6 or less. The^3 He is implanted in the IDP grains dur-
ing their exposure to the solar wind. Using this technique,
one can look for variations in the infall rate of extraterrestrial
particulates over time, and such variations are seen, some-
times correlated with impact events on the Earth.
A largely unseen part of the solar system is the solar
wind, an ionized gas that streams continuously into space
from the Sun. The solar wind is composed primarily of pro-
tons (hydrogen nuclei) and electrons with some alpha par-
ticles (helium nuclei) and trace amounts of heavier ions. It
is accelerated to supersonic speed in the solar corona and
streams outward at a typical velocity of 400 km sec−^1. The
solar wind is highly variable, changing with both the solar
rotation period of∼25 days and with the 22 year solar cycle,
as well as on much more rapid time scales. As the solar wind
expands outward, it carries the solar magnetic field with it
in a spiral pattern caused by the rotation of the Sun. The so-
lar wind was first inferred in the late 1940s by L. Biermann
based on observations of cometary plasma tails. The theory
of the supersonic solar wind was first described by E. N.
Parker in 1958, and the solar wind itself was detected in
1962 by theExplorer 10spacecraft in Earth orbit, and the
Mariner 2spacecraft while en route to a flyby of Venus.
The solar wind interaction with the planets and the other
bodies in the solar system is also highly variable, depend-
ing primarily on whether or not the body has its own in-
trinsic magnetic field. For bodies without a magnetic field,
such as Venus and the Moon, the solar wind impinges di-
rectly on the top of the atmosphere or on the solid surface,
respectively. For bodies like the Earth or Jupiter, which
do have magnetic fields, the field acts as a barrier and de-
flects the solar wind around it. Because the solar wind is ex-
panding at supersonic speeds, a shock wave, or bow shock,
develops at the interface between the interplanetary solar
wind and the planetary magnetosphere or ionosphere. The
planetary magnetospheres can be quite large, extending out
∼12 planetary radii upstream (sunward) of the Earth, and
50–100 radii from Jupiter. Solar wind ions can leak into the
planetary magnetospheres near the poles, and these can
result in visible aurora, which have been observed on the
Earth, Jupiter (Fig. 10), and Saturn. As it flows past the
planet, the interaction of the solar wind with the planetary
magnetospheres results in huge magnetotail structures that
often extend over interplanetary distances.
All the jovian planets, as well as the Earth, have sub-
stantial magnetic fields and thus planetary magnetospheres.
Mercury has a weak magnetic field, but Venus has no de-
tectable field. Mars has a patchy field, indicative of a past
magnetic field at some point in the planet’s history, but it has
no organized magnetic field at this time. TheGalileospace-
craft detected a magnetic field associated with Ganymede,
the largest of the Galilean satellites. However, no magnetic
field was detected for Europa or Callisto. The Earth’s Moon
has no magnetic field.
The most visible manifestation of the solar wind is
cometary plasma tails, which result when the evolving gases
in the cometary comae are ionized by sunlight and by charge
exchange with the solar wind and then accelerated by theFIGURE 10 The auroral ring over the north polar region of
Jupiter, as imaged by theHubble Space Telescope. Several of the
bright spots correspond to “footprints” of the Galilean satellites
and their interaction with Jupiter’s magnetosphere.