Philips Atlas of the Universe

(Marvins-Underground-K-12) #1

THE SOLAR SYSTEM


Planetary orbit

Leda

Pasiphaë
Ananke

Sinope

Carme

Himalia
Lysithea

Elara

Io

Europa

Ganymede

Callisto

Amalthea Metis

Thebe Adrastea

centred on Io’s orbit. When the Ulysses space probe
swung round Jupiter, on 9 February 1992, there was appre-
hension about what might happen during the transit of Io’s
torus, though in the event Ulysses emerged unscathed.
Why is Io so active? It seems that the interior is
churned and heated by gravitational flexing by Jupiter and
the other Galileans; the orbit of Io is somewhat eccentric,
so that the tidal stresses vary. All the same, it is strange
that Io should be so active while Europa, less than twice
the distance of Io from Jupiter, is so inert. Incidentally, Io
lies in the midst of Jupiter’s radiation zones, so that it may
qualify as the most lethal world in the Solar System.
The movements of the Galileans can be followed from
night to night. They may pass into Jupiter’s shadow, and
be eclipsed; they may be occulted by the planet; they
and their shadows may pass in transit across Jupiter’s
disk, so that they can be seen as they track slowly along –
the shifts in position become evident after only a few
minutes. During transits, Io and Europa are usually hard
to find except when near the limb, but the less reflective
Ganymede and Callisto show up as grey spots. The
shadows are always jet-black.
The fifth satellite, Amalthea, was discovered by E. E.
Barnard in 1892, with the aid of the great Lick refractor.
Amalthea was imaged by Voyager 1, and found to be
irregular in shape; the surface is reddish, so that it may
have been coloured by contamination from Io. There are
two bowl-shaped craters, two bright features which seem
to be mountains, and a medley of ridges and troughs. No
close-range images were obtained of the other small inner
satellites, Metis, Adrastea and Thebe.
All the outer satellites are very small; only Himalia is
as much as 100 kilometres (60 miles) in diameter.
The members of the inner group, including Himalia, have
direct motion, but those moving further out are retrograde,
and presumably ex-asteroids.
Other small satellites were discovered between 2000
and 2003, and the total is now 53. Of these, 11 have been
officially named: Themisto, Harpalke, Praxidike, Iocaste,
Chaldene, Isonoe, Erinome, Taygete, Kalyke, Megaclite
and Callirrhoe. All are below 10 kilometres (6 miles) in
diameter. Themisto has direct motion, and has an orbit
lying between those of Callisto and Leda; the others are
retrograde, and move beyond the orbit of Elara. Their
orbits are so strongly influenced by the Sun that they are
not even approximately circular, and no two cycles are
alike. No doubt many more small asteroidal satellites
await discovery.

SATELLITES OF JUPITER
Name Distance from Orbital period, Orbital Incl., Orbital ecc. Diameter, Density, Escape vel., Mean opp.,
Jupiter, km days ° km water = 1 km/s mag.
Metis 127,900 0.290 0 0 40 3? 0.02? 17.4
Adrastea 128,980 0.298 0 0 26  20  16 3? 0.01? 18.9
Amalthea 181,300 0.498 0.45 0.003 262  146 143 3? 0.16? 14.1
Thebe 221,900 0.675 0.9 0.013 110  90 3? 0.8? 15.5
Io 421,600 1.769 0.04 3660  3637  3631 3.55 2.56 5.0
Europa 670,900 3.551 0.47 0.009 3130 3.04 2.10 5.3
Ganymede 1,070,00 7.155 0.21 0.002 5268 1.93 2.78 4.6
Callisto 1,880,000 16.689 0.51 0.007 4806 1.81 2.43 5.6
Leda 11,094,000 238.7 26.1 0.148 8 3? 0.1? 20.2
Himalia 11,480,000 250.6 27.6 0.158 186 3? 0.1? 14.8
Lysithea 11,720,000 259.2 29.0 0.107 36 3? 0.01? 18.4
Elara 11,737,000 259.7 24.8 0.207 76 3? 0.05? 16.7
Ananke 21,200,000 631* 147 0.17 30 3? 0.01? 18.9
Carme 22,600,000 692* 164 0.21 40 3? 0.02 18.0
Pasiphaë 23,500,000 735* 145 0.38 40 3? 0.02 17.7
Sinope 23,700,000 758* 153 0.28 35 3? 0.01? 18.3
(* = retrograde)

 The volcanoes of Io
from Voyager. Activity, seen
along the horizon, seems
to be constant; the crust
is certainly unstable, and
material is sent up from
the volcanoes to a height
of hundreds of kilometres
above the surface.

▼ Satellites of Jupiter.
The four large satellites,
the Galileans, have almost
circular orbits, and their
inclinations to the Jovian
equator are low. This also
applies to the small inner
satellites. The outer satellites
fall into two groups; Leda,
Himalia, Lysithea and Elara
have prograde motion, while
Ananke, Carme, Pasiphaë
and Sinope have retrograde
motion. The orbits of these
outer satellites are so
affected by solar gravitation
that they are not even
approximately circular.

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