Transcript The Other Jovians

Title Frame
Activity 2:
The Other Jovians
Module 15:
The Jovian Planets
Summary:
In this Activity, we will investigate
(a) comparative statistics for the four Jovian planets
(b) properties - and the underlying factors determining
the properties of these planets, extended from our
Jupiter Activity,
(c) spacecraft missions, and
(d) clues about the history of the Solar System.
With a little artistic licence we
look inwards to the ‘backlit’
Jovian planets.
(a) Comparative Statistics
The value of comparative studies
• Some of the observed properties of planets lead to
testable predictions about what conditions might be
experienced at their surfaces and in their atmospheres.
These include . . .
The value of comparative studies
• Other properties (and some of those in the previous
slide) lead to possible conclusions about the formation
and development of the Solar System in the outer region
of the gas giants. These include . . .
Orbit Comparisons (i)
A perspective view of the almost-circular orbits.
Note how our inner Solar System shrinks in comparison.
Uranus
Saturn
Jupiter
Sun to Mars
Neptune
Here we are looking down on the Solar System from about
30 AU away. In fact, from here we would not detect any
size to the planets; the Sun would appear as a star about
one-thousandth of its current intensity; and Jupiter and
Saturn would only appear as average stars to the naked
eye.
Neptune
Uranus
Saturn
Jupiter
Sun to Mars
space is certainly a lonely place ...
In the previous frame we emphasized the insignificance, in
size and brightness, of the planets when viewed with their
orbits to scale. Often their size is exaggerated - as in this
frame - giving the wrong impression about their potential for
mutual attraction (or worse, collision!).
Uranus
Saturn
Jupiter
Sun to Mars
Neptune
Orbit Comparisons (ii)
However, with this diagram in front of us, we will add the
orbit sizes (relative to the Earth-Sun unit of 1AU) and
durations to the nearest Earth-year.
Neptune
165 years
Uranus
84 years
Saturn
Jupiter
30 years
12 years
Sun to Mars
5.2 AU
All orbits are anti-clockwise when
north is ‘up/out of the screen’
9.5 AU
19.2 AU
30.1 AU
Orbit Comparisons (iii)
Comparing circularity of orbits,
viewed from directly above and
shown to scale
J
0.048
Compare with the grey circles
S
0.056
U
0.046
N
0.009
1.3o 2.5o
0.8o
1.8o
Eccentricities all close to
zero
The same direction of orbits,
the approximate circularity
and common orbital planes,
all point to a common origin
mechanism
Comparing inclination of
orbits, viewed edge on
Comment - science at work
• It was once thought that orbits had to be circular.
Benefitting from the careful observations of others, Kepler
and Newton showed that the ellipse was the natural orbit
and the circle was but one example (that itself needed the
explanation!).
• That most of the planetary orbits are so close to circular
suggests a well ordered mechanism for the original
condensation of the planetary material orbiting the newly
forming Sun, (without much interference from external
bodies such as comets, passing stars etc.)
• Scientific theories arise from regular patterns such as
seen in the last frame, but must also be able to explain
irregularities such as you will see in the next frame.
Size and Rotational properties
Sizes to scale
Earth
Diameter (km)
142,985
120,537
51, 119
49,528
(Earth=1)
11.2
9.4
4
3.9
Mass (Earth=1)
318
95
14.5
17
17h14m
16h07m
.024
.026
98°
30°
Rotation* period
9h55m
(body of planet)
10h39m
Oblateness
.065
.108
Tilt of axis to
orbital plane
3°
27°
*As is usual throughout solar system, rotation directions are the same as
orbit directions with the technical exception of Uranus with its high axial tilt
(b) Properties - Consistencies and Irregularities
Before we move on to more detailed examination of the
gas giants, are some of the inter-relationships of the
previous frame what we might expect?
• Yes, the higher rotation rate planets have higher oblateness.
This, along with the relatively low, but consistent mass with
planet size, suggest, as we saw with Jupiter, small rocky
cores with overlying liquid and gaseous atmospheres.
• Even in the small images used so far, the variation in
atmospheric detail and colour invites speculation about the
causes. As well as composition, we may expect axial tilt
(which causes our familiar seasons) to be a contributing
factor to weather systems and, already, Uranus stands out
as nearly featureless.
• If Uranus’ tilt was 90° we could not say whether its rotation
was clockwise or anticlockwise compared with its anticlockwise orbital direction.
• So, though its 98° tilt technically means its rotation and orbital
directions differ, not too much can be made of this. It is believed
that the whole Uranian system was perturbed by some passing
body early in its formation history. However, we might look to its
different seasonal heating effects to help explain its featureless
appearance.
Satellites and Rings
Satellites and Ring systems are covered in detail in the
next Module on Satellites & Rings of the Jovian Planets.
However one cannot look at Saturn without noticing its
spectacular rings, and a planet’s system of satellites also
provides information about its environment now and in its
formative period:
Satellites and Rings
Retrograde*
Rings#
Planet
Satellites
Over
1000km
Jupiter
63**
4
14
3 tenuous
Saturn
31**
5
6
7 prominent
Uranus
27**
4
5
11 faint and dark
Neptune
13**
1
1
4 faint and dark
Most regular satellite orbits and rings are near-circular and inclined at a similar
angle to the planet’s axial tilt; even Uranus’!
* Retrograde means orbiting in a direction opposite to that of the parent planet.
In the few cases above, they are small, outer satellites with high orbital
inclinations and eccentricities.
# Ring
system numbers relate to appearance from a distance. Close detail show
subtle influence of associated satellites.
** Note that many new satellites have been announced
since 1997 and are still being discovered. We’ll learn
about these in the Activity Minor Jovian Satellites & Rings
Photo Gallery
White oval and
turbulence in Jupiter’s
atmosphere
Saturn’s thin rings, shadows
and almost featureless belts
in atmosphere
False colour and enhancement
reveals clouds in Uranus’
otherwise featureless
atmosphere
Increasing Saturn
detail showing ovals,
swirls and storms
Neptune’s dark spots and
‘scooter’ clouds
NASA/JPL Voyager 2 images
Magnetic Fields
Planets are shown (not to scale) with their rotational axes tilted to their orbital planes
Comparing the magnetic fields of the four Jovian planets
with that of Earth . . .
• The magnetic axes (where a compass needle would point)
are tilted with respect to the rotational axis (except for
Saturn). The fields are measured by the magnetometer of
passing spacecraft such as Voyager 2.
Magnetic Fields
• For Uranus and Neptune they are offset from the planet
centre. For Neptune this could mean the magnetic axis is
slowly reversing (as it has done for the Earth in the past).
For Uranus the misalignment could be a further result of the
past perturbation that caused its dramatic axial tilt.
• The field strength gives information about the size and
nature of internal materials such as iron and metallic
hydrogen as shown in the next frame . . .
Internal Structure
Jupiter
Saturn
Uranus
Neptune
Rocky core
Compressed water
Liquid Hydrogen
Liquid metallic Hydrogen
Liquid Hydrogen & Helium
Relative masses and sizes of cores and higher layers is
deduced from oblateness, rotation rate, overall mass and
magnetic field of the planets.
Internal Structure
Jupiter
Saturn
Uranus
Neptune
Rocky core
Compressed water
Liquid Hydrogen
Liquid metallic Hydrogen
Liquid Hydrogen & Helium
Massive Jupiter is thought to be most similar in composition
to the original nebula from which the Sun and planets
condensed. Lower mass and other factors have been
invoked to explain differences in the other three planets.
Temperature and radiation
Temperature at cloud tops
Albedo
Jupiter
Saturn
Uranus
Neptune
-110oC
-180oC
-216oC
-220oC
.52
.47
.51
.41
Energy radiated
nearly twice
compared with that received from Sun
%H:%He:%other elements
in atmosphere by mass
85:14:1
~three times
96:3:1
less than
82:15:3
greater than
79:18:3
Voyager found that Saturn’s atmosphere had less helium than
expected. One theory that explains both the lower helium in the
atmosphere and Saturn’s excess energy radiation is that, since
Saturn’s formation, Helium has been slowly dropping toward the core
and its gravitational energy is converted to the observed excess heat.
Temperature and radiation
Temperature at cloud tops
Albedo
Jupiter
Saturn
Uranus
Neptune
-110oC
-180oC
-216oC
-220oC
.52
.47
.51
.41
Energy radiated
nearly twice
compared with that received from Sun
%H:%He:%other elements
in atmosphere by mass
85:14:1
~three times
96:3:1
less than
greater than
82:15:3
79:18:3
Neptune’s excess radiated energy is also attributed to slow
gravitational contraction.
Uranus’ lack of an internal energy source is a further contributor to its
featureless weather systems.
General Descriptions
Jupiter: The most massive planet, comprising 71% of all the
planetary matter in our Solar System. With its high rotation rate,
internal energy source, and impurities which colour its
atmosphere at different depths, it exhibits a spectacularly
detailed turbulent atmosphere with belts, storms, eddies and
small ovals. It has a gravitational influence on objects such as
comets and asteroids if they pass close enough.
Saturn: With a slightly smaller size and rotation rate than
Jupiter; and less than a third of its mass, but a stronger internal
source of energy, Saturn exhibits more subtle variations of
Jupiter’s belts and storms in a similar three layered
atmosphere.
General Descriptions
Uranus: Less than half the diameter of Saturn and twice as
distant from the Sun, Uranus is much colder and lacking a
strong internal energy source. Though it has high speed winds
its atmosphere is generally featureless. Traces of methane give
it its blue-green tint. Uranus’ unique feature is its 98° tilt to its
orbit plane. That its ring system and most of its satellite orbits
are circular and in its equatorial plane suggests it was
perturbed early in its formation to a stable but inclined system.
Neptune: Slightly smaller but more massive than Uranus and
with an internal heat source driving a high-wind atmosphere
with rotating storms seen as spots and clouds. The blue colour
of the atmosphere is because of its methane content which, like
Earth’s air, scatters blue light more than red.
(c) Spacecraft Missions
Voyager 2, launched August 1977, reached Jupiter in July
1979. It went on to photograph Saturn (August 1981), Uranus
(January 1986) and Neptune (August 1989), before heading
out of the Solar System. Most of the photos in this Activity are
from Voyager 2.
The Galileo spacecraft, launched October 1989, reached
Jupiter in December 1995, dropping a probe into Jupiter’s
atmosphere. It went on to an extensive photographic tour of
Jupiter’s satellites. Galileo’s mission ended in September 2003.
[See previous and following Activities.]
The Cassini-Huygens
Mission was launched in
October 1997 to arrived at
Saturn in July 2004. It has
begun its 4 year mission,
which will include over 30
orbits of Saturn and its
moons.
Cassini’s trajectory included 4 gravity assists:
two at Venus, and one each at Earth and Jupiter
The Cassini-Huygens mission will study Saturn’s composition and
atmosphere, magnetosphere, rings and satellites – specifically
Titan. The Cassini spacecraft orbiter has 12 instruments, while the
Huygens probe, which will descend to Titan, has another 6
instruments.
For information about the mission, visit
http://saturn.jpl.nasa.gov/
The Cassini Spacecraft
At 2150 kg, the spacecraft carries instruments for imaging,
remote sensing and measuring magnetic fields and particles.
Cassini needs 600-700 watts to
operate the science instruments and
transmit their data. It must be able to
produce power reliably for 11 or
more years at up to 1.6 billion km
from the Sun. Power is provided by
3 Radioisotope Thermoelectric
Generators requiring 33kg of
plutonium in total - a controversial
aspect in the case of any launch or
mission failure.
Cassini’s Huygens Probe
The Huygen’s Probe, supplied by the
European Space Agency, is planned to be
released from Cassini in December 2004. It
will study the clouds, atmosphere, and
surface of Saturn’s satellite Titan.
The probe will enter and brake in Titan’s
atmosphere and parachute a fully
instrumented robotic laboratory down to
the surface.
It is designed for a maximum descent time
of 2.5 hours and will spend at least 3
additional minutes (and possibly a half hour
or more) on Titan’s surface.
Huygens Probe
release over Titan
(d) Clues about the history of the Solar System
The near circular orbits of all the Sun’s family of planets; in
the same direction and (apart from Pluto) generally close to
the same plane; and (apart from Venus, Uranus and Pluto)
the rotation of the planets in the same direction, are strong
evidence that all formed along with the Sun from a
condensing nebula of gas and dust.
[That we now see dust disks around hundreds of other stars
and planets around tens of others is additional evidence that
this is a normal occurrence for, at least, the Sun’s type of
star.]
The Jovian gas giants, being the most massive planets and
furthest from the heating effects of the Sun, should have
retained the original material of the solar nebula better than
the inner planets.
Their further study (by the Cassini spacecraft) and the study
of their satellites (in the following Activities) thus adds to our
knowledge of the possible formation and evolution of our
Solar System.
In the next Module we will look at the satellites and
rings of the Jovian gas giant planets.
Image Credits
NASA:
http://www.nasa.gov
Indexed status of all NASA spacecraft
http://www.hq.nasa.gov/office/oss/missions/index.htm
Hubble Space Telescope images indexed by subject
http://oposite.stsci.edu/pubinfo/subject.html
Now return to the Module 15 home page, and
read more about the Jovian planets in the
Textbook Readings.
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