An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 9:30
Icy Worlds of the Outer Solar System
- Neptune's giant moon
- Young surface with cryovolcanism & geysers
- Pluto & Eris
- Dwarf Planets
- Very similar to Triton in their properties
- Trans-Neptunian Objects:
- Family of icy bodies orbiting beyond Neptune
- Kuiper Belt Objects
- Plutinos (3:2 Resonance with Neptune)
- Scattered Disk Objects
Triton: Neptune's icy moon
Triton is the only giant moon of Neptune:
Cold, icy surface:
- Diameter of 2710 km (0.21 REarth)
- Mean density of ~2.05 g/cc
- Interior is probably an icy mantle over a rocky core.
Triton has a geologically young surface with very few impact craters.
- Temperature 34 K (-398º F)
- Surface is covered in frozen N2, CH4,
CO2, H2O, and CO
- Thin Nitrogen (N2) Atmosphere
Cryovolcanoes & Geyers
The smooth plains are repaved by cryovolcanic flows:
Geysers of N2 gas from the interior:
- Liquid material oozing through cracks in the icy crust.
- Interior heated by tides
Help feed the thin N2 atmosphere of Triton.
- Plumes of ices & dark particles shooting many km
- Swept downwind, making dark streaks.
Origin of Triton
Triton is in a circular retrograde orbit around Neptune.
A collision model has been proposed to explain this:
- Triton started out as a free object
- Collided with a moon of Neptune, destroying the smaller moon.
- Collision slowed Triton enough to capture it into a
retrograde, eccentric orbit.
Tides from Neptune melted the interior and eased it into its present
This tidal heating gives Triton is current-day geologic activity, and
explains its young surface.
Pluto was discovered in 1930 by Clyde Tombaugh of the Lowell Observatory
who was carrying out a search for a putative "Planet X" beyond Neptune.
It was immediately declared the "9th Planet"
Did not fit the pattern of the other 8 planets, but so far as anyone
knew at the time, it was also unique in its orbit, and so it was still
considered a planet.
- Orbit: a=39.48 AU, P=366.7 years
- Orbit was very tilted relative to the Ecliptic (17°),
and very eccentric (e=0.249).
Pluto is a small, icy world:
- Diameter: 2390 km (19% REarth)
- Mass: 0.0021 MEarth
- Density: 2.03 g/cc (Rocky core with ice mantle)
Icy Surface & Thin Atmosphere:
- Temperature: 35-45 K (-378 to -396°F)
- Surface is covered with frozen N2 mixed with
CH4 and traces of CO and H2O.
- Thin nitrogen atmosphere.
All based on studies from the Earth, since no spacecraft have visited
Pluto has three moons:
- 1 Large Moon: Charon
- 2 Small Moons: Nix & Hydra
- Discovered in 1978 at the Naval Observatory
- Orbit: 19,570 km
- Period: 6.3872 days
- Diameter: 1205 km
Pluto & Charon Rotate & Revolve synchronously:
- Pluto & Charon always keep same face towards each other.
- Example of a very strong 1:1 tidal resonance that locks both
Charon's and Pluto's rotations to their mutual orbit.
Nix & Hydra
Two small outer moons discovered in 2005 using the Hubble
- Nix: 48700 km orbit, 25.5 day period, about 40 km across
- Hydra: 64800 km orbit, 38.2 day period, about 160 km across
Eris: A 10th Planet?
A candidate "tenth planet" was announced in August of 2005
by Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and
David Rabinowitz (Yale).
The provisional name was 2003UB313 (the discovery team code-named it
"Xena"). It was assigned the name Eris in August 2006.
Eris is larger than Pluto:
Orbits beyond Neptune:
- Diameter: 2400 km (0.19% REarth)
- Mass: 0.0028 MEarth (1.27x Pluto)
- Density: 2.3 g/cc
It also has a Pluto-like composition as revealed by spectroscopy, and it
has 1 small moon, named Dysnomia. We know Eris' mass because we can
measure the orbit of its moon Dysnomia (otherwise we'd only have rough
- Semi-major axis: a=68AU
- Period: P=560 years
- Very Elliptical orbit: e=0.44
- Orbit Tilt: i=45°
It is currently the largest Trans-Neptunian Object known, and bigger
The discovery of Eris re-opened the debate over whether or not Pluto
should properly be defined as a Planet. In August 2006, the IAU
reclassified Pluto and Eris as "Dwarf Planets". We'll discuss what a
Dwarf Planet is in an upcoming lecture.
The Icy Worlds
Pluto and Eris are now recognized as the largest members of a new class
of distant icy worlds:
Most are found beyond the orbit of Neptune, and hence are refered
to generically as Trans-Neptunian Objects or TNOs for short.
- Found only in the outer Solar System
- Densities of 1-2 g/cc, so composed mostly of ices with a small
amount of rock
- Very cold and covered with N2 and Methane (CH4) ices.
Class of icy bodies that orbit the Sun in the space beyond Neptune's
- Range in distance from 30AU outward from the Sun.
- May extend as far out as 100 AU.
Roughly divided into three sub-classes:
These classes are distinguished by the properties of their orbits.
- Kuiper-Belt Objects (KBOs)
- Plutinos ("little Plutos")
- Scattered Disk Objects
Kuiper Belt Objects (KBOs)
Most Trans-Neptunian objects are in the Kuiper Belt:
- Flattened region between 30 and 50 AU from the Sun
- Bounded by the 3:2 and 2:1 resonant orbits with Neptune.
- Scattered KBOs have long elliptical orbits, and are the
likely source of short-period comets.
First KBO was discovered in 1992:
Total Mass of the Kuiper Belt is estimated to be about
0.03MEarth, or about 2.5x the mass of the Moon.
- About 1000 are now known.
- Expect ~70,000 objects >100 km across
- Largest are >1000 km across
David Jewett, one of the leading experts on KBOs, maintains a very nice
webpage with all the latest information.
Kuiper-Belt Objects are distinguished by their orbits into dynamical
The latter, Scattered Disk Objects, may have once been part of the main
Kuiper Belt, but have been scattered into high, elliptical orbits by
gravitational interactions with Neptune, and are not really KBOs proper.
- Classic KBOs: in belt between 30-50 AU
- Plutinos: objects in a 3:2 orbital resonance with Neptune
- Scattered Disk Objects: have very elliptical orbits, and
semi-major axes extending out to >100AU
Resonant Objects: Plutinos & Twotinos
An important subset of the KBOs are the Resonant Objects, which
are divided into two basic groups:
Currently, resonant objects, primarily Plutinos,
comprise about 25% of known Trans-Neptunian objects.
- In 3:2 resonant orbits with Neptune (2 orbits for every 3 orbits of
- This resonance defines the inner edge of the Kuiper Belt.
- Pluto is the largest Plutino
- In 2:1 resonant orbits with Neptune (1 orbit for every 2 of Neptune)
- This resonance is thought to defined the outer edge of the Kuiper Belt.
The existance of the resonant objects is evidence of an outward
migration of Neptune during the late phases of the formation of the
Solar System. Current estimates are that during the last 10-100Myr (or
so) of the last stages of the Solar System's formation, Neptune moved
outward as much as 5 AU to its present orbit. As Neptune moved slowly
outward, its gravity swept icy planetesimals and larger objects, like
Pluto, into the 3:2 and 2:1 mean motion resonances, where they get
trapped and migrate outward in lock-step with Neptune. Pluto would have
started out in a circular orbit, but as Neptune swept it into a
resonance and then outwards, orbital dynamics theory predicts that its
eccentricity and inclination get "pumped up", giving it the large
eccentricity we see today. This is analogous to what we saw in the previous lecture where Jupiter migrated
inwards and swept up asteroids into resonances during the same
late stages of Solar System formation.
Leftover Raw Materials
The icy Trans-Neptunian Objects are thought to be the leftover
primordial material from the formation of the solar system.
Understanding their properties is therefore of great interest to
understanding the origin of the Solar System.
These objects will be the target of the New Horizons mission launched on
2006 January 19. So far the spacecraft is performing well, and is
on-course for a Pluto Pluto fly-by in July 2015, followed by an extended
mission to explore various Kuiper Belt Objects from 2016-2020.
Readings in Universe: Chapter 16, sections
16-8 through 16-10; Chapter 17, section 17-8
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Updated: 2006 November 24
Copyright © Richard W. Pogge,
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