Transcript Orbit inclined 17º from Ecliptic, with a high eccentricity

Pluto, the Kuiper Belt, and TransNeptunian Objects
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What about Pluto?
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Discovery of Pluto
Calculations indicated that Neptune’s orbit was being
purturbed => another planet beyond Neptune.
Found in 1930 by Clyde Tombaugh. Too small to
account for the perturbations. They were later shown
not to exist – no need to invoke planet beyond Neptune!
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Pluto: basic data
Semi-major axis
Orbital period
Inclination
Axis tilt
Eccentricity
Diameter
Mass
Density
Temp
Uranus
19.2AU
84 yrs.
0.8°
98°
0.04
4.0 DEarth
15 MEarth
1.3 g/cm3
55K
Neptune
30.1
165
1.8°
30°
0.01
3.9
17
1.6
55K
Pluto
39.5
248
17°
120°
0.25
0.19
0.0025
2.0
44K
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Orbit inclined 17º from Ecliptic, with a high
eccentricity (sometimes inside Neptune's orbit)
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HST
hemispheric
images
Bright regions were speculated to
be impact craters where ice has
been exposed.
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Mean density: rock/ice
Spectroscopy: thin atmosphere of
predominantly nitrogen, showing
large pressure variations over the
years. Thought to be seasonal.
Pluto and Charon
• Smaller mass ratio than any
planet/moon, same density.
• Both objects tidally locked
• Binary orbit gave precise
mass estimates of both
• Rare eclipses in 1985-1991
gave diameters from timing
measurements. Then could
work out precise densities.
Angular separation
here is only 0.9”
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Now five moons found
Charon (New
Horizons)
Hubble image showing Pluto, Charon,
Nix and Hydra
Nix and Hydra found in 2005, Kerberos
2011, Styx 2012, all with Hubble
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New Horizons
• Launched January 2006, flew past in July 2015.
• Goals are to understand surface composition,
temperature, thin atmosphere of Pluto, study
Charon, look for more satellites, examine one
other Kuiper Belt object.
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Plain of frozen nitrogen (“Sputnik Planum”), no impact craters
found, age <10 million years.
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Sputnik Planum may be
impact basin, possibly
1-3 km lower than
surroundings. Ice may
be from cryo-volcanoes or
atmospheric freeze-out
At northern edge there
appear to be flowing glaciers.
Flyover
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Other areas show a range of ages, up to >4 billion years old. A long
history of geologic activity. But crater-dating uncertain because of
uncertainty in population of impactors in outer Solar System. 12
Elevation maps of
the two candidate
ice volcanoes
50 km
One of two possible ice volcanoes. If
so, internal heat source is unknown!
Although nitrogen-dominated,
frozen hydrocarbons give thin
atmosphere blue color. Atmosphere
loss rate is low, so it’s permanent,
but should still vary seasonally. May
be fed by ice volcanoes.
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Charon
• Long fractures. Also may have ice volcanoes. IR spectra show ice
crystals of water and ammonia. Sunlight would soon break these
down if not replenished. Surface has age variations. Charon flyover.
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Moon system
• Moon orbits all prograde.
• Only Charon is tidally locked.
• Others spin rapidly, Nix retrograde. Spins are slower closer to Pluto.
• Hydra spin period only 10.3 hours. Nix and Hydra spin chaotically,
due to combined gravity of Pluto and Charon.
• Charon’s gravity may be preventing Pluto from tidally locking the
other moons.
• All presumably formed by fragmenting collision with Pluto?
Although why not more moons? Speculation that some merged.
• Moons’ orbit animation. Nix chaotic spin animation
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What is Pluto…?
• Pluto and Charon are similar to Triton in density (mixtures
of rock and ice)
• Characteristics of objects formed in the outer solar system.
• The discovery of more, similar objects beginning in early
1990’s means that Pluto is just one of the “TransNeptunian Objects – icy bodies orbiting beyond Neptune.
The “Kuiper Belt” is a zone from 30 to 50 AU containing
great majority of TNOs.
• Orbit inclinations can be typically 20°.
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• If orbit determined and distance from Sun and Earth known,
then measurement of amount of sunlight reflected (optical)
and reradiated (infrared) allows size and albedo (fraction of
light reflected) to be found.
• Other sizes from stellar occultations, again if orbit is known.
• Some are binaries and masses can be found, thus densities.
• May be 35,000 TNOs larger than D=100 km. >1200
known. Probably many more smaller ones. Smallest found
has R about 1 km.
• Pluto probably wouldn’t have been called a planet if
discovered now!
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Kuiper Belt and TNOs are leftover planetesimals thought to
be scattered outward by the late outward migration of
Neptune. Total mass of Kuiper Belt 0.06 - 0.25 Earth masses.
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Eris and other large TNOs
Orbit of Eris – the most massive known TNO
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Eris and Dysnomia
from HST
Other large Trans-Neptunian Objects
Makemake
Haumea
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Reminder of definitions
• A planet is a spherical object orbiting a star, is not a
star itself, and has swept out its path
• A dwarf planet is a spherical object orbiting a star that has not
swept out its path (Pluto, Eris, Ceres, a few other TNOs), and is
not a satellite. Note Pluto and Eris are also TNOs, Pluto is also
a KBO, and Ceres is an asteroid.
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Mass and Size Functions
Whenever you have a large number of objects with various
masses, useful to describe the number as a function of mass,
N(M), or size, N(R). Constrains theories of their origin. Useful
for Kuiper Belt, Asteroids, impact craters, Saturn’s ring
particles, stars, gas clouds, galaxies.
Often have many small objects and a few large ones, which
we can try to describe with a “power law” mass function:
N(M) α M-β
Gives relative importance of large and small objects (not total
numbers – this depends on constant of proportionality)
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A Stellar Mass Function
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For KBOs with measured radii, find
N(R) α R-4
If they all have the same density then M/R3 = constant, so
M α R3, so R-4 α M-4/3, and
N(M) α M-4/3
Now can ask, for example, how many are there of mass M1
vs. 10xM1?
N(M1)/N(10xM1) = M1-4/3/(10xM1)-4/3 = 104/3 = 21.5
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Can also ask: is most of the mass in larger KBO objects or
smaller ones? For example, how much mass in objects of
mass M1 vs. objects of mass 10xM1?
If you have N objects of mass M, the total mass is MxN.
So for N=N(M), total mass is MxN(M) α MxM-4/3 or M-1/3.
So relative mass is
M1-1/3/(10xM1)-1/3 = 101/3 = 2.2
So somewhat more mass in lower mass objects. Recall β
was -4/3. Note if β > -1, more mass in higher mass objects.
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The mass function is a constraint on our understanding of the origin
of the population. KBOs represent leftovers from planet building
process. Sizes affected by
• growth through collisions (and gravitational focusing when they
get massive enough),
• shattering by collisions, which depends on speed and
composition (held together by material strength, or by gravity
(rubble piles)?),
• ejection from Solar System, etc.
Mass function helps us understand the importance of each process.
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True color, HST
Animation
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