Planetolog´ıa comparada: Interiores, atmósferas

Planetologı́a comparada:
Interiores, atmósferas
∗ Interiores de los planetas terrestres y la Luna. Fuentes de energı́a interna. Achatamiento
polar. Interiores de otros objetos ricos en hielos.
∗ Interiores de los planetas gigantes.
∗ Distintos tipos de atmósferas en el sistema solar.
∗ Capacidad de un objeto planetario de retener una atmósfera.
∗ Condiciones fı́sicas de las atmósferas: Equilibrio hidrostático, perfiles de temperaturas.
∗ Atmósferas de los planetas terrestres y Titán.
∗ Atmósferas de los planetas gigantes.
∗ Atmósferas extremadamente tenues: casos de Plutón, Tritón, Io.
CTE 1 - clase 13
1
Interiores de los planetas terrestres
∗ El achatamiento polar esta dado por la relación: = (RE − RP )/RE y es un paramétro importante
para deducir la distribución interna del material.
∗ Fuentes de calor interior:
(1) decaimiento radioactivo de los radioisótopos contenidos en los minerales.
(2) Calor primordial proveniente de la acreción del material. Energı́a potencial del material:
M2
EP ∼ G
R
la cual se transforma en energı́a cinética (aceleración de las partı́culas al confluir hacia el centro) y,
finalmente, en calor disipado en colisiones mutuas.
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2
Interiores de objetos ricos en hielos: caso de los satélites galileanos
de Júpiter
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3
Interiores de los planetas jovianos
∗ El hidrógeno en el interior de los planetas más masivos, Júpiter y Saturno, se convierte
en una variedad sólida conductora, debido a las altas presiones allı́ reinantes, que se
denomina hidrógeno metálico.
CTE 1 - clase 13
4
formation and otller density instabilities, large impacts, the decay
of raclionuclides, and tidal interactions. We can also ask how heat
is distributed within the interior over timc, what is kno\.vn as a
As our closest neighbor in space, the Moon IV
the attention of g:eologist~ ~tudying other pi
Proceso de diferenciación interior
de un planeta
10).
planet's thermal history. We can estimate the rates of change of
(sec Chapter
'j'he Moon is the largest satd
parent body, in tlle solar system, and it has lOll!
if it were a planet in its own right Apollo an
remote sensing, and surface seismic data shO'
has been internaJlv differentiated into a ru~t, II
bly a small core (P~qure 7), Seismic data and g
show that the Innar maria are relatively thin
mum thickncss) and perched on a globally COil
rich crust This crust is thinner on the central
a) Interior inicial homogéneo.
b) La energı́a cinética impartida
55 km, but may be up to 100 km tl1ick on tlle
por
los impactos
Following
its formation,
believed to have rderrite el material
impact of a Mars-sized body with Earth, the Me
y provoca la segregación del más
to a period of heavy bombardment tl13.t result
melting. Opinions diller on whnher the mel
denso hacia el núcleo. La corteza
extensive (a magma "ocean") ur regional (I
Whatever the
melting was
accol liviano que,
esexact
eldetaib,
material
más
tional crystallization and separation of plagiocJa
como
escoria,
queda flotando
density stlicate
mineral. una
This floated
to tlIe
primary crust, which survives today as the luna
sobre el manto. c) El decaimiento
ure 8) and the residual upper-mantle layers. T
below the crust
denser than the
underlyil
dewere
isótopos
radioactivos
mantiene
probably caused them to sink to\\ard the inter
el During
manto
parcialmente
derretido
to form a core.
this latter
period, partl
mantic created magmas that flooded the Il
(magma): el más liviano asciende
hacia la superficie provocando
derrames de lava y volcanes.
~lIr
FigUl'C 3. Differentiation is the process by which a body's initially
CTE
homogeneous primitive materials (a) become segregated in planetary
interiors. A number of dif 'erent mechanisms contribut<: to
differentiation. For example, the kinetic energy delivered by impact
1 bombardment
- clase 13 (b) can cause widespread - eveJl global -melting of near­
surface layers. Denser mar.:ria! collects and sinks to form a core (c), aJld
Figure 4. PlaJ1etary bodies are commonly divided
compositionaHy discrete layers (left half), such as
dominated core, a mantle rich in iron- and magm
5
ain~' ~ 111
d can he
agen ami
'occurr 'd
. metallic­
nkdc 'per
'1 sho\\u"
III tal/I(
ji
solve ,It
. solar sy\
,,,ledge of
morucsllf
',and [he
. -section
ionfi'om
: ro<.:k or
o probe,
essure of
bstantial
are than
is is :tho
Saturn,
<l',~'
comes mostly from the slow decay of radioactive iso­
Jupiter, Saturn, and Neptune have detectable internal sources of
heat. This has proved to be one of the most interesting revela­
'11 Uj]l!!; of heat would be difficult to detect from space,
tions of modern planetary science.
·lu~e it is overwhelmed by sunlight's warming effect on
Some clues to tlle ol'igin of the giant-planet heat flows can be
icql/anet surfaces.
obtained by computing the speciflc luminosity of each planet,
that is, the average power released per unit of planetary mass
II. lile' outer solar system, the situation is different. There
. lilT healin)!: contributes much less to a planet's
atmospheric
(Figureis9).
The luminosity
of the
Sun,
by the ther­
monuckar
fusion ofhycL'ogen,
enormous
compared
with
:tn)'powered
hydrogen,
which powers the Sun, is not possible insi
'I't', I\Korporated when the planets formed. Such a modest
Fuentes internas de calor
other solar-system object. At the other extreme are primitive
planets beLJusc their c,l!culated central tempCl
nowhw:: nt'.lr tlie 10,000,000° K required for h dre
rocky bodies, such as the carbonaceous chondrite lllcrcorites,
whose internal heat comes only from the radioactive decay of
to occur, or el en the 500,000° K needed for deutcrit
what little uranium, potassium, and thorium they contain. The
Therdore, b)' diminanon, only one process cOlild I
Earth's specific luminosity lies very close to the earbonaceous­
bie for tlle luminosities on upiter, SJtUrJ1, and, 'cprun
liberated when mass in a gravitationally bound object
chondrite value, suggesting that most of its internaJ heat derives
from radioactive decay.
to thc center of attraction, that is, when the object bel
Among the giant planets, only anomalous Uranus lies close to
centralIv condensed. In effect, potential enugy beco
the very loll' power-to-mass ratio of carbonaceous chondrites
energy. This process causes the tempcrallire 11··rhit
body to rise. Gaseous stars like the 011n become so
Jupiter, Saturn, and Neptune release far more energy per kilo­
gram of mass, so they must derive tlleir luminosities from some­
contract that hydrogen fusion begins to occur. At cl13
F;glm8. Heat flow in the Jovian planets. The blue rays denote
interior (red), this is the heat observed escaping at the sW'face of
tbing other than radioactive decay.
The thermonuclear
of detected
cannot
radiate
accumulating heat into space fast \
~Kidcnt solar energy, some of which (dashed) is scattered back into
each planet.
No interior heathlsion
has been
COOling
from the
Uranns,
contraction
stops.
In liquid bodies (giant plane
'PIlC by the planet's atmosphere. The remaining fraction is absorbed
even though Voyager 2 flew by at close rangethe
i.n 1986.
However,
it
3 r---------~-----------,
/Od convcrted into infrared energy (yellow)10-deeper
in the atmosphere.
must surely be present because of radioactivesolid
decaybodies
in the planet's
(terrestrial planets), pressures depend c
Tog ther with the infrared energy flowing out from the planet's deep
preSW1lably chondritic (rocky) core.
on temperature, so rhey contr,lCt as they slowly cae
Sun
pl~u)ets and brown dwarfs evolve in this manner, sloll
INTERIORS OF THE Gif\NT PLANETS
heat to space. This slow, self-regulated coupling of
E
L
2
and cooling, called the Ke[;Jin-Helmholtz mahllllisl
""
..2
represents the 1,1 t phase ofthe giant planets' vioknt
~
...'lJ 10- 6 ­
perature origin. By itself, the Kell'lll-Helmholtz mec'
.Q
be insufficient to cxplain Saturn's luminosity. The c
13CJ
may be coming from the sinking of dense componel
~
>­
or perhaps ice) toward the planet's center.
f--­
Vi
Although tlle escape of primordial heat frOIll JuP
0
z
and Nephl11e is ver\' s[ow, it appears to be enough tc
~
:::) 10- 9 ­
the hydrogen Ia)'ers throughout much of their in!
--J
Jupiter Saturn
U
C,lUses the hot, liquid hydrogen to convect al the I'
u::
G
centimeters per second. Thus, the convective OVer!
f­
lJJ
Neptune
a..
cloud tops of Jupiter and SaturtJ probabl\- has dee
Uranus •
'­
and may extend virtuallv to these planets' centers I
cc Earth
Decp inside Jupiter and Saturn, convection crea
10-12 L ­
--'
tains powcrful dvnamos in the metallic-hydrogenz,
in turn generate immense rn~lgnerie tields (sec (
Figul"e 9. With the exception of Uranus, dIe Jovian planets emit more
Uranus ~lpjlears to lack the heat flow required to i
energy per unit of mass than other solar-system bodies do. The value
labeled CC(carbonaceous chondrite) is that expected for a body of
deep-seated convection, so perhaps it is no coincide
approximately chondritic (rocky) composition.
planet's enigmatic magnetic field difters markedly it
rry f!'Om those ofJupiter and SaturtJ. Researchers hal
to simulate the conditions inside Uranus and Nepn
shock-compression experiments on water and orh
considered likely to exist in the planers' inreriors. T
tions show tllat high pressure and temperature cau
increase in the materials' electrical conductivity. Th,
though neither Uranus nor l 'eptune has a Illetalli
layer, slow convection within their inrerior fluids Ill;
enuugh for their global magl1etic fields observed by
One clue to dynamic processes at work inside a
comes, oddly enough, h'om noting how the orbits a
evolve over time. The connection is as follows: A 1110
tional 6e1d creates smaJl, periodic tidal distortions in
its parenr body (as the Moon does 011 Eart11). The I
Figlwe 10. The gravitational attraction of a nearby sateJlite raises
bulges are nor directly "under" the satellite, because
symmetric tidal bulges on a giant planet. Because d1e planet is rotating
planets rotate raster than their major moons orbit t
with respect to the satellite, fluid CLU'rents (red arrows) ccur in its
10). As the bulges slide backward in JOJlgirude to rw
interior. This motion results in a net loss of cncrgy, as dissipated heat,
satellite, they encounter resistJnce and generate "fi-i,
dnt very graduaJly slows the planet's rotation. A fracrion of this
foml of f1uid currents in the planet's interior. Cor
energy also serves to speed up the satellite, thus moving its orbit
displacement or phase lag develops between I\her
outward. (The bulges' size and angular displacement, Or phase lag,
from d1e planet-satellite line are shown much exaggerated.)
should be and where rhey are, and its magnitude is
•
•
•
1
V)
•
•
•
...
∗ Mecanismo de Kelvin-Helmholz: Lenta contracción de los planetas en su fase final y
migración de los materiales más densos hacia el centro. Energı́a gravitacional ⇒ Energı́a
térmica:
GM 2
∆E ∼ − 2 ∆R
R
CTE 1 - clase 13
CHAPTER fOURTEEN
6
Capacidad de un planeta de retener gases
De acuerdo a la teorı́a cinética de los gases, la velocidad cuadrática media de las moléculas gaseosas
de masa µ está dada por:
s
v̄ =
3kT
µ
Por otro lado, la velocidad de escape de un planeta es:
s
ve =
2GMP
RP
Es obvio que la condición mı́nima para que el planeta retenga las moléculas gaseosas es que v̄ < ve.
De esta desigualdad, obtenemos la condición de que la masa del planeta MP debe ser mayor que:
1/2 3k 3/2 T 3/2
2
MP >
4πρP
2G
µ
3
donde ρP es la densidad media del planeta: MP = 4/3πRP
ρP .
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7
∗ También hay que tener en cuenta qué sustancias se encuentran en estado gaseoso a
una cierta temperatura
∗ En realidad, las moléculas gaseosas escapan desde la exósfera, que es la capa más
externa de la atmósfera donde la trayectoria libre media de las moléculas es mayor que las
dimensiones del planeta. Debido a que las moléculas gaseosas no tienen todas la misma
velocidad v̄, sino una distribución de velocidades maxwelliana, siempre habrá moléculas
con velocidades mayores que la de escape del planeta.
CTE 1 - clase 13
8
tmospheres that span
nse air masses. The Componentes principales de algunas atmósferas
ferences. Mercury's is
occasionally hidden
Atmospheric Compositions Compared
cent of Earth is
II
Planet
Molecule
Abundance
Fraction
face of Venus is never
(bars)
Venus
CO 2
N2
rate to the surface. There
s either removes them to
erior.
Ar
H 2O
Earth
N2
E T STATES
°2
ed from similar materi<lls
esses, the atmospheres of
e same gases - though in
ions (Table I). The most
re are nitrogen (N 2 ) and
r more than 98 percent of
re present in the <ltmos­
are not the predominant
n composition appears to
does it dominate the gas
CTE 1 - clase 13
portant role in determin-
AI"
Hp
CO 2
Mal"s
CO 2
N2
Ar
H 2O
86.4
3.2
0.0063
0.009
of total
rtl if I
I IJn I
Ll
0.96
0.035
0.000070
0.000 I00
0.78
0.21
0.01
0.94
0.000355
0.77
0.21
0.01
0.0093
0.00035
0.0062
0.00018
0.00010
3.9xI0-7
0.95
0.027
0.016
0.00006
.l
III
III
I
Jar
In j
\1r~
Table 1. The atmospheres of Earth, Venus, aod Mars contain
many of the s:une gases - hut in very different absolute and
relative abundances. SOUle values are lower limits only, rcllen;'
the past escape of gas to space and other factors.
ra
'J,e~)
i
IUl
I J I
mOll III
.11 lUll
9
Distintos tipos de atmósferas planetarias
Paisajes planetarios de Mercurio, Venus, Tierra y Marte con sus diferentes atmósferas.
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10
Visión desde la superficie marciana
CTE 1 - clase 13
11
r studies is to understand what the his­
f gas has been, and what the atmos­
e throughout time,
, the gases help to determine the sur­
planet. In their absence, the tempera­
alance between sunlight absorbed at
(heat) energy emitted by the surface
solar energy that is absorbed, or the
Sun, rhe hotter the surface will be, If
mosphere, this energy balance would
out 255 0 1<.. This is about 30 0 Kless
ace temperatu re and 18 0 K below the
bands) that the other planets do not share. In addition, the upper
atmospheres of Venus and Earth exhibit a striki.ug diurnal cycle (day· night
pairs of ClIl'\'es). Arrows indicate the cooler surface temperamres that would
occur in the absence of any greenhouse warming.
Distribución de la energı́a solar incidente
Solar
radiation
100%
AU:teclto •
16
7 = 33%
Reflected to
5" ce (2 )
S"attered by
clouds (47)
r, absorb infrared energy emi tted by
ly and thereby trap it in the atmosually is lost back to space, but nor as
e ;][mospheric temperature increases,
t similar to what happens in a green­
very easily, but heat has a difficult time
erature rises, The planetary-scale ver­
ed the greenhouse effect, Carbon diox­
e Earth's atmosphere to retain heat,
re, The atmosphere of Venus contains
of CO 2 - 260,000 times more than
ere. Consequently, CO 2 (and other
mperature of Venus to about 750 0 K,
odlerwise be, By contrast, the Mart­
Ol1.ly 6 millibars of CO 2 and very small
As a result, greenhol1se warming
raises dle average surface temperature
).
CTE 1 - clase 13
Reflected
to space
by air and
by ground
Absorbed in
atmosphere
(22%)
Scattered to
ground (21)
Absorbed by
ground (24)
Figure 3. The interaction of sunlight with the atmospheric gases, clouds,
and surface of Earth has many avenues. Ultimately, about one-third of
t.he solar energy reaching our globe is reflected back to space.
ATMOSPHERES
or TlfE TERRESTRIAL PLANETS
12
Condiciones fı́sicas de las atmósferas
Equilibrio hidrostático:
dP = −gρdh
(1)
h: altura sobre el nivel del mar; g: aceleración de la gravedad.
Ecuación de estado de un gas ideal:
P = ρ kT
µ
(2)
µ: masa de las moleculas gaseosas.
Si asumimos ρ, g y T constantes, podemos hallar la altura de escala H de la atmósfera.
De las ecuaciones (1) y (2) obtenemos:
P = +gρH
=⇒
kT
H=
µg
Ejemplo para la atmósfera terrestre: Altura de escala del H2O (µ = 18 × 1, 67 × 10−24
g), asumimiendo una temperatura T = 270 K
=⇒ H = 12, 6 km.
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13
Disminución de la densidad con la altura
Si diferenciamos la ecuación de estado de los gases ideales obtenemos:
kT
dρ
µ
Combinando esta ecuación con la de equilibrio hidrostático nos queda:
dP =
kT
dρ
−gρdh =
µ
dρ
µg
dh
= − dh = −
ρ
kT
H
Z
ρ
ρo
dρ
=−
ρ
ln
CTE 1 - clase 13
Z
h
0
dh
H
ρ
h
=−
ρo
H
14
=⇒
ρ = ρoe−h/H
Ejemplo: ¿Cuánto disminuye la concentración de H2O a 20 km de altura en la
atmósfera de la Tierra?
ρ
= e−20/12,6 = 0, 20
ρo
¿Y a 40 km?
ρ
= e−40/12,6 = 0, 04
ρo
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15
Caso de la atmósfera de la Tierra: perfil de temperaturas
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16
pushed
e CO 2
back to
ch as at
natubo
f CO 2
resen ts
nd loss
Reciclado de gases entre la atmósfera y la corteza
CO 2 in the
atmosphere
I
Infusion
into ocean
I
,
Formation
of sediments
~
\
Chemical weathering
Of~
Release by
voIcanrsn1
atures,
xide is
wed to
or sed­
h make
s), car­
mply by
Figure 16. On Earth, carbon dioxide cycles between land and sky.
with a
Gaseous COl is removed from rhe atmosphere to form sea-floor
CTE 1 - clase 13
y, cold
sediments; plate tectonism C<luses these sediments to be subducted
)
17
Atmósferas de Venus y Marte: perfiles de temperaturas y presiones
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18
Perfiles de temperaturas de los planetas jovianos
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19
Titán: La misión Cassini-Huygens
∗ Composición de la atmósfera (tropósfera): N2 (95%), CH4 (4,9%), trazas de otros
hidrocarburos, p.ej. etano, propano, acetileno, y otros gases como el cianuro de hidrógeno
(HCN), cianoacetileno, etc.
∗ Presión superficial: 1,45 atm.
Trayectoria de la nave Cassini-Huygens.
CTE 1 - clase 13
Maniobra de descenso del módulo Huygens
en la superficie de Titán.
20
La atmósfera de Titán: perfil de temperaturas y presiones
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21
Cuerpos con atmósferas tenues: Casos de Io, Tritón y Plutón
Composición:
dióxido de azufre (SO2),
monóxido de azufre (SO),
cloruro de sodio (NaCl)
Presión superficial:
0, 3 − 3 × 10−9 atm., hasta
40 × 10−9 atm. cerca de
plumas volcánicas.
Superfie de Io reciclada.
Se observa el volcán activo
Prometeo (disco oscuro a la
izquierda del centro.
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22
La tenue atmósfera de Tritón
∗ Composición: nitrógeno (N2), trazas de metano (CH4)
∗ Presión superficial: 1, 4 × 10−5 atm.
∗ Tritón es geológicamente activo: muestra geysers potenciados por la débil radiación
solar que vaporiza el nitrógeno sólido del subsuelo.
Zona polar de Tritón mostrando una escarcha
de nitrógeno congelado altamente reflector.
(albedo = 0,76). Radio = 1353 km
CTE 1 - clase 13
Nubes tenues asoman sobre el horizonte
de Tritón.
23
El caso de Plutón
∗ Composición: nitrógeno (N2), metano (CH4), monóxido de carbono (CO).
∗ Presión superficial: 6, 5 − 24 × 10−6 atm.
El alto albedo de Plutón (0,49-0,66) también sugiere una atmósfera tenue que condensa
formando una escarcha altamente reflectora.
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24