From NEOs to the Planet(s) X

From NEOs
to the Planet(s) X
Nuno Peixinho
THE BRUSH, HURRY
UP! A SHOOTING
STAR IS COMING!
Nilus: Origone / Bertrand Editores
We all love to talk
about catastrophes!
Lets start with the Moon
Moon (Luna)
Date of discovery: maybe around 4000 BC when Adam looked into the sky for the first time
• Just by looking at it we see that impacts by minor bodies are or were frequent.
• First image of the “far/dark side” on October 7, 1959, by Luna 3.
• The crater asymmetry between each side is still a source of debate.
Complex Crater
What do we see?
• Large number of impact craters:
• Simple craters: D<10-15 km.
• Complex craters: D>10-15 km.
• Basins: D>200-300 km.
• Even recent volcanic deposits:
• 50-100 million years ago!
• Note that volcanism presumably
Basin
Simple Crater
ended 1-1.5 thousand of million
years ago!
Volcanic deposits
[Lunar Reconnaissance Orbiter, 2014]
What about on Earth?
Meteoroids and meteor showers
• Comets release dust during their close passages to the Sun. Once entering Earth’s atmosphere,
that dust creates meteor showers.
Meteoroids and meteor showers
• Comets release dust during their close passages to the Sun. Once entering Earth’s atmosphere,
that dust creates meteor showers.
How fast do they travel?
• Meteors(oids) close to Earth travel at a speed of 10 to 70 km/s.
• On a planet/body without atmosphere nothing stops it before hitting the groud.
How much energy do they carry?
• Energy of 1 kg at 30 km/s: E = 1/2 mv
• Energy of 1 kg of TNT: E = 4.7 MJ
• Energy of 1 kg of dynamite: E = 7.5 MJ
c
2=
450 Mega Joule
Why small objects do not affect us?
• On Earth, the atmosphere creates a strong aerodynamic drag:
It’s not friction
that does it!
• that drag tends to stop them, since they lose energy;
• the strong ram pressure on them tends to break them apart;
• being supersonic, the shock-wave heats them up to 25 000 ºC, vaporizing them.
• What is the biggest thing our atmosphere can “stop”?
• Roughly (very roughly!), when the air column beneath the object equals it’s mass.
Meteor volume: Vm= 4/3 π a3
a
ρ
Meteor mass: Mm = 4/3 π a3 ρ
a
H
Air column volume: Vair = π a2 H
ρo
Air column mass: Mair = π a2 H ρo
Thanks to Jewitt and his great courses!
So:
π a2 ρo H≈4∕3 π a3 ρ
a≈ρo H∕ρ
If, on Earth:
H =10 km (atmosphere thickness)
ρo =1kg/m3 (air density)
and for the meteor:
ρ =3000 kg/m3 (rock density)
We have:
ρo
a≈3 m
The atmosphere protects us
from meteors with a few
meters diameter.
Thanks to Jewitt and his great courses!
What happens if the object is bigger?
• Luckily, the ram pressure is so strong that it usually breaks apart.
• Typically, only objects larger than 30-50 m will hit the ground creating big damage.
Pancake formation
Ram pressure
fragmentation
Spreading
How do they fragment themselves?
• The objects is subjected to a huge ram pressure:
Pram
• Examples:
F
⇡ a2 ⇢ v 2
2
=
=
=
⇢
v
A
⇡ a2
Football:
Pram≈1kg/m3 x 40 km/h ≈1kg/m3 x 10 m/s = 10 N/m2
Cosmic impact:
Pram≈1kg/m3 x 35000 km/h ≈1kg/m3 x 10000 m/s = 1000 bar
• If the pressure is larger than the cohesion strength… the object breaks.
v
How high do they break apart? (I)
• Atmospheric density diminishes with height:
⇢(h) = ⇢o e
H
h
H
h
• When the ram pressure P
ram
equal the cohesion strength S the body breaks:
Pram (h) = ⇢(h) v = ⇢o e
2
) h = H ln
✓
⇢o v
S
2
◆
h
H
v2 = S
How high do they break apart? (II)
• Examples:
Iron meteor
S =108 N/m2
v = 10 km/s = 104 m/s
ρo = 1kg/m3
h = H ln (1x108/108) = H ln (1) = 0
Ground burst!
Comet fall
S =104 N/m2
v = 10 km/s = 104 m/s
ρo = 1kg/m3
h = H ln (1x108/104) = H ln (104) = 100 km
Very high burst!
What about the crater?
• Knowing the kinetic energy of the meteor we can make a “guesstimate” of the crater size:
1
Ec = mm v 2
✓2
◆
1 4
=
a3 ⇥m v 2
2 3
r
Vesf era =
4
3
Ep = me g r
✓4
◆
3
r
⇥
e
= 3
gr
2
r
r3
• Supposing all the kinetic energy is used by the ejecta moving against gravity at a
hight r:
2
3
⇥m a3 v 2 =
2
3
vs
u
u ⇥ v 2 a3
m
4
⇥e g r ) r = t
⇥e g
What about the crater?
• Knowing the kinetic energy of the meteor we can make a “guesstimate” of the crater size:
2
3
⇥m a3 v 2 =
2
3
vs
u
u ⇥ v 2 a3
m
4
⇥e g r ) r = t
⇥e g
Example
a =1000 m
v = 30 km/s = 30000 m/s
ρm = ρe = 3000 kg/m3
g = 9.8 m/s2
r ≈ √√1017 ≈17 km
What is the danger after all?
•
Objects of 5-10 m diameter: once a year.
•
Objects of 50 m diameter: once every 1000 years.
•
Objects of 1 km diameter: once every half million years.
•
Objects of 5 km diameter: once every 10 million years.
Big events I
•
65 million years ago a 10 km object hit Earth, in Yucatan, Mexico, leading to the mass
extinction of the Cretaceous period.
•
Note: that might be the most spread explanation, but it is not the only one.
Big events II
•
In 1908, an object of unknown size “exploded” in Tunguska, Russia, devastating an
area of 2000 km2: maybe 60-190 m bursting at 5-10 km altitude.
Where do they
come from?
Mostly from the (Main) Asteroid Belt
• Discovered by Piazzi in 1801.
• Mostly between Mars and Jupiter.
• More than 700 000 have orbits well
established.
•M
total
∼ 3.5 x 1021 kg (∼0.0006 M⊕).
• Those with perihelia between 0.983
and 1.017 AU are called Near-Earth
Asteroids (NEAs) or Near-Earth
Objects (NEOs).
• Note: the subtlety with the NEO/NEA
thing is that a dead comet may be a
NEO but not a NEA.
Asteroid families and types
• Lots of dynamical families:
• Flora, Hungaria, Cybele, Eos, Pallas, Koronis, Hilda, Themis…
• Non-uniform distribution:
• Kirkwood gaps!
DeMeo & Carry 2014
Mars
Earth
Jupiter
Asteroid families and types
• Main composition types:
• C - Carbonaceous (75%)
• S - Stony (17%)
• M - Metallic
• Note I: the classification of asteroids is currently very complex.
• Note II: the link between families of meteorites and families of asteroids is not well established!
Orbital resonances and Kirkwood gaps
• Two bodies are in an orbital resonance if the ratio between their orbital periods
equals the ration between two integers.
It might be stable:
It might be unstable:
Orbital resonances and Kirkwood gaps
• Two bodies are in an orbital resonance if the ratio between their orbital periods
equals the ration between two integers.
orbital period / yr
number of asteroids per 0.01 AU
2.5
3.0
3.5
4.0
4.5
5
6
7 8 9 10 12
50
7:3
3 :1
5:2
2:1
25
0
1.9
2.1
2.3
2.5
2.7 2.9 3.1 3.3
4.0
5.0
semimajor axis / AU
© The Open University
Orbital resonances and Kirkwood gaps
• Two bodies are in an orbital resonance if the ratio between their orbital periods
equals the ration between two integers.
Chaos!
To mess with our heads: comets among asteroids
• In 2006, Hsieh & Jewitt, discover 3 comets
among asteroids.
• These “Main Belt Comets” (MBCs) or
“Active Asteroids” (AA) raised new
questions:
• How did their ice survived for so long?
• If they were captured comets how to
explain their too circular orbits?
• Are they a lost source of Earth’s water?
• Bottom line: sooner or later we will have a
fight about the definition of asteroid.
Hsieh & Jewitt 2006
Trojan asteroids
• Lagrange points - in the three body problem
there are five points where a small mass will
not feel any net force:
• L , L and L are aligned with the two
masses m and m (unstable points);
• L and L are 60° ahead and behind the
1
2
3
1
4
2
5
mass m2, respectively (stable points).
• Jupiter has more than 6000 Trojans.
• Uranus has 1.
• Neptune has 9.
• Mars has 7
• The Earth has 1 in L4 (2010TK7)
Centaurs
And I never finished
my PhD!
• In 1977, Kowall, discovers the first Centaur: 1977UB (2060 Chiron)
• Chiron was visible in photographic plates dating back to 1895!
“Planet-X”
Pluto
• Apparently, another planet was needed to explain all
the orbital perturbations on Uranus. Neptune was not
enough.
• Beginning of 20th Century, Persival Lowell searched
for “Planet-X” without success,
• In 1930, Clyde Tombaugh finds Pluto, but quite far
from the predicted region and too small for those
needed perturbations. He kept searching for the “real”
Planet-X for 13 more years..
• In 1993, Standish, demonstrates that the alleged
perturbations were just… errors:
• Pluto was discovered by persistence and
luck.
How I hate
theoretical
astronomers!
Pluto
Pluto, Charon, Nix, Hydra, Kerberos and Styx
• In 2005, Canup demonstrated that
the Pluto-Charon system was
formed by a collision.
• In 2005, two more satellites were
discovered, and two more in 2011
and 2012:
Nix
Hydra
Kerberos
Styx
•
•
•
•
Better than a thousand words…
Kuiper Belt
Kuiper belt objects / Trans-Neptunian objects
Some are big, but most are small.
Nix
Namaka
Hydra
Styx
Kerberos
Haumea
Hi’iaka
Pluto
Charon
Weywot
Makemake
Quaoar
Sedna
Dysnomia
Eris
1000 km
3000 km
Moon
Earth
There are also many families
• Note I: there are no strict definitions for each dynamical family
• Note II: a good taxonomy based on surface composition is one of the main subbjets of study
And in 2019… 2014MU69
“Planet nine” and others
The “tenth planets” and the “end” of Pluto
• Since the discovery of the Kuiper belt, Pluto situation became threaten.
• 14 November 2003, the discovery of the “first” 10th planet in announced: Sedna (2003VB
• 27 July 2005, the “second” 10th planet is announced: Haumea (2003EL )
• 29 July 2005, the “third” 10th planet is announced: Eris (2003UB )
• First measures indicated Eris as being larger than Pluto and the battle was on!
• None of them is now classified as planet!
• Eris is actually a little smaller than Pluto.
61
313
12)
The hypothesis of “planet nine”
• In 2008, Patryk Lykawka, and Tadashi Mukai argue for the existence of a trans-Neptunian planet…
but nobody cares because they are not US Americans.
• In 2014, Chad Trujillo and Scott S. Sheppard discuss the possible existence of a massive transNeptunian planet
• In 2016, Konstantin
Batygin and Mike E.
Brown make a better
argument for the
existence of a massive
trans-Neptunian planet,
now nicknamed “planet
nine”.
•
The 8 m Subaru
telescope will be
used to attempt to
detect it.
P = 10 000 – 20 000 yr
q = 200 AU
Q =1200 AU
i = 30°
Oort Cloud
The Oort cloud
• In 1950, Oort argued for the existence of a far away “cloud”, around the solar system that should
be the source of all comets (Öpik already had suggested in 1932).
• Made by the large number of
objects ejected by the giant planets
• The cloud should be between the 10
000 and 100 000 AU (∼1 light-year).
• Maybe there is an “inner Oort
cloud” slightly decoupled from the
rest (Sedna and 2012VP113 are there).
• Nearby start may push those
objects so they become comets in the
inner solar system.
• Number of comets ∼ 1012
The Oort cloud
• In 1950, Oort argued for the existence of a far away “cloud”, around the solar system that should
be the source of all comets (Öpik already had suggested in 1932).
• Made by the large number of
objects ejected by the giant planets
• The cloud should be between the 10
000 and 100 000 AU (∼1 light-year).
• Maybe there is an “inner Oort
cloud” slightly decoupled from the
rest (Sedna and 2012VP113 are there).
• Nearby start may push those
objects so they become comets in the
inner solar system.
• Number of comets ∼ 1012
How did they
get there?
Migration
PAST MIGRATION AND MIXING
Contamination
by comets?
5 AU
Ejection
15 AU
30 AU
48 AU
30 AU
48 AU
Mixing
Exogenous
water?
PRESENTLY
NEOs
Shooting
stars
5 AU
Ejection
Jupiter
Neptune
Asteroid belt
Centaurs
Comets
Kuiper belt
How do they
evolve?
Surface evolution
MODELING
EXPERIMENTS
Reddening
Ice and dust
Energetic particles
bombardment
Collision
Collisions
Complete
resurfacing
Cometary
activity
Crust
formation
Thompson et al. 1987
...
Kaňuchová et al. 2012
Temporary
atmosphere
Luu & Jewitt 1996
...
Delsanti et al. 2004
Internal evolution
Cooling time-scale
τ∼r2/K
Kwater∼10-6 m2s-1 [thermal diffusivity]
Potato (r=3 cm): τ∼1000 s ∼15 min
During the solar system lifetime
τSS∼1017 s
r∼300 km
Internal evolution
• However... there might be
radiogenic heating by 26Al.
• Liquid water was possible for 5
Myr even in the smaller objects
(D∼20-60 km).
• Would that be enough to create
bio-organic reactions?
Merk & Prialnik 2006
How do we study
these things?
Surface photometry / spectroscopy
Methanol ice
Albédo Géométrique
Water ice
Pholus and model
Spectroscopy
Wavelength (μm)
Adapted from Cruikshank et al. 1998
Surface photometry / spectroscopy
Methanol ice
Albédo Géométrique
Water ice
Normalized reflectivity
Color photometry
J
V
R
K
Pholus and model
Red
B
H
Spectroscopy
I
Neutral
Wavelength (μm)
Wavelength (Å)
Adapted from Cruikshank et al. 1998
Surface colors
Photometry
A non-spherical rotating body reflecting the sunlight will create a lightcurve.
Δm=2.5log(a/b)
Prot
We can determine the rotation period and estimate the asymmetry.
Photometry
A non-spherical rotating body reflecting the sunlight will create a lightcurve.
Pcritico =
3
G⇥
Stellar occultations
2003: detetam-se alterações na
atmosfera de N2 de Plutão.
2006: setetam-se os primeiros KBOs
hectométricos por ocultação estelar.
Stellar occultations
2014: a ring system is detected around Chariklo.
Questions?