Comets, other solar systems, origins of life, our grand conclusion

In our last installment, we turn to "everything else that we haven't already discussed." Basically, comets are the moles of the solar system: they can turn up anywhere, from near the atmosphere of the sun to the farthest regions. Knowledge of comets will complete our "inventory" & then we will turn to the synthesis of it all and go for the "big picture". Already, more than 30 other stars are known to have planets in orbit about them, and so far our system is still unique. How does life form and what allows it to "stick" on a particular planet?

Let's start with

comets

more small leftovers of solar system formation
what is in your housemates' refrigerator??
what can it tell you about them?
rocky bodies are asteroids, primarily inner (hotter) solar nebula
icy bodies are comets, from the colder nebula, mostly remain outer SS

Cometary Orbits

seemed to be 2 populations of these things
short-period more inclined but still in plane
long-period from anywhere they liked -- seemed spherical
Orbital elements exercise -- pick out populations easily
short period: Halley-family P=20-200 yr, Jupiter-family P<=20
LPC have larger inclinations
q is diagnostic of behavior
also hard to determine semi-major axis
if "bound" must be ellipse, otherwise escapes, though may be hard to tell
spherical Oort cloud
40000-100000 AU
likely ejected by giant planets during planetesimal phase
Kuiper belt
from Neptune 30 - 10000 AU
some objects discovered -- 270 as of today
as orbits change, discovered or "recovered"
interaction in inner solar system (esp. Jupiter), change, throw out
loss of matter affects orbit, too

"Anatomy" of a comet -- the nucleus is the heart of it all

NUCLEUS
small (<10km usually), irregularly shaped
heterogeneous mix of frozen gases (ice) & "grains"
distant comets as much as 300 km (KBO's, compare to Pluto)
"fragile" meaning easily torn apart, very porous
chunk of ice, 3-10 K, too cold to see, little gravity
COMA (100,000 km)
ices sublime & form an escaping "atmosphere"
not uniform -- from individual "vents"
appears within 5 AU of sun, although depends on the gas
takes dust too (slower moving -- recall vmean depends on mass)
we must infer the nuclear comp from the coma (bane & blessing)
photodestruction, ionization, chemical reactions
TAILS (few x 10⁶ km)
radiation pushes gas tail opposite sun, some ionized
ion tail always radial to Sun
dust tail curves behind in orbit
shape depends on Force/mass ratio on particle

scale
Hyakutake=UMass campus; Hale-Bopp = Pioneer Valley
coma -- halfway to the moon
tail 0.1 AU

grain of rice, size of a house, about a mile (to UMass)
large building (Grad Tower), to Washington DC, 8 Earths side-by-side

Gas composition derived from spectroscopy
products (daughters)
native molecules (parents, or mothers)
Dust composition -- 2 kinds
silicate
CHON particles
Gas relative abundances fairly uniform from one to another
Gas/Dust ratio is highly variable

"primitive" C, O, N
T>20 K to deplete N
T<50 K to retain everything else

Viewing comets is tricky business

to the naked eye, comets appear fuzzy
photos allow more impressive tail (note stars trail) -- incredible vacuum
predictions depend on brightness increases (not accurate)
viewing geometry effects ("shortened" tails)

What have we learned?

populations:
Oort cloud (10¹¹ comets) from outer SS
Kuiper Belt probably indigenous, with much influence from Neptune
change in orbit can split comets as well as perturb
Comet Shoemaker-Levy 9 passed 25000 vs 119000 Roche limit of Jupiter
comets do hit planets !
compute "historical" and "future" orbit -- less than radius of Jupiter??!
at least 20 fragments collided July 16-22 1994
SL-9 (1994), dinosaurs/iridium layer (60 Myr)
low tensile strength?
Comet Shoemaker-Levy 9 ripped apart, Mathilde still exists
is tensile strength the only difference between comets & asteroids?
as comets "age" (lose the ices), they become "rockier"
SL9 has dust but no appreciable gas -- "old" comet?
might some of the NEO's be "dead comets"?
can contribute rich array of molecules, some organic
chemistry in the coma complicates relation back to solar nebula, BUT
inventory similar to ISM, and maybe some ices are preserved as such?
formation on grain surfaces more difficult in protosolar nebula
good old isotope ratios to tell us

Now to spring from there to

Life as we know it...

requires energy
is comprised of organic material -- H, C, O, and N (most abundant)
probably needs water
needs other elements, too -- probably N, P, S

is all the same & all different
20 amino acids, 5 nucleotides
RNA, then DNA

reproduces itself in some way
can evolve by mutation or selection

Earth and life are inseparable, life alters the environment

What factors influence the existence of life? (create a list)

source of organic material
presence of atmosphere
moderate temperatures
presence of liquid water
stable conditions
time to develop
catastrophic events
life in extremes -- bottom of ocean, km's under surface
to wipe it ALL out, have to go deep

Life was established within 1 billion years of Earth's formation.

• initial proto-Earth was very hot and bombarded
  large bombardments could vaporize all liquid & "sterilize"
  probably no life until end of heavy bombardment 3.8 Gyr ago

  microorganisms developed early, but that was it for another 2 Gyr

  isotopic evidence in sediments as early as 3.9 Gyr

  direct fossil evidence 3.5 Gyr

  Rise of oxygen about 2.2 Gyr ago

  Multicellular life around 1.4 Gyr ago

  600 Myr ago, animals

Unlikely scenarios:

Extraterrestrial origin (formed elsewhere & was transported)
interstellar medium or another planet
random or "directed"
local inorganics (clay organisms) evolved into organics

in-situ formation
Miller-Urey experiment took gases & zapped them, formed amino acids
taken as evidence that organics could arise spontaneously
transport from elsewhere (beyond frost line in organic ices)
comets & asteroids
cometary or interstellar dust
sources of energy -- autotrophs & heterotrophs
auto: chemical or solar energy utilized (non-organic)
hetero: consume organic matter for energy
Pieces of evidence:
fossils of course -- 3.5 Gyr
isotopes in sediments (photosynthesis prefers ¹²C) 3.9 Gyr

At the end, we look back:

1. Energy is clearly available & sufficient in lots of places
2. Carbon is in lots of places, and in many forms (comets & ISM)
3. Other elements are also there (comets & ISM, asteroids, meteorites)
4. Water is the "waterloo"?

Extrasolar planetary systems

Detection mechanisms
direct imaging -- see the planet itself (IR excess)
astrometric detection -- see motion of star on sky
photometry -- change in brightness
occultation -- watching for planet to go in front of star
*** radial velocity (from spectra) -- motion of star ***

more than 30 planets
the smaller, the more (Jupiter, Saturn)

big planets REALLY close to the star
high-eccentricity