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
- 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
- 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
- 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
Composition in the context of formation: Volatile & Refractory
- 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
- 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
- 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?
- 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)
An outline of life origin on Earth:
- 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.
Ok, fine, but where/how did it start in the first place:
- 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
More likely is a terrestrial organic origin:
- 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
At the end, we look back:
- Energy is clearly available & sufficient in lots of places
- Carbon is in lots of places, and in many forms (comets & ISM)
- Other elements are also there (comets & ISM, asteroids, meteorites)
- 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 ***
see also: Mass Radius Plot
- more than 30 planets
- the smaller, the more (Jupiter, Saturn)
- big planets REALLY close to the star