Venus’ lack of water
The Runaway Greenhouse
Earth’s gain of O2
Evolution according to the geological record
Titan’s dwindling CH4
What’s happening here.
Venus formed closer to the Sun, where H2O was
depleted compared to the building blocks of Earth.
Venus was warm enough to catastrophically loose all
it’s water in a Runaway Greenhouse event.*
* This further suggests that there is a tipping point for which
atmospheres can irreversibly reach an entirely diﬀerent state.
Convection involves the rise of hot air, which is more buoyant than the overlying air.
This occurs fast enough that little heat is exchanged and the process is adiabatic.
Assume ideal gas: (1)
Include (2): (3)
First law of thermodynamics:
For adiabatic motion (dq=0):
And hydrostatic equilibrium:
(radiative equilibrium) Optical depth due to vapor alone
Dry lapse rate
Flux not conserved
Assume hydrostatic equilibrium:
Write condensable mass density:
Assume grey atmosphere:
H is the humidity, Ps the saturation pressure, Pv is the
partial pressure of the vapor , k the absorption
coefficient, mv and m the mean molecular weights of the
vapor and the atmosphere, and g the gravity
Optical depth at Trc
Consider: K= 0.1 cm2/g (appropriate for 8-20 um window
H = 100%
m = 29 g (N2) and mv = 18 g (H2O)
g = 10 m/s
Po = 8 mm Hg
Fmax = 0.63 cal cm-2 min-1
T = 260 K
Average incident sunlight on Earth is 0.5 cal cm-2 min-1
Average incident sunlight on Earth is 0.9 cal cm-2 min-1
But the albedos of Earth is 0.3 and for Venus is 0.78
So… both Venus and Earth absorb 0.3
They are subcritical.
But what if Venus was not as cloudy as in the past
Note: runaway greenhouses grow from below
Atmosphere is transparent at short wavelengths
This prohibits vast cloud cover above the Trc level
Surface has a large condensable reservoir
When vaporized the condensable is optically thick at
However, this study indicates that an atmosphere can reach a tipping point,
beyond which a planet irreversibly ends up in a radically diﬀerent state.
Titan Ganymede Callisto
R=2575 km R=2631 km R=2410 km
d=1.88 gm/cm3 d=1.93 gm/cm3 d=1.83 gm/cm3
Formed from diﬀerent ices, perhaps with more carbon?
Acquired an atmosphere but lost it through impacts. (Griﬃth & Zahnle 1995)
How can a molecule or atom transition between 2 states?
LTE: the occupation is set by collisions and follows a Boltzmann distribution
Non-LTE: spontaneous and stimulated emission can depopulate states
Lunine, Stevenson and Yung 1982, Sagan & Dermott 1982
The atmosphere has the equivalent of 5 m of methane. It is close to saturation.
Just running out of methane after billions of years
Idea: CH4 outgassed early on, heated e.g. by accretion
Consequence: the surface has 0.5 km of organics*
Has a recent bout of geological activity & outgassing
Idea: Titan’s interior is freezing now, which circularizes the orbit
Advantage: Explains Titan’s non-circular orbit
Consequence: there are not a lot of organics coating the surface
Is in some sort of equilibrium with subsurface CH4
Idea: Subsurface aquifers or methanogens balance CH4 loss
Consequence: There’s more than meets the eye.
* The byproducts of CH4 & N2 photolysis
PCA analysis of Cassini Data
Griﬃth, in preparation
Composition of abundant elements likely established by
thermochemical equilibrium because hot and “surfaceless”
Secondary molecules aﬀected by photochemistry
Complications: magnetospheric eﬀects on atmospheric structure
and chemistry, disentangling host star & planet signals, and eﬀects
of clouds on measurements of gas abundances and temporature
If hot, potentially somewhat in thermochemical equilibrium, but
surface interaction/eﬀects are inevitable.
Complicated non-equilibrium eﬀects inevitable and perhaps most
In situ measurements of V, E, M, J, A, T
Surface, Atmosphere, Magnetosphere, Ionosphere,
Spatially resolved observations (e.g. 1 meter by HiRISE)
Detailed information of the Sun’s attributes too
Greater breadth of conditions to test processes
E.g. The C/O ratio in giant planets
More photons (TMT)
More spectral coverage (JWST)
But not spatially resolved, nor in situ… or not…
A = Asteroid (Bennu, a carbonaceous asteroid) Mission: OSIRIS-Rex (University of Arizona)
The StarChip can be mass-produced at the cost of an iPhone and be sent on
missions in large numbers to provide redundancy and coverage
Light beam to propel gram-scale ‘nanocrafts’ to 20% speed of light could reach
Alpha Centauri (4.37 light years) within about 20 years of its launch.
StarChip: cameras, photon thrusters, power supply, navigation and communication
equipment, and constituting a fully functional space probe.
Lightsail: Advances in nanotechnology are producing increasingly thin and light-
weight metamaterials, promising to enable the fabrication of meter-scale sails no
more than a few hundred atoms thick and at gram-scale mass.
2. Light Beamer
The rising power and falling cost of lasers, consistent with Moore’s law, lead to
signiﬁcant advances in light beaming technology. Meanwhile, phased arrays of
lasers (the ‘light beamer’) could potentially be scaled up to the 100 gigawatt level.
To be lead by Pete Worden, the former director of NASA AMES Research Center