Effect of Rotation Rate onTerrestrial Planet Characterization

James Cho and Sara Seager(DTM, Carnegie Institution of Washington)

• Soon, Earth-like extrasolar planets will be detected and characterized. TPF will even want to ascertain habitability on them by looking for biomarkers. 

• The preferred spectral bands for characterization are based on Earth's atmospheric signature with present-day attributes (climate, circulation, etc.).

• Planets with eccentric orbits that temporarily leave the habitable zone, high obliquities, or tidally-locked (or very slow rotation rates) could still have suitable temperatures for liquid water to stably exist. 

• The fraction and pattern of cloud cover will differ on exo-Earths with different circulations (due to differences in stellar flux, planet size, rotation rate, energy partition, topography, land-ocean-ice fraction, etc.).

• Spectral line strengths will be definitely different, making interpretations difficult.

So how can we attack this problem?

INTRODUCTION

APPROACH

• We are using a state-of-the-art general circulation model (CCM3)– to study the coupling to land/ocean/ice, orbital parameters, and expected signatures– model is extensively used (and tested) for Earth’s weather and climate studies– solves the primitive equations (nonlinear hydro equations) in full spherical geometry– 18 active atmospheric layers and 4 active land layers in the vertical direction.

• Running multi-centennial (< 1000 yr) climate simulations at 128x64x18+4 (T42) resolution and multi-decadal runs at T85 resolution.

• A full parameter space exploration with varying land/ocean configuration and orbital conditions is currently underway, including for giant planets.

• This work will augment the definition of the habitable zone and help to redefine the strength and depth of spectral signatures to be characterized. In this talk, we present effects of planetary rotation rate on characterization.

TALE OF TWO EARTHS (main dynamical result)

NET RADIATION AT TOP OF ATMOSPHERE:

• Two rotation rates: Period = 24 and 6 hrs.

• Net Rad = (Down SW Flux) – (Up LW Flux)

• In general, equator gains heat and poleloses heat in the net.

• Faster rotating earth leads to:

- Reduced global net cooling (implied more vigorous motion)

- Smaller equator-to-pole net flux gradient and corresponding reduced N-S transport

Rotation is a key controlling parameter!

P = 1 DAY REGULAR EARTH

FAST EARTHP = 0.25 DAY

JET STREAMS

• Arrows indicate jet locations and widths.

• Faster rotation (purple) leads to greaternumber of (narrower) jets from pole to pole.

• Equatorial jet, associatedwith Hadley circulationnarrows with rotationrate.

ORIGIN OF JETS

• Stratification movement is nearly

horizontal

• Rotation flows line up vertically

(“barotropic”)

• Spherical geometry variable rotation strength

over the surface

Planetary atmosphere is a stably-stratified rotating fluid,which is highly turbulent and anisotropic.

Log

(Ene

rgy)

2

6

1

Log (Wavenumber, n)10

-4

cooling

eddiesjets

CLOUD DISTRIBUTION (0 to 1)

P = 100 days P = 4 days

P = 1 day P = 0.25 day

CLOUD PHYSICS

• Clouds reflect SW visible radiation coming down from above and traps LW IR radiation coming up from below. Net radiation drives atmospheric motion, which in turn redistributes the clouds.

• Generally tall, narrow convective clouds are located in the tropics and short, wide stratus clouds in the extratropics.

• Moist adiabatic adjustment (convective clouds) and conditional diagnostics (large-scale clouds) -- based on relative humidity, temperature variability, and altitude -- used in the GCM to form new clouds.

• Present day Earth cloud distribution reasonably well-modeled in the gross sense;

however, microphysics still not well-understood, as well as its feedback on large-scale dynamics.

• Cloud distributions shown are in each case self-consistent with net flux, OLR, and surface temperature distributions, giving good confidence of correctness.

“TWO-FACED SPECTRA” FOR TWO EARTHS: (P = 1 and 100 day)

EQUATOR VIEW NORTH POLE VIEW

ABSORPTION COMPARISON (POLE VIEW)

• Clouds muffle absorption lines, hence abundance and temperatureassessment becomes difficult.

• Viewing orientation of the planet is very important.

SURFACE TEMPERATURE (220 K to 310K)

P = 100 days P = 4 days

P = 1 day P = 0.25 day

CURRENT AND FUTURE WORK

• Complete parameter space exploration (e.g., distance from star, eccentricity, obliquity, density, size of planet, thermal structure, composition, etc.) underway:

– “Aqua-planet”, to eliminate orographic effects (temperature gradients and planetary/gravity waves).

– “Exo-Pangaea”, one and two, placed at various locations on the planet

– Accurate, high-resolution spectra from resulting temperature and species distribution are generated. 

– More realistic ocean coupling for > 1000 year calculations will be included.

SUMMARY

• Atmospheric circulation have NOT been explored and is required for efforts to characterize Earth-sized extrasolar planets and ascertain their habitability. 

• Simple1-D, homogeneous modeling will not be adequate since signature is dominated by localized regions on the globe. This is a generic feature.

• These calculations show orbital parameters (e.g., rotation rate) acutely influence key fields, such as the cloud distribution and surface temperature.

• This shows extrasolar giant planets, which is currently being put through similar

theoretical scrutiny, should also exhibit same type of behavior.

• Very large number of parameters and effects, not necessarily typically of Solar System planets, interact to produce very complex behaviors. Well-designed and analyzed simulations should give robust, enlightening, and perhaps “surprising”, planet characteristics.

(Cho & Seager, in prep.)