60 research outputs found
Effects of Ozone Levels on Climate Through Earth History
Molecular oxygen in our atmosphere has increased from less than a part per million in the Archean Eon, to a fraction of a percent in the Proterozoic, and finally to modern levels during the Phanerozoic. While oxygen itself has only minor radiative and climatic effects, the accompanying ozone has important consequences for Earth climate. Using the Community Earth System Model (CESM), a 3-D general circulation model, we test the effects of various levels of ozone on Earth's climate. When CO2 is held constant, the global mean surface temperature decreases with decreasing ozone, with a maximum drop of ~3.5 K at near total ozone removal. By supplementing our GCM results with 1-D radiative flux calculations, we are able to test which changes to the atmosphere are responsible for this temperature change. We find that the surface temperature change is caused mostly by the stratosphere being much colder when ozone is absent; this makes it drier, substantially weakening the greenhouse effect. We also examine the effect of the structure of the upper troposphere and lower stratosphere on the formation of clouds, and on the global circulation. At low ozone, both high and low clouds become more abundant, due to changes in the tropospheric stability. These generate opposing short-wave and long-wave radiative forcings that are nearly equal. The Hadley circulation and tropospheric jet streams are strengthened, while the stratospheric polar jets are weakened, the latter being a direct consequence of the change in stratospheric temperatures. This work identifies the major climatic impacts of ozone, an important piece of the evolution of Earth's atmosphere.</p
Habitability and Water Loss Limits on Eccentric Planets Orbiting Main Sequence Stars
A planet's climate can be strongly affected by its orbital eccentricity and
obliquity. Here we use a 1-dimensional energy balance model modified to include
a simple runaway greenhouse (RGH) parameterization to explore the effects of
these two parameters on the climate of Earth-like aqua planets - completely
ocean-covered planets - orbiting F-, G-, K-, and M-dwarf stars. We find that
the range of instellations for which planets exhibit habitable surface
conditions throughout an orbit decreases with increasing eccentricity. However,
the appearance of temporarily habitable conditions during an orbit creates an
eccentric habitable zone (EHZ) that is sensitive to orbital eccentricity and
obliquity, planetary latitude, and host star spectral type. We find that the
fraction of a planet's orbit over which it exhibits habitable surface
conditions is larger on eccentric planets orbiting M-dwarf stars, due to the
lower broadband planetary albedos of these planets. Planets with larger
obliquities have smaller EHZs, but exhibit warmer climates if they do not enter
a snowball state during their orbits. We also find no transient runaway
greenhouse state on planets at all eccentricities. Rather, planets spend their
entire orbits either in a RGH or not. For G-dwarf planets receiving 100% of the
modern solar constant and with eccentricities above 0.55, an entire Earth ocean
inventory can be lost in 3.6 Gyr. M-dwarf planets, due to their larger incident
XUV flux, can become desiccated in only 690 Myr with eccentricities above 0.38.
This work has important implications for eccentric planets that may exhibit
surface habitability despite technically departing from the traditional
habitable zone as they orbit their host stars.Comment: 22 pages, 9 figures, 2 tables, accepted for publication in the
Astrophysical Journa
STARRY: Analytic Occultation Light Curves
We derive analytic, closed form, numerically stable solutions for the total
flux received from a spherical planet, moon or star during an occultation if
the specific intensity map of the body is expressed as a sum of spherical
harmonics. Our expressions are valid to arbitrary degree and may be computed
recursively for speed. The formalism we develop here applies to the computation
of stellar transit light curves, planetary secondary eclipse light curves, and
planet-planet/planet-moon occultation light curves, as well as thermal
(rotational) phase curves. In this paper we also introduce STARRY, an
open-source package written in C++ and wrapped in Python that computes these
light curves. The algorithm in STARRY is six orders of magnitude faster than
direct numerical integration and several orders of magnitude more precise.
STARRY also computes analytic derivatives of the light curves with respect to
all input parameters for use in gradient-based optimization and inference, such
as Hamiltonian Monte Carlo (HMC), allowing users to quickly and efficiently fit
observed light curves to infer properties of a celestial body's surface map.Comment: 55 pages, 20 figures. Accepted to the Astronomical Journal. Check out
the code at https://github.com/rodluger/starr
THOR 2.0: Major Improvements to the Open-Source General Circulation Model
THOR is the first open-source general circulation model (GCM) developed from
scratch to study the atmospheres and climates of exoplanets, free from Earth-
or Solar System-centric tunings. It solves the general non-hydrostatic Euler
equations (instead of the primitive equations) on a sphere using the
icosahedral grid. In the current study, we report major upgrades to THOR,
building upon the work of Mendon\c{c}a et al. (2016). First, while the
Horizontally Explicit Vertically Implicit (HEVI) integration scheme is the same
as that described in Mendon\c{c}a et al. (2016), we provide a clearer
description of the scheme and improved its implementation in the code. The
differences in implementation between the hydrostatic shallow (HSS),
quasi-hydrostatic deep (QHD) and non-hydrostatic deep (NHD) treatments are
fully detailed. Second, standard physics modules are added: two-stream,
double-gray radiative transfer and dry convective adjustment. Third, THOR is
tested on additional benchmarks: tidally-locked Earth, deep hot Jupiter,
acoustic wave, and gravity wave. Fourth, we report that differences between the
hydrostatic and non-hydrostatic simulations are negligible in the Earth case,
but pronounced in the hot Jupiter case. Finally, the effects of the so-called
"sponge layer", a form of drag implemented in most GCMs to provide numerical
stability, are examined. Overall, these upgrades have improved the flexibility,
user-friendliness, and stability of THOR.Comment: 57 pages, 31 figures, revised, accepted for publication in ApJ
Thermal tides in neutrally stratified atmospheres: Revisiting the Earth's Precambrian rotational equilibrium
Rotational dynamics of the Earth, over geological timescales, have profoundly
affected local and global climatic evolution, probably contributing to the
evolution of life. To better retrieve the Earth's rotational history, and
motivated by the published hypothesis of a stabilized length of day during the
Precambrian, we examine the effect of thermal tides on the evolution of
planetary rotational motion. The hypothesized scenario is contingent upon
encountering a resonance in atmospheric Lamb waves, whereby an amplified
thermotidal torque cancels the opposing torque of the oceans and solid
interior, driving the Earth into a rotational equilibrium. With this scenario
in mind, we construct an ab initio model of thermal tides on rocky planets
describing a neutrally stratified atmosphere. The model takes into account
dissipative processes with Newtonian cooling and diffusive processes in the
planetary boundary layer. We retrieve from this model a closed-form solution
for the frequency-dependent tidal torque which captures the main spectral
features previously computed using 3D general circulation models. In
particular, under longwave heating, diffusive processes near the surface and
the delayed thermal response of the ground prove to be responsible for
attenuating, and possibly annihilating, the accelerating effect of the
thermotidal torque at the resonance. When applied to the Earth, our model
prediction suggests the occurrence of the Lamb resonance in the Phanerozoic,
but with an amplitude that is insufficient for the rotational equilibrium.
Interestingly, though our study was motivated by the Earth's history, the
generic tidal solution can be straightforwardly and efficiently applied in
exoplanetary settings.Comment: 20 pages (+14 for appendices), 6 figure
The Peculiar Atmospheric Chemistry of KELT-9b
The atmospheric temperatures of the ultra-hot Jupiter KELT-9b straddle the
transition between gas giants and stars, and therefore between two
traditionally distinct regimes of atmospheric chemistry. Previous theoretical
studies assume the atmosphere of KELT-9b to be in chemical equilibrium. Despite
the high ultraviolet flux from KELT-9, we show using photochemical kinetics
calculations that the observable atmosphere of KELT-9b is predicted to be close
to chemical equilibrium, which greatly simplifies any theoretical
interpretation of its spectra. It also makes the atmosphere of KELT-9b, which
is expected to be cloudfree, a tightly constrained chemical system that lends
itself to a clean set of theoretical predictions. Due to the lower pressures
probed in transmission (compared to emission) spectroscopy, we predict the
abundance of water to vary by several orders of magnitude across the
atmospheric limb depending on temperature, which makes water a sensitive
thermometer. Carbon monoxide is predicted to be the dominant molecule under a
wide range of scenarios, rendering it a robust diagnostic of the metallicity
when analyzed in tandem with water. All of the other usual suspects (acetylene,
ammonia, carbon dioxide, hydrogen cyanide, methane) are predicted to be
subdominant at solar metallicity, while atomic oxygen, iron and magnesium are
predicted to have relative abundances as high as 1 part in 10,000. Neutral
atomic iron is predicted to be seen through a forest of optical and
near-infrared lines, which makes KELT-9b suitable for high-resolution
ground-based spectroscopy with HARPS-N or CARMENES. We summarize future
observational prospects of characterizing the atmosphere of KELT-9b.Comment: Accepted by ApJ. 9 pages, 6 figures. Corrected minor errors in
Figures 1a and 1b (some line styles were switched by accident), text and
conclusions unchanged, these minor changes will be updated in final ApJ proo
The 3-dimensional architecture of the Upsilon Andromedae planetary system
The Upsilon Andromedae system is the first exoplanetary system to have the
relative inclination of two planets' orbital planes directly measured, and
therefore offers our first window into the 3-dimensional configurations of
planetary systems. We present, for the first time, full 3-dimensional,
dynamically stable configurations for the 3 planets of the system consistent
with all observational constraints. While the outer 2 planets, c and d, are
inclined by about 30 degrees, the inner planet's orbital plane has not been
detected. We use N-body simulations to search for stable 3-planet
configurations that are consistent with the combined radial velocity and
astrometric solution. We find that only 10 trials out of 1000 are robustly
stable on 100 Myr timescales, or about 8 billion orbits of planet b. Planet b's
orbit must lie near the invariable plane of planets c and d, but can be either
prograde or retrograde. These solutions predict b's mass is in the range 2 - 9
and has an inclination angle from the sky plane of less than 25
degrees. Combined with brightness variations in the combined star/planet light
curve ("phase curve"), our results imply that planet b's radius is about 1.8
, relatively large for a planet of its age. However, the eccentricity
of b in several of our stable solutions reaches values greater than 0.1,
generating upwards of watts in the interior of the planet via tidal
dissipation, possibly inflating the radius to an amount consistent with phase
curve observations.Comment: 17 pages, 10 figures, accepted for publication in ApJ; revised
statement in Section 1.1, references added, results unchange
The THOR+HELIOS general circulation model: multi-wavelength radiative transfer with accurate scattering by clouds/hazes
General circulation models (GCMs) provide context for interpreting
multi-wavelength, multi-phase data of the atmospheres of tidally locked
exoplanets. In the current study, the non-hydrostatic THOR GCM is coupled with
the HELIOS radiative transfer solver for the first time, supported by an
equilibrium chemistry solver (FastChem), opacity calculator (HELIOS-K) and Mie
scattering code (LX-MIE). To accurately treat the scattering of radiation by
medium-sized to large aerosols/condensates, improved two-stream radiative
transfer is implemented within a GCM for the first time. Multiple scattering is
implemented using a Thomas algorithm formulation of the two-stream flux
solutions, which decreases the computational time by about 2 orders of
magnitude compared to the iterative method used in past versions of HELIOS. As
a case study, we present four GCMs of the hot Jupiter WASP-43b, where we
compare the temperature, velocity, entropy, and streamfunction, as well as the
synthetic spectra and phase curves, of runs using regular versus improved
two-stream radiative transfer and isothermal versus non-isothermal layers.
While the global climate is qualitatively robust, the synthetic spectra and
phase curves are sensitive to these details. A THOR+HELIOS WASP-43b GCM
(horizontal resolution of about 4 degrees on the sphere and with 40 radial
points) with multi-wavelength radiative transfer (30 k-table bins) running for
3000 Earth days (864,000 time steps) takes about 19-26 days to complete
depending on the type of GPU.Comment: 31 pages, 24 figures, accepted for publication at MNRA
- …