37 research outputs found
A False Positive For Ocean Glint on Exoplanets: the Latitude-Albedo Effect
Identifying liquid water on the surface of planets is a high priority, as
this traditionally defines habitability. One proposed signature of oceans is
specular reflection ("glint"), which increases the apparent albedo of a planet
at crescent phases. We post-process a global climate model of an Earth-like
planet to simulate reflected lightcurves. Significantly, we obtain glint-like
phase variations even though we do not include specular reflection in our
model. This false positive is the product of two generic properties: 1) for
modest obliquities, a planet's poles receive less orbit-averaged stellar flux
than its equator, so the poles are more likely to be covered in highly
reflective snow and ice, and 2) we show that reflected light from a
modest-obliquity planet at crescent phases probes higher latitudes than at
gibbous phases, therefore a planet's apparent albedo will naturally increase at
crescent phase. We suggest that this "latitude-albedo effect" will operate even
for large obliquities: in that case the equator receives less orbit-averaged
flux than the poles, and the equator is preferentially sampled at crescent
phase. Using rotational and orbital color variations to map the surfaces of
directly imaged planets and estimate their obliquity will therefore be a
necessary pre-condition for properly interpreting their reflected phase
variations. The latitude-albedo effect is a particularly convincing glint false
positive for zero-obliquity planets, and such worlds are not amenable to
latitudinal mapping. This effect severely limits the utility of specular
reflection for detecting oceans on exoplanets.Comment: 5 pages, 3 figures, ApJL accepte
Thermal Phases of Earth-Like Planets: Estimating Thermal Inertia from Eccentricity, Obliquity, and Diurnal Forcing
In order to understand the climate on terrestrial planets orbiting nearby
Sun-like stars, one would like to know their thermal inertia. We use a global
climate model to simulate the thermal phase variations of Earth-analogs and
test whether these data could distinguish between planets with different heat
storage and heat transport characteristics. In particular, we consider a
temperate climate with polar ice caps (like modern Earth), and a snowball state
where the oceans are globally covered in ice. We first quantitatively study the
periodic radiative forcing from, and climatic response to, rotation, obliquity,
and eccentricity. Orbital eccentricity and seasonal changes in albedo cause
variations in the global-mean absorbed flux. The responses of the two climates
to these global seasons indicate that the temperate planet has 3 times the bulk
heat capacity of the snowball planet due to the presence of liquid water
oceans. The temperate obliquity seasons are weaker than one would expect based
on thermal inertia alone; this is due to cross-equatorial oceanic and
atmospheric energy transport. Thermal inertia and cross-equatorial heat
transport have qualitatively different effects on obliquity seasons, insofar as
heat transport tends to reduce seasonal amplitude without inducing a phase lag.
For an Earth-like planet, however, this effect is masked by the mixing of
signals from low thermal inertia regions (sea ice and land) with that from high
thermal inertia regions (oceans), which also produces a damped response with
small phase lag. We then simulate thermal lightcurves as they would appear to a
high-contrast imaging mission (TPF-I/Darwin) and consider the inverse problem
of estimating thermal inertia based solely on time-resolved photometry.
[Abridged]Comment: 14 pages, 12 figures, ApJ accepte
The JANUS X-Ray Flash Monitor
JANUS is a NASA small explorer class mission which just completed phase A and
was intended for a 2013 launch date. The primary science goals of JANUS are to
use high redshift (6<z<12) gamma ray bursts and quasars to explore the
formation history of the first stars in the early universe and to study
contributions to reionization. The X-Ray Flash Monitor (XRFM) and the Near-IR
Telescope (NIRT) are the two primary instruments on JANUS. XRFM has been
designed to detect bright X-ray flashes (XRFs) and gamma ray bursts (GRBs) in
the 1-20 keV energy band over a wide field of view (4 steradians), thus
facilitating the detection of z>6 XRFs/GRBs, which can be further studied by
other instruments. XRFM would use a coded mask aperture design with hybrid CMOS
Si detectors. It would be sensitive to XRFs/GRBs with flux in excess of
approximately 240 mCrab. The spacecraft is designed to rapidly slew to source
positions following a GRB trigger from XRFM. XRFM instrument design parameters
and science goals are presented in this paper.Comment: submitted to Proc. SPIE, Vol. 7435 (2009), 7 pages, 8 figure
Colors of a Second Earth II: Effects of Clouds on Photometric Characterization of Earth-like Exoplanets
As a test-bed for future investigations of directly imaged terrestrial
exoplanets, we present the recovery of the surface components of the Earth from
multi-band diurnal light curves obtained with the EPOXI spacecraft. We find
that the presence and longitudinal distribution of ocean, soil and vegetation
are reasonably well reproduced by fitting the observed color variations with a
simplified model composed of a priori known albedo spectra of ocean, soil,
vegetation, snow and clouds. The effect of atmosphere, including clouds, on
light scattered from surface components is modeled using a radiative transfer
code. The required noise levels for future observations of exoplanets are also
determined. Our model-dependent approach allows us to infer the presence of
major elements of the planet (in the case of the Earth, clouds and ocean) with
observations having S/N in most cases and with high confidence if
S/N . In addition, S/N enables us to detect the
presence of components other than ocean and clouds in a fairly
model-independent way. Degradation of our inversion procedure produced by cloud
cover is also quantified. While cloud cover significantly dilutes the magnitude
of color variations compared to the cloudless case, the pattern of color
changes remains. Therefore, the possibility of investigating surface features
through light curve fitting remains even for exoplanets with cloud cover
similar to the Earth's.Comment: 33 pages, 16 figures, accepted for publication in ApJ (discussion,
references, and description of data reduction added, typo fixed
Surface and Temporal Biosignatures
Recent discoveries of potentially habitable exoplanets have ignited the
prospect of spectroscopic investigations of exoplanet surfaces and atmospheres
for signs of life. This chapter provides an overview of potential surface and
temporal exoplanet biosignatures, reviewing Earth analogues and proposed
applications based on observations and models. The vegetation red-edge (VRE)
remains the most well-studied surface biosignature. Extensions of the VRE,
spectral "edges" produced in part by photosynthetic or nonphotosynthetic
pigments, may likewise present potential evidence of life. Polarization
signatures have the capacity to discriminate between biotic and abiotic "edge"
features in the face of false positives from band-gap generating material.
Temporal biosignatures -- modulations in measurable quantities such as gas
abundances (e.g., CO2), surface features, or emission of light (e.g.,
fluorescence, bioluminescence) that can be directly linked to the actions of a
biosphere -- are in general less well studied than surface or gaseous
biosignatures. However, remote observations of Earth's biosphere nonetheless
provide proofs of concept for these techniques and are reviewed here. Surface
and temporal biosignatures provide complementary information to gaseous
biosignatures, and while likely more challenging to observe, would contribute
information inaccessible from study of the time-averaged atmospheric
composition alone.Comment: 26 pages, 9 figures, review to appear in Handbook of Exoplanets.
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The Influence of Jazz and Popular Music on the American Trombone Concerto, A Selected Study of the Solo Trombone and Large Ensemble Works of Richard Peaslee, James Pugh, Howard Buss, and Dexter Morrill
Ionic Liquids Separating Rubber Latex from Guayule
Danger to rubber trees (Hevea brasiliensis) from South American leaf blight fungus imperils the world’s source of natural latex for essential rubber products. Avoiding latex allergies also requires a non-Hevea latex source. The present methods for removing latex entrapped in the individual cells of guayule plants require environmentally hazardous chemicals. This study proposes a new method for latex extraction from guayule using various ionic liquids (ILs) to dissolve cell walls and release latex, as substantiated by Fourier transform infrared spectroscopy (FTIR) data