Drawing from over 20 years of teaching experience in the US, ranging from pre-kindergarten to post-graduate, Affolter illustrates personal, practical, and theoretical ways for teachers to grapple ...with the complexities of race and racism within their own schools and communities and develop as inclusive anti-racist teachers.
The Mastcam-Z Camera is a stereoscopic, multispectral camera with zoom capability on NASA’s Mars-2020
Perseverance
rover. The Mastcam-Z relies on a set of two deck-mounted radiometric calibration ...targets to validate camera performance and to provide an instantaneous estimate of local irradiance and allow conversion of image data to units of reflectance (R
∗
or I/F) on a tactical timescale. Here, we describe the heritage, design, and optical characterization of these targets and discuss their use during rover operations. The Mastcam-Z primary calibration target inherits features of camera calibration targets on the Mars Exploration Rovers, Phoenix and Mars Science Laboratory missions. This target will be regularly imaged during flight to accompany multispectral observations of the martian surface. The primary target consists of a gold-plated aluminum base, eight strong hollow-cylinder Sm
2
Co
17
alloy permanent magnets mounted in the base, eight ceramic color and grayscale patches mounted over the magnets, four concentric, ceramic grayscale rings and a central aluminum shadow post (gnomon) painted with an IR-black paint. The magnets are expected to keep the central area of each patch relatively free of Martian aeolian dust. The Mastcam-Z secondary calibration target is a simple angled aluminum shelf carrying seven vertically mounted ceramic color and grayscale chips and seven identical, but horizontally mounted ceramic chips. The secondary target is intended to augment and validate the calibration-related information derived from the primary target. The Mastcam-Z radiometric calibration targets are critically important to achieving Mastcam-Z science objectives for spectroscopy and photometric properties.
Context.
Observations by the
Kepler
satellite have revealed a gap between larger sub-Neptunes and smaller super-Earths that atmospheric escape models had predicted as an evaporation valley prior to ...discovery.
Aims.
We seek to contrast results from a simple X-ray and extreme-ultraviolet (XUV)-driven energy-limited escape model against those from a direct hydrodynamic model. The latter calculates the thermospheric temperature structure self-consistently, including cooling effects such as thermal conduction. Besides XUV-driven escape, it also includes the boil-off escape regime where the escape is driven by the atmospheric thermal energy and low planetary gravity, catalysed by stellar continuum irradiation. We coupled these two escape models to an internal structure model and followed the planets’ temporal evolution.
Methods.
To examine the population-wide imprint of the two escape models and to compare it to observations, we first employed a rectangular grid, tracking the evolution of planets as a function of core mass and orbital period over gigayear timescales. We then studied the slope of the valley also for initial conditions derived from the observed
Kepler
planet population.
Results.
For the rectangular grid, we find that the power-law slope of the valley with respect to orbital period is −0.18 and −0.11 in the energy-limited and hydrodynamic model, respectively. For the initial conditions derived from the
Kepler
planets, the results are similar (−0.16 and −0.10). While the slope found with the energy-limited model is steeper than observed, the one of the hydrodynamic model is in excellent agreement with observations. The reason for the shallower slope is caused by the two regimes in which the energy-limited approximation fails. The first one are low-mass planets at low-to-intermediate stellar irradiation. For them, boil-off dominates mass loss. However, boil-off is absent in the energy-limited model, and thus it underestimates escape relative to the hydrodynamic model. The second one are massive compact planets at high XUV irradiation. For them, the energy-limited approximation overestimates escape relative to the hydrodynamic model because of cooling by thermal conduction, which is neglected in the energy-limited model.
Conclusions.
The two effects act together in concert to yield, in the hydrodynamic model, a shallower slope of the valley that agrees very well with observations. We conclude that a hydrodynamic escape model that includes boil-off and a more realistic treatment of cooling mechanisms can reproduce one of the most important constraints for escape models, the valley slope.
The CREMA collaboration is pursuing a measurement of the ground-state
hyperfine splitting (HFS) in muonic hydrogen
(
\mu
μ
p)
with 1 ppm accuracy by means of pulsed laser spectroscopy to determine
...the two-photon-exchange contribution with
2\times10^{-4}
2
×
10
−
4
relative accuracy. In the proposed experiment, the
\mu
μ
p
atom undergoes a laser excitation from the singlet hyperfine state to
the triplet hyperfine state, then is quenched back to the singlet
state by an inelastic collision with a H
_2
2
molecule. The resulting increase of kinetic energy after the collisional
deexcitation is used as a signature of a successful laser transition
between hyperfine states. In this paper, we calculate the combined
probability that a
\mu
μ
p
atom initially in the singlet hyperfine state undergoes a laser
excitation to the triplet state followed by a collisional-induced
deexcitation back to the singlet state. This combined probability has
been computed using the optical Bloch equations including the inelastic
and elastic collisions. Omitting the decoherence effects caused by the
laser bandwidth and collisions would overestimate the transition
probability by more than a factor of two in the experimental
conditions. Moreover, we also account for Doppler effects and provide
the matrix element, the saturation fluence, the elastic and inelastic
collision rates for the singlet and triplet states, and the resonance
linewidth. This calculation thus quantifies one of the key unknowns of
the HFS experiment, leading to a precise definition of the requirements
for the laser system and to an optimization of the hydrogen gas target
where
\mu
μ
p
is formed and the laser spectroscopy will occur.
Slope streaks are albedo features that form frequently on equatorial Martian slopes. Most slope streaks are dark relative to surrounding terrains, a minor fraction is bright, and there are rare ...transitioning streaks that exhibit a contrast reversal partway downslope. Their formation mechanisms and physical surface properties are not well understood. New observations acquired by the Colour and Stereo Surface Imaging System (CaSSIS) on board ESA's ExoMars Trace Gas Orbiter (TGO) provide insights into slope streaks' surface microstructure, roughness and particle size ranges. Using multiple phase angle observations, we show that dark slope streaks are substantially rougher and possibly more porous than their bright counterparts, which are likely composed of more compact regolith. Color data acquired in the four wavelength bands suggest that dark streaks are spectrally similar to bright streaks but are composed of larger particles. The comparison of our orbital results to the laboratory measurements of Martian regolith analogs indicates that particles within dark slope streaks may be up to a factor of four larger than the granular material of the surrounding terrains. At one study site in Arabia Terra, using complementary imagery from other orbiters, we identify a case where dark slope streaks turned fully bright in a twenty-year period. These and CaSSIS observations suggest that bright slope streaks are old dark slope streaks, likely formed by deposition of dust or decomposition of surface aggregates into smaller particles.
Display omitted
•We report first CaSSIS observations of Martian slope streaks.•We identified a site where dark streaks turned fully bright in a twenty-year period.•Dark streaks are possibly composed of more porous and rough Martian soil.•Laboratory measurements of Martian simulant indicate particle size effects.•Bright streaks are more compact but of similar grain sizes relative to the surrounding terrains.
The CREMA collaboration is pursuing a measurement of the ground-state hyperfine splitting (HFS) in muonic hydrogen (
\mu
μ
p) with 1 ppm accuracy by means of pulsed laser spectroscopy. In the ...proposed experiment, the
\mu
μ
p atom is excited by a laser pulse from the singlet to the triplet hyperfine sub-levels, and is quenched back to the singlet state by an inelastic collision with a H
_2
2
molecule. The resulting increase of kinetic energy after this cycle modifies the
\mu
μ
p atom diffusion in the hydrogen gas and the arrival time of the
\mu
μ
p atoms at the target walls. This laser-induced modification of the arrival times is used to expose the atomic transition. In this paper we present the simulation of the
\mu
μ
p diffusion in the H
_2
2
gas which is at the core of the experimental scheme. These simulations have been implemented with the Geant4 framework by introducing various low-energy processes including the motion of the H
_2
2
molecules, i.e. the effects related with the hydrogen target temperature. The simulations have been used to optimize the hydrogen target parameters (pressure, temperatures and thickness) and to estimate signal and background rates. These rates allow to estimate the maximum time needed to find the resonance and the statistical accuracy of the spectroscopy experiment.
The CREMA collaboration is pursuing a measurement of the ground-state
hyperfine splitting (HFS) in muonic hydrogen ($\mu$p) with 1 ppm accuracy by
means of pulsed laser spectroscopy. In the proposed ...experiment, the $\mu$p atom
is excited by a laser pulse from the singlet to the triplet hyperfine
sub-levels, and is quenched back to the singlet state by an inelastic collision
with a H$_2$ molecule. The resulting increase of kinetic energy after this
cycle modifies the $\mu$p atom diffusion in the hydrogen gas and the arrival
time of the $\mu$p atoms at the target walls. This laser-induced modification
of the arrival times is used to expose the atomic transition. In this paper we
present the simulation of the $\mu$p diffusion in the H$_2$ gas which is at the
core of the experimental scheme. These simulations have been implemented with
the Geant4 framework by introducing various low-energy processes including the
motion of the H$_2$ molecules, i.e. the effects related with the hydrogen
target temperature. The simulations have been used to optimize the hydrogen
target parameters (pressure, temperatures and thickness) and to estimate signal
and background rates. These rates allow to estimate the maximum time needed to
find the resonance and the statistical accuracy of the spectroscopy experiment.
Piano Lessons Affolter, Tara L.
White Women Getting Real about Race,
2013
Book Chapter
Despite the assertion that American society is becoming postracial, they can look at many schools across their nation and realize that they are still living in a White-dominated world that only ...masquerades as a color-blind place. Theater programs provide a remarkable opportunity to view a microcosm of this larger masquerade. As a high school English and theater teacher the author learned to struggle against practicing color blindness, but that learning was slow, painful, and demanding of her in ways she had never imagined. Auditions for the next play were coming up, and she made the decision to direct an all-Black play by August Wilson entitled The Piano Lesson. The Piano Lesson is the fourth play in the cycle. As a college student the author had admired Wilson's work but had never really considered staging one of these plays at a high school.
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