The atmospheres of Mars and Venus

Of all the planets which may exist in the Universe, only nine have been studied by man. As a result, one cannot classify planets with the same confidence that one has in classifying stars; there is no theory of planetary evolution comparable in development to the theory of stellar evolution. Nevertheless, many of the goals of planetary science and stellar astronomy are the same: to classify objects according to their most fundamental properties in order to understand their present physical state and their evolution. From this point of view, the terrestrial planets comprise a group which can usefully be considered together. By comparing the similarities and differences between them, we may hope to gain insight into the evolution of the entire group

An information approach to the dynamics in farm income: implications for farmland markets

The valuation of farmland is a perennial issue for agricultural policy, given its importance in the farm investment portfolio. Despite the significance of farmland values to farmer wealth, prediction remains a difficult task. This study develops a dynamic information measure to examine the informational content of farmland values and farm income in explaining the distribution of farmland values over time

Sublimation pit distribution indicates convection cell surface velocities of ∼10 cm per year in Sputnik Planitia, Pluto

The ∼10^6 km^2 Sputnik Planitia, Pluto is the upper surface of a vast basin of nitrogen ice. Cellular landforms in Sputnik Planitia with areas in the range of a few × 10^2–10^3 km^2 are likely the surface manifestation of convective overturn in the nitrogen ice. The cells have sublimation pits on them, with smaller pits near their centers and larger pits near their edges. We map pits on seven cells and find that the pit radii increase by between 2.1 ± 0.4 × 10^(−3) and 5.9 ± 0.8 × 10^(−3) m m^(−1) away from the cell center, depending on the cell. This is a lower bound on the size increase because of the finite resolution of the data. Accounting for resolution yields upper bounds on the size vs. distance distribution of between 4.2 ± 0.2 × 10^(−3) and 23.4 ± 1.5 × 10^(−3)m m^(−1). We then use an analytic model to calculate that pit radii grow via sublimation at a rate of 3.6_(−0.6)^(+2.1)×10^(−4) m yr^(−1), which allows us to convert the pit size vs. distance distribution into a pit age vs. distance distribution. This yields surface velocities between 1.5_(−0.2)^(+1.0) and 6.2_(−1.4)^(+3.4) cm yr^(−1) for the slowest cell and surface velocities between 8.1_(−1.0)^(+5.5) and 17.9_(−5.1)^(+8.9) cm yr^(−1) for the fastest cell. These convection rates imply that the surface ages at the edge of cells reach ∼4.2–8.9 × 10^5 yr. The rates are comparable to rates of ∼6 cm yr^(−1) that were previously obtained from modeling of the convective overturn in Sputnik Planitia (McKinnon et al., 2016). Finally, we investigate the surface rheology of the convection cells and estimate that the minimum ice viscosity necessary to support the geometry of the observed pits is of order 10^(16)–10^(17) Pa s, based on the argument that pits would relax away before growing to their observed radii of several hundred meters if the viscosity were lower than this value

Sublimation pit distribution indicates convection cell surface velocities of ∼10 cm per year in Sputnik Planitia, Pluto

The ∼10^6 km^2 Sputnik Planitia, Pluto is the upper surface of a vast basin of nitrogen ice. Cellular landforms in Sputnik Planitia with areas in the range of a few × 10^2–10^3 km^2 are likely the surface manifestation of convective overturn in the nitrogen ice. The cells have sublimation pits on them, with smaller pits near their centers and larger pits near their edges. We map pits on seven cells and find that the pit radii increase by between 2.1 ± 0.4 × 10^(−3) and 5.9 ± 0.8 × 10^(−3) m m^(−1) away from the cell center, depending on the cell. This is a lower bound on the size increase because of the finite resolution of the data. Accounting for resolution yields upper bounds on the size vs. distance distribution of between 4.2 ± 0.2 × 10^(−3) and 23.4 ± 1.5 × 10^(−3)m m^(−1). We then use an analytic model to calculate that pit radii grow via sublimation at a rate of 3.6_(−0.6)^(+2.1)×10^(−4) m yr^(−1), which allows us to convert the pit size vs. distance distribution into a pit age vs. distance distribution. This yields surface velocities between 1.5_(−0.2)^(+1.0) and 6.2_(−1.4)^(+3.4) cm yr^(−1) for the slowest cell and surface velocities between 8.1_(−1.0)^(+5.5) and 17.9_(−5.1)^(+8.9) cm yr^(−1) for the fastest cell. These convection rates imply that the surface ages at the edge of cells reach ∼4.2–8.9 × 10^5 yr. The rates are comparable to rates of ∼6 cm yr^(−1) that were previously obtained from modeling of the convective overturn in Sputnik Planitia (McKinnon et al., 2016). Finally, we investigate the surface rheology of the convection cells and estimate that the minimum ice viscosity necessary to support the geometry of the observed pits is of order 10^(16)–10^(17) Pa s, based on the argument that pits would relax away before growing to their observed radii of several hundred meters if the viscosity were lower than this value

Venus lower atmosphere heat balance

Pioneer Venus observations of temperatures and radiative fluxes are examined in an attempt to understand the thermal balance of the lower atmosphere. If all observations are correct and the probe sites are typical of the planet, the second law of thermodynamics requires that the bulk of the lower atmosphere heating must come from a source other than direct sunlight or a thermally driven atmospheric circulation. Neither the so-called greenhouse models nor the mechanical heating models are consistent with this interpretation of the observations. One possible interpretation is that two out of the three probe sites are atypical of the planet. Additional lower atmosphere heat sources provide another possible interpretation. These include a planetary heat flux that is 250 times the earth's, a secular cooling of the atmosphere, and a chemically energetic rain carrying solar energy from the clouds to the surface. Other data make these interpretations seem unlikely, so measurement error remains a serious possibility

Experimentally Observed Instability of a Laminar Ekman Flow in a Rotating Basin

In studying the axi-symmetric flow induced by source-sink distributions in a rotating cylindrical basin in the absence of radial barriers, a highly organized pattern of instability has been observed in the laminar Ekman layer along the bottom of the basin. The instability manifests itself in the form of almost perfectly concentric cylindrical sheets or curtains of water which rise as sharply defined vertical jets from the Ekman layer and penetrate the entire depth of fluid. A less sharply defined downward motion between the curtains completes the circulation celis thus developed. At some maximum critical radius, the curtains usually disappear, and the flow at larger radii is a stable, laminar Ekman flow. Quantitative observations of ring spacing and critical radius are reported for experiments in which angular velocity, flow rate, viscosity and total depth of water were varied over experimentally available ranges

The Co-Evolution of Mars’ Atmosphere and Massive South Polar CO₂ Ice Deposit

A Massive CO₂ Ice Deposit (MCID) that rivals the mass of Mars’ current, 96% CO₂ atmosphere was recently discovered to overlie part of Mars’ southern H₂O cap [1]. The MCID is layered: a top layer of 1-10 m of CO₂, the Residual South Polar Cap (RSPC) [2], is underlain by ~10-20 m of H₂O ice, followed by up to three 100s-meter-thick layers of CO2 ice, separated by two layers of ~20-40 m of H₂O ice [3] (Fig. 1). Previous studies invoked orbital cycles to explain the layering, assuming the H₂O ice insulates and seals in the CO₂, allowing it to survive periods of high obliquity [3,4]. We also model that orbital cycles [5] drive the MCID’s development, but instead assume the MCID is in continuous vapor contact with the atmosphere rather than sealed. Pervasive meter-scale polygonal patterning and km-scale collapse pits observed on the sub-RSPC H₂O layer [1,3] are consistent with it being fractured and permeable to CO₂ mass flux. Using currently observed optical properties of martian polar CO₂ ice deposits [6], our model demonstrates that the present MCID is a remnant of larger CO₂ ice deposits laid down during epochs of decreasing obliquity that are eroded, liberating a residual lag layer of H₂O ice, when obliquity increases. With these assumptions, our energy balance model ex-plains why only the south polar cap hosts an MCID, why the RSPC exists, and the observed MCID stratigraphy. We use our model to calculate Mars’ pressure history and the age of the MCID

How hummingbirds hum: acoustic holography of hummingbirds during maneuvering flight

Hummingbirds make a characteristic humming sound when they flaptheir wings. The physics and the biological significance of hummingbird aeroacoustics is still poorly understood. We used acoustic holography and high-speed cameras to determine the acoustic field of six hummingbirds while they either hovered stationary in front of a flower or maneuvered to track flower motion. We used a robotic flower that oscillated either laterally or longitudinally with a linear combination of 20 different frequencies between 0.2 and 20 Hz, a range that encompasses natural flower vibration frequencies in wind. We used high-speed marker tracking to dissect the transfer function between the moving flower, the head, and body of the bird. We also positioned four acoustic arrays equipped with 2176 microphones total above, below, and in front of the hummingbird. Acoustic data from the microphones were back-propagated to planes adjacent to the hummingbird to create the first real-time holograms of the pressure field a hummingbird generates in vivo. Integration of all this data offers insight into how hummingbirds modulate the acoustic field during hovering and maneuvering flight