Dust Devil Tracks

Dust devils that leave dark- or light-toned tracks are common on Mars and they can also be found on the Earth’s surface. Dust devil tracks (hereinafter DDTs) are ephemeral surface features with mostly sub-annual lifetimes. Regarding their size, DDT widths can range between ∼1 m and ∼1 km, depending on the diameter of dust devil that created the track, and DDT lengths range from a few tens of meters to several kilometers, limited by the duration and horizontal ground speed of dust devils. DDTs can be classified into three main types based on their morphology and albedo in contrast to their surroundings; all are found on both planets: (a) dark continuous DDTs, (b) dark cycloidal DDTs, and (c) bright DDTs. Dark continuous DDTs are the most common type on Mars. They are characterized by their relatively homogenous and continuous low albedo surface tracks. Based on terrestrial and martian in situ studies, these DDTs most likely form when surficial dust layers are removed to expose larger-grained substrate material (coarse sands of ≥500 μm in diameter). The exposure of larger-grained materials changes the photometric properties of the surface; hence leading to lower albedo tracks because grain size is photometrically inversely proportional to the surface reflectance. However, although not observed so far, compositional differences (i.e., color differences) might also lead to albedo contrasts when dust is removed to expose substrate materials with mineralogical differences. For dark continuous DDTs, albedo drop measurements are around 2.5 % in the wavelength range of 550–850 nm on Mars and around 0.5 % in the wavelength range from 300–1100 nm on Earth. The removal of an equivalent layer thickness around 1 μm is sufficient for the formation of visible dark continuous DDTs on Mars and Earth. The next type of DDTs, dark cycloidal DDTs, are characterized by their low albedo pattern of overlapping scallops. Terrestrial in situ studies imply that they are formed when sand-sized material that is eroded from the outer vortex area of a dust devil is redeposited in annular patterns in the central vortex region. This type of DDT can also be found in on Mars in orbital image data, and although in situ studies are lacking, terrestrial analog studies, laboratory work, and numerical modeling suggest they have the same formation mechanism as those on Earth. Finally, bright DDTs are characterized by their continuous track pattern and high albedo compared to their undisturbed surroundings. They are found on both planets, but to date they have only been analyzed in situ on Earth. Here, the destruction of aggregates of dust, silt and sand by dust devils leads to smooth surfaces in contrast to the undisturbed rough surfaces surrounding the track. The resulting change in photometric properties occurs because the smoother surfaces have a higher reflectance compared to the surrounding rough surface, leading to bright DDTs. On Mars, the destruction of surficial dust-aggregates may also lead to bright DDTs. However, higher reflective surfaces may be produced by other formation mechanisms, such as dust compaction by passing dust devils, as this may also cause changes in photometric properties. On Mars, DDTs in general are found at all elevations and on a global scale, except on the permanent polar caps. DDT maximum areal densities occur during spring and summer in both hemispheres produced by an increase in dust devil activity caused by maximum insolation. Regionally, dust devil densities vary spatially likely controlled by changes in dust cover thicknesses and substrate materials. This variability makes it difficult to infer dust devil activity from DDT frequencies. Furthermore, only a fraction of dust devils leave tracks. However, DDTs can be used as proxies for dust devil lifetimes and wind directions and speeds, and they can also be used to predict lander or rover solar panel clearing events. Overall, the high DDT frequency in many areas on Mars leads to drastic albedo changes that affect large-scale weather patterns

Non-destructive evaluation of isotropic plate structures by means of mode filtering in the frequency-wavenumber domain

Non-Destructive Testing (NDT) techniques are prevalent in the aerospace, green energy and automotive industries. These techniques, including ultrasound, despite their use, still have limitations surrounding speed and resolution. This work presents the development of a band-pass mode filtering technique in the frequency wavenumber domain for the purpose of damage detection in isotropic materials. Data was captured in the temporal and spatial domain using a 3D Scanning Laser Doppler Vibrometer (SLDV) with piezoelectric transducers exciting the structure with a variety of steady state signals ranging in frequency from 75 kHz to 400 kHz and a frequency modulated signal from 75 kHz to 400 kHz. A thickness map was created based on the frequency and wavelength of the A0 Lamb wave mode. The technique was demonstrated on two aluminium specimens with dimensions of 400 mm by 400 mm with a thickness range of 0.5 mm to 8 mm with distinct geometric features. Through using multi-frequency excitation combined with mode based filters an estimation of thickness was achieved with a mean percentage thickness error of 15%. Circular thickness reductions with a diameter of 10 mm were clearly identified at the maximum plate thickness of 8 mm. This technique was shown to perform better than wavenumber filtering by allowing data from multi frequency steady state excitation to be combined into a single resulting thickness map. This improvement was shown to be particularly important at greater thicknesses

Track D Social Science, Human Rights and Political Science

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138414/1/jia218442.pd

Particulate organic carbon mass distribution at the Bermuda Atlantic Time-series Study (BATS) site

The use of N-heterocyclic carbenes (NHCs) in chemistry has developed rapidly over the past twenty years. These interesting compounds are predominantly employed in organometallic chemistry as ligands for various metal centres, and as organocatalysts able to mediate an exciting range of reactions. However, the sheer number of NHCs known in the literature can make the appropriate choice of NHC for a given application difficult. A number of metrics have been explored that allow the electronic properties of NHCs to be quantified and compared. In this review, we discuss these various metrics and what they can teach about the electronic properties of NHCs. Data for approximately three hundred NHCs are presented, obtained from a detailed survey of the literature

Dissolved Organic Carbon as a Component of the Biological Pump in the North Atlantic Ocean [and Discussion]

The North Atlantic is characterized by strong seasonality in mixed layer depths, resulting in winter recharge of surface layer nutrients and the spring phytoplankton bloom. This is the classical textbook model of seasonal cycles of oceanic biogeochemical processes, but in fact the North Atlantic is the exception rather than the rule. In much of the temperate and subpolar regions of the basin, the vernal accumulation of biomass is accompanied by a marked drawdown of inorganic carbon in the water column and pulses of particle flux to the seafloor. In the classical model, the decline of the CO<latex>$_{2}$</latex> is balanced by accumulation of biogenic carbon and particle export. The main export mechanisms include sinking of ungrazed but possibly senescent phytoplankton and zooplankton grazing and egestion. Carbon budgets based on observations from the Joint Global Ocean Flux Study North Atlantic Bloom Experiment and Bermuda Atlantic Time Series cannot be closed using the elements of the classical model. That is, the CO<latex>$_{2}$</latex> drawdown cannot be balanced by biomass accumulation and exports estimated by sediment traps. There are at least three possible routes toward reconciliation: (i) trap estimates are in error and systematically biased; (ii) spatial variability aliasses the observations making budgeting impossible without recourse to coupled three-dimensional models; and/or (iii) the classical model must be abandoned and replaced by a concept in which the accumulation and export of dissolved organic carbon assumes a major role in the North Atlantic carbon balance. At Bermuda, where the most complete data set exists, the weight of the evidence favours the first and third possibilities