Radar sounder evidence of thick, porous sediments in Meridiani Planum and implications for ice‐filled deposits on Mars

Meridiani Planum is one of the most intensely studied regions on Mars, yet little is known about the physical properties of the deposits below those examined by the Opportunity rover. We report the detection of subsurface echoes within the Meridiani Planum deposits from data obtained by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument. The delay time between the surface and subsurface returns is indicative of materials with a real dielectric constant of 3.6 ± 0.6. The real dielectric constant is strongly modulated by bulk density. Newly derived compaction relationships for Mars indicate that the relatively low dielectric constant of the Meridiani Planum deposits is consistent with a thick layer of ice‐free, porous, basaltic sand. The unique physiographic and hydrologic setting of Meridiani Planum may have provided an ideal sediment trap for eolian sands. The relatively low gravity and the cold, dry climate that has dominated Mars for billions of years may have allowed thick eolian sand deposits to remain porous and only weakly indurated. Minimally compacted sedimentary deposits may offer a possible explanation for other nonpolar region units with low apparent bulk dielectric constants

Radar Sounding of the Medusae Fossae Formation Mars: Equatorial Ice or Dry, Low-Density Deposits?

The equatorial Medusae Fossae Formation (MFF) is enigmatic and perhaps among the youngest geologic deposits on Mars. They are thought to be composed of volcanic ash, eolian sediments, or an ice-rich material analogous to polar layered deposits. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument aboard the Mars Express Spacecraft has detected nadir echoes offset in time-delay from the surface return in orbits over MFF material. These echoes are interpreted to be from the subsurface interface between the MFF material and the underlying terrain. The delay time between the MFF surface and subsurface echoes is consistent with massive deposits emplaced on generally planar lowlands materials with a real dielectric constant of ∼2.9 ± 0.4. The real dielectric constant and the estimated dielectric losses are consistent with a substantial component of water ice. However, an anomalously low-density, ice-poor material cannot be ruled out. If ice-rich, the MFF must have a higher percentage of dust and sand than polar layered deposits. The volume of water in an ice-rich MFF deposit would be comparable to that of the south polar layered deposits

Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars

The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155◦ E and east MFF between 209 and 213◦ E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD dat

Re-Imagining School Feeding : A High-Return Investment in Human Capital and Local Economies

Analysis shows that a quality education, combined with a guaranteed package of health and nutrition interventions at school, such as school feeding, can contribute to child and adolescent development and build human capital. School feeding programs can help get children into school and help them stay there, increasing enrollment and reducing absenteeism. Once children are in the classroom, these programs can contribute to their learning by avoiding hunger and enhancing cognitive abilities. The benefits are especially great for the poorest and most disadvantaged children. As highlighted in the World Bank’s 2018 World Development Report (World Bank 2018), countries need to prioritize learning, not just schooling. Children must be healthy, not hungry, if they are to match learning opportunities with the ability to learn. In the most vulnerable communities, nutrition-sensitive school meals can offer children a regular source of nutrients that are essential for their mental and physical development. And for the growing number of countries with a “double burden” of undernutrition and emerging obesity problems, well-designed school meals can help set children on the path toward more healthy diets. In Latin America, for example, where there is a growing burden of noncommunicable diseases (NCDs), school feeding programs are a key intervention in reducing undernutrition and promoting healthy diet choices. Mexico’s experience reducing sugary beverages in school cafeterias, for example, was found to be beneficial in advancing a healthy lifestyle. A large trial of school-based interventions in China also found that nutritional or physical activity interventions alone are not as effective as a joint program that combines nutritional and educational interventions. In poor communities, economic benefits from school feeding programs are also evident—reducing poverty by boosting income for households and communities as a whole. For families, the value of meals in school is equivalent to about 10 percent of a household’s income. For families with several children, that can mean substantial savings. As a result, school feeding programs are often part of social safety nets in poor countries, and they can be a stable way to reliably target pro-poor investments into communities, as well as a system that can be scaled up rapidly to respond to crises. There are also direct economic benefits for smallholder farmers in the community. Buying local food creates stable markets, boosting local agriculture, impacting rural transformation, and strengthening local food systems. In Brazil, for example, 30 percent of all purchases for school feeding come from smallholder agriculture (Drake and others 2016). These farmers are oftentimes parents with schoolchildren, helping them break intergenerational cycles of hunger and poverty. Notably, benefits to households and communities offer important synergies. The economic growth in poor communities helps provide stability and better-quality education and health systems that promote human capital. At the same time, children and adolescents grow up to enjoy better employment and social opportunities as their communities grow

Recruitment and Consolidation of Cell Assemblies for Words by Way of Hebbian Learning and Competition in a Multi-Layer Neural Network

Current cognitive theories postulate either localist representations of knowledge or fully overlapping, distributed ones. We use a connectionist model that closely replicates known anatomical properties of the cerebral cortex and neurophysiological principles to show that Hebbian learning in a multi-layer neural network leads to memory traces (cell assemblies) that are both distributed and anatomically distinct. Taking the example of word learning based on action-perception correlation, we document mechanisms underlying the emergence of these assemblies, especially (i) the recruitment of neurons and consolidation of connections defining the kernel of the assembly along with (ii) the pruning of the cell assembly’s halo (consisting of very weakly connected cells). We found that, whereas a learning rule mapping covariance led to significant overlap and merging of assemblies, a neurobiologically grounded synaptic plasticity rule with fixed LTP/LTD thresholds produced minimal overlap and prevented merging, exhibiting competitive learning behaviour. Our results are discussed in light of current theories of language and memory. As simulations with neurobiologically realistic neural networks demonstrate here spontaneous emergence of lexical representations that are both cortically dispersed and anatomically distinct, both localist and distributed cognitive accounts receive partial support