On the history of the lunar orbit

A frequency-dependent model of tidal friction is used in the determination of the time rate of change of the lunar orbital elements and the angular velocity of the Earth. The variational equations consider eccentricity, the solar tide on the Earth, Earth oblateness, and higher-order terms in the Earth's tidal potential. A linearized solution of the equations governing the precission of the Earth's rotational angular momentum and the lunar ascending node is found. This allows the analytical averaging of the variational equations over the period of relative precession which, though large, is necessarily small in comparison to the time step of the numerical integrator that yields the system history over geological time. Results for this history are presented and are identified as consistent with origin of the Moon by capture. This model may be applied to any planet-satellite system where evolution under tidal friction is of interest.     

Strain analysis using deformed line distributions

This paper describes a new technique of strain analysis using passively deformed linear markers which were initially uniformly or symmetrically distributed in a plane. The axial ratio of the strain ellipse is determined from the quartiles or other percentiles of the deformed distribution. The technique permits statistical testing of assumptions and rapid reconstruction of the undeformed state.     

Vorticity measure for non steady deformations—a discussion

The dynamic recrystallisation mechanisms in a magnesium alloy have been investigated during uniaxial large strain compression at T = 0.7?0.95 T_m (T_m = melting temperature). Previous work at T = 0.45?0.64 T_m has shown that the metal is a useful analogue for minerals with a high plastic anisotropy.The high strain microstructures were akin to those in high grade tectonites. Initial grain sizes and shapes had been completely transformed with the recrystallised grainsize related solely to the flow stress. Two processes were responsible for the transformation, grain boundary migration and the formation of new high angle boundaries by progressive misorientation across subgrain boundaries. Estimates of the grain boundary migration velocity suggest that solute-escape migration does not occur.This type of recrystallisation mechanism where grain boundary migration and subgrain rotation are equally involved has not previously been described. A ‘recrystallisation mechanism diagram’ has been produced to show the relationship of this mechanism, to other recrystallisation mechanisms. In the diagram a particular mechanism is defined in terms of the relative contribution of the following processes; subgrain rotation, subgrain growth, grain boundary bulging and grain growth. Consideration of normalised experimentally defined stress grain size relationships reveals that there is a systematic relationship between the normalised stress grain size relationship and the type of recrystallisation mechanism, as defined by the ‘mechanism diagram’.The implications of these results to the interpretation of upper mantle olivine palaeoblast grain size profiles (Ave Lallement et al.) are discussed.     

Long term wind erosion on Mars

Wind erosion and deposition are powerful agents of surface change on Mars. Erosion is sensitive to the atmospheric density, so feedback between orbit variations and atmospheric density can enhance the sensitivity of erosion rates to orbital parameters. We use statistics derived from a 1 Gyr integration of the spin axis of Mars, coupled with runs of the NASA Ames 3-D general circulation model (GCM) at a variety of orbital conditions and pressures, to explore this feedback. We find that wind erosion in the GCM is associated with two factors: baroclinic zone winds, and strong cross-equator solstice flows. Both of these factors are influenced by topography, producing an asymmetry in the erosion pattern between the north and the south. The erosion model, averaged over 1 Gyr, produces potential erosion rates of in the north and in the south, which increase by an order of magnitude in an early 28 mbar atmosphere. The stability of these potential erosion patterns over geological time indicates that the lowland regions of Mars are continuously eroded, and that wind is capable of revealing ancient sedimentary deposits, or eroding substantial deposits that may have otherwise been preserved.     

Enhanced runout and erosion by overland flow at low pressure and sub-freezing conditions: Experiments and application to Mars

We present the results of laboratory experiments to study the sediment transport and erosional capacity of water at current martian temperature and pressure. We have performed laboratory simulation experiments in which a stream of water flowed over test beds at low temperature (∼−20 °C) and low pressure (∼7 mbar). The slope angle was 14° and three sediment types were tested. We compared the erosive ability, runout and resulting morphologies to experiments performed at ambient terrestrial temperature (∼20 °C) and pressure (∼1000 mbar), and also to experiments performed under low pressure only. We observed that, as expected, water is unstable in the liquid phase at low temperature and low pressure, with boiling and freezing in competition. Despite this, our results show that water at low temperature and low pressure has an equivalent and sometimes greater erosion rate than at terrestrial temperature and pressure. Water flows faster over the sediment body under low temperature and low pressure conditions because the formation of ice below the liquid-sediment contact inhibits infiltration. Flow speed and therefore runout distance are increased. Experiments at low pressure but Earth-ambient temperature suggest that flow speeds are faster under these conditions than under Earth-ambient pressure and temperature. We hypothesise that this is due to gas bubbles, created by the boiling of the water under low atmospheric pressure, impeding liquid infiltration. We have found that both basal freezing and low pressure increase the flow propagation speed - effects not included in current models of fluvial activity on Mars. Any future modelling of water flows on Mars should consider this extra mobility and incorporate the large reduction in fluid loss through infiltration into the substrate.     

Climate change and the water–energy–food nexus: insights from policy and practice in Tanzania

The threat of climate change is emerging at a time of rapid growth for many economies in sub-Saharan Africa (SSA). Dominant narratives comprising ambitious development plans are common and often based around sectors with strong inter-dependencies that are highly exposed to climate variability. Using document analysis and key informant interviews, this article examines how climate change is addressed in policy, how it is being mainstreamed into water, energy and agriculture sector policies and the extent to which cross-sectoral linkages enable coordinated action. These questions are addressed through a case study of Tanzania, highlighting broader lessons for other developing countries, particularly those in SSA facing similar challenges. The article finds that, while the agriculture and water sectors are increasingly integrating climate change into policies and plans in Tanzania, practical coordination on adaptation remains relatively superficial. Publication of the Tanzania National Adaptation Plan of Action (NAPA) in 2007 marked a step change in the integration of climate change in sectoral policies and plans; however, it may have reinforced a sectoral approach to climate change. Examining the policies for coherence highlights overlaps and complementarities which lend themselves to a coordinated approach. Institutional constraints (particularly structures and resources) restrict opportunities for inter-sectoral action and thus collaboration is confined to ad hoc projects with mixed success to date. The results highlight the need for institutional frameworks that recognize and address these constraints to enable development goals to be pursued in a more sustainable and climate-resilient manner.

KEY POLICY INSIGHTS

• The NAPA has been successful at encouraging climate change mainstreaming into sectoral policies in Tanzania; however, the cross-sectoral collaboration crucial to implementing adaptation strategies remains limited due to institutional challenges such as power imbalances, budget constraints and an ingrained sectoral approach.

• Collaboration between nexus sectors in Tanzania is largely through ad hoc projects with limited progress on establishing deeper connections to enable collaboration as a process. Regular cross-sectoral planning meetings and consistent annual budgets could provide a platform to enhance cross-sectoral coordination.

• Plans to develop hydropower and agriculture are prevalent across sub-Saharan Africa. Insights from Tanzania highlight the importance of institutional and policy frameworks that enable cross-sectoral coordination.