The Norwegian Height System NN1954 Revisited

Vertical reference systems based on national precise levelling networks have been realised in Europe since the 1860’s. The heights are related to gravity, which by convention may be measured values or amodel. The Norwegian Height System was adjusted in 1956 when the southern part of the country had been levelled once. Because of the lack of measured gravity, Clairaut’s formula for gravity was used in the orthometric correction. The intention was to establish an orthometric height system, but this was never tested or confirmed. In the literature the system is often referred to as orthometric, but some times as normal orthometric. This study shows that the Norwegian Height System was realised by a strongly deformed net due to lack of land uplift corrections. The derived heights are neither normal nor orthometric. When the land uplift is handled in a proper way, the heights are close to normal heights. The remaining small differences are shown to depend on the free air anomaly

Biosignatures from Earth-Like Planets Around M Dwarfs

Coupled one-dimensional photochemical-climate calculations have been performed for hypothetical Earth-like planets around M dwarfs. Visible, near-infrared and thermal-infrared synthetic spectra of these planets were generated to determine which biosignature gases might be observed by a future, space-based telescope. Our star sample included two observed active M dwarfs, AD Leo and GJ 643, and three quiescent model stars. The spectral distribution of these stars in the ultraviolet generates a different photochemistry on these planets. As a result, the biogenic gases CH4, N2O, and CH3Cl have substantially longer lifetimes and higher mixing ratios than on Earth, making them potentially observable by space-based telescopes. On the active M-star planets, an ozone layer similar to Earth's was developed that resulted in a spectroscopic signature comparable to the terrestrial one. The simultaneous detection of O2 (or O3) and a reduced gas in a planet's atmosphere has been suggested as strong evidence for life. Planets circling M stars may be good locations to search for such evidence.Comment: 34 pages, 10 figures, Astrobiology, in pres