Recent Viking Orbiter observations of the Martian polar layered deposits, thought to be thick accumulations of dust, have provided new insights into the mechanisms of topographic evolution of these terrains. Here we report the discovery of wavelike topographic relief on north polar surfaces previously supposed to be flat plains. These undulations have wavelengths of a few kilometers and amplitudes of probably only a few tens of meters. The crests and troughs extend laterally up to hundreds of kilometers and have radii of curvature in plan of several hundred kilometers. Much steeper slopes, exhibiting fine terracing due to layering in the deposits, occur within troughs that separate areas of undulating plains and that parallel in strike the topographic grain of the undulations. In the model presented here, both the undulations on the plains surfaces and the layers that appear in cross section on steeper slopes are attributed to the same basic cause—periodic episodes of dust accumulation. These are probably associated with variations in global dust storm intensity and perennial polar ice cap size, brought about by periodic changes in the eccentricity of the Martian orbit and the orientation of its spin axis with respect to the ecliptic. In earlier work it has been proposed that each layer formed in a single cycle of deposition. Specifically, it was argued that dust from global dust storms is swept from the atmosphere by the accumulating annual ice and retained in the place of deposition by a matrix of perennial ice. Here we propose a mechanism for generating the undulating topography at the edge of the advancing or receding perennial ice cap in an environment of periodic deposition of dust. We have recognized several families of topographic undulations with markedly different orientations. In regions where two differently oriented sets of undulations overlap, the wave crests and troughs break up into mounds and shallow basins. We attribute this type of landscape to the occurrence of two passages of the perennial ice cap edge across the same area with the ice cap margin oriented differently on the two occasions. This implies past variations in both the size of the perennial polar cap and the location of its center if we assume that the shape of the perennial cap has not varied drastically from circular. Also reported here is evidence for a genetic relationship between undulating plains and the curvilinear defrosted bands which are the dominant physiographic features of the summer residual polar cap. We establish here that most of these curvilinear features are topographic troughs, and we propose that they are derived from a primary undulating landscape by an insolation‐controlled process. First, areas of steeper slopes within the undulating terrain lose their perennial ice cover because of a change in insolation conditions. Then, these slopes evolve into topographic troughs because in the absence of perennial ice cover they are unable to trap deposited dust. We attribute the planimetric complexity of the troughs—sha...

Eight active galactic nuclei (AGNs; seven type 1 seyferts and one QSO) are reported. They are 14th and 15th mag objects with broad hydrogen lines and redshifts in the range 0.026 < z < 0.138. The AGNs were discovered in a systematic program to identify the fainter X-ray sources of the HEAO 1 survey. All are located in or near the X-ray positions of the HEAO 1 scanning modulation collimator, the HEAO 1 Large Area Sky Survey, and other X-ray experiments. The AGNs are H0307-730, PKS 0558-504, H0707-495, HI 143-182, HI839-786, HI934-513, H2106-099, and H2132-626. The X-ray and optical luminosities and many of the optical emission features of these AGNs resemble other bright and nearby Seyfert 1 galaxies. However, three of the galaxies exhibit very strong Fe n emission, and two additionally show very weak forbidden line spectra similar to I Zw 1. One of the I Zw 1 types, the previously unidentified radio source PKS 0558-504, is a QSO with unusually narrow hydrogen lines for a highluminosity object (M v =-25.1). The other, H0707-495, has an Fe n/H/? emission ratio that is similar to the largest known values. Another galaxy, H2106-099, has relatively strong emission lines of Fe x and Fe vu, and it is a variable X-ray source. Finally, the one for which we have X-ray spectral measurements, H1839-786, exhibits an unusually hard power-law spectrum with an X-ray energy index of 0.1. We have detected weak radio emission from the two Seyfert galaxies accessible from the VLA (H2106-099 and HI 143-182). Photometric magnitudes and the intensities of selected emission lines are given for each of the AGNs.

No abstract

We report 22 emission-line active galactic nuclei as bright, hard x-ray sources. All of them appear to be new classifications with the exception of one peculiar IRAS source which is a known quasar and has no published spectrum. The objects were located on the basis of positional data from the HEAO-1 scanning modulation collimator and the HEAO-1 Large Area Sky Survey. A few of these AGN had been detected by the Uhuru and Ariel V surveys, and four are also IRAS sources. This sample exhibits a rich diversity in optical spectral properties and luminosities, ranging from a powerful broad-absorption-line quasar to a weak nucleus embedded in a nearby NGC galaxy. Two cases confer x-ray luminosities in excess of 10 47 erg/s. However, there is a degree of uncertainty in the x-ray identification for the AGN fainter than F~16.5. Optically, several AGNs exhibit very strong Fell emission, continuing this tendency among the x-ray selected objects from HEAO-1. One Seyfert galaxy with substantial radio flux is an exception to the common association of strong Fe II emission and radio-quiet AGN. The previously recognized IRAS quasar shows extreme velocities in the profiles of the forbidden lines; the O III pair is broadened to the point that the lines are blended (FWHM-3500 kms-1). Several of these AGN show evidence of intrinsic obscuration, illustrating the effectiveness of hard x-ray surveys in locating AGN through high column density.

No abstract