The Borealis quadrangle is a quadrangle on Mercury surrounding the north pole down to 65° latitude. It was mapped in its entirety by the MESSENGER spacecraft, which orbited the planet from 2008 to 2015, excluding areas of permanent shadow near the north pole. Only approximately 25% of the quadrangle was imaged by the Mariner 10 spacecraft during its flybys in 1974 and 1975. The quadrangle is now called H-1.Map of the H-1 (Borealis) Quadrangle of Mercury
It contains the Goethe Basin, whose diameter of at least makes it the sixth-largest impact basin observed on Mariner 10 images(Murray and others, 1974; Boyce and Grolier, 1977; Strom, 1977). The west half of the mapped area (between long 100° and 190° W.) is dominated by older craters and by intercrater plains material that lies between and within them. Younger crater materials, intermediate plains material, and small patches of smooth plains material are superposed on all other units. The crater Verdi, in diameter, is the largest of the younger craters. Its extensive ejecta blanket and secondary crater field are superposed on plains materials and older craters.
The east half of the mapped area (between long 0° and 100° W.) is characterized by smooth plains material (Murray and others, 1974). This unit covers vast expanses of Borealis Planitia, a depression about in diameter that has an irregular arcuate west boundary. This depression is located over the site(s) of one or several old impact structures (Boyce and Grolier, 1977).
Adjacent quadrangles to the south of Borealis are Victoria quadrangle (0° to 90° W), Shakespeare quadrangle (90° to 180° W), Raditladi quadrangle (180° to 270° W), and Hokusai quadrangle (270° to 0° W). It is opposite the Bach quadrangle at the south pole.
Mariner 10 images
In the Borealis region, Mariner 10 images are available for only the western hemisphere, from long 0° to about long 190° W. Mercury was in darkness beyond long 190° W. on March 29, 1974, when the first Mariner 10 flyby acquired the most useful photographs of the region. Most of the photographs used for geologic mapping were acquired by the departing spacecraft during the first pass (Mercury I). The Mercury II encounter provided no usable images of the map area; two low-oblique photographs suitable for geologic mapping were acquired during the third flyby on March 17, 1975. No stereoscopic photographic pairs are available for the Borealis region.
Because the terminator was a few degrees away from the 0°-180° meridian at the time of the first encounter, photographs of the region were acquired under a wide range of lighting conditions. These conditions and the large obliquity of the photographs hampered geologic interpretation of surface materials in the map area, as they did in the Kuiper (De Hon and others, 1981), Victoria (McGill and King, 1983), and Shakespeare (Guest and Greeley, 1983) quadrangles to the south.
Climate Mercury's equatorial plane is inclined less than 2° to its orbital plane (Klaasen, 1976; Murray and others, 1981, p. 28); its rotation period of 58.64 terrestrial days is in two-thirds resonance with its orbital period of 87.97 terrestrial days (Colombo, 1965; Colombo and Shapiro, 1966). The resulting lag and orbital eccentricity create a variation of mean temperature not only with latitude, as on the Earth, but also with longitude. However, because of Mercury's relatively slow rotational period, diurnal variations in temperature probably greatly exceed mean-temperature variations along latitude and longitude, even in the high latitudes. Its pronounced orbital eccentricity (0.2563) causes the apparent solar intensity at Mercury to vary by more than a factor of 2 throughout a Mercurian year, corresponding to about a 20 percent change in equilibrium temperature. Further, conservation of orbital angular momentum and spin-orbit coupling cause considerable variation in the length of daylight. Dawns and sunsets are prolonged by the long transit time of the Mercurian horizon across the solar disk, so that daylight is lengthened and nighttime reduced by several terrestrial days at sunset and vice versa at sunrise (Robert Wildey, U.S. Geological Survey, oral commun., 1982). Despite these considerations and despite the daily range in surface temperatures of several hundred kelvins, the subsurface temperature in the polar regions always remains well below freezing (Murray, 1975).
Stratigraphy Within the Borealis region, three widespread plains units are recognized largely by their obvious differences in crater density, which is closely related to relative age (Soderblom and Boyce, 1972). From most heavily cratered (oldest) to least cratered (youngest), these units are intercrater plains material, intermediate plains material, and smooth plains material. Visual identification is confirmed and refined by actual crater counts. If one uses the lunar surface as a frame of reference, the crater density of Mercurian plains in the Borealis region is bracketed by that of the lunar uplands, the most heavily cratered lunar surface, and that of Oceanus Procellarum, a moderately cratered lunar mare surface. The curve for the lunar uplands was derived from crater counts in the region northwest of crater Tsiolkovskiy, between crater Mendeleev and Mare Smithii. The curve for the southeastern part of Oceanus Procellarum was obtained in an area centered near lat 2°00' N. and long 31°00'W., south of the crater Kunowsky. Ocean Procellarum has long been considered close to the "average lunar mare" (Hartmann, 1966, 1967); its crater density is intermediate between those of the heavily cratered Mare Tranquillitatis and the lightly cratered Mare Serenitatis.
Material of Borealis Planitia was not included in the smooth plains count because images of the area were blurred by spacecraft motion, and so reliable crater counts could not be obtained. However, smooth plains south of lat 65° N. in the Shakespeare quadrangle, in the crater Strindberg and in Suisei Planitia, are included in these counts. The plains materials that lie outside Borealis Planitia are distributed in irregular belts, which are subparallel to the terminator and to one another. Eastward from long 190° W., the following belt pattern is observed: intercrater plains material, intermediate plains material, and again intercrater plains material. All three belts extend southward into the Shakespeare quadrangle (Guest and Greeley, 1983).
Distinguishing one type of plains material from another by variations in roughness and crater density is highly dependent on the resolution and lighting conditions of individual Mariner frames (Schaber and McCauley, 1980). This constraint is well documented for the Moon (Masursky and others, 1978, p. 80–81) and for Mars (Boyce and others, 1976). In the Borealis region, where intercrater and intermediate plains materials were imaged at an increasingly low sun angle close to the terminator, the number of observable small craters increase with decreasing distance from the terminator and concomitantly decreasing sun angle. This discrepancy in the apparent abundance of craters occurs only for craters that have small diameters and can be obviated by counting only craters larger than in diameter.
Older plains materials The intercrater plains material is the oldest recognizable map unit in the Borealis region. It lies between large craters from about long 155° to long 190° W., and it also occurs between clusters of closely packed and overlapping large craters west of crater Gauguin and south and southeast of crater Mansart. The unit was described originally by Trask and Guest, who considered it to be the most widespread unit on Mercury; Strom reported that this material covers one-third of the surface viewed by Mariner 10. The principal morphologic characteristic of the intercrater plains material is the high density of superposed craters 5 to 10 km in diameter, which are commonly shallow and elongate; probably they are secondary craters derived from nearby large primary craters that are superposed on the unit. As one group, the large craters and associated intercrater plains form some of the heavily cratered terrain defined by Trask and Guest.
The relative age and nature of intercrater plains material are as uncertain in the Borealis region as they are elsewhere on Mercury. Strom noted the similarity in surficial morphologies between mercurian intercrater plains and pre-Imbrian pitted plains south-southwest of Mare Nectaris on the Moon (Wilhelms and McCauley, 1971; Scott, 1972). The pits in the lunar pre-Imbrian pitted plains are similar to the small secondaries that pepper the surface of Mercurian intercrater plains material. On the Moon, pre-Imbrian pitted plains material embays the Janssen Formation (Scott, 1972), whose base is defined as the base of the Nectarian System (Stuart-Alexander and Wilhelms, 1975). However, the crater density of the intercrater plains material in the Borealis region matches that of an area on the far side of the Moon, in the region northwest of crater Tsiolkovskiy bounded by crater Mendeleev and Mare Smithii. This area is dominated by pre-Nectarian unmantled terra and pre-Nectarian and Nectarian craters (Wilhelms and El- Baz, 1977). The similarity in crater density of intercrater plains material on Mercury and of pre-Nectarian terrain on the Moon is geologically significant, inasmuch as it shows that the oldest recognizable surfaces on both Mercury and the Moon went through similar stages of crustal cratering, but not necessarily at the same absolute geologic time. Differences in crater density as well as embayment relations in the Borealis region show that the intercrater plains material and the smoother intermediate plains material are younger than many craters in the area northeast of crater Turgenev, and older than smooth plains material in Borealis Planitia.
The relative age of intercrater plains material has a bearing on its origin. If very old, intercrater plains material may consist of anorthosite derived from a magma ocean such as may have existed on the Moon (Wood and others, 1970). If emplaced during later stages of mercurian evolution, it may consist of basin ejecta or lava flows. However, planetwide, the morphologic evidence for an impact origin rather than a volcanic one is not compelling. Whether or not either hypothesis is eventually substantiated, the emplacement of intercrater plains material likely began during an early stage of intense accretionary bombardment (Guest and O’Donnell, 1977) and lasted until the time of formation of intermediate plains material.
This general conclusion seems to be supported in the Borealis region by the relative scarcity of craters between 30 km and 60 km in diameter. This scarcity may indicate resurfacing by crater overlap and blanketing by crater ejecta or resurfacing by lava flows. Craters ≥60 km in diameter on Mercury also are relatively scarce compared to similar craters in the lunar uplands northwest of crater Tsiolkovskiy. The reduced density of large craters and basins on Mercury relative to the Moon could be either a function of different crater-population rates on these bodies or an effect of different crustal histories (Schaber and others, 1977).