In June 1962, Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences led by Vladimir Kotelnikov became first to bounce radar signal off Mercury and receive it, starting radar observations of the planet.[118][119][120] Three years later radar observations by Americans Gordon Pettengill and R. Dyce using 300-meter Arecibo Observatory radio telescope in Puerto Rico showed conclusively that the planet's rotational period was about 59 days.[121][122] The theory that Mercury's rotation was synchronous had become widely held, and it was a surprise to astronomers when these radio observations were announced. If Mercury were tidally locked, its dark face would be extremely cold, but measurements of radio emission revealed that it was much hotter than expected. Astronomers were reluctant to drop the synchronous rotation theory and proposed alternative mechanisms such as powerful heat-distributing winds to explain the observations.[123]
Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury's orbital period, and proposed that the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.[124] Data from Mariner 10 subsequently confirmed this view.[125] This means that Schiaparelli's and Antoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every second orbit and recorded them, but disregarded those seen in the meantime, when Mercury's other face was toward the Sun, because the orbital geometry meant that these observations were made under poor viewing conditions.[115]
Ground-based optical observations did not shed much further light on the innermost planet, but radio astronomers using interferometry at microwave wavelengths, a technique that enables removal of the solar radiation, were able to discern physical and chemical characteristics of the subsurface layers to a depth of several meters.[126][127] Not until the first space probe flew past Mercury did many of its most fundamental morphological properties become known. Moreover, recent technological advances have led to improved ground-based observations. In 2000, high-resolution lucky imaging observations were conducted by the Mount Wilson Observatory 1.5 meter Hale telescope. They provided the first views that resolved surface features on the parts of Mercury that were not imaged in the Mariner mission.[128] Later imaging has shown evidence of a huge double-ringed impact basin even larger than the Caloris Basin in the non-Mariner-imaged hemisphere. It has informally been dubbed the Skinakas Basin.[129] Most of the planet has been mapped by the Arecibo radar telescope, with 5 km resolution, including polar deposits in shadowed craters of what may be water ice
Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury's orbital period, and proposed that the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.[124] Data from Mariner 10 subsequently confirmed this view.[125] This means that Schiaparelli's and Antoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every second orbit and recorded them, but disregarded those seen in the meantime, when Mercury's other face was toward the Sun, because the orbital geometry meant that these observations were made under poor viewing conditions.[115]
Ground-based optical observations did not shed much further light on the innermost planet, but radio astronomers using interferometry at microwave wavelengths, a technique that enables removal of the solar radiation, were able to discern physical and chemical characteristics of the subsurface layers to a depth of several meters.[126][127] Not until the first space probe flew past Mercury did many of its most fundamental morphological properties become known. Moreover, recent technological advances have led to improved ground-based observations. In 2000, high-resolution lucky imaging observations were conducted by the Mount Wilson Observatory 1.5 meter Hale telescope. They provided the first views that resolved surface features on the parts of Mercury that were not imaged in the Mariner mission.[128] Later imaging has shown evidence of a huge double-ringed impact basin even larger than the Caloris Basin in the non-Mariner-imaged hemisphere. It has informally been dubbed the Skinakas Basin.[129] Most of the planet has been mapped by the Arecibo radar telescope, with 5 km resolution, including polar deposits in shadowed craters of what may be water ice
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