 A flat map projection of the dark Nicholson Regio. | |
| Feature type | Regio |
|---|---|
| Coordinates | 33°06′S6°24′W / 33.10°S 6.40°W |
| Diameter | 3,900 kilometres (2,400 mi) |
| Eponym | Seth Barnes Nicholson |
Nicholson Regio is a large, dark, surface feature on Jupiter's moon Ganymede. It is a roughly-oval feature composed of several dark, ancient areas that are dissected or interrupted by younger sulci.
The International Astronomical Union (IAU) follows the rule that dark areas on Ganymede (regiones) should be named after astronomers who made significant contributions to the study of Jupiter and its moons. Nicholson Regio is named after Seth Barnes Nicholson, an American astronomer who discovered four of Jupiter's minor moons, Sinope, Lysithea, Carme and Anake. [1] The name was approved by the IAU in 1979. [2]
Nicholson Regio is a massive, bean-shape area that covers most of the southern portion of Ganymede's Jupiter-facing hemisphere.
The dark area of Nicholson Regio is divided into three rough segments by the Arbela Sulcus and Dardanus Sulcus. Arbela Sulcus cuts the regio from its southwest to its northeast, while Dardanus Sulcus cuts from the north to its southeast.
The narrow unnamed sulcus separates Nicholson Regio from another regio called Barnard Regio to the north, while the Harpagia Sulcus separates Nicholson from Melotte Regio to the east. The Borsippa Sulcus marks the northeastern boundary of Nicholson regio, while the southwestern boundary of the regio is marked by the rest of Arbela Sulcus. [3]
The dark areas to the southeast of the regio (which are probably physically related to Nicholson Regio) are officially not considered part of Nicholson.
Nicholson Regio is part of several sections of Ganymede called quadrangles. Nicholson Regio is so massive that it is part of four quadrangles, the Dardanus, Misharu, Nabu, and Namtar quadrangles (designated as Jg6Jg10Jg11 and Jg14 respectively). [4] Nicholson Regio also resides on the side of Ganymede that always faces Jupiter, caused by the moon's synchronous rotation. This means that an observer on Nicholson Regio will always see Jupiter.
Like the other dark regions on Ganymede, Nicholson Regio is believed to be one of most ancient parts of the surface on the moon. [5] [6] The dark regions on Ganymede are equivalent to the brighter highlands on the Earth's Moon in terms of age as both are considered the oldest parts of their respective moons. [7] Nicholson Regio is a very rugged place with a tumultuous history. Its dark areas are dominated by extremely rough terrain, cut by thousands of fractures, scarps, and other tectonic disruptions that crisscross its surface everywhere. Occasionally, smoother, linear, plank-like strips of bright terrain sharply cut across the dark areas which can easily be noticed due to how different they are from the surrounding plains. [8]
The western part of Nicholson Regio contains the darkest sections of the feature and they probably contain some of its oldest terrain. In enhanced images, it has a brownish-black color and is less red than other regiones elsewhere on Ganymede. This section is saturated with many ray craters, halo craters, and regular craters, although none of them have been given official names.
The Dardanus Sulcus sharply divides Nicholson Regio into its western and eastern lobe. It is younger and brighter compared to all dark areas of the regio. To the south of Nicholson Regio where Dardanus Sulcus terminates is the crater Nabu.
The eastern part of Nicholson Regio is also very rugged. On the side that borders Harpagia Sulcus, numerous grooves run parallel to each other, resembling a tilted stack of books, occasionally pockmarked by small craters. [9] However, this section is lighter in color than the western side. The brighter shade suggests that the dark material has been partially erased by brighter, newer materials which is a common occurrence on Ganymede. Like its western lobe, the eastern side of Nicholson Regio is full of craters. The named craters in that region include Damkina, Khumba, Salu and Seker. The brightest named crater in the region is the ray crater Enkidu which lies to its east. Enkidu straddles the boundary between the dark regio and the lighter terrain outside.
The Arbela Sulcus is a major feature on the eastern lobe of Nicholson Regio. The sulcus snakes its way in the middle of the regio for thousands of kilometers. It is smoother and younger than the rest of Nicholson Regio and it was probably created by the stretching and faulting of the moon's icy crust. [10] Arbela Sulcus is so smooth compared to it surrounding that it is highly noticeable in images. Arbela Sulcus further divides the eastern lobe of Nicholson Regio into a northeastern part and southeastern part.
A 2020 study by Hirata, Suetsugu, and Ohtsuki suggests that Ganymede may have been struck by a massive asteroid or comet approximately 4 billion years ago. The impact was likely so immense that it created a global-scale multi-ring structure. By analyzing the concentric patterns of furrow systems in Ganymede’s dark regions—including those found in Nicholson Regio—the researchers inferred that Nicholson, Barnard, Galileo, Marius, Perrine Regiones are probably remnants of a single ancient basin. This structure is analogous to Callisto’s multi-ring basins like Valhalla or Asgard, but even larger than any of them. Over billions of years, tectonic activity and active resurfacing by younger, brighter grooved terrains (now seen as Ganymede's sulci terrain) fragmented these dark regions and obscured much of the original ring pattern. Today, only traces of this colossal basin remain. If confirmed by future missions, this impact would rank among the largest known in the Solar System. [11]
Nicholson Regio is one of the most studied and photographed parts of Ganymede. It is so massive that it can be seen even by Earth-based telescopes such as the Hubble space telescope. Like many other regiones, it can also be seen by amateur telescopes provided that they have enough resolving power to do so.
Voyager 1 became the first probe to send back clear images of Nicholson regio in March 1979 as it flew by Jupiter.
Galileo was able to image the regio several times as it orbited Jupiter from December 1995 to September 2003. As of 2025, the highest available resolution images of Nicholson Regio were taken by Galileo.
In December 2000, the Cassini space probe was able to image Nicholson Regio in low resolution.
The New Horizons probe was able to photograph about half of Nicholson Regio in February 2007. The eastern half of the regio was in darkness during the probe's quick visit.
The Juno spacecraft was barely able to photograph Nicholson Regio during its single close flyby of Ganymede. Most of the regio was behind the moon's southern limb during Juno's Perijove 34 flyby in June 2021. Only the northern part of Nicholson regio was photographed.
The Jupiter Icy Moons Explorer (Juice), a space probe operated and launched by the European Space Agency's (ESA), is scheduled to arrive at Jupiter in July 2031, [13] and in December 2034, it is expected to enter into a low orbit around Ganymede at a distance of just 500 kilometres (310 mi). [14] Juice is expected to be able to capture even more detailed images of Nicholson Regio.
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