The Marius Hills are a set of volcanic

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

 domes

Dome (geology)

In structural geology, a dome is a deformational feature consisting of symmetrically-dipping anticlines; their general outline on a geologic map is circular or oval...

 located in Oceanus Procellarum

Oceanus Procellarum

Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...

 on Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

. The domes are thought to have formed from lavas fairly more viscous

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 than those that formed the Lunar mare

Lunar mare

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...

. These domes average approximately 200–500 m (656.17-1640.43 ft) in height. The Marius Hills take their name from the nearby 41 km (25.5 mi) diameter crater Marius

Marius (crater)

Marius is a lunar crater located on the Oceanus Procellarum. The surface to the west and north of this crater contains a large number of lunar domes spread across an area over a hundred kilometers in diameter that may be of volcanic origin dubbed the Marius Hills...

. These hills represent the highest concentration of volcanic features on the Moon.

Geography and geology

An abundance of domes, cones, and volcanic rilles and channels is characteristic of the Marius Hills. The Lunar Reconnaissance Orbiter

Lunar Reconnaissance Orbiter

The Lunar Precursor Robotic Program is a program of robotic spacecraft missions which NASA will use to prepare for future human spaceflight missions to the Moon. Two LPRP missions, the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite , were launched in June 2009...

 photographed a pit that could be a skylight in an underground lava tube

Lava tube

Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like...

, indicating that part of its roof has collapsed, as what often happens after lava tubes cease to be active.

Data from the Lunar Reconnaissance Orbiter has been used to identify two different varieties of domes among the Marius Hills: (1) large, irregularly shaped domes and (2) smaller domes with steep sides and diameters of about 1–2 km (0.62-1.24 mi). Another feature, possibly pyroclastic

Pyroclastic rock

Pyroclastic rocks or pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic...

, or primarily volcanic in composition, has a roughly circular shape and steep sides. Bright, high albedo

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

 boulders have been shown to be characteristic of lava flows in the Marius Hills. This suggests blocky lava with a high silica content formed these features. This hypothesis, however, is not supported by data obtained from the Clementine lunar orbiter. Analysis of lower albedo, or less reflective boulders suggest that many domes in the area may contain two layers of material: (1) an upper layer of thin, dark material covering (2) a layer of thick, bright material.

The Marius Hills region was at one time considered a possible landing site for a lunar landing mission during the American

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 Apollo program (eventually becoming the alternative site for Apollo 15

Apollo 15

Apollo 15 was the ninth manned mission in the American Apollo space program, the fourth to land on the Moon and the eighth successful manned mission. It was the first of what were termed "J missions", long duration stays on the Moon with a greater focus on science than had been possible on previous...

), with the possibility of gaining insight about the volcanic history of the Moon from domes in the area.

Marius Hills "pit"

The pit discovered by the Lunar Reconnaissance Orbiter (LRO) has been the subject of much research and speculation. There is a possibility that this feature could be a skylight in an underground lava tube. The depth of this pit is estimated to be 80 and 88 m (262.5 and 288.7 ft) and it's width is estimated to be several hundreds of meters.