Ina is a peculiar small depression ("
crater
Crater may refer to:
Landforms
*Impact crater, a depression caused by two celestial bodies impacting each other, such as a meteorite hitting a planet
*Explosion crater, a hole formed in the ground produced by an explosion near or below the surfac ...
" in
IAU
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...
nomenclature
Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal naming conventions, conventions of everyday speech to the i ...
) on the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, in
Lacus Felicitatis
Lacus Felicitatis (Latin ''fēlīcitātis'', "Lake of Happiness") is a small patch of the lunar surface that has been inundated by flows of lava, leaving a level patch with a lower albedo than the surrounding ground. It is located in Terra Niviu ...
. It is , wide and 64 m deep (from the deepest point of the bottom to the highest point of the rim).
[
Ina is remarkable for several dozens low hills with flat or rounded tops and very sharp rounded boundaries, looking like drops of ]mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
.[ Their surface looks like the usual surface of the Moon while the space between them is very different. Ina is the most prominent of several dozen similar features on the Moon. Their origin is unclear.][
]
Discovery, exploration and naming
Ina was discovered on photographs taken in 1971 by the crew of Apollo 15
Apollo 15 (July 26August 7, 1971) was the ninth crewed mission in the United States' Apollo program and the fourth to Moon landing, land on the Moon. It was the first List of Apollo missions#Alphabetical mission types, J mission, with a ...
from lunar orbit.[ It could have been found 5 years earlier on images taken by ]Lunar Orbiter 4
Lunar Orbiter 4 was a robotic U.S. spacecraft, part of the Lunar Orbiter program, Lunar Orbiter Program, designed to orbit the Moon, after the three previous orbiters had completed the required needs for Project Apollo, Apollo mapping and site s ...
had a photographic flaw not prevented this.[ At the end of 1972 Ina was observed and photographed by the crew of ]Apollo 17
Apollo 17 (December 7–19, 1972) was the final mission of NASA's Apollo program, the most recent time humans have set foot on the Moon or traveled beyond low Earth orbit. Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on ...
.[ After Apollo it was reconnoitred by several orbiting spacecraft beginning in 2009 when the ]Lunar Reconnaissance Orbiter
The Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric polar mapping orbit. Data collected by LRO have been described as essential for planning NASA's future human and robotic missions t ...
obtained photos of Ina with resolution
Resolution(s) may refer to:
Common meanings
* Resolution (debate), the statement which is debated in policy debate
* Resolution (law), a written motion adopted by a deliberative body
* New Year's resolution, a commitment that an individual mak ...
of about 0.5 m/pixel and with varying illumination angles.[
On a topophotomap published in 1974 by ]NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
this feature was given the Latin female name Ina, in accordance with the convention to give small lunar craters human first names.[ In 1979 this name was adopted by the ]International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
.[ In Apollo-era publications the feature is called ''D-caldera'' due to its shape, and at the time it was believed to be a unique feature on the moon.][
Two neighbouring features were named in 1976. They are the small craters Osama on Ina's southwestern edge and Dag to its northwest (both 400 m in diameter). The widest hill in the eastern part of Ina (650 m wide) was named Mons Agnes in 1979.
]
Description
Ina is located on top of a rounded upland (dome) 300 m high and 15 km in diameter.[ It is situated on an elongated ]plateau
In geology and physical geography, a plateau (; ; ), also called a high plain or a tableland, is an area of a highland consisting of flat terrain that is raised sharply above the surrounding area on at least one side. Often one or more sides ha ...
( horst) about 30 km wide.[ This plateau stands in the middle of ]Lacus Felicitatis
Lacus Felicitatis (Latin ''fēlīcitātis'', "Lake of Happiness") is a small patch of the lunar surface that has been inundated by flows of lava, leaving a level patch with a lower albedo than the surrounding ground. It is located in Terra Niviu ...
‒ a small lunar lake between Mare Serenitatis
Mare Serenitatis (Latin ''serēnitātis'', the "Sea of Serenity") is a lunar mare located to the east of Mare Imbrium on the Moon. Its diameter is .
Geology
Mare Serenitatis is located within the Serenitatis basin, which is of the Nectarian ep ...
, Mare Vaporum
Mare Vaporum (Latin ''vapōrum'', the "sea of vapors") is a lunar mare located between the southwest rim of Mare Serenitatis and the southeast rim of Mare Imbrium. It was named by Giovanni Battista Riccioli in 1651.''Autostar Suite Astronomer ...
and Mare Imbrium
Mare Imbrium (Latin ''imbrium'', the "Sea of Showers" or "Sea of Rains", "Sea of Tears") is a vast lava plain within the Imbrium Basin on the Moon and is one of the larger craters in the Solar System. The Imbrium Basin formed from the colli ...
.
Ina is a D-shaped depression 2.9 km × 1.9 km wide. It has an elevated rim 600–1000 m wide and 30–40 m high. The eastern part of the rim is 10 m higher than the western. Its outer slope is very gently sloping (1°–3°) and lacks a distinct edge while the inner slope is very steep (tens of degrees) and has a very sharp border with the depression. The deepest point of the depression is located somewhat to the northwest of its center. This point is 30 m deeper than the edges of the depression and 64 m deeper than the highest point of the rim.[
There are 2 clearly distinct types of surface inside Ina: hills and lowlands. The surface of the hills looks like the usual surface of ]Lacus Felicitatis
Lacus Felicitatis (Latin ''fēlīcitātis'', "Lake of Happiness") is a small patch of the lunar surface that has been inundated by flows of lava, leaving a level patch with a lower albedo than the surrounding ground. It is located in Terra Niviu ...
while the surface of the lowlands is very different.[
There are several dozen hills within Ina. They are diverse in size and have rounded ]amoeba
An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by extending and ret ...
-like edges, like drops of mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
.[ Many of them are interconnected with other hills or with the edge of the depression.][ They are rather low (5–25 m, usually 10–15 m][). Their tops are flat or slightly rounded while their slopes are steep. The edges of the hills are usually very sharp. Often they are surrounded by a little moat. The boundary between the hills and the lowland has the same appearance as the outer boundary of the depression. The surface of the hills is very smooth compared to the lowlands. In addition, although the sample size is not great, the hills do have an ]impact crater
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters ...
density intermediate between the fresh lowlands and the ancient neighboring plains of Lacus Felicitatis.[
Ina's lowlands are much rougher than its hills and show lots of small irregular relief features whose height is no more than several meters.][ Some craters are also discernible there.][ Some of the lowlands contain small patches with very bright tone. These are outcrops of scattered rocks in size. Such patches are located mainly near the border between the lowlands and the hills, especially in the lowest places.][
Ina's lowlands are bright bluish-grey. The hills are darker and brown (the usual lunar surface colour).][ The lowlands resemble freshly exposed ]basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
with high titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
content like the basalt seen in some young impact craters. Ina is surrounded by a weak dark halo.[ The surface of its surroundings is slightly more blue than the more distant surface.][
]
Analogs
Ina is the most prominent, large and well-known representative of a class of features called irregular mare patch
An irregular mare patch also known as an IMP, is a smooth, rounded, slightly mounded area, generally about 500 meters wide, occurring in the lunar maria.Wood, Charles. "Strange Little IMPs." ''Sky and Telescope,'' February 2015 issue.
Discovery
D ...
es,[Wood, Charles. "Strange Little IMPs." ''Sky and Telescope,'' February 2015 issue.] which are also known as "meniscus hollows" for the similarity of their edges to a convex meniscus
Meniscus may refer to:
*Meniscus (anatomy), crescent-shaped fibrocartilaginous structure that partly divides a joint cavity
*Meniscus (liquid)
The meniscus (plural: ''menisci'', from the Greek for "crescent") is the curve in the upper surface ...
.[ Several dozen such features are known on the ]Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, all located on the maria
Maria may refer to:
People
* Mary, mother of Jesus
* Maria (given name), a popular given name in many languages
Place names Extraterrestrial
* 170 Maria, a Main belt S-type asteroid discovered in 1877
* Lunar maria (plural of ''mare''), large, ...
. They are hypothesized to be volcanic in origin but several alternative hypotheses exist (see Origin
Origin(s) or The Origin may refer to:
Arts, entertainment, and media
Comics and manga
* Origin (comics), ''Origin'' (comics), a Wolverine comic book mini-series published by Marvel Comics in 2002
* The Origin (Buffy comic), ''The Origin'' (Bu ...
below) and their origin is far from certain.[ Somewhat similar but distinct features, called '' hollows'', are rather common on ]Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
. These features differ from lunar meniscus hollows in having bright halos; they are also more widespread, often bigger and usually located in impact craters.[
Image:14284-Moon-Maskelyne-LRO-20141012.jpg, Ina-like feature in lunar ]Mare Tranquillitatis
Mare Tranquillitatis (Latin ''tranquillitātis'', the Sea of Tranquillity or Sea of Tranquility; see spelling differences) is a lunar mare that sits within the Tranquillitatis basin on the Moon. It is the first location on another world to be ...
. Width of the image is 3 km.
Image:Hollows in Raditladi 250.png, "Hollows" on Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
. Colors are enhanced. Width of the image is 7 km.
Interpretations
Age
The surface of Ina's lowlands seems to be much younger than the surface of its hills and outer surroundings. Evidence of this includes the light color and low concentration of craters on these lowlands. The Moon's surface darkens over time and multiple meteorite impacts dot it with craters, blur sharp edges of all relief features and make slopes more gentle.[ Ina seems to be one of the youngest features on the Moon.][ The surface of the hills is much older: its age seems to be roughly equal to the age of the usual surface of Lacus Felicitatis (more than 1 billion years][) but the slopes and edges of these hills are young: they couldn't maintain their steepness and sharpness even for 50 million years.][ The dome on which Ina sits seems to be somewhat younger than its surroundings, judging from crater concentration.][
On the other hand, there is a lower estimate for the age of the tops of Ina's hills: 33±2 million years (based on crater counts with the result of 137 craters/km2 for craters >10 m in diameter).][
The latest research suggests Ina is not so young.] The study looked at well-studied volcanoes on Earth that might be similar to Ina. Ina appears to be a pit crater on a shield volcano, a gently sloping mountain similar to the Kilauea volcano in Hawaii. Kilauea has a pit crater similar to Ina known as the Kilauea Iki crater, which last erupted in 1959.
As lava from that eruption solidified, it created a highly porous rock layer inside the pit with underground vesicles as large as three feet in diameter and surface void space as deep as two feet. That porous surface, Head and his colleagues say, is created by the nature of the lava erupted in the late stages of events like this. As the subsurface lava supply starts to diminish, it erupts as "magmatic foam" - a bubbly mixture of lava and gas. When that foam cools and solidifies it forms the highly porous surface.
The researchers suggest that an Ina eruption would also have produced magmatic foam. And because of the Moon's decreased gravity and nearly absent atmosphere the lunar foam would have been even fluffier than on Earth so it is expected that the structures within Ina are even more porous than on Earth.
It is the high porosity of those surfaces that throws off date estimates for Ina, both by hiding the buildup of regolith and by throwing off crater counts.
A highly porous surface, the researchers say, would allow loose rock and dust to filter into surface void space, making it appear as though less regolith has built up. That process would be perpetuated by seismic shaking in the region, much of which is caused by ongoing meteor impacts. "It's like banging on the side of a sieve to make the flour go through," Head said. "Regolith is jostled into holes rather than sitting on the surface, which makes Ina look a lot younger."
Porosity could also skew crater counts. Laboratory experiments using a high-speed projectile cannon have shown that impacts into porous targets make much smaller craters. Because of Ina's extreme porosity, the researchers say, its craters are much smaller than they would normally be, and many craters might not be visible at all. That could drastically alter the age estimate derived from crater counts.
The researchers estimate that the porous surface would reduce by a factor of three the size of craters on Ina's mounds. In other words, an impactor that would make a 100-foot-diameter crater in lunar basalt bedrock would make a crater of a little over 30 feet in a foam deposit.
Taking that scaling relationship into account, the team gets a revised age for the Ina mounds of about 3.5 billion years old. This age is similar to the surface age of the volcanic shield that surrounds Ina, and places the Ina activity within the timeframe of common volcanism on the Moon
Volcanism on the Moon is represented by the presence of volcanoes, pyroclastic deposits and vast lava plains on the lunar surface. The volcanoes are typically in the form of small domes and cones that form large volcanic complexes and isolated edi ...
.
The researchers believe this work offers a plausible explanation for Ina's formation without having to invoke the puzzling billion-year pause in volcanic activity.
"We think the young-looking features in Ina are the natural consequence of magmatic foam eruptions on the Moon," Head said. "The landforms created by these foams simply look a lot younger than they are."
Origin
Ina's origin is unclear as of 2015.[ Firstly it was interpreted as a ]caldera
A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
of a very low ancient volcano.[ Another version considers it as a result of powerful ejection of some gases (volcanic or even radiogenic), which removed ]regolith
Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
. In that case the hills are places where the original regolith was preserved. The other hypothesis says that the hills are lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...
flows, inflated during growth under some dense coating.[ All these versions have flaws.][ In particular, Ina does not seem to have a ring of volcanic ejecta, and the volcanic activity on the Moon seems to have ceased a long time ago.][
According to one another version, Ina appeared (and continues to form) due to collapse of the regolith into some underground cavities. These may be ancient ]lava tube
A lava tube, or pyroduct, is a natural conduit formed by flowing lava from a volcanic vent that moves beneath the hardened surface of a lava flow. If lava in the tube empties, it will leave a cave.
Formation
A lava tube is a type of lava ca ...
s[ or the result of evaporation of volatile compounds.][ In that case the bright rocky places on Ina's floor are where the regolith has already fallen while darker parts of the lowlands are where collapse is incomplete collapsing, and the hills are the slowly diminishing remains of the primigenial surface.][
]
References
External links
Ina at The Moon Wiki
*
*
*
*
*
*
*
Interactive map of Ina
Other interactive maps of Ina (elevation, slope and other)
Interactive map of the Moon, centered on Ina
* {{cite book
, chapter =Unusual Features
, title =Apollo over the Moon. A view from orbit
, editor =H. Masursky
, editor2 =G. W. Colton
, editor3 =F. El-Baz
, date =1978
, url =http://www.hq.nasa.gov/office/pao/History/SP-362/contents.htm
, chapter-url =http://www.hq.nasa.gov/office/pao/History/SP-362/ch7.1.htm
, archive-url =https://web.archive.org/web/20141007191724/http://www.hq.nasa.gov/office/pao/History/SP-362/ch7.1.htm
, archive-date=2014-10-07
— (contains parallel-view stereopair of Ina)
* Panoramic photos from Apollo 15 showing Ina
AS15-P-0176
an
AS15-P-0181
Map of the region with names of surface features
Supplement to an article of Braden et al. (2014) with photos of Ina's analogs
Geological features on the Moon