Liars In Public Places: How A Lying Lunar Volcano Hides Its True Age
By Judith E Braffman-Miller
“The truth does not change because it is, or is not, believed by a majority of the people,” said the Italian Dominican friar, philosopher, mathematician, poet, and cosmological theorist, Giordano Bruno (1548-1600)–who was ultimately martyred for his beliefs. Some lies, of course, are worse than others and lying about one’s age is not considered to be a particularly sinister lie. Indeed, while orbiting Earth’s Moon in 1971, the astronauts aboard Apollo 15 photographed a bewitching, bothersome, and bewildering geological feature–a strange, bumpy, D-shaped depression approximately two miles long and a mile wide, that has sung a sirens’ song of tantalizing mystery to scientists ever since. Some scientists have proposed that the bizarre feature, named Ina, is evidence of a volcanic eruption on Earth’s Moon within the past 100 million years–a billion years or so after most volcanic activity on the Moon is thought to have come to an end. However, new research released by geologists in March 2017, indicates that Ina may be deceiving scientists about its age, and is really not so young after all.
The new research, published in the journal Geology, concludes that the strange feature was really formed by an eruption around 3.5 billion years ago, making it approximately the same age as the dark volcanic deposits streaking the Moon’s near-side. It’s this very same peculiar form of lava that erupted from Ina that helps hide its real age, the planetary scientists propose.
“As interesting as it would be for Ina to have formed in the recent geologic past, we just don’t think that’s the case. The model we’ve developed for Ina’s formation puts it firmly within the period of peak volcanic activity on the Moon several billion years ago,” explained Dr. Jim Head in a March 28, 2017, Brown University Press Release. Dr. Head is co-author of the paper and a professor in Brown University’s Department of Earth, Environmental, and Planetary Sciences in Providence, Rhode Island.
Footprints Left In Lunar Dust
Earth’s Moon is the only body beyond Earth that we have walked upon, leaving our footprints behind in the lunar dust–a silent testimony that we were once there. Earth’s companion Moon-world has been with us almost from the very beginning, when our Solar System was still forming, approximately 4.5 billion years ago.
As human beings, we apparently have a need to understand and explain our origins. The universal drama of natural phenomena cannot be explained entirely as exclusively scientific–it is, instead, something generally human. Through magical, bewitching myths and remarkable, lovely tales that involve superhuman heroes and heroines, as well as gods and goddesses, ancient pre-scientific civilizations tried to understand the mysterious, elusive complexities of the Cosmos. Earth’s Moon has always presented humanity with a special fascination, long inspiring our very human imaginations to transcend some very frustrating limitations and–as we hunt for that which may exist beyond our Earth-bound lives–help us to progress towards an understanding of who we really are, in the Cosmic scheme of things, in all our bewildering human complexity. The fabrication of ancient gods and goddesses mimics our lovely Moon’s gentle tug on human life existing on our planet. In this particular context, it may be lacking in wisdom to completely dismiss ancient, wild myths–tossing them aside as merely the inventions of those who lived in an unsophisticated, long-forgotten, and long-lost past.
There are more than 100 moons orbiting the eight major planets of our Solar System. The majority of them are icy, small worldlets, that contain only a relatively sparse amount of rocky material–and most of these frozen little objects do their mesmerizing dance around the quartet of gaseous, giant planets inhabiting the distant regions of the outer Solar System–Jupiter, Saturn, Uranus, and Neptune. In dramatic contrast, the warm and well-lit inner region of our Solar System–where are own Earth is situated–is almost moonless. Of the small, solid quartet of relatively tiny terrestrial planets–Mercury, Venus, our Earth, and Mars–only Earth can boast the existence of a large and significant Moon. Mercury and Venus are entirely without a companion moon, and Mars is circled by an interesting, but small, duo of lumpy, misshapen moons, dubbed Phobos and Deimos. The two potato-shaped moons of Mars are frequently considered to be asteroids that escaped from the Main Asteroid Belt, situated between Mars and Jupiter, that were ultimately captured by the gravitational embrace of their adopted planet very long ago.
A moon is a natural satellite in orbit around a larger body–such as a planet–that, in turn, orbits a star. The moon is kept in place both by the gravity of the object that it orbits, as well as by its own gravity. Some planets have moons; some do not. Some dwarf planets–such as Pluto–possess moons. Indeed, one of Pluto’s quintet of moons, Charon, is almost 50% the size of Pluto itself, and some planetary scientists think that Charon is actually a big chunk of Pluto that was ripped away in a monumental, disastrous collision with another object when our Solar System was much younger. Also, some asteroids are circled by very tiny moons of their own!
Volcanism On Earth’s Moon
Vast plains of basaltic lava coat much of the lunar surface. Early astronomers, using primitive instruments, erroneously thought that these plains were seas of water on the Moon. Therefore, these dark regions were named mare (pronounced mahr-ay), which is Latin for “sea”. Furthermore, other volcanic features also appear within the lunar mare–such as sinuous rilles, dark mantle deposits, and small volcanic cones and domes. However, the majority of these volcanic features are somewhat small, and they account for only an insignificant percentage of the volcanic record left to scar the surface of the Moon.
Volcanism on the Moon is different in several ways from volcanism on Earth. First, there is the matter of age. Volcanism on our own planet is a continuing process, and many of Earth’s volcanoes are actually quite youthful on geologic timescales–frequently less than a few hundred thousand years old. In dramatic contrast, most of the volcanism on the Moon appears to have occurred between 3 and 4 billion years ago. Mare samples usually are about 3,500,000,000 years old.
Even the most youthful mare flows have approximate ages of almost 1 billion years. These “young” rocks have not been sampled directly or dated. Therefore, the age of these rocks has not been precisely determined. By comparison, the oldest rock on Earth has been dated at about 3.9 billion years old. The most ancient sea floor basalts on our planet are a “mere” 200 million years old–a brief span on geologic timescales. Because the Moon shows no sign of recent volcanic or geologic activity, it is sometimes referred to as a “dead” object.
The locations of mare volcanism show yet another significant difference from the volcanism that occurs on our planet. Earth’s volcanoes primarily are situated within long linear mountain chains, such as the Andes, that mark the edge of a lithospheric plate. Mountain chains such as the Hawaiian Islands trace out past plate movements over a hotspot on Earth’s mantle.
But the lunar mare usually occur at the bottom of ancient, large impact craters. Therefore, most mare are almost circular in shape. In addition, lunar mountain chains form the edges of these impact basins and have a tendency to encircle the lunar mare. There is also no evidence that plate tectonics ever occurred on the Moon.
The lunar mare are most frequently found on only one side of the Moon–covering almost one-third of the lunar near-side–but a mere 2% of the far-side. However, the lunar surface is much higher on the far-side, and the crust is usually much thicker there as well. Therefore, the most important factors controlling volcanism on the Moon appear to be surface elevation and crustal thickness.
There are some major physical differences between volcanism on our planet and its lunar companion. First, lunar gravity is only one-sixth that of Earth’s. This causes the forces that are driving the lava flows to be significantly weaker on the Moon than on Earth. The flat, smooth mare surfaces suggest that the lavas that formed them were rather runny. They could flow easily and spread out over large areas of the lunar surface. In addition, the comparatively weak gravity could be responsible for explosive eruptions that can hurl debris on the Moon further than on the Earth. Eruptions on the Moon, therefore, should smear lavas out into a broad flat layer, in contrast to the cone-shaped features on our planet. Second, Earth’s dry Moon has essentially no dissolved water. In fact, the so-called “seas” are dry as bone. In dramatic contrast, water is one of the starring players in Earth’s lavas. Water also is important in driving violent volcanic eruptions on Earth. Therefore, the lack of water on the Moon should significantly affect lunar volcanism. This is because, without water, explosive and violent volcanic eruptions are much less likely to occur on Earth’s Moon than on Earth. Indeed, lunar lava should flow quietly, smoothly, and even gently on the Moon’s surface.
Ina: A Lying Lunar Volcano
Ina is situated near the summit of a gently sloped mound of basaltic rock. This location has caused many planetary scientists to propose that it was probably the caldera of an ancient volcano on the Moon. However, it has been difficult to determine just how ancient Ina really is. While the flanks of the volcano appear to be billions of years old, the Ina caldera itself looks considerably younger. One misleading sign of Ina’s so-called “youth” is its bright appearance relative to its environment. The brightness indicates that Ina is of tender years, and has not had sufficient time to accumulate a great quantity regolith. Regolith is the layer of loose rock and dust that builds up on the surface as time goes by.
In addition, there are about 80 distinctive mounds found on Ina. These mounds are smooth hills composed of rock, with some reaching as high as 100 feet, dominating the landscape within the caldera. The mounds show far fewer impact craters when compared to the surrounding area–another sign of relative youth. Old surfaces are scarred by more craters than younger surfaces. This is because, over time, it’s expected that a surface should accumulate more and more craters, as a result of impacting objects. The craters should be of various sizes and accumulate at fairly constant rates. Planetary scientists, knowing this, use the number and size of craters to calculate the relative age of a given surface. In 2014, a team of planetary scientists performed a very careful crater-count on Ina’s mounds, and came to the conclusion that they must have been formed by lava that erupted to the surface within the last 50 to 100 million years–thus indicating youth.
“That was a really puzzling finding. I think most people agree that the volcano Ina sits on was formed billions of years ago, which means there would have been a pause in volcanic activity for a billion years or more before the activity that formed Ina. We wanted to see if there might be something about geologic structure within Ina that throws off our estimation of its age,” Dr. Head explained in the March 28, 2017 Brown University Press Release.
In order to solve the mystery of Ina’s true age, the team of scientists looked at well-studied volcanoes on Earth that were similar to Ina. Ina is a pit crater on a shield volcano, a gently sloping mountain that is comparable to Kilauea volcano in Hawaii. Kilauea displays a pit crater similar to Ina that has been named the Kilauea Iki crater, which erupted back in 1959.
The lava that was shot out from that eruption solidified. As it did so, it formed a very porous layer of rock inside the pit, with subsurface vesicles reaching three feet in diameter, as well as surface void space two feet deep. Dr. Head and his colleagues propose the porous surface is formed by the type of lava erupted during the later stages of events like this particular eruption. As the subsurface lava supply began to diminish, it erupted what has been called “magmatic foam”, which is a frothy combination of lava and gas. As it cools off, magmatic foam turns into a solid, and as it does so it creates the highly porous surface.
Dr. Head and his team propose that an Ina eruption would also have manufactured magmatic foam. Furthermore, because of the Moon’s comparatively weak gravitational grasp, and almost nonexistent atmosphere, the lunar magmatic foam would have been fluffier than that seen on Earth. As a result, the scientists suspect that the structures hidden within Ina are even more porous than on Earth.
It’s the high porosity of those surfaces that presents problems in respect to date estimates for Ina, both by hiding the buildup of regolith and by throwing off crater counts.
A very porous surface, according to Dr. Head and his colleagues, would cause dust and loose rock to invade the surface void space. This would make it appear as though less regolith has accumulated. That process would be perpetuated by seismic jostling in the area, much of which is the result of continuing meteor impacts. “It’s like banging on the side of a sieve to make the flour go through. Regolith is jostled into holes rather than sitting on the surface, which makes Ina look a lot younger,” Dr. Head explained in the March 28, 2017 Brown University Press Release.
This high porosity would also affect crater counts. Laboratory experiments using a high-speed projectile cannon have demonstrated that impacts pelting porous targets create much tinier craters. Because of Ina’s high porosity, the planetary scientists propose, its craters are considerably smaller than they would normally be. Furthermore, many of the craters might not be visible at all. This would drastically alter the age estimate derived from crater counts.
The team of planetary scientists then went on to estimate that this highly porous surface would diminish, by a factor of three, the size of craters pockmarking Ina’s mounds. This is because an impactor, that would excavate a 100-foot-diameter crater in lunar basalt bedrock, would form a crater of only a little over 30 feet in the magmatic foam deposit. Taking this crater-size-reduction into consideration, the planetary scientists arrived at a revised age for the Ina mounds of approximately 3.5 billion years. This age is similar to the surface age of the volcanic shield that surrounds Ina–and also places the Ina activity within the time frame of calculated volcanism on the Moon.
The planetary scientists think that this research presents a plausible explanation for Ina’s formation without having to resort to the mysterious billion-year absence of volcanic activity.
Le Qiao, a doctoral student visitng Brown University from the Chinese University of Geosciences, is the lead author of the research.
Dr. Head said in the March 28, 2017 Brown University Press Release that “We think the young-looking features in Ina are the natural consequence of magmatic foam eruptions on the Moon. These landforms created by these foams simply look a lot younger than they are.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, newspapers, and magazines. Although she has written on a variety of topics, she especially loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.