Was There a Time When the Moon was Habitable?

Artist's concept of a terraformed moon. According to a new study, the Moon may have had periods of habitability in its past where it had an atmosphere and liquid water on its surface. Credit: Ittiz

To put it simply, the Earth’s Moon is a dry, airless place where nothing lives. Aside from concentrations of ice that exist in permanently-shaded craters in the polar regions, the only water on the moon is believed to exist beneath the surface. What little atmosphere there is consists of elements released from the interior (some of which are radioactive) and helium-4 and neon, which are contributed by solar wind.

However, astronomers have theorized that there may have been a time when the Moon might have been inhabitable. According to a new study by an astrophysicist and an Earth and planetary scientist, the Moon may have had two early “windows” for habitability in the past. These took place roughly 4 billion years ago (after the Moon formed) and during the peak in lunar volcanic activity (ca. 3.5 billion years ago).

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New Insights Into What Might Have Smashed Uranus Over Onto its Side

Uranus
A new study indicates that a massive impact may be why Uranus orbits on its side. Credit: NASA/JPL/Voyager mission

The gas/ice giant Uranus has long been a source of mystery to astronomers. In addition to presenting some thermal anomalies and a magnetic field that is off-center, the planet is also unique in that it is the only one in the Solar System to rotate on its side. With an axial tilt of 98°, the planet experiences radical seasons and a day-night cycle at the poles where a single day and night last 42 years each.

Thanks to a new study led by researchers from Durham University, the reason for these mysteries may finally have been found. With the help of NASA researchers and multiple scientific organizations, the team conducted simulations that indicated how Uranus may have suffered a massive impact in its past. Not only would this account for the planet’s extreme tilt and magnetic field, it would also explain why the planet’s outer atmosphere is so cold.

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How Many of Earth’s Moons Crashed Back Into the Planet?

Artist's concept of a collision between proto-Earth and Theia, believed to happened 4.5 billion years ago. Credit: NASA

For decades, scientists have pondered how Earth acquired its only satellite, the Moon. Whereas some have argued that it formed from material lost by Earth due to centrifugal force, or was captured by Earth’s gravity, the most widely accepted theory is that the Moon formed roughly 4.5 billion years ago when a Mars-sized object (named Theia) collided with a proto-Earth (aka. the Giant Impact Hypothesis).

However, since the proto-Earth experienced many giant-impacts, several moons are expected to have formed in orbit around it over time. The question thus arises, what happened to these moons? Raising this very question, a team an international team of scientist conducted a study in which they suggest that these “moonlets” could have eventually crashed back into Earth, leaving only the one we see today.

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Through The Nuclear Looking Glass: The Moon & The Bomb

continents
An impact between a Mars-sized protoplanet and early Earth is the most widely-accepted origin of the Moon. Did smaller impacts seed the formation of continents? (NASA/JPL-Caltech)

For centuries, scientists have been attempting to explain how the Moon formed. Whereas some have argued that it formed from material lost by Earth due to centrifugal force, others asserted that a preformed Moon was captured by Earth’s gravity. In recent decades, the most widely-accepted theory has been the Giant-impact hypothesis, which states that the Moon formed after the Earth was struck by a Mars-sized object (named Theia) 4.5 billion years ago.

According to a new study by an international team of researchers, the key to proving which theory is correct may come from the first nuclear tests conducted here on Earth, some 70 years ago. After examining samples of radioactive glass obtained from the Trinity test site in New Mexico (where the first atomic bomb was detonated), they determined that samples of Moon rocks showed a similar depletion of volatile elements.

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Isotopic Evidence of the Moon’s Violent Origins

Artist’s impression of an impact of two planet-sized worlds (NASA/JPL-Caltech)

Scientists have uncovered a history of violence hidden within lunar rocks, further evidence that our large, lovely Moon was born of a cataclysmic collision between worlds billions of years ago.

Using samples gathered during several Apollo missions as well as a lunar meteorite that had fallen to Earth (and using Martian meteorites as comparisons) researchers have observed a marked depletion in lunar rocks of lighter isotopes, including those of zinc — a telltale element that can be “a powerful tracer of the volatile histories of planets.”

The research utilized an advanced mass spectroscopy instrument to measure the ratios of specific isotopes present in the lunar samples. The spectrometer’s high level of precision allows for data not possible even five years ago.

Scientists have been looking for this kind of sorting by mass, called isotopic fractionation, since the Apollo missions first brought Moon rocks to Earth in the 1970s, and Frédéric Moynier, PhD, assistant professor of Earth and Planetary Sciences at Washington University in St. Louis — together with PhD student, Randal Paniello, and colleague James Day of the Scripps Institution of Oceanography — are the first to find it.

The team’s findings support a now-widely-accepted hypothesis — called the Giant Impact Theory, first suggested by PSI scientists William K. Hartmann and Donald Davis in 1975 — that the Moon was created from a collision between early Earth and a Mars-sized protoplanet about 4.5 billion years ago. The effects of the impact eventually formed the Moon and changed the evolution of our planet forever — possibly even proving crucial to the development of life on Earth.

(What would a catastrophic event like that have looked like? Probably something like this:)

Read more: What’s the Moon Made Of? Earth, Most Likely.

“This is compelling evidence of extreme volatile depletion of the moon,” said Scripps researcher James Day, a member of the team. “How do you remove all of the volatiles from a planet, or in this case a planetary body? You require some kind of wholesale melting event of the moon to provide the heat necessary to evaporate the zinc.”

In the team’s paper, published in the October 18 issue of Nature, the researchers suggest that the only way for such lunar volatiles to be absent on such a large scale would be evaporation resulting from a massive impact event.

“When a rock is melted and then evaporated, the light isotopes enter the vapor phase faster than the heavy isotopes, so you end up with a vapor enriched in the light isotopes and a solid residue enriched in the heavier isotopes. If you lose the vapor, the residue will be enriched in the heavy isotopes compared to the starting material,” explains Moynier.

The fact that similar isotopic fractionation has been found in lunar samples gathered from many different locations indicates a widespread global event, and not something limited to any specific regional effect.

The next step is finding out why Earth’s crust doesn’t show an absence of similar volatiles, an investigation that may lead to clues to where Earth’s surface water came from.

“Where did all the water on Earth come from?” asked Day. “This is a very important question because if we are looking for life on other planets we have to recognize that similar conditions are probably required. So understanding how planets obtain such conditions is critical for understanding how life ultimately occurs on a planet.”

“The work also has implications for the origin of the Earth,”  adds Moynier, “because the origin of the Moon was a big part of the origin of the Earth.”

Read more on the Washington University news release and at the UC San Diego news center.

Inset image: Cross-polarized transmitted-light image of a lunar rock. Photo by James Day, Scripps/UCSD