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Giant Comets Could Endanger Life on Earth

By David Reneke

Giant comets could endanger life on Earth, and there is new evidence of water in the Moon’s interior.

Astronomers have warned that the discovery of hundreds of giant comets in the outer planetary system over the past two decades make these objects a much greater hazard to life than asteroids. These centaurs appear as pinpricks of light in even the largest telescopes because they are so distant from the Earth. Saturn’s 200 km-wide moon Phoebe seems likely to be a centaur that was captured by that planet’s gravity at some time in the past, so until spacecraft are sent to visit other centaurs our best idea of what they look like comes from images obtained by the Cassini space probe orbiting Saturn. NASA’s New Horizons spacecraft, having flown past Pluto earlier this year, will conduct an approach to a 45 km-wide trans-Neptunian object at the end of 2018.

Centaurs move in unstable orbits crossing the paths of the massive outer planets Jupiter, Saturn, Uranus and Neptune. The planetary gravitational fields can occasionally deflect these objects towards the Earth.

Centaurs are typically 50–100 km across, or larger, and a single such body contains more mass than the entire population of Earth-crossing asteroids found to date. Calculations of the rate at which centaurs enter the inner solar system indicate that one will be deflected onto a path crossing the Earth’s orbit about once every 40–100,000 years.

Known severe upsets of the terrestrial environment and interruptions in the progress of ancient civilisations all indicate the arrival of a centaur around 30,000 years ago. This giant comet would have strewn the inner planetary system with debris ranging in size from dust all the way up to lumps several kilometres across.

Specific episodes of environmental upheaval around 10,800 BCE and 2300 BCE, identified by geologists and palaeontologists, are also consistent with this new understanding of cometary populations. Some of the greatest mass extinctions in the distant past, such as the death of the dinosaurs 65 million years ago, may similarly be associated with this giant comet hypothesis.

This work suggests we need to look beyond our immediate neighbourhood too, and look out beyond the orbit of Jupiter to find centaurs. If we are right, then these distant comets could be a serious hazard, and it’s time to understand them better.

New Evidence of Water in the Moon’s Interior

A new study of satellite data has found that volcanic deposits distributed across the surface of the Moon contain unusually high amounts of trapped water compared with the surrounding terrain.

The finding of water in these ancient deposits, which are believed to consist of glass beads formed by the explosive eruption of magma coming from the deep lunar interior, bolsters the idea that the lunar mantle is surprisingly water-rich.

Scientists had assumed for years that the interior of the Moon had been largely depleted of water and other volatile compounds. That began to change in 2008, when a research team including Brown University geologist Alberto Saal detected trace amounts of water in some of the volcanic glass beads brought back to Earth from the Apollo 15 and 17 missions to the Moon.

In 2011, further study of tiny crystalline formations within those beads revealed that they actually contain similar amounts of water as some basalts on Earth. This suggests that the Moon’s mantle, or parts of it at least, contain as much water as Earth’s.

The researchers found evidence of water in nearly all of the large “pyroclastic” deposits that had been previously mapped across the Moon’s surface, including deposits near the Apollo 15 and 17 landing sites where the water-bearing glass bead samples were collected.

The key question is whether those Apollo samples represent the bulk conditions of the lunar interior or instead represent unusual or perhaps anomalous water-rich regions within an otherwise “dry” mantle.

The growing evidence for water inside the Moon suggest that water did somehow survive, or that it was brought in shortly after the impact of asteroids or comets before the Moon had completely solidified.

One of the reasons scientists had assumed that the Moon’s interior should be dry is that it seems unlikely that any of the hydrogen needed to form water could have survived the heat of that impact.

In addition to shedding light on the water story in the early solar system, the findings could also have implications for future lunar exploration. The volcanic beads don’t contain a lot of water but the deposits are large, and the water could potentially be extracted.

Other studies have suggested the presence of water-ice in shadowed regions at the lunar poles, but the pyroclastic deposits are at locations that may be easier to access. I guess anything that helps save future lunar explorers from having to bring lots of water from home is another “giant leap for mankind”.


David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com