Scientists have proposed a new idea in the long-running debate over the Moon formation.
What is certain is that some sort of impact from another body freed material from the young Earth and the resulting debris coalesced into today’s Moon.
But the exact details of the impactor’s size and speed have remained debatable.
In a report online to be published in Icarus, researchers suggest that the crash happened with a much larger, faster body than previously thought.
Such theories need to line up with what we know about the Moon, about the violent processes that set off the creation of moons, and what computer simulations show about the more sedate gravitational “gathering-up” that finishes the job.
In recent years, scientists’ best guess for how the Moon formed has been that a relatively slowly moving, Mars-sized body called Theia crashed into the very young Earth.
What is certain is that some sort of impact from another body freed material from the young Earth and the resulting debris coalesced into today's Moon
That would have heated both of them up and released a vast cloud of molten material, much of which cooled and clumped together to give rise to the Moon.
That would suggest that the Moon is made up of material from both the early Earth and from Theia, which should be somewhat different from one another.
What complicates that story is a number of observations of “isotopic compositions” – the ratios of naturally-occurring variants of some atoms – taken from the Earth and from lunar samples.
While the Moon has an iron core like Earth, it does not have the same fraction of iron – and computer models supporting the Theia impact idea show just the same thing.
However, the ratio of the Earth’s and the Moon’s oxygen isotopes is nearly identical, and not all scientists agree on how that may have come about.
Confounding the issue further, scientists reporting in Nature Geoscience in March said that a fresh analysis of lunar samples taken by the Apollo missions showed that the Moon and the Earth shared an uncannily similar isotope ratio of the metal titanium.
That, they said, gave weight to the idea that the Moon was somehow cleaved from the Earth itself.
Now, Andreas Reufer, of the Center for Space and Habitability in Bern, Switzerland, and colleagues have run computer simulations that suggest another possibility: that a far larger and faster-moving body made an even more glancing blow with the young Earth.
They said this body would have lost only a small amount of material and most of it would have continued on after the “hit-and-run”.
That results in a much hotter disc of debris from the collision, but matches up with what would be needed to make a Moon-sized body.
The authors suggest that since most of what became the Moon would have been liberated by the impact from the Earth, similarities between the isotope fractions should be more pronounced.
More analyses of different elements within lunar samples – and a great deal more computer simulations that result in a Moon like our own – will be needed to settle the debate.
A huge dome of freshwater that is developing in the western Arctic Ocean has been detected by British scientists.
The bulge is some 8,000 cubic km in size and has risen by about 15 cm since 2002.
The scientists think it may be the result of strong winds whipping up a great clockwise current in the northern polar region called the Beaufort Gyre.
This would force the water together, raising sea surface height, the group tells the journal Nature Geoscience.
“In the western Arctic, the Beaufort Gyre is driven by a permanent anti-cyclonic wind circulation. It drives the water, forcing it to pile up in the centre of gyre, and this domes the sea surface,” explained lead author Dr. Katharine Giles from the Centre for Polar Observation and Modelling (CPOM) at University College London.
“In our data, we see the trend being biggest in the centre of the gyre and less around the edges,” said Dr. Katharine Giles.
Dr. Katharine Giles and colleagues made their discovery using radar satellites belonging to the European Space Agency (Esa).
These spacecraft can measure sea-surface height even when there is widespread ice cover because they are adept at picking out the cracks, or leads, that frequently appear in the frozen floes.
The data (1995-2010) indicates a significant swelling of water in the Beaufort Gyre, particularly since the early part of the 2000s. The rising trend has been running at 2 cm per year.
A lot of research from buoys and other in-situ sampling had already indicated that water in this region of the Arctic had been freshening.
This freshwater is coming in large part from the rivers running off the Eurasian (Russian) side of the Arctic basin.
Winds and currents have transported this freshwater around the ocean until it has been pulled into the gyre. The volume currently held in the circulation probably represents about 10% of all the freshwater in the Arctic.
Of interest to future observations is what might happen if the anticyclonic winds, which have been whipping up the bulge, change behavior.
“What we seen occurring is precisely what the climate models had predicted,” said Dr. Katharine Giles.
“When you have clockwise rotation – the freshwater is stored. If the wind goes the other way – and that has happened in the past – then the freshwater can be pushed to the margins of the Arctic Ocean.
“If the spin-up starts to spin down, the freshwater could be released. It could go to the rest of the Arctic Ocean or even leave the Arctic Ocean.”
If the freshwater were to enter the North Atlantic in large volumes, the concern would be that it might disturb the currents that have such a great influence on European weather patterns. These currents draw warm waters up from the tropics, maintaining milder temperatures in winter than would ordinarily be expected at northern European latitudes.
The creation of the Beaufort Gyre bulge is not a continuous development throughout the 15-year data-set, and only becomes a dominant feature in the latter half of the study period.
This may indicate a change in the relationship between the wind and the ocean in the Arctic brought about by the recent rapid decline in sea-ice cover, the CPOM team argues in its Nature Geoscience paper.
It is possible that the wind is now imparting momentum to the water in ways that were not possible when the sea-ice was thicker and more extensive.
“The ice is now much freer to move around,” said Dr. Katharine Giles.
“So, as the wind acts on the ice, it’s able to pull the water around with it. Depending on how ridged the surface of ice is or how smooth the bottom of the ice is – this will all affect the drag on the water. If you have more leads, this also might provide more vertical ice surfaces for the wind to blow against.”
One consequence of less sea-ice in the region is the possibility that winds could now initiate greater mixing of the different layers in the Arctic Ocean.
Scientists are aware that there is a lot of warm water at depth.
At present, this deep water’s energy is unable to influence the sea-ice because of a buffer of colder, less dense water lying between it and the floes above.
But if this warm water were made to well up because of wind-driven changes at the surface, it could further accelerate the loss of seasonal ice cover.
The CPOM team is now investigating the likelihood of this happening with Cryosat-2, Esa’s first radar satellite dedicated to the study of the Polar Regions.
“We now have the means to measure not only the ice thickness but also to monitor how the ocean under the ice is changing,” says Dr. Seymour Laxon, director of CPOM and co-author of the study, “and with CryoSat-2, we can now do so over the entire Arctic Ocean.”
