Tag Archives: Moon

How the Moon Was Formed

The moon is the result of a primordial collision between a Mars-sized planetary body and our planet, but little else beyond this is certain about the silvery world we can see whenever we look at the sky. Despite 61 space missions, six of which were landings, where astronauts collected moon rock samples, there are a whole host of questions. Particularly – exactly how much of the moon, a natural satellite is actually made up from Earth, and how much from this strange, lost world – and what was it like?

Researchers working in France and Israel might have enough evidence to answer one of those questions, finding more about the smaller planetary body that hit us. Apparently, it’s not all that different from home – with a composition that was likely similar to Earth. The latest models proposed suggest that the moon’s current composition is explained best if whatever happened to strike the Earth also formed nearby, in the early days of our solar system – a time of extreme chaos. It is believed that Jupiter alone caused a number of violent collisions between the other worlds in our solar system, before being dragged to its current orbit by Saturn. Two subsequent studies have proposed that both of these worlds continued to accumulate new material as they were continually bombarded with smaller protoplanets. Unlike the moon, however, Earth managed to gather more of this cosmic dust.

The most supported explanation is the “giant impact hypothesis,” which states that the moon first came into being about 4.5 billion years ago, the remains of an accident in which our planet was struck by an object approximately the size of a planet, roughly one tenth the mass of Earth. Attempted recreations of what happened and recent studies of the moon rock samples have suggested that our moon should consist primarily of remains from this mysterious impactor, which planetary scientists call Theia. If this is the case, it would explain why so many of the moon rocks resemble the minerals composing the Earth’s mantle.

The main problem faced by the researchers is that so often, planets bear very distinct compositions from each other. Mars, Mercury and the big asteroids like Vesta, for all that they have in common, also have slightly differing ratios of their respective elements. If this world known as Theia first developed in a remote part of the solar system, then its makeup would be significantly different from that on Earth, and therefore the bulk of the moon should not be so similar to the Earth’s mantle. It had to have struck Earth with a glancing blow, anything stronger could have destroyed it well before the first life forms began appearing on its surface.

In their effort to solve the problem, Alessandra Mastrobuono-Battisti and Hagai Perets at the Israel Institute of Technology both pored over data from a series of simulations – covering the scenarios of 40 different artificial solar systems. The effort applied more computer power than had been utilized by any previous study. The model simulated the growth of the planetary bodies we’re familiar with and then set them off in a game of cosmic billiards, consistently striking each other with their orbits.

When they developed this new simulation, they considered that any planets found farther away from our sun typically contain a greater relative abundance of oxygen isotopes, consistent with chemical mixes observed within the Earth, as well as the moon and Mars. Therefore any such planetesimals that were to form near our planet would contain similar chemical traces. “If they are living in the same neighborhood, they will be made of roughly the same material,” said Perets.

The team described their work in the journal Nature: the majority of these interplanetary collisions were about 20 to 40 percent—big, occurring among bodies that had formed from similar distances near the sun, which was responsible for their similarities in makeup. It’s far less likely that Theia would have sailed a long distance before impacting the planet, and the study lends credence to this idea.

Not all is readily explained in their work. There’s still a wealth of the element tungsten, which needs to be explained. This element, categorized as a siderophile – iron-loving, is expected to sink towards planetary cores as time progresses, something that would affect its variability on planets throughout the solar system, even if they did in fact form at the same time, as Earth and Mars are believed to. Bodies develop their That’s because bodies of cores with different rates, based on a number of variables including size. Although a little mixing as a result of the impact is inevitable, much of Theia’s tungsten-rich mantle material would be flung into orbit from the explosion and eventually incorporated into the moon, making the amounts of tungsten on Earth and the moon considerably different.

In the two independent studies which appeared separately in the journal Nature, Thomas Kruijer at the University of Münster in Germany and Mathieu Touboul at the University of Lyon in France compared the ratio from two tungsten isotopes— both tungsten-184 and tungsten-182 – found in both moon rocks as well as within the Earth’s mantle. According to the teams, these moon rocks contain slightly more tungsten-182 than has been found on Earth.

This is quite an exciting find, especially since this particular isotope of tungsten is actually the result of radioactive decay from an isotope derived of the element hafnium. Its half-life is rather short, approximately nine million years. Because the iron-loving tungsten has a tendency for sinking towards a planet’s core, the hafnium isotope remains closer to the surface, where over an extended period of time, it transforms into tungsten-182. That means there’s an excess of tungsten-182 within a planet’s mantle contrasted with tungsten-184 and other naturally occurring isotopes.

The difference of this isotope on both the Earth and the moon is comparatively small: the two studies rate this level at somewhere between 20 to 27 parts per million. Even a shift this minimal would mean a great deal of chemical fine-tuning, said Kruijer, meaning that chance occurrences are a bit unlikely. “Varying the tungsten by only a percent or so has a dramatic effect,” he says. “The only solution is if the mantle of proto-Earth had similar tungsten-182 content to Theia, and the core of the impactor directly merged with Earth’s.”

That’s not so likely, however. Although most of Theia’s core, considerably heavier than the mantle, shall remain a part of planet Earth, the mantle itself will continue to mix with that of the Earth as it continues to be flung through orbit. As the moon gradually accretes, more chemical mixing will take place. Although the ratio of Theia’s core and its mantle material that actually transforms into parts of the moon is left up to random chance, there must have been at least a small degree of core material, according to Kruijer. Touboul’s team arrived at a similar conclusion: Were these differences in the tungsten actually the result of a random mixture when Theia’s shards sloshed across the Earth, our moon would be a great deal different than it presently is.

Therefore, the simplest and most plausible solution, according to the authors of the study, is known as the “late veneer” hypothesis, positing that the Earth along with the proto-moon began with tungsten isotope ratios that were similar to each other. Since Earth was a much larger and more massive body, it would continuously draw in more planetesimals following the impact, giving the mantle new material to build a crust. The veneer from those planetary fragments would then have significantly more tungsten-184 compared to tungsten-182, and the moon would then contain the same ratio due to the impact.

“This looks like solid data,” Fréderic Moynier, a cosmochemist and astrophysicist at the Institut de Physique du Globe de Paris, said in an email. “It fits with the present theory of late veneer, which is simply based on the elemental abundance of the siderophile elements (such as tungsten): there are simply too many siderophile elements in the present Earth’s mantle (they should all be in the core) and therefore they must have been brought to Earth after core formation via meteorite impacts.”

There is one remaining mystery to be solved. For the moon to remain identical to the Earth’s tungsten ratio, Theia and Earth would have had to begin with similar degrees of tungsten in their composition. Where did it all go? While there are a number of questions to arise from the study, ones that future studies hope to answer in the near future, it seems like a bit more light is waxing over the story of the moon’s origins.

James Sullivan
James Sullivan is the assistant editor of Brain World Magazine and a contributor to Truth Is Cool and OMNI Reboot. He can usually be found on TVTropes or RationalWiki when not exploiting life and science stories for another blog article.

NASA’s Next Destination is an Asteroid

For the last few decades, ever since the gradual decline of interest in the American space program, scientists and engineers have sought a way to get to Mars, a rather lofty goal that might be reachable within the next 20 years, but may perhaps be one of the longest planetary flights that astronauts have yet to undergo, with a great deal of equipment to be tested out first for safely landing on Mars’ thin atmosphere alone. While NASA still hopes to reach the red planet by 2035, their spaceflight program has a different short-term goal for next decade, one that might even be attractive to at least some of the types of people who were adamant against trips to the moon – putting an asteroid in lunar orbit.

You read it right. The plan unveiled yesterday by NASA officials has been called by some as the plan to give the moon its own moon – to capture an asteroid and safely place it within the orbit of our moon for closer study, allowing astronauts to land on it and sample it from the safety of a nearby orbit. Its the goal of the agency’s latest effort known as the Asteroid Redirect Mission (ARM), which hopes to bring about new technology that will be crucial in the mission to Mars and other deep space missions in the future.

NASA’s decision to kick off ARM was postponed for several months, while two individual teams investigated how to best pursue the mission objectives. In its original proposal, known as Option A, ARM proposed using a “grab and bag” approach, a robotic space tug is used to trap a small asteroid in its entirety, cover it in a protective sheath and then guide it towards the moon’s orbit. Option B costs about $100 million more, whitch will snatch the boulder directly, but was the winning option, as it provided more more operational flexibility, according to NASA’s associate administrator Robert Lightfoot.

In a recent conference call he took with reporters, Lightfoot noted that their decision was partially based in the fact that our telescopic surveys have not yet sighted any asteroids that are both small enough and slow-moving to fulfill Option A. Any asteroids suitable for harvesting would also be extremely difficult to categorize from Earth with the existing technology. Option A would therefore be “a one-shot deal,” had he chosen to carry it out, with many potential shortcomings should they choose the wrong target, whereas Option B would have a lot more to choose from.

“From what we know of the asteroids we’ve been to, they have boulders on the surface,” said Lightfoot. “I’m going to have multiple targets when I get there, is what it boils down to.” Implementing Option B would mean that ARM would then be allowed to capture and deliver a space boulder as large as 12 feet across and then carry it off to high lunar orbit.

This might sound to you like searching the entire beach for a grain of sand, considering the size of asteroid belts existing in our solar system, but NASA already has three potential candidates lined up for ARM: asteroids Itokawa, Bennu and 2008 EV5. The Japanese space agency’s craft Hayabusa paid a visit to Itokawa in 2005, and NASA’s OSIRIS-REx already plans to visit and extract new samples from asteroid Bennu, which it will reach by the year 2019. No spacecraft has yet orbited the 2008 EV5, and NASA has placed it on the top of their list for ARM targets. They hope to make their official target selection by 2019, according to Lightfoot.

“[2008 EV5] has been extensively observed” using infrared and radio telescopes,” said Lindley Johnson, the program executive at NASA’s Near-Earth Object Program. Scientists have been using the observations of Johnson’s team for determining the asteroid’s orbit, and also the size, shape, rate of spin and composition. 2008 EV5 has been described as ‘a slowly spinning 400-meter-wide walnut, with a prominent ridge wrapped around its middle.’ It is a carbonaceous asteroid, which means that it’s a composite of rock and also molecular organic compounds and water-rich minerals, containing much of the substance found in the primitive nebula, out of which the solar system condensed from. For astronauts who are privileged to walk on its surface, it may be the closest they’ll ever get to walking on an early primordial Earth. Bennu is also a carbonaceous asteroid, and very soon scientists may be able to hold a piece of the solar system from when it first originated once the OSIRIS-REx returns.

Science, however, is only ARM’s ulterior motive. Their goal, as stated, is to both test and create new potential technologies to improve spaceflight, particularly NASA’s Space Launch System heavy-lift rocket, the Orion deep-space crew capsule and also an advanced solar-electric propulsion engine that would be best suited for long-haul cargo trips such as the Mars mission. NASA has also sold the missions as being a potential step forward in showing how a spacecraft is capable of changing the orbits of asteroids that may pose threats to life on Earth, part of its “Redirect” program.

According to Lightfoot, NASA’s current plan hopes to launch the robotic tug into space by 2020, where it will spend two years navigating for its target. The robotic tug might spend up to 400 days near its target as it selects the right boulder. Once the tug makes its selection, it will then use the extra mass from its target to function as a “gravity tractor,” in which it will orbit the asteroid in a way that will slightly alter its trajectory. While this orbital shift may be slight, it will still be in the measurable range of ground-based instruments, and could prepare NASA for making stronger orbital shifts in the future, effectively averting an Armageddon-like scenario. Once the boulder is in its grasp, the robotic tug will then travel to high lunar orbit, where it shall anticipate the arrival of two astronauts via an Orion capsule, an encounter that could happen as soon as the end of 2025. The astronauts will then dock the robotic tug as they conduct spacewalks, investigating the boulder before it returns to Earth, meaning the astronauts will spend approximately an entire month in space preparing the target.

This all might sound exciting, but it’s actually a far cry from what was initially proposed with ARM, and both policy makers and engineers alike are treating it with mild enthusiasm, if not outright disdain. It’s also not without scientists who are critical of the mission. Among them is Mark Sykes, director of the Planetary Science Institute who is unsure of whether, despite the fairly low risks, this mission is of much practical value.

“It is not at all clear how this mission is necessary to advance the stated objective of sending humans to Mars,” Sykes says. “Or even its vicinity.”

There is also concern related to what the mission might mean for NASA’s public relations. The mission may stave off some recent criticism that the agency has not dedicated enough time to space travel. However, some scientists such as NASA’s Advisory Council chair Steve Squyres of Cornell University, feel that the mission may be little more than a tactic – fulfilling political expectations while putting off funding that could be better spent on going to Mars.

“If you’re going to spend $1.25 billion plus launch vehicle costs to do something,” said Squyres, during a council meeting back in January, “and you get the most important things by not going after the rock, don’t go after the rock.”

There are quite a few needs to be met before ARM is fully ready for its mission, such as a solar electric engine that may not be ready by the time of the launch.

The ARM mission was born back in 2010, shortly after President Obama canceled plans for a second manned lunar mission, deciding instead to have astronauts step on an asteroid by the year 2025. At the time, some experts suspected that this was a quicker way to bring humans to Mars, if they would first pick near targets such as the Martian moons Phobos and Deimos which are very reminiscent of asteroids. Unfortunately, NASA’s budget at the time did not have sufficient enough funds for building either the new heavy-lift rockets or deep-space crew capsules fast enough to fulfill that deadline. Sending a manned capsule to an asteroid within its native orbit would simply not be possible by the year 2025. However, there was a slight loophole in the president’s choice of words. Suppose NASA brought the asteroid to the astronauts using that funding? Thus, NASA’s research teams put together ARM.

James Sullivan
James Sullivan is the assistant editor of Brain World Magazine and a contributor to Truth Is Cool and OMNI Reboot. He can usually be found on TVTropes or RationalWiki when not exploiting life and science stories for another blog article.