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Scientists Pin Down Timing of Lunar Dynamo’s Demise

A conventional compass would be of little use on the moon, which today lacks a global magnetic field. But the moon did produce a magnetic field billions of years ago, and it was likely even stronger than the Earth’s field today. Scientists believe that this lunar field, like Earth’s, was generated by a powerful dynamo — the churning of the moon’s core. At some point, this dynamo, and the magnetic field it generated, petered out.

Now scientists from MIT and elsewhere have pinned down the timing of the lunar dynamo’s end, to around 1 billion years ago. The findings appeared in the journal Science Advances.

The new timing rules out some theories for what drove the lunar dynamo in its later stages and favors one particular mechanism: core crystallization. As the moon’s inner iron core crystallized, the liquid core’s electrically charged fluid was buoyantly stirred, producing the dynamo.

“The magnetic field is this nebulous thing that pervades space, like an invisible force field,” says Benjamin Weiss, professor of earth, atmospheric, and planetary sciences at MIT. “We’ve shown that the dynamo that produced the moon’s magnetic field died somewhere between 1.5 and 1 billion years ago, and seems to have been powered in an Earth-like way.”

Weiss’ co-authors on the paper are co-lead authors Saied Mighani and Huapei Wang, as well as Caue Borlina and Claire Nichols of MIT, along with David Shuster of the University of California at Berkeley.

Over the past few years, Weiss’ group and others have discovered signs of a strong magnetic field, of around 100 microteslas, in lunar rocks as old as 4 billion years. For comparison, Earth’s magnetic field today is around 50 microteslas.

In 2017, Weiss’s group studied a sample collected from NASA’s Apollo project, and found traces of a much weaker magnetic field, below 10 microteslas, in a moon rock they determined to be about 2.5 billion years old. Their thinking at the time was that perhaps two mechanisms for the lunar dynamo were at play: The first could have generated a much stronger, earlier magnetic field around 4 billion years ago, before being replaced by a second, more long-lived mechanism that sustained a much weaker field, through to at least 2.5 billion years ago.

“There are several ideas for what mechanisms powered the lunar dynamo, and the question is, how do you figure out which one did it?” Weiss says. “It turns out all these power sources have different lifetimes. So if you could figure out when the dynamo turned off, then you could distinguish between the mechanisms that have been proposed for the lunar dynamo. That was the purpose of this new paper.”

Most of the magnetic studies lunar samples from the Apollo missions have been from ancient rocks, dating to about 3 billion to 4 billion years old. These are rocks that originally spewed out as lava onto a very young lunar surface, and as they cooled, their microscopic grains aligned in the direction of the moon’s magnetic field. Much of the moon’s surface is covered in such rocks, which have remained unchanged since, preserving a record of the ancient magnetic field.

However, lunar rocks whose magnetic histories began less than 3 billion years ago have been much harder to find because most lunar volcanism had ceased by this time.

“The past 3 billion years of lunar history has been a mystery because there’s almost no rock record of it,” Weiss says.

Nevertheless, he and his colleagues identified two samples of lunar rock, collected by astronauts during the Apollo missions, that appear to have suffered a massive impact about 1 billion years ago and as a result were melted and welded back together in such a way that their ancient magnetic record was all but erased.

The team took the samples back to the lab and first analyzed the orientation of each rock’s electrons, which Weiss describes as “little compasses” that either align in the direction of an existing magnetic field or appear in random orientations in the absence of one. For both samples, the team observed the latter: random configurations of electrons, suggesting that the rocks formed in an extremely weak to essentially zero magnetic field, of no more than 0.1 microteslas.

The team then determined the age of both samples using a radiometric dating technique that Weiss and Shuster were able to adapt for this study.

The team put the samples through a battery of tests to see whether they were indeed good magnetic recorders. In other words, once they were reheated by some massive impact, could they have still been sensitive enough to record even a weak magnetic field on the moon, if it existed?

To answer this, the researchers placed both samples in an oven and blasted them with high temperatures to effectively erase their magnetic record, then exposed the rocks to an artificially generated magnetic field in the laboratory as they cooled.

The results confirmed that the two samples were indeed reliable magnetic recorders and that the field strength they initially measured, of 0.1 microteslas, accurately represented the maximum possible value of the moon’s extremely weak magnetic field 1 billion years ago. Weiss says a field of 0.1 microteslas is so low that it’s likely the lunar dynamo ended by this time.

The new findings line up with the predicted lifetime of core crystallization, a proposed mechanism for the lunar dynamo that could have generated a weak and long-lived magnetic field in the later part of the moon’s history. Weiss says that prior to core crystallization, a mechanism known as precession may have powered a much stronger though shorter-lived dynamo. Precession is a phenomenon by which the solid outer shell of a body such as the moon, in close proximity to a much larger body such as the Earth, wobbles in response to the Earth’s gravity. This wobbling stirs up the fluid in the core, the way swishing a cup of coffee stirs up the liquid inside.

Around 4 billion years ago, the infant moon was likely much closer to the Earth than it is today, and much more susceptible to the planet’s gravitational effects. As the moon moved slowly away from the Earth, the effect of precession decreased, weakening the dynamo and the magnetic field in turn. Weiss says it’s likely that around 2.5 billion years ago, core crystallization became the dominant mechanism by which the lunar dynamo continued, producing a weaker magnetic field that continued to dissipate as the moon’s core eventually fully crystallized.

The group is looking next to measure the direction of the moon’s ancient magnetic field in hopes of gleaning more information about the moon’s evolution.

This research was supported, in part, by NASA.

Source: mit.edu

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NASA has banned fighting and littering on the moon

New details of the agreement signed by representatives of a number of countries on the development of the moon and the extraction of minerals within the framework of the Artemis program have appeared. Reported by the National Aeronautics and Space Administration (NASA).

So, astronauts involved in the mission are prohibited from littering and fighting on the territory of a natural satellite of the Earth.

So, we present to you the new rules for being on the Moon:

Everyone comes in peace;

Confidentiality is prohibited, all launched objects must be identified and registered;

All travel participants agree to help each other in case of emergencies;

All received data is transferred to the rest of the participants, and space systems must be universal;

Historic sites must be preserved and all rubbish must be disposed of;

Rovers and spacecraft should not interfere with other participants.

“”It is important not only to go to the moon with our astronauts, but also that we bring our values ​​with us,” said Mike Gold, acting head of NASA’s international and inter-agency relations.

According to him, violators of the above rules will be asked to “just leave” the territory of the moon.

The effect of these principles so far applies to eight signatory countries of the agreement: the USA, Australia, Canada, Italy, Japan, Luxembourg, the United Arab Emirates and the United Kingdom. Countries other than China can join if they wish.

 It should be noted that at the moment NASA is prohibited from signing any bilateral agreements with the PRC leadership.

The first NASA mission to the moon, known as “Artemis 1”, is scheduled for 2021 without astronauts, and “Artemis 2” will fly with a crew in 2023.

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Methane snow found on the tops of Pluto’s equatorial mountains

Scientists believe that it arose as a result of the accumulation of large amounts of methane at an altitude of several kilometers above the surface of the planet.

In the images of the Cthulhu region – a dark region in the equatorial regions of Pluto – planetary scientists have found large reserves of methane snow that covers the peaks of local mountains and hills. It formed quite differently from how snow forms on Earth, astronomers write in the scientific journal Nature Communications.

“The white caps on the tops of Pluto’s mountains did not arise from the cooling of air currents that rise along the slopes into the upper atmosphere, as it happens on Earth, but from the accumulation of large amounts of methane at an altitude of several kilometers above Pluto’s surface. This gas condensed on the mountain tops, “the scientists write.

We owe almost everything we know about Pluto to the New Horizons interplanetary station. It was launched in January 2006, and in mid-July 2015 the station reached the Pluto system. New Horizons flew just 13 thousand km from the dwarf planet, taking many photographs of its surface. 

New Horizons data indicated an interesting feature of Pluto – in its depths, a giant subglacial ocean of liquid water can be hidden. It can be a kind of engine of those geological processes, traces of which can be seen on the surface of a dwarf planet. Because of this discovery of New Horizons, many discussions began among planetary scientists. Scientists are trying to understand how such a structure could have arisen, as well as to find out the appearance of Pluto in the distant past.

Members of the New Horizons science team and their colleagues from France, led by planetary scientist from NASA’s Ames Research Center (USA) Tanguy Bertrand, have discovered another unusual feature of Pluto. They studied the relief of one of the regions of the dwarf planet – the Cthulhu region. This is what astronomers call a large dark region at Pluto’s equator, which is whale-like in shape and is covered in many craters, mountains and hills.

Snow in Pluto’s mountains

By analyzing images of these structures taken by the LORRI camera installed on board New Horizons, astronomers have noticed many blank spots on the slopes of the highest mountain peaks. Having studied their composition, scientists have found that they consist mainly of methane.

Initially, planetary scientists assumed that these are deposits of methane ice. However, Bertrand and his colleagues found that the slopes and even the tops of Pluto’s equatorial mountains are actually covered not only with ice, but also with exotic methane snow that forms right on their surface.

Planetary scientists came to this conclusion by calculating how methane behaves in Pluto’s atmosphere. In doing so, they took into account how the molecules of its gases interact with the sun’s rays and other heat sources. It turned out that at the equator of Pluto, at an altitude of 2-3 km from its surface, due to the special nature of the movement of winds, unique conditions have formed, due to which snow is formed from methane vapor.

Unlike Earth, where such deposits are formed as a result of the rise of warm air into the upper atmosphere, on Pluto this process goes in the opposite direction – as a result of contact of the cold surface of the peaks and slopes of mountains with warm air masses from the relatively high layers of the dwarf planet’s atmosphere.

Previously, as noted by Bertrand and his colleagues, scientists did not suspect that this was possible. The fact is that they did not take into account that due to the deposition of even a small amount of methane snow and ice, the reflectivity of the peaks and slopes of mountains in the Cthulhu region increases. As a result, their surface temperature drops sharply, and snow forms even faster.

Scientists suggest that another mysterious feature of Pluto’s relief could have arisen in a similar way – the so-called Tartarus Ridges, located east of the Sputnik plain. A distinctive feature of this mountainous region is strange peaks that are shaped like skyscrapers or blades. Bertrand and his colleagues suggest that these peaks are also methane ice deposits that grow “from top to bottom.”

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First exoplanet discovered in another galaxy

The Chandra X-ray Space Telescope has found the first potential exoplanet in another galaxy. The planet orbits in a binary star system in a galaxy 23 million light-years away and was discovered by the eclipse of its star, a compact, ultra-powerful X-ray source.

A new method for searching for exoplanets, which is also suitable for studying other galaxies, is to register a decrease in the brightness of powerful X-ray sources (degenerate stars), which is caused by the transit of the planet. Thus, it was possible to detect the object M51-ULS-1b in the spiral galaxy M51 (the “Whirlpool” galaxy in the constellation of the Dog Hounds), which may be a gas giant or a brown dwarf in the X-ray binary system. An article by astronomers at the Harvard-Smithsonian Center for Astrophysics about this possible discovery appeared in September 2020 and is still available as a preprint at arXiv.org; it will probably be published later in the Monthly Notices of the Royal Astronomical Society (MNRAS).

The Whirlpool Galaxy, or M51, is a spiral galaxy 23 million light years distant. It is distinguishable through binoculars (apparent stellar magnitude about + 8ᵐ), and it is easy to find it in the northern sky near the extreme star of the Big Dipper bucket, although formally the site belongs to the neighboring constellation of Hounds.

It is one of the first extragalactic objects to be photographed in the middle of the 19th century. The galaxy has a companion – the neighboring dwarf galaxy NGC 5195, which it gradually eats, so a pair of galaxies looks spectacular in the pictures. It is assumed that the painting by Van Gogh “Starry Night” depicts this very object, which was then well known for publishing astronomical sketches in magazines.

The Maelstrom nebula (possibly) in a painting by Van Gogh (1889).

Extragalactic planets are objects in star systems or lonely planets outside our Galaxy. Most of the about 6,000 exoplanets discovered today orbit around stars at distances of up to hundreds of light years, that is, they belong to the nearest galactic environs. 

More distant stars, even within the Galaxy, are beyond the scope of studying their planetary systems. Moreover, this applies to objects in other galaxies at distances of millions of light years (for example, the distance to the center of our Galaxy is 25 thousand light years, and the nearest giant galaxy Andromeda is located at a distance of 2.5 million light years). Nevertheless, several extragalactic candidate planets are known. 

They were all discovered by the method of gravitational microlensing (distortion of the trajectory of light rays from a distant light source in the gravitational field of a star and its planetary system). 

This is an indirect method, and at extragalactic distances there is practically no possibility to independently confirm the discovery of an exoplanet by other means. Object M51-ULS-1b became the first extragalactic planet, which was discovered by the method of transit, standard for the study of “near” exoplanets – observations of periodic “eclipses” by the planet of its star in the process of moving in orbit. 

The transit method is one of two popular methods for exploring nearby exoplanets, along with the radial velocity method. Currently, the TESS space telescope is in orbit, the main task of which is to monitor several thousand of the nearest stars in the entire celestial sphere and search for their planets in this way (for more details about this NASA project, there is a separate article on our website). 

A similar problem was previously solved by the Kepler space telescope, which has already completed its work. The main difference between the two projects is that TESS monitors almost the entire sky sector-by-sector, exploring stars at distances of up to 100-200 light years, while Kepler focused on a small area, but captured stars at distances of up to 3 thousand light years (there is also note on the site). 

But extragalactic distances are orders of magnitude greater, and precise observations of the brightness of ordinary stars even in neighboring galaxies are not yet possible. 

Therefore, only superbright objects are suitable for research in other galaxies (not necessarily stars that are bright in the optical range). So far, these are X-ray sources, which are most often binary systems, where a compact object (black hole or neutron star) actively absorbs the matter of the companion star.

X-ray image of the galaxy M51 by the Chandra telescope and the position of the X-ray source M 51-ULS at the edge of the young star cluster in a detailed Hubble image. Di Stefano et al. (2020).


There can be up to several hundred such objects in galaxies. In the above Chandra image of M51 (left), they appear as bright dots. If the system contains a large exoplanet, then it can cause a short-term full or partial drop in the brightness of the source in the X-ray range, similar to the optical transit that telescopes can track.

 The very first exoplanets discovered in the mid-1990s in our Galaxy were also found near such exotic objects. Subsequently, when the number of exoplanets began to be measured in thousands, interest for obvious reasons shifted to planets in star systems similar to the Sun (or even better – in their “zones of potential habitability” and preferably closer, for example, near Proxima Centauri).

A group at the Harvard-Smithsonian Center for Astrophysics searched for transit events among 2,624 archived light curves for more than two hundred X-ray sources in spiral galaxies M51 , M101, and  M104, according to the Chandra orbiting telescope . 

Two other objects here are also familiar to astrolamists – these are the photogenic galaxies “Pinwheel” in Ursa Major (M101) and “Sombrero” in Virgo (M104), well oriented for observations in relation to us. One of the cases found is in good agreement with the light curve in the single transit model. It belongs to the X-ray source designated M51-ULS-1 – a young massive binary system closer to the outskirts of the galaxy M51. 

The object that caused the source to completely darken for 20-30 minutes could theoretically belong to several classes, including rocky or gas planets, as well as stars – white dwarfs or M-class stars (ordinary stars are red dwarfs). 

The properties of the light curve, according to the authors, exclude the “stellar” nature of this object, which received the “exoplanetary” designation M51-ULS-1b… It is assumed that it is slightly smaller than Saturn and may be a hot gas giant or a substellar object – a brown dwarf. It moves in an orbit of a large radius (according to estimates – tens of astronomical units) and at one time survived a supernova explosion in this binary system, which led to the formation of a compact X-ray object. 

The authors of the work suggest that the method can be used to search for exoplanets both in other galaxies and in the Milky Way, and its accuracy will increase with the quality of data from orbiting telescopes.

Whirlpool Galaxy M51 and companion by Hubble
Galaxies M51 (“Whirlpool”) and NGC 5195 – image from the Hubble telescope . NASA / ESA / STScI .

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