Did the Moon Ever Have a Magnetosphere?

In a few years, NASA will be sending astronauts to the Moon for the first time since the Apollo Era (1969-1972). As part of the Artemis Program, the long-term goal is to create the necessary infrastructure for a “sustained program of lunar exploration.” The opportunities this will present for lunar research are profound and will likely result in new discoveries about the formation and evolution of the Moon.

In particular, scientists are hoping to investigate the long-standing mystery of whether or not the Moon had a magnetosphere. In anticipation of what scientists might find, an international team of geophysicists led by the University of Rochester examined samples of lunar material brought back by the Apollo astronauts. Based on the composition of these samples, the team determined that the Moon’s dynamo was short-lived.

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A Lunar Farside Telescope Could Detect Exoplanets Through Their Magnetospheres

It’s difficult to do radio astronomy on Earth, and it’s getting harder every day. Our everyday reliance on radio technology means that radio interference is a constant challenge, even in remote areas. And for some wavelengths even the Earth’s atmosphere is a problem, absorbing or scattering radio light so that Earth-based telescopes can’t observe these wavelengths well. To overcome these challenges, astronomers have proposed putting a radio telescope on the far side of the Moon.

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Super-Earth Conditions Simulated in the Lab to Discover if They’re Habitable

Deep inside planet Earth, there is a liquid outer core and a solid inner core that counter-rotate with each other. This creates the dynamo effect that is responsible for generating Earth’s planetary magnetic field. Also known as a magnetosphere, this field keeps our climate stable by preventing Earth’s atmosphere from being lost to space. So when studying rocky exoplanets, scientists naturally wonder if they too have magnetospheres.

Unfortunately, until we can measure an exoplanet’s magnetic fields, we are forced to infer their existence from the available evidence. This is precisely what researchers at the Sandia National Laboratories did with its Z Pulsed Power Facility (PPF). Along with their partners at the Carnegie Institution for Science, they were able to replicate the gravitational pressures of “Super-Earths” to see if they could generate magnetic fields.

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Electrons Can Get Accelerated to Nearly the Speed of Light As They Interact With the Earth’s Magnetosphere

Electrons serve many purposes in physics.  They are used by some particle accelerators and they underpin our modern world in the silicon chips that run the world’s computers.  They’re also prevalent in space, where they can occasionally be seen floating around in a plasma in the magnetospheres of planets.  Now, a team from the German Research Centre for Geosciences (GFZ) lead by Drs Hayley Allison and Yuri Shprits have discovered that those electrons present in the magnetosphere can be accelerated up to relativistic speeds, and that could potentially be hazardous to our increasing orbital infrastructure.

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Mars Doesn’t Have Much of a Magnetosphere, But Here’s a Map

Even though Earthling scientists are studying Mars intently, it’s still a mysterious place.

One of the striking things about Mars is all of the evidence, clearly visible on its surface, that it harbored liquid water. Now, all that water is gone, and in fact, liquid water couldn’t survive on the surface of the Red Planet. Not as the planet is now, anyway.

But it could harbour water in the past. What happened?

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Detecting Exoplanets Through Their Exoauroras

At present, scientists can only look for planets beyond our Solar System using indirect means. Depending on the method, this will involve looking for signs of transits in front of a star (Transit Photometry), measuring a star for signs of wobble (Doppler Spectroscopy), looking for light reflected from a planet’s atmosphere (Direct Imaging), and a slew of other methods.

Based on certain parameters, astronomers are then able to determine whether a planet is potentially-habitable or not. However, a team of astronomers from the Netherlands recently released a study in which they describe a novel approach for exoplanet-hunting: looking for signs of aurorae. As these are the result of interaction between a planet’s magnetic field and a star, this method could be a shortcut to finding life!

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The Moon’s Magnetosphere Used to be Twice as Strong as the Earth’s

For decades, scientists have held that the Earth-Moon system formed as a result of a collision between Earth and a Mars-sized object roughly 4.5 billion years ago. Known as the Giant Impact Hypothesis, this theory explains why Earth and the Moon are similar in structure and composition. Interestingly enough, scientists have also determined that during its early history, the Moon had a magnetosphere – much like Earth does today.

However, a new study led by researchers at MIT (with support provided by NASA) indicates that at one time, the Moon’s magnetic field may have actually been stronger than Earth’s. They were also able to place tighter constraints on when this field petered out, claiming it would have happened about 1 billion years ago. These findings have helped resolve the mystery of what mechanism powered the Moon’s magnetic field over time.

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Now That NASA’s Missing IMAGE Satellite Has Been Found, Talking To It Is Going To Be Difficult

It’s easy to imagine the excitement NASA personnel must have felt when an amateur astronomer contacted NASA to tell them that he might have found their missing IMAGE satellite. After all, the satellite had been missing for 10 years.

IMAGE, which stands for Imager for Magnetopause-to-Aurora Global Exploration, was launched on March 25th, 2000. In Dec. 2005 the satellite failed to make routine contact, and in 2007 it failed to reboot. After that, the mission was declared over.

NASA’s IMAGE satellite. Credit: NASA

It’s astonishing that after 10 years, the satellite has been found. It’s even more astonishing that it was an amateur who found it. As if the story couldn’t get any more interesting, the amateur astronomer who found it—Scott Tilly of British Columbia, Canada—was actually looking for a different missing satellite: the secret ZUMA spy satellite launched by the US government on January 7, 2018. (If you’re prone to wearing a tin foil hat, now might be a good time to reach for one.)

NASA’s half-ton IMAGE satellite being launched from Vandenberg Air Force Base on March 25th, 2000. IMAGE was the first satellite designed to actually “see” most of the major charged particle systems in the space surrounding Earth. Image: NASA

After Tilly contacted NASA, they hurried to confirm that it was indeed IMAGE that had been found. To do that, NASA employed 5 separate antennae to seek out any radio signals from the satellite. As of Monday, Jan. 29, signals received from all five sites were consistent with the radio frequency characteristics expected of IMAGE.

In a press release, NASA said, “Specifically, the radio frequency showed a spike at the expected center frequency, as well as side bands where they should be for IMAGE. Oscillation of the signal was also consistent with the last known spin rate for IMAGE.”

“…the radio frequency showed a spike at the expected center frequency…” – NASA Press Release confirming the discovery of IMAGE

Then, on January 30, the Johns Hopkins Applied Physics Lab (JHUAPL) reported that they had successfully collected telemetry data from the satellite. In that signal was the ID code 166, the code for IMAGE. There were probably some pretty happy people at NASA.

So, now what?

A diagram of NASA’s IMAGE satellite. Image: NASA

NASA’s next step is to confirm without a doubt that this is indeed IMAGE. That means capturing and analyzing the data in the signal. That will be a technical challenge, because the types of hardware and operating systems used in the IMAGE Mission Operations Center no longer exist. According to NASA, “other systems have been updated several versions beyond what they were at the time, requiring significant reverse-engineering.” But that should be no problem for NASA. After all, they got Apollo 13 home safely, didn’t they?

If NASA is successful at decoding the data in the signal, the next step is to attempt to turn on IMAGE’s science payload. NASA has yet to decide how to proceed if they’re successful.

IMAGE was the first spacecraft designed to “see the invisible,” as they put it back then. Prior to IMAGE, spacecraft examined Earth’s magnetosphere by detecting particles and fields they encountered as they passed through them. But this method had limited success. The magnetosphere is enormous, and simply sampling a small path—while better than nothing—did not give us an accurate understanding of it.

During its mission, IMAGE did a lot of great science. In July 2000, a spectacular solar storm caused auroras as far south as Mexico. IMAGE captured these images of those poweful auroras. Credit: NASA

IMAGE was going to do things differently. It used 3-dimensional imaging techniques to measure simultaneously the densities, energies and masses of charged particles throughout the inner magnetosphere. To do this, IMAGE carried a payload of 7 instruments:

  • High Energy Neutral Atom (HENA) imager
  • Medium Energy Neutral Atom (MENA) imager
  • Low Energy Neutral Atom (LENA) imager
  • Extreme Ultraviolet (EUV) imager
  • Far Ultraviolet (FUV) imager
  • Radio Plasma Imager (RPI)
  • Central Instrument Data Processor (CIDP)

These instruments allowed IMAGE to not only do great science, and to capture great images, but also to create some stunning never-seen-before movies of auroral activity.

This is a fascinating story, and it’ll be interesting to see if NASA can establish meaningful contact with IMAGE. Will it have a treasure trove of unexplored data on-board? Can it be re-booted and brought back into service? We’ll have to wait and see.

This story is also interesting culturally. IMAGE was in service at a time when the internet wasn’t as refined as it is currently. NASA has mastered the internet and public communications now, but back then? Not so much. For example, to build up interest around the mission, NASA gave IMAGE its own theme song, titled “To See The Invisible.” Yes, seriously.

But that’s just a side-note. IMAGE was all about great science, and it accomplished a lot. You can read all about IMAGE’s science achievements here.

Earth and Venus are the Same Size, so Why Doesn’t Venus Have a Magnetosphere? Maybe it Didn’t Get Smashed Hard Enough

For many reasons, Venus is sometimes referred to as “Earth’s Twin” (or “Sister Planet”, depending on who you ask). Like Earth, it is terrestrial (i.e. rocky) in nature, composed of silicate minerals and metals that are differentiated between an iron-nickel core and silicate mantle and crust. But when it comes to their respective atmospheres and magnetic fields, our two planets could not be more different.

For some time, astronomers have struggled to answer why Earth has a magnetic field (which allows it to retain a thick atmosphere) and Venus do not. According to a new study conducted by an international team of scientists, it may have something to do with a massive impact that occurred in the past. Since Venus appears to have never suffered such an impact, its never developed the dynamo needed to generate a magnetic field.

The study, titled “Formation, stratification, and mixing of the cores of Earth and Venus“, recently appeared in the scientific journal Earth and Science Planetary Letters. The study was led by Seth A. Jacobson of Northwestern University, and included members from the Observatory de la Côte d’Azur, the University of Bayreuth, the Tokyo Institute of Technology, and the Carnegie Institution of Washington.

The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com
The Earth’s layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com

For the sake of their study, Jacobson and his colleagues began considering how terrestrial planets form in the first place. According to the most widely-accepted models of planet formation, terrestrial planets are not formed in a single stage, but from a series of accretion events characterized by collisions with planetesimals and planetary embryos – most of which have cores of their own.

Recent studies on high-pressure mineral physics and on orbital dynamics have also indicated that planetary cores develop a stratified structure as they accrete. The reason for this has to do with how a higher abundance of light elements are incorporated in with liquid metal during the process, which would then sink to form the core of the planet as temperatures and pressure increased.

Such a stratified core would be incapable of convection, which is believed to be what allows for Earth’s magnetic field. What’s more, such models are incompatible with seismological studies that indicate that Earth’s core consists mostly of iron and nickel, while approximately 10% of its weight is made up of light elements – such as silicon, oxygen, sulfur, and others. It’s outer core is similarly homogeneous, and composed of much the same elements.

As Dr. Jacobson explained to Universe Today via email:

“The terrestrial planets grew from a sequence of accretionary (impact) events, so the core also grew in a multi-stage fashion. Multi-stage core formation creates a layered stably stratified density structure in the core because light elements are increasingly incorporated in later core additions. Light elements like O, Si, and S increasingly partition into core forming liquids during core formation when pressures and temperatures are higher, so later core forming events incorporate more of these elements into the core because the Earth is bigger and pressures and temperatures are therefore higher.

“This establishes a stable stratification which prevents a long-lasting geodynamo and a planetary magnetic field. This is our hypothesis for Venus. In the case of Earth, we think the Moon-forming impact was violent enough to mechanically mix the core of the Earth and allow a long-lasting geodynamo to generate today’s planetary magnetic field.”

To add to this state of confusion, paleomagnetic studies have been conducted that indicate that Earth’s magnetic field has existed for at least 4.2 billion years (roughly 340 million years after it formed). As such, the question naturally arises as to what could account for the current state of convection and how it came about. For the sake of their study, Jacobson and his team considering the possibility that a massive impact could account for this. As Jacobson indicated:

“Energetic impacts mechanically mix the core and so can destroy stable stratification. Stable stratification prevents convection which inhibits a geodynamo. Removing the stratification allows the dynamo to operate.”

Basically, the energy of this impact would have shaken up the core, creating a single homogeneous region within which a long-lasting geodynamo could operate. Given the age of Earth’s magnetic field, this is consistent with the Theia impact theory, where a Mars-sized object is believed to have collided with Earth 4.51 billion years ago and led to the formation of the Earth-Moon system.

This impact could have caused Earth’s core to go from being stratified to homogeneous, and over the course of the next 300 million years, pressure and temperature conditions could have caused it to differentiate between a solid inner core and liquid outer core. Thanks to rotation in the outer core, the result was a dynamo effect that protected our atmosphere as it formed.

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

The seeds of this theory were presented last year at the 47th Lunar and Planetary Science Conference in The Woodlands, Texas. During a presentation titled “Dynamical Mixing of Planetary Cores by Giant Impacts“, Dr. Miki Nakajima of Caltech – one of the co-authors on this latest study – and David J. Stevenson of the Carnegie Institution of Washington. At the time, they indicated that the stratification of Earth’s core may have been reset by the same impact that formed the Moon.

It was Nakajima and Stevenson’s study that showed how the most violent impacts could stir the core of planets late in their accretion. Building on this, Jacobson and the other co-authors applied models of how Earth and Venus accreted from a disk of solids and gas about a proto-Sun. They also applied calculations of how Earth and Venus grew, based on the chemistry of the mantle and core of each planet through each accretion event.

The significance of this study, in terms of how it relates to the evolution of Earth and the emergence of life, cannot be understated. If Earth’s magnetosphere is the result of a late energetic impact, then such impacts could very well be the difference between our planet being habitable or being either too cold and arid (like Mars) or too hot and hellish (like Venus). As Jacobson concluded:

“Planetary magnetic fields shield planets and life on the planet from harmful cosmic radiation. If a late, violent and giant impact is necessary for a planetary magnetic field then such an impact may be necessary for life.”

Looking beyond our Solar System, this paper also has implications in the study of extra-solar planets. Here too, the difference between a planet being habitable or not may come down to high-energy impacts being a part of the system’s early history. In the future, when studying extra-solar planets and looking for signs of habitability, scientists may very well be forced to ask one simple question: “Was it hit hard enough?”

Further Reading: Earth Science and Planetary Letters