There are a few interesting stories lurking in this news item, but lets start with the top level – a new study revises the minimum age of the Moon to 4.46 billion years, 40 million years older than the previous estimate. That in itself is interesting, but not game-changing. It’s really a tweak, an incremental increase in precision. How scientists made this calculation, however, is more interesting.

The researchers studied zircon crystals brought back from Apollo 17. Zircon is a crystal silicate that often contains some uranium. These crystals would have formed when the magma surface of the Moon cooled. The current dominant theory is that a Mars-sized planet slammed into the proto-Earth about four and a half billion years ago, creating the Earth as we know it. The collision also threw up a tremendous amount of material, with the bulk of it coalescing into our Moon. The surface of both worlds would have been molten from the heat of the collision, but it is easier to date the Moon because the surface is better preserved. The surface of the Earth undergoes constant turnover of one type or another, while the lunar surface is ancient. So dating the Moon tells us something about the age of the Earth also.

The method of dating employed in this latest study is called atom probe tomography. First they use an ion beam microscope to carve the tip of a crystal to a sharp point. Then they use UV lasers to evaporate atoms off the tip of the crystal. These atoms pass through a mass spectrometer, which uses the time it takes to pass through as a measure of mass, which identifies the element. The researchers are interested in the proportion of uranium to lead. Uranium is a common element found in zircon, and it also undergoes radioactive decay into lead at a known rate. In any sample you can therefore use the ratio of uranium to lead to calculate the age of that sample. Doing so yielded an age of 4.46 billion years old – the new minimum age of the Moon. It’s possible the Moon could be older than this, but it can’t be any younger.

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The James Webb Space Telescope spectroscopic analysis of K2-18b, an exoplanet 124 light years from Earth, shows signs that the atmosphere may contain dimethyl sulphide (DMS). This finding is more impressive when you know that DMS on Earth is only produced by living organisms, not by any geological process. The atmosphere of K2-18b also contains methane and CO2, which could be compatible with liquid water on the surface. Methane is also a possible signature of life, but it can also be produced by geological processes. This is pretty exciting, but the astronomers caution that this is a preliminary result. It must be confirmed by more detailed analysis and observation, which will likely take a year.

According to NASA:

K2-18 b is a super Earth exoplanet that orbits an M-type star. Its mass is 8.92 Earths, it takes 32.9 days to complete one orbit of its star, and is 0.1429 AU from its star. Its discovery was announced in 2015.

This planet was discovered with the transit method, so we have some idea of its radius and therefore density. It’s surface gravity is likely about 12 m/s^2 (Earth’s is 9.8). It has a hydrogen-rich atmosphere, which would explain the methane without the need for life. It orbits a red dwarf, and is likely tidally locked, or in a tidal resonance orbit. It receives an amount of radiation from its star similar to Earth. The big question – is K2-18 b potentially habitable?

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I know you don’t need one more thing to worry about, but I have already written about the growing problem of space debris. At least this update is about a mission to help clear some of that debris – ClearSpace-1. This is an ESA mission which they contracted out to a Swiss company, Clearspace SA, who is making a satellite whose purpose is to grab large pieces of space junk and de-orbit it.

The problem this mission is trying to solve is the fact that we have put millions of pieces of debris into various orbits around the Earth. While space is big, usable near-Earth orbits are finite, and if you put millions of pieces of debris there zipping around at fast speeds, there will be collisions. The worst-case scenario is what’s called a Kessler cascade, in which a collision causes more debris which then increases the probability of further collisions which increases the amount of debris, and the cycle continues. This won’t be an event so much as a process that unfolds over a long period of time. But it has the potential of rendering Earth orbit increasingly dangerous to the point of being unusable. It also makes the task of cleaning up orbit exponentially more difficult.

The Clearspace craft looks like a mechanical squid with 4 arms which can grab tightly onto a large piece of space debris. The planned first mission with target the upper stage of Vespa, part of the ESA Vega launcher. This is a 112 kg target. Once it grabs the debris it with then undergo a controlled deorbit. The mission is planned for 2026, and if successful will be the first mission of its kind. This is a proof-of-concept mission, because removing one piece of large debris is insignificant compared to how much debris is already up there. But we need to demonstrate that the whole system works, and we also need to confirm how much each such mission will cost.

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Dark matter is one of the greatest current scientific mysteries. It’s a fascinating story playing out in real time, although over years, so you have to be patient. Future generations might be able to binge the dark matter show, but not us. We have to wait for each episode to drop. Another episode did just drop, in the form of an analysis of the massive relic galaxy NGC 1277, but let’s get caught up before we watch this episode.

The term “dark matter” was coined by astronomer Fritz Zwicky in 1933 as one possible explanation for the rotation of the Coma Galaxy Cluster. The galaxies were essentially moving too quickly, implying that there was more gravity (and hence more matter) present in the cluster than was observed. This matter could not be seen, therefore it was dark. The notion was a mere footnote, however, until the 1970s when astronomer Vera Rubin analyzed the rotation curves of many individual galaxies. She found that galaxies were rotating too quickly. The stars should be flying apart because there was insufficient gravity to hold them together (or alternatively they should be rotating more slowly). There must be more gravity that can be seen. The notion of dark matter was therefore solidified, and has been a matter of debate ever since.

Half a century after Rubin confirmed the existence of dark matter, we still don’t know what it is. It must be some kind of particle that does not interact much with other stuff in the universe, does not give off or reflect radiation, but possesses significant mass and therefore gravity. There are candidate particles, such as wimps (weakly interacting massive particles), MACHOs (massive astrophysical compact halo object), axions (particles with a tiny amount of mass but could be very common) or perhaps even several particles currently not accounted for in the standard model of particle physics.

This is one of the exciting things about dark matter – when we figure out what dark matter is, it could break the standard model, pointing the way to a new and deeper understanding of physics. But how certain are we that dark matter exists? To a degree the existence of dark matter is an argument from ignorance – it is a placeholder filling in a gap in our knowledge. We can only infer its existence because we cannot explain with our current models of gravity how stuff is moving in the universe. Perhaps our current models of gravity are wrong?

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It’s always exciting when a scientific institution announces that they are going to make an announcement. Earlier this week we were told that there was going to be a major announcement today (June 29th) regarding a gravitational wave discovery. The goal of the pre-announcement is to generate buzz and media attention, although I almost always find the reveal to be disappointing. I guess we are too programmed by movie plotlines where such reveals are truly earthshattering. So I have learned to moderate my expectations (a generally good strategy to avoid disappointment).

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) team released five papers late last night in the Astrophysical Journal Letters – I think my moderated expectations were pretty much on target. This is awesomely cool science, but didn’t shatter my world. I can see why the scientists were so excited, however. This was the culmination of 15 years of investigation. The bottom line discovery is that spacetime is constantly rippling, in line with Einstein’s predictions of General Relativity. Let’s get into the details.

Gravitational wave astronomy is a new window onto the universe. Most of the recent news has been made by the Laser Interferometer Gravitational-wave Observatory (LIGO). This is a large instrument, with two powerful lasers at right angles to each other, with each arm about 4 km long firing through a vacuum pipeline. Where the lasers cross they create an interference pattern. The slightest disturbance in the lasers can change the interference, and therefore it is a very sensitive detector. The primary challenge is isolating LIGO from background noise and filtering it out. What is left is a signal produced by gravitational waves, ripples in spacetime created by massive gravitational events. LIGO is able to detect high frequency gravitational waves formed by the collision of black holes and/or neutron stars with each other.

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What will be the ultimate fate of our universe? There are a number of theories and possibilities, but at present the most likely scenario seems to be that the universe will continue to expand, most mass will eventually find its way into a black hole, and those black holes will slowly evaporate into Hawking Radiation, resulting in what is called the “heat death” of the universe. Don’t worry, this will likely take 1.7×10¹⁰⁶ years, so we got some time.

But what about objects, like stellar remnants, that are not black holes? Will the ultimate fate of the universe still contain some neutron stars and cold white dwarfs that managed to never get sucked up by a black hole? To answer this question we have to back up a bit and talk about Hawking Radiation.

Stephen Hawking famously proposed this idea in 1975 – he was asked if black holes have a temperature, and that sent him down another type of hole until Hawking Radiation popped out as the answer. But what is Hawking Radiation? The conventional answer is that the vacuum of space isn’t really nothing, it still contains the quantum fields that make up spacetime. Those quantum field do not have to have zero energy, and so occasionally virtual particles will pop into existence, always in pairs with opposite properties (like opposite charge and spin), and then they join back together, cancelling each other out. But at the event horizon of black holes, the distance at which light can just barely escape the black hole’s gravity, a virtual pair might occur where one particle gets sucked into the black hole and the other escapes. The escaping particle is Hawking Radiation. It carries away a little mass from the black hole, causing it to glow slightly and evaporate very slowly. This evaporation gets quicker as the black hole becomes less massive, until eventually it explodes in gamma radiation.

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ESA’s Gaia orbital telescope has recently discovered two new black holes. This, in itself, is not surprising, as that is Gaia’s mission – to precisely map the three-dimensional position of two billion objects in our galaxy, using three separate instruments. The process is called astrometry, and the goal is to produce a highly accurate map of the galaxy. The two new black holes, Gaia BH1 and Gaia BH2, have two features that make them noteworthy. The first is that these are the two closest blackholes to the Earth every discovered, at 1560 and 3800 light years away (on galactic terms, that’s close). More important, however, is that they represent a new type of black hole.

Black holes are the most massive single objects in the universe. They are made from stellar remnants that have sufficient mass that the inward force of gravity is greater than any outward force of the matter itself. If the stellar remnant is >2.16 solar masses, it will become a black hole. Less than that, down to about 1.4 solar masses, and you have a neutron star. Stars of 20 solar masses or more are large enough to leave a stellar remnant behind after they nova to create a black hole. Black holes can also be formed when two neutron stars collide, the resulting mass being greater than the 2.16 limit. Or, a neutron star can have a close companion and it can gravitationally draw mass from the companion star until it reaches the black hole limit.

There are also supermassive black holes, usually formed at the center of galaxies from gobbling up more and more stars. The most massive discovered so far is 66 billion solar masses.

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NASA recently discovered a 50 meter wide asteroid whose orbit will come close to Earth. They estimate a close approach in 2046, which will likely bring the asteroid within 1.1 million miles of the Earth, about four times the distance of the moon. However, there is always uncertainty in calculating orbits, and the farther into the future you try to project their path, the more uncertainty there is. At this point in time NASA estimates a one in 560 chance that the asteroid, dubbed 2023 DW, will hit the Earth in 2046.

Orbits are calculated through multiple observations of the object along its orbit. We have to see how it is moving, and the longer the observation the greater the precision. For recently discovered objects, like this asteroid, there is more uncertainty in the orbital calculations, which is why NASA cannot completely rule out an impact. Because it is a near-Earth object, however, they will continue to make observations, refining their calculated orbit, and reducing the uncertainty.

Interestingly, the current scale for designating the risk of an object hitting the Earth, the Palermo scale, is not based on a simple percentage probability, but on the probability relative to the background rate of impacts. A Palermo scale of 0 means that the chance of a particular object hitting the Earth is no different than the background rate of impacts. The scale is also logarithmic, so a Palermo rating of 1 means a chance of impact 10 times the background rate, 2 is 100 times. 2023 DW has a Palermo rating of -2.17.

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I recently received an e-mail question from an SGU listener about the speed of gravity. They were questioning a statement they heard by Neil DeGrasse Tyson that if the sun were magically plucked from existence, the Earth would not feel the effects for 8 minutes and 20 seconds – the time it takes for light to travel from the sun to the Earth. This blew their mind, writing: “That statement doesn’t make sense to me. What DeGrasse is saying is that we don’t actually orbit the sun but a point in space where the sun was 8 minutes 20s ago.”

Actually, that’s exactly right. He did get it. We see the sun as it was 8 minutes 20 seconds ago. We also feel the sun as it was at that time – in every way. The speed of light is more than the maximal relative velocity that energy travels through the universe, it is the speed at which reality propagates throughout the universe. No effect can exceed the speed of light – not information, matter, energy, or force.

We can use the General Relativity conception of gravity, that matter curves space time and that matter and energy travel in a straight line through curved space. You have likely seen the typical graphical representation of this, with a heavy object like a planet or star distorting a grid of spacetime like a heavy ball resting on a stretched piece of fabric. This is actually just a conceptual aid, not an accurate depiction. It is missing one dimension – space is represented as a two-dimensional fabric stretching into a third dimension. You need to add a dimension to represent reality, which is three dimensions. Some experts say that three dimensional space is being curved in a fourth dimension, but others say you can work the math to describe the curvature of 3D space without invoking a 4th spacial dimension. I’m not sure what the current consensus is – I’ll have to look deeper when I have time (if anyone has a good reference, please share it in the comments).

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Scientists love mysteries, because that is where new discoveries lay. It is nice to find evidence consistent with existing theories, providing further confirmation, but it’s exciting to find evidence that cannot be explained with existing theories. Astronomers may have found such a mystery in the dwarf planet Quaoar – it has a ring where one shouldn’t be.

When we think of planetary rings we of course think first of Saturn, which has by far the largest and most impressive ring system in the solar system. But all the gas giants have their own rings, including Jupiter, Uranus, and Neptune. None of the smaller rocky planets have detectable rings. This is likely not a coincidence and relates to how rings form and how long they will last. But smaller bodies can have rings. Saturn’s moon Rhea may have a faint ring of its own. There is even an asteroid, Chariklo, which has two faint rings.

Rings are basically linear clouds of dust, ice, and other material that spreads out in an orbit around a planetary body. They form in one of two ways. For large planets like Saturn, if a moon’s orbit decays to the point where it gets within the Roche limit (the point at which tidal forces are so great they tear apart any large object), then the moon will break apart into debris that forms a ring. But anything that spreads dust around a planet can also feed a ring. For example, when meteorites hit the moons of Jupiter they throw up dust which can feed its faint ring system. Because the material that forms rings are close to their host planet they also tend to slowly rain down to the surface, so they have a limited lifetime. Saturn will eventually lose its beautiful rings. But new rings may also form. When Phobos gets too close to Mars one day it will break up and become a Martian ring system.

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