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When comets hit white dwarfs

A white dwarf is a star that's run out of fuel and is slowly cooling down... but still very hot thanks to the energy it got from gravity crushing it down. White dwarfs should have atmospheres that are almost pure hydrogen and helium, since heavier elements quickly sink down further. But about a quarter have noticeable amounts of heavier elements in their atmosphere. How did those get there?

Alexander Stephan and other scientists at UCLA argue that they come from comets! More precisely, large icy objects like those in the Kuiper belt of our Solar System, beyond the orbit of Pluto.

But it takes work to explain how so many of these objects hit white dwarfs.

The theory is that these white dwarfs are in binary star systems. When the star that becomes the white dwarf begins to die it emits a lot of gas and loses mass - we know that's how it works. So, the Kuiper belt objects orbiting it start to move further out. There are lots of these things. So, some will interact gravitationally with the other star in binary system and get thrown this way and that... and eventually some will hit the white dwarf!

The scientists did detailed computer simulations to check that this could account for what we see. Even more exciting: sometimes Neptune-like planets will hit the white dwarf! And indeed we see some white dwarfs that have a lot more heavy elements in their atmosphere.

By running large Monte Carlo simulations, Stephan and collaborators demonstrate that this scenario can successfully produce accretion of both Neptune-like planets and Kuiper-belt-analog objects. Their simulation results indicate that ~1% of all white dwarfs should accrete Neptune-like planets, and ~7.5% of all white dwarfs should accrete Kuiper-belt-analog objects.

While these fractions are broadly consistent with observations, it’s hard to say with certainty whether this model is correct, as observations are scant. Only ~200 polluted white dwarfs have been observed, and of these, only ~15 have had detailed abundance measurements made. Next steps for understanding white-dwarf pollution certainly must include gathering more observations of polluted white dwarfs and establishing the statistics of what is polluting them.

Also, 7.5% is a lot less than 25%.

I got this from here:

http://aasnova.org/2017/07/28/throwing-icebergs-at-white-dwarfs/

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The corpse of a Martian dune

Dunes are born and dunes die. The dark patch here is the 'corpse' of a dune on Mars. It's in the process of dissolving, as the wind patterns that originally formed it have changed, and its sand is blown away. It's one of several dune corpses just upwind of a dune field in Holden crater - a large old crater on Mars, which once had water in it.

The whole scene here is 600 meters × 600 meters in size.

This is from +Lori Fenton's blog, which is full of great pictures:

http://cosmicdiary.org/lfenton/2017/11/27/the-corpse-of-a-dune/

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Asteroid families

Many asteroids belong to families. These formed when bigger asteroids collided and broke into pieces. Asteroids in a family have similar orbits. But finding these families takes work!

This picture makes it look easy. But it's hard. In reality, the different families don't come in different colors. ( ͡° ͜ʖ ͡°) There are lots of asteroids that don't belong to families, not shown on this chart. And there's another even more important reason!

We can describe an orbit using some numbers. An orbit is an ellipse. The semimajor axis is half the length of this ellipse. The eccentricity says how stretched-out the ellipse is: it's 0 for a circle. The inclination is an angle that describes the orbit's tilt. Most planets have inclination almost zero, because they move close to a plane called the ecliptic.

So, you might try to find asteroid families by taking lots of asteroids and making a chart of their semimajor axis, eccentricity and inclination. Would that work? This picture makes it seem so!

But it wouldn't work. You'd get random junk - no obvious families.

To find the families, you have to correct for the fact that orbits keep changing! Look at this picture:

https://commons.wikimedia.org/wiki/File:Asteroid_osculating_vs_proper_elements.png

At left you see a chart of the eccentricity and inclination of lots of asteroids. Random junk! At right you see a corrected chart. Now you see asteroid families!

How does this 'correction' business work? It was invented by the Japanese astronomer Kiyotsugu Hirayama in 1918. He noticed that asteroid orbits change in a roughly periodic way over thousands of years due to the gravitational pull of the planets... but one can create ideal unchanging orbits by correcting for this fact.

Hirayama didn't have a computer, so he had to do these computations by hand, approximately! He succeeded in finding several families of asteroids this way: the Koronis, Eos, and Themis families, and later the Flora and Maria families.

By now we can do much better, and find many more families... and more asteroids in each family. For example, the Eos family was formed about 1.1 billion years ago between Jupiter and Mars. Hirayama found 19 asteroids in this family. The biggest, which gives the family its name, is Eos, named after the Greek goddess of the dawn. It's almost 100 kilometers across! But now we know almost 300 members of this family.

Here's what Wikipedia says about this business of correcting orbits:

The proper orbital elements of an orbit are constants of motion of an object in space that remain practically unchanged over an astronomically long timescale. The term is usually used to describe the three quantities:

proper semimajor axis (a_p),
proper eccentricity (e_p), and
proper inclination (i_p).

The proper elements can be contrasted with the osculating Keplerian orbital elements observed at a particular time or epoch, such as the semi-major axis, eccentricity, and inclination. Those osculating elements change in a quasi-periodic and (in principle) predictable manner due to such effects as perturbations from planets or other bodies, and precession (e.g. perihelion precession). In the Solar System, such changes usually occur on timescales of thousands of years, while proper elements are meant to be practically constant over at least tens of millions of years.

I suspect the math here is quite interesting. Unfortunately Wikipedia doesn't go into details.

https://en.wikipedia.org/wiki/Proper_orbital_elements

For table of asteroid families, go here:

https://en.wikipedia.org/wiki/Asteroid_family

The picture is from here:

https://thenaturalhistorian.com/2014/05/14/rewinding-the-clock-an-asteroid-family-history-age-solar-system/

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A visitor from outside the Solar System!

It came from the direction of the star Vega in the constellation Lyra. It shot toward us at 26 kilometers per second. That's much faster than the escape velocity of the Solar System. So it wasn't orbiting the Sun. It's an interloper from interstellar space! We've never seen such a thing in our Solar System before.

As it fell toward the Sun it picked up speed. It shot past the Sun at 88 kilometers per second. It took a sharp turn... and now it's leaving.

It's called A/2017 U1. It was discovered on October 19th. Rob Weryk, a postdoc at the University of Hawaii Institute for Astronomy, was the lucky fellow. He spotted it using a telescope at the University of Hawaii. Every night this telescope helps NASA search for potentially dangerous near-Earth objects. This was his lucky night.

It came fairly close to Earth: 24 million kilometers, about 60 times the distance to the Moon. It was never a threat. It's an intriguing puzzle!

Weryk immediately realized this was an unusual object. "Its motion could not be explained using either a normal solar system asteroid or comet orbit," he said. Weryk contacted Institute for Astronomy graduate Marco Micheli, who had the same realization using his own follow-up images taken at the European Space Agency's telescope on Tenerife in the Canary Islands. But with the combined data, everything made sense. Said Weryk, "This object came from outside our solar system."

"This is the most extreme orbit I have ever seen," said Davide Farnocchia, a scientist at NASA's Center for Near-Earth Object Studies (CNEOS) at the agency's Jet Propulsion Laboratory in Pasadena, California. "It is going extremely fast and on such a trajectory that we can say with confidence that this object is on its way out of the solar system and not coming back."

What is it? At first people thought it was a comet and called it C/2017 U1. But on October 25, incredibly detailed photos taken at the Very Large Telescope in the deserts of Chile showed it had no tail. So, it's probably made of rock. It was renamed A/2017 U1, becoming the first comet to be reclassified as an asteroid. But it's not a normal asteroid, so it may eventually get a new name. The International Astronomical Union doesn't have rules for naming this sort of entity.

If it's a rock that reflect 10% of the light that hits it, it would be roughly 160 meters in diameter.

On October 25th another telescope, the William Herschel Telescope, saw that it's red. This is a big clue, because objects way out in the Kuiper belt, beyond Pluto, tend to be red. That's because they're covered with tholins - a messy and mysterious mix of complex organic chemicals formed by billion-year-long exposure to radiation.

It's on its way out now, and astronomers are watching it carefully, desperately trying to squeeze a bit more information out of this encounter. How does a rock escape another solar system? How long has this object been shooting through the icy depths of interstellar space before it reached us? How many of these things are there?

"We have been waiting for this day for decades," said CNEOS Manager Paul Chodas. "It's long been theorized that such objects exist -- asteroids or comets moving around between the stars and occasionally passing through our solar system -- but this is the first such detection. So far, everything indicates this is likely an interstellar object, but more data would help to confirm it."

The quotes are from NASA's webpage:

https://www.nasa.gov/feature/jpl/small-asteroid-or-comet-visits-from-beyond-the-solar-system

and so is the animated gif.

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The beauty of Saturn at night

I wish I could go to Saturn, and gaze past its rings to see the dimly lit moon Mimas suspended above this huge planet at night.

Luckily Cassini did it for me.

Mimas is a moon of many resonances. It orbits Saturn twice each time the larger moon Tethys goes around: that's called a 2:1 resonance. It also orbits twice while the the tiny moonlet Pandora goes around 3 times: that's a 2:3 resonance.

Pandora is a shepherd moon that lives amid the rings and helps craft them. But Mimas also helps shape Saturn's rings: it clears material from the Cassini Division, the gap between Saturn's two widest rings. Why? Because it's in a 1:2 resonance with stuff orbiting at the inner edge of the Cassini division. In other words, that stuff orbits twice for each orbit of Mimas... and repeated pulls and pushes from Mimas knock it away.

Similarly, Mimas is in a 1:3 resonance with stuff in the the gap between Saturn's B and C rings.

Notice something funny? Resonance can stabilize orbits - as with Mimas and Tethys, that have somehow gotten locked into a 2:1 resonance. But an object can also destabilize the orbit of something else that it's in resonance with. I've never understood this. I think it requires calculations to really understand it - words are not enough. While I'm sure people have done these calculations, I've never seen them presented clearly in a way that demystifies the 'paradox' of how resonance can both stabilize and destabilize orbits, depending on the details of the situation. Maybe when I retire I'll have time to go through these calculations and explain them to everyone.

It would be even better if I could hear the resonances. If I could hear them, Mimas would be an octave above Tethys, a fifth below Pandora, an octave below the Cassini division, and an octave and a fifth below the gap between the B and C rings. The music of the spheres... a very deep, harmonious chord.

But for now, I can relax by gazing past Saturn's rings to its dimly lit moon Mimas, suspended above this huge planet at night.

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Kilonova

By the time we actually saw it, it was a ball of gas the size of Neptune's orbit, expanding at 1/5 the speed of light. But the gamma rays reached us sooner, and the gravitational waves even earlier.

We suspected it before, but now we have seen how gold is made. And platinum, and iodine, and xenon, and many others. In a kilonova: the collision of two neutron stars.

Luckily this event occurred a safe distance away: 130 million light years, in another galaxy.

Greg Egan's novel Diaspora describes the disaster that would happen if neutron stars collided closer to home. After we spread into space, so we have a fighting chance.

Check out his post and his website for more about how neutron star binaries produce gravitational waves.

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The chirp of death

Almost exactly twenty years ago, my novel Diaspora was published. The story begins about a thousand years in the future — just before Earth’s biosphere is ravaged by the gamma-ray burst created when a pair of neutron stars a mere 100 light-years away from us collide. This event doesn’t happen without warning: a gravitational wave observatory tracks the decay of the neutron stars’ orbit in the run-up to the collision.

As anyone who’s read the news today will know, twenty years later, we’ve actually observed gravitational waves from exactly this kind of collision — as well as the gamma rays, light and radio waves it produced. Needless to say, since it was about 130 million light years away, rather than 100 light years, we won’t be suffering any ill effects.

Back in 1997, I wrote a little Java applet to simulate the effects of the gravitational waves produced by this kind of source. Today, since more and more browsers refuse to let their users run Java, I rewrote the applet in JavaScript, which most browsers support. The applet is at:

http://www.gregegan.net/DIASPORA/04/04.html

One thing I learned when I was writing the applet back in 1997 was that, although the general-relativistic calculations describing the gravitational waves from a binary system are fairly complicated (even in the linearised version of GR, which is good enough for most purposes prior to the collision itself), you can get the correct form of some of the relevant equations from simple heuristic arguments and ordinary old Newtonian orbital mechanics. You only really need GR to get the constants of proportionality exactly right. [For anyone with Misner, Thorne and Wheeler’s book Gravitation, this is covered in Chapters 35 and 36.]

For example, suppose you want to know the rate at which a binary pair of neutron stars is radiating energy in the form of gravitational waves. Each star is experiencing centrifugal acceleration of a_i ω^2, where a_1 and a_2 are the distances of the two stars from the centre of mass of the system and ω is the angular frequency of the orbit. The amplitude, A_i, of the gravitational radiation produced by each star is proportional to the product of its acceleration and the mass of the star, m_i, and inversely proportional to the distance r at which the wave is measured

A_i = m_i a_i ω^2 / r

A free-falling body isn’t really accelerating in the general-relativistic view of things, but in linearised GR, where you pretend that special relativity applies and that space-time curvature is like an extra field, you can think of a free-falling body as accelerating, and just as the amplitude of electromagnetic radiation from an accelerating object is proportional to the product of that acceleration and the object’s charge, the amplitude of gravitational radiation from an accelerating object is proportional to the product of the acceleration and the object’s mass.

Now, the tricky thing is that the two stars will always be accelerating in opposite directions, so their gravitational radiation will be 180° out of phase. What’s more, by the definition of “centre of mass”, the distances a_1 and a_2 of the stars from the centre of mass obey the equation:

m_1 a_1 = m_2 a_2

This means the amplitudes of the gravitational radiation from both stars will also be more or less equal (assuming we are observing them from far enough away that r is essentially the same in both cases):

A_1 ≈ A_2

Two waves of equal amplitude, 180° out of phase, will cancel out!

But what prevents this cancellation is the fact that, in general, the two stars will not be equidistant from the observer, and though the difference in r won’t be significant, the time lag between the waves due to the different distances they travel will introduce a further phase difference. This will be proportional to the distance between the stars, a_1+a_2, and inversely proportional to the wavelength of the radiation ... and so proportional to the angular frequency, ω. So we have a total amplitude with the proportionality:

A ~ m_1 a_1 (a_1+a_2) ω^3 / r

If we define:

a = a_1+a_2

and

μ = m_1 m_2 / (m_1+m_2)

this simplifies to:

A ~ μ a^2 ω^3 / r

The power of radiation of amplitude A is proportional to A^2, and if we integrate over a sphere around the source, with surface area proportional to r^2, we end up with a total luminosity of:

L ~ μ^2 a^4 ω^6

But by Kepler’s Third Law, we have:

ω^2 ~ M/a^3

where M = m_1+m_2, and so:

L ~ M^3 μ^2 / a^5

The actual result from a careful calculation in linearised GR is:

L = (32/5) (G^4 / c^5) M^3 μ^2 / a^5

Now, a warning: don’t take this heuristic too literally! This argument makes it sound as if an observer positioned on the axis of the orbit, always equidistant from both stars, would measure no gravitational radiation, but that isn’t actually the case. We have ignored the fact that these waves are tensors, which introduces further subtleties, but nonetheless we did end up with the correct form for the gravitational luminosity.

If you ponder this result, you can see why there is an intense “chirp” of gravitational radiation as the two stars approach the moment of collision. As their separation, a, decreases, the rate at which energy is lost rises rapidly, which in turn drives the separation down ever faster, as well as increasing the frequency of the orbit and raising the “pitch” of the radiation.

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Explosion equal to 1000 novas makes gravitational waves!

Now gravitational wave astronomy is getting really interesting.

On August 17th, three days after the Virgo detector in Italy saw its first gravitational wave event, the LIGO detectors in the USA saw another.

Virgo did not see this one - it was in Virgo's "blind spot". But this fact helped LIGO locate the source.

At that moment, all we knew is that something had happened in a large patch of sky, the size of several hundred full moons.

But about two seconds later, two satellites detected a short gamma-ray burst from this patch of sky!

When night fell in Chile, many wonderful telescopes located in the high desert there started staring at this patch of sky. The Swope 1-meter telescope was the first to see something: a new point of light very close to the galaxy NGC 4993. Shortly afterward, the VISTA telescope saw infrared light coming from the same spot. And as night marched west across the globe, two telescopes in Hawaii picked up where those in Chile left off. They saw rapid changes as the explosion expanded and cooled down.

We now have a pretty good guess as to what happened. Distance estimates from the gravitational wave data and other observations agree that the source was at the same distance as NGC 4993, a galaxy about 130 million light-years from Earth.

That may sound far — but it's the closest gravitational wave event we've seen yet! It's also one of the the closest gamma-ray bursts ever seen. That's good news for science.

What creates short gamma rays bursts? The leading candidate is the merging of two neutron stars. This should create an explosion 1000 times brighter than a typical nova — called a kilonova. It should also create a burst of gravitational waves.

This is the first kilonova we've seen in detail — not just the gamma rays, but gravitational waves and light. Thanks to this, we know the kilonova shot out a blob of radioactive elements, moving as fast as 1/5 the speed of light!

Spectroscopes show that this blob contained cesium and tellurium. We actually expected such heavy elements to be produced during a neutron star collision. In fact, theorists have believed for a while that about half the elements heavier than iron were made this way. But this is the first time we've seen it happen! So this is a big win.

If you want to impress your friends, you can casually say that scientists have finally seen r-process nucleosynthesis. The r is for "rapid" — it's where nuclei grow by capturing lots of fast neutrons that are shooting around. It happens in neutron star collisions but also in some supernovae. The other main method for making heavy elements is the s-process, where s is for "slow". That happens in large stars as they live out their last days before blowing up and shooting the stuff into space.

Stephen Smartt, who did some of the spectroscopy, said:

When the spectrum appeared on our screens I realised that this was the most unusual transient event I’d ever seen. I had never seen anything like it. Our data, along with data from other groups, proved to everyone that this was not a supernova or a foreground variable star, but was something quite remarkable.

For more, check out the news below.

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How to move the Sun

Suppose we wanted to move the Solar System. How could we do it?

Okay, first things first: why would we want to?

Well, our Sun will eventually become a red giant. In just about 1.1 billion years it will become 10% brighter – enough to boil the Earth's oceans and create a runaway greenhouse effect. If we could move the Earth farther from the Sun, that would buy us time. But it would be even cooler to carry the Earth to a brand new star. And to keep it from freezing en route, we could try to move the whole Solar System.

Of course this seems like a wacky idea. But a billion years ago, the whole concept of intelligent life was a wacky idea. Heck, back then they didn't even have the idea of an 'idea'. So a lot can happen in a billion years.

One way to move the Solar System is a Shkadov thruster. The Russian physicist Leonid Shkadov came up with this idea in 1987. Russian physicists have had some impressively bold thoughts, and this is a great example.

The idea is to build an enormous mirror or 'light sail'. If you did it right, the push of sunlight would balance the pull of gravity towards the Sun, so it wouldn't fall in and it wouldn't fly away.

With this mirror in place, more sunlight would shine out into space in one direction than another! This would push the Sun, which would drag the Solar System with it.

The acceleration would be very tiny. At best, after a million years the Sun would be moving at just 20 meters per second... and it would have moved 0.03 light-years. That's a respectable distance, but nowhere near the closest star.

But with a constant acceleration, the distance traveled grows as the square of the time (at least until special relativity kicks in). So, after a billion years, the speed would be 20 kilometers per second... and the Sun would have moved 34,000 light-years! That's a third the diameter of the Milky Way!

Of course, a billion years would be pushing it, since we're expecting the oceans to boil away just 100 million years after that. You don't want a last-minute rush to hand off the Earth to a new star! Luckily, we won't need to go nearly this far to reach a nice new star.

Building a Shkadov thruster won't be easy.

For starters, it will take a lot of material! Viorel Badescu, a physicist at the Polytechnic University of Bucharest in Romania, estimated the mirror would have to weigh 1/10,000th of the Earth's mass. That's 600,000,000,000,000,000 tonnes. The easiest way to get this stuff might be to mine the planet Mercury.

Hey, I've got an idea! Let's start with an easier project, as a kind of warmup. Let's stop global warming.

What really pisses me off about modern politics is that we're spending so much energy fighting about stupid stuff instead of thinking big.

For more on the Shkadov thruster:

http://geekisnotadirtyword.com/the-shkadov-thruster-or-how-to-move-an-entire-solar-system/

The Shkadov thruster is just one kind of stellar engine. For others, try this:

https://en.wikipedia.org/wiki/Stellar_engine

Also try this:

• V. Badescu and R. B. Cathcart, Use of class A and class C stellar engines to control Sun movement in the galaxy, Acta Astronautica 58 (2006), 119–129. Available at https://www.dynamical-systems.org/zwicky/stellarengines.pdf

Ah, those Eastern Europeans, with their big ideas and their disdain for little words like 'the'.

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Three eyes are better than two

Until recently, gravitational waves had only been seen by the two LIGO detectors in Washington and Louisiana. But there are other detectors - and the best is Virgo, shown below. It's near Pisa, in Italy. Laser light bounces back and forth through two very long pipes. When a gravitational wave zips past, it stretches and squashes space... affecting the time it takes for light to move through these pipes... so Virgo can detect it.

And it did! It happened on August 14th, but it was just announced 3 days ago.

This event was also seen by the two LIGO detectors. This is really great! With previous gravitational waves, we had trouble telling where they came from. But if you know how long it takes a wave to reach three different locations, you can be almost sure which direction it came from. A third detector also reduces the chance of mistaking random noise for a real event.

The first paper on this event only gives a few basic facts. It seems that two black holes of masses about 25 and 30 solar masses collided with each other in a distant galaxy about 1.6 billion light years away. We don't know which galaxy, but we can pin it down to a much smaller region of the sky than we could with just LIGO.

The detector in Louisiana was the first to receive the signal. The detector in Washington saw it 8 milliseconds later, and Virgo saw it 14 milliseconds later. These differences in timing let us triangulate the source, giving a very accurate estimate of its location, with a 90% chance of it being in a region of of just 60 square degrees - 20 times more accurate than if we just had LIGO.

This signals the full arrival of gravitational wave astronomy. Science demands replication. Now different teams of scientists can detect gravitational waves and check each other's work.

It's been a long time coming! I've been hearing about Virgo for decades. It's almost strange to hear it's finally working. In 2011 they turned it off and spent 5 years improving it. Now it's much more sensitive - not as good as LIGO, about 1/3 as sensitive, but still very good.

One cute fact: Virgo contains the biggest chunk of almost nothing in all of Europe!

The laser beams in Virgo bounce back and forth in two pipes 3 kilometers long and 1.2 meters in diameter. The air in these pipes has been sucked out very carefully, so changes in temperature and such don't affect the light. So this is the largest ultra-high vacuum installation in Europe, with 6,800 cubic meters of space at one trillionth the ordinary air pressure!

I bet the only thing better on this planet is LIGO, where each installation has two pipes 4 kilometers long.

Here's the paper. If you work on gravitational wave astronomy, it help to have a last name beginning with A:

B. P. Abbott, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley, C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya, C. Affeldt, M. Afrough, B. Agarwal, M. Agathos, K. Agatsuma, N. Aggarwal, O. D. Aguiar, L. Aiello, A. Ain, P. Ajith, B. Allen, G. Allen, A. Allocca, P. A. Altin, A. Amato, A. Ananyeva, S. B. Anderson, W. G. Anderson, S. V. Angelova, S. Antier, S. Appert, K. Arai, M. C. Araya, J. S. Areeda, N. Arnaud, K. G. Arun, S. Ascenzi, G. Ashton, M. Ast, S. M. Aston, P. Astone, D. V. Atallah, P. Aufmuth, C. Aulbert, K. AultONeal, C. Austin, A. Avila-Alvarez, S. Babak, P. Bacon, M. K. M. Bader, S. Bae, P. T. Baker, F. Baldaccini, G. Ballardin, S. W. Ballmer, S. Banagiri, J. C. Barayoga, S. E. Barclay, B. C. Barish, D. Barker and 1048 more authors, GW170814: A three-detector observation of gravitational waves from a binary black hole coalescence, Physical Review Letters 116 (September 25, 2018), 061102. Available at https://arxiv.org/abs/1709.09660.
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An ancient lake on Mars

As you kept hearing about that Mars rover Curiosity crawling around Gale Crater, did you find yourself getting a bit bored? Everyone loves the idea of life on Mars, but all it ever found was rocks.

Geology is not glamorous.... until the results come in. It's like detective work. You dig up rocks, you study them, you dig up more rocks... and eventually you learn about the past.

Gale Crater was once a lake! The picture here is a recreation of what that lake might have looked like. And we now know quite a lot about that lake, thanks to Curiosity.

The rocks Curiosity found were sedimentary: they're made of mud, and they have layers!

On one side of Gale Crater there are rocks with thick layers, maybe laid down by a stream flowing into the lake. On the other side are rocks with thinner layers - maybe from a calmer part of the lake.

The rocks have a lot of iron in them. In the rocks nearer the top of Gale Crater the iron is more oxidized. Why would that be? The upper layer of a lake often has more oxygen in the water.... here on Earth, at least! Mars now has a very thin atmosphere that's mostly carbon dioxide. But once it had a thicker atmosphere, with more oxygen. So the upper layer of the lake could have had more oxygen in the water!

This is just part of the wonderful slow detective work that Curiosity is helping us do. Amanda Doyle explains in more detail:

In a recent paper published in the journal Science, “Redox stratification of an ancient lake in Gale crater,” Stony Brook University geoscientist Joel Hurowitz and his colleagues used more than three years of data retrieved from the rover to paint a picture of ancient conditions at Gale Crater, the lowest point in a thousand kilometers. The site, a 150-mile kilometer crater formed during an impact around 3.8 billion years ago, once flowed with rivers ending in a lake. The sedimentary rocks laid down by these rivers and onto the lakebed tell the story of how the environment changed over time.

Curiosity landed on a group of sedimentary rocks known as the Bradbury group. The rover sampled a part of this group called the Sheepbed mudstones, as well as rocks from the Murray formation at the base of the 5-kilometer high peak at the center of the crater known as Mount Sharp. Both types of rocks were deposited in the ancient lake, but the Sheepbed rocks are older and occur lower in the stratigraphic layers of rocks. Comparing the two types of rocks can lead to interesting revelations about the paleoenvironment.

Rocks that form at the same time in the same area can nevertheless display differences in composition and other characteristics. These different groupings are known as facies and the Murray formation is split into two facies. One is comprised mainly of hematite and phyllosilicate, and given the name HP, while the other is the magnetite-silicate facies, known as MS.

“The two Murray facies were probably laid down at about the same time within different parts of the lake,” explained Hurowitz. “The former laid down in shallow water, and the latter in deeper water.”

The near-shore HP facies have thicker layers in the rocks compared to the thin layers of the deeper water MS facies. This difference in layer thickness is because the river flowing into the lake would have slowed down and dumped some of its sedimentary material at the lake shore. The flow would then have spread into the lake and dropped finer material into the deeper parts of the lake.

The different mineralogy of the two facies was caused by the lake becoming separated into two layers. Ultraviolet (UV) radiation along with low levels of atmospheric oxygen penetrated the upper part of the lake and acted as oxidants on molecules in the water. These ions of iron (Fe2+) and manganese (Mn2+) were brought to the lake via seepage of groundwater through the lake floor.

When the UV and oxygen interacted with these, they lost electrons, meaning that they had become “oxidized.” The oxidized iron and manganese precipitated into minerals — hematite and manganese oxide — that eventually made up the rocks sampled by Curiosity in the HP facies. However, the UV and oxygen didn’t reach all the way to the lake floor, so the iron and manganese wasn’t oxidized in the deeper part of the lake, and instead became the mineral known as magnetite, making up the MS facies.

The difference in oxidation of the two facies in the Murray formation due to differences in layers of the lake is known as redox stratification. Identifying redox stratification in the ancient lake shows that there were two completely different types of potential habitat available to any microbial life that might have been present.

The researchers also discovered that the Murray formation has a high concentration of salts, which provide clues relating to evaporation of the lake, and thus the end of the potential habitat. High salinity is a result of water evaporating and leaving salts behind. However, evaporation leaves other tell-tale signs such as desiccation cracks — similar to what you see when mud dries and cracks — and none of these signs appear in the Murray formation. This indicates that the evaporation occurred at a later period of time and that the salts seeped through layers overlying the Murray formation before becoming deposited in the Murray rocks.

“Curiosity will definitely be able to examine the rocks higher up in the stratigraphy to determine if lake evaporation influenced the rocks deposited in it,” said Hurowitz. “In fact, that’s exactly what the rover is doing as we speak at the area known as Vera Rubin Ridge.”

Yes, it's still roving around, learning more!

The quote is from here:

• Amanda Doyle, Ancient lake on Mars was hospitable enough to support life, Astrobiology, 18 September 2017, https://www.astrobio.net/news-exclusive/ancient-lake-mars-hospitable-enough-support-life/

and I recommend looking at the pictures, to enjoy imagining a Martian lake in more detail!

#astronomy
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