New stuff from really old rocks.

A recent article in caught my (and several other people’s) eye. Chi Ma, et al., “Panguite, (Ti⁴⁺,Sc,Al,Mg,Zr,Ca)_1.8 O₃, a new ultra-refractory titania mineral from the Allende meteorite: Synchrotron micro-diffraction and EBSD,” American Mineralogist , July 2012, v. 97, no. 7, p. 1219-1225.  Now I’m not a geologist.  Most of the “meteorites” that I study are still in space, and I don’t know the author at all.  Why am I excited about it?  Therin lies the story…

On February 8, 1969 thousands of rocks fell from the sky over an area some 300 km² is size near the village of Pueblito de Allende in Chihuahua Mexico.  Now known as the Allende meteorite, it stands as one of the most famous and important meteorites in modern times.  Why is that?  Well Allende’s main claim to fame is that it extremely primitive, or in other words, it is really honking old, and is basically unchanged from the earliest times of the Solar System.  It is known as a carbonaceous chondrite, a class of meteorite that are very dark and rich in volatiles like water and (sometimes) organics.  Some carbonaceous chondrites have been known to sweat water when heated.

The fact that these meteorites exhibit that trait indicates something striking – they were never part of a very large parent body (asteroid).  If that was the case the heat generated by the formation of slamming all these small rocks together, and the mutual heat generated by the natural decay of radioactive nucleotides would have radically changed the composition of these rocks.

Allende in particular is know to have in it many, tiny, little white bits trapped within the generally dark matrix that makes up the bulk of Allende meteorites.  These little white bits are called calcium-aluminium inclusions, or CAIs. 

Above is a slice of Allende with a couple prominent CAIs visible.  It is these CAIs that hold the Solar System’s clock.  They were the (some of) the very first high temperature solids to form in the Solar System.  When someone says that the Solar System is 4.57 billion years old, they are really saying that these first solids (the CAIs) formed that long ago (age “zero” for a rock is when it crystalizes/becomes solid).  Essentially, meteorites like Allende are the left over building blocks of the Solar System.  Put enough together you get Mercury.  Put enough together you get Mars.  Put enough together and you get the Earth.  They are the most primitive bits of Solar System solids, older than any rock we can find on large bodies like the Earth, Mars, or even the Moon.  These CAIs have basically the same elemental composition as the early Sun (excluding gasses of course), and CAI bearing meteorites like Allende preserve these pre-Solar System grains - samples of what the Solar System itself was like almost 4.6 billion years ago!

Now, back to the Chi Ma, et al., 2012 paper.  Take apart the title and you have the story: they found a brand new high temperature mineral, now officially named “Panguite,” in samples of Allende.  The same Allende that fell in Mexico in 1969, was collected and has been studied for over 40 years!  Allende, one of the most famous and well studied meteorites in history still has many secrets to reveal – and that’s one of the things that makes planetary science, astronomy, and science in general, great to me:  It always has some new way of surprising you, whether it is bizarre, unexpected features on the first images of a new world or brand new minerals being found in rocks that have been continually studied for half a century.

Science just plain rocks.  Sorry, had to make the obligatory geology joke there.

Panguite is ready for its close up as seen in Figure 2 from Chi Ma, et al., 2012.

A Black Hole Made of Water?

So while reading through twitter the other day I noticed a few folks posting that if you took the mass of a Black Hole and spread it out through its entire volume you would find that the density of this smeared-out black hole would have the same density of water.  This tripped my astronomer-senses right away.  A 1 solar mass black hole (that is a black hole with the same mass as our sun) has a classical, non-rotating radius of about 3 kilometers.  A 3 km ball of water won’t spontaneously collapse into a black hole, as evident by the the fact that our oceans and Jupiter’s moon Europa are not black holes right now.  While this statement was wrong, it got me thinking – “Could a Black Hole be massive enough for it to be true?”

In a classical sense this problem isn’t too tough to work out, I just need to find how the volume of a black hole scales with its mass and figure out where that gives a gross density of about 1000 kg/m³, the density of water (1 g/cm³ for those using cgs units!).

To make things easy on myself I’m taking a classic, non-rotating black hole, and assuming that the radius in question is the Event Horizon, where the local escape velocity equals the speed of light.   Take the equation of escape velocity v_e:

 

with G being Newton’s Gravitational Constant (6.67×10⁻¹¹ m³ kg⁻¹ s⁻²), M being the mass of the world one is trying to escape, and r the radius of that place.

Setting v_e equal to the speed of light c (3×10⁵ km/s) and solve for r one finds the radius for which the mass is enough to keep even light from escaping.  This is the Schwarzschild Radius.

 

Now this is a pretty small radius for most masses - as it should be since we don’t see ordinary things collapsing into black holes all around us!  A Black Hole with the mass of the Sun ends up with a radius of about 3 km.  An Earth-mass black hole would be about the size of a single playing die, and a person-mass Black Hole would be much smaller than a proton!

Now I can use that equation to find the radius of a Black Hole of a given mass.  To find the gross density rho I need to divide the Mass M by the volume of a sphere of radius r_s. 

Plugging in the Schwarzschild Radius we get

And finally using the the numerical values for all these constants we get the fairly simple

So the gross density enclosed within the event horizon of a black hole scales as 1/M².  Solving for the density of water (1000 kg/m³) I find that a black hole with a mass of about 2.7 x 10³⁸ kg would do it at a radius of about 4 x 10¹¹ km.  This is over the mass of 100 million Suns in a sphere about 90 times larger than the orbit of Neptune.  Huge, but there are Super Massive Black Holes that dwell at the center of galaxies that can achieve this mass.  While own Milky Way Galaxy has a central black hole with the mass of about 4.5 million Suns, too small for the water-density hypothesis, the most massive Galactic Black holes that we have detected check in at thousands of millions of solar masses!  These monsters (under this simple analysis) would indeed have a gross density that would be much less than water.  To visualize this I made up a quick and dirty plot showing Black Hole density on the y-axis and the Black Hole mass on the x-axis.  My simple density equation is the diagonal red line, while the strait line shows the density of water for comparison.  They cross right at a Black Hole mass of 2.7 x 10³⁸ kg. 

So while my astronomer-sense did kick in to point out that the statement about the gross density of a Black Hole being about the same as water is incorrect in general, there are known Black Holes that are massive enough for this to be true!  While the Black Holes left over after the death of a super-massive star or the Black Holes in the center of the Milky Way and Andromeda galaxies are far more dense than water, the super massive Black Holes found at the heart of giant elliptical galaxies, for example NGC 4889 located in the Coma Cluster end up being much less dense than water.

Flag Day!

For Flag Day I pulled out a few extraterrestrial flags that humans have placed on other worlds.

On The Moon: 

Have to start off with the big one, Apollo 11’s flag.  The flags placed by the Apollo astronauts remain the only flags ever planted on the surface of another world by a human being.  Also they are the only ones actually on flag poles!  Whether these flags are still around today is debatable – a number of them were knocked over during lift-off of the Lunar Excursion Module’s (LEM) Ascent stage, as they were placed too close to the LEM.  Buzz Aldrin has mentioned that he saw the flag get knocked over when he and Neil Armstrong left the Moon.

This blurry image is a screen capture I made from of video footage from the video camera left behind on the Moon after Apollo 17 left.  In this case the flag remained standing, and can be seen as the blurry rectangle on the right of the image.  Even that flag however may not really be in that great of shape.  For 40 years they have been exposed to the extreme day/night heating cycles of the Moon, vicious UV light from the Sun, and potentially micrometeoritic bombardment, if the fabric is still there at all it may very well be essentially bleached white!

More flags than just the US flag are on the Moon.  In the above image from the Soviet Union’s Luna 17 Lander (bringing with it the Lunokhod Rover).  The Soviet Union’s flag can bee seen on the right hand side of the image.

While not a soft landing, India’s flag arrived on the moon on the side of the Moon Impact Probe, from the Chandrayaan-1 orbiter.  Japan and China both have also had “hard” landings on the Moon – but I haven’t been able to track down a clear image of national flags on either Japan’s Hiten and Selene/Kaguya probes, or on China’s Chang'e 1.

On Mars

Moving even further away from Earth, here is the flag carried on the body of the Viking 2 Lander on the surface of Mars.  The Viking probes were a pair of orbiter and lander probes that pretty much were the backbone of Mars data until the 1990’s.

In the mid 1990’s, Mars Pathfinder landed on Mars with the flag decal seen above in this pre launch image, but to my knowledge there are no images of the flag on Mars since the camera mast was above the decal and couldn’t see it.  I think the same is true for the Mars Expedition Rovers, Spirit and Opportunity

The Phoenix Mars Lander was able to include this flag here while taking images of the polar regions of Mars where it landed in 2008.

On Venus:

No images of the Soviet Union's flag itself from the surface of Venus, but here it is painted on the side of the Venera 13 Lander, which would eventually land on Venus in 1982, and managed to operate for 127 minutes (about 4 times longer than planned!) in the harsh (460C/900F and 90ATM) condition on the surface of Venus.

And beyond…

Finally, while this flag did not end up on the surface of any world, I think it deserves a bit of special mention here.  This is John Casani, Voyager Project Manager, holding a small flag that was to be folded and sewn into the thermal blankets of the Voyager spacecraft in 1977.  The Voyagers are the two most distant objects from Earth ever made by humans (not counting radio signals!) and are now at 18 and 14.7 billion kilometers from Earth.  You can even follow them on twitter – @NASAVoyager2 updates it’s distance and engineering tasks!

I’m still putting together a final collection, a few other probes which may have placed flags on other worlds that I’m interested in finding out about are NEAR Shoemaker (soft-crashed onto asteroid 433 Eros in 2001), Galileo (burned up in Jupiter’s atmosphere in 2003).  I don’t think there was a flag on the Deep Impact impactor that collided with comet 9P/Tempel, and my understanding is that ESA doesn’t include national flags on their missions (the Huygens lander on Saturn’s moon Titan for example).  If you’ve got info on these or any others hit me up on twitter @rocksinspace.

Killer rocks from space!

I had an excellent topic raised today at the Adler Planetarium's Space Visualization Lab about everyone's favorite killer asteroid, 99942 Apophis (the asteroid formerly known as 2004 MN₄). Indeed there is a small chance of Apophis having a rather intimate encounter with our home planet on April 13, 2036.  By slim, I really mean slim.  As of earlier this year the "odds" of Apophis hitting us were being cited at about 1:250,000.  For perspective, that's about the same odds as rolling all 6's on 7 playing dice.

It all comes down to how Apophis passes the Earth on it’s next close passage (no real chance of impact associated with this one!) in 2029.  If Apophis misses the Earth just right it could pass through an exceedingly narrow gravitation keyhole, which would alter the orbit of the asteroid just enough to make a possible impact in 2036.  That said, it isn’t anything to get worked up about, the odds of the asteroid missing us are probably even better than what are being cited due to how incredibly hard it is to pin down the exact positions of the Earth and Apophis with enough accuracy to really predict the impact.

Now how can that be?  I mean it is just gravity after all.  It is true that just about any second year physics major in the country could take on the problem of the Sun and Apophis, and solve the system such that they could predict at all times where it would be in its orbit.  It is also true that adding just a single additional body to that system (say Jupiter, or Earth) makes it such that no single person can exactly solve the resulting system!  This “n-body” problem is at the heart of the very complex calculations needed to predict if an impact can take place.  Since it can’t be worked out analytically, the orbits of asteroids in the solar system must be solved numerically, through simulating the orbits.  Work out all the forces involved, let the solar system move for a very short period of time, work out the changed forces, evolve the system some more, and on and on.  The resulting solution is only as good as the computer and code that is trying to solve it!

What type of accuracy is needed?  A whole lot.  It takes just 7 minutes for the Earth to travel a distance equal to its diameter.  Every 7 minutes that go by Earth is in a completely new portion of space.  One needs to find out if the asteroid shares that space within that 7 minute window.  To complicate matters even more, to get that level of precision, much more than just point-particle Newtonian gravity must be taken into account.  For example, the Sun isn’t a perfect sphere, and is slowly losing mass.  The masses of the Planets and other asteroids are only known to a finite amount, and our own Milky Way Galaxy exerts a tidal force on the solar system.  Largest of all the uncertainties is the way in which the asteroid itself absorbs and reradiates sunlight (the Yarkovsky effect).  Depending on the thermal make-up of the asteroid and how it is spinning this force can significantly alter its orbit, even of fairly short time scales.

In short, it is really hard to nail down exactly where an asteroid will be at a particular time, which creates enough uncertainty to be unable to rule out an impact for objects like Apophis.  As time goes on (and the projected time errors grow smaller) I fully expect the impact chance to continue to drop, although you’ll never see that on cover of the New York Times, as “Asteroid to miss the Earth – everything is fine” probably won’t move many copies.

Worse case, Apophis is only about 270 meters across, enough to cause some significant damage, but not any sort of Doomsday Scenario.

This figure by J. Giorgini (JPL) shows the results of evolving the orbit of Apophis under dynamical conditions, and assumed properties of the asteroid.  The "Nominal" solution shown in red is the "most probable" position of the asteroid on April 13, 2036 - about 0.3 AU from the Earth.  The blue shows the 3-sigma range of the position uncertainty which does encounter the Earth, indicating the simulation produces a small possibility of impact on this date.  The unknown physical properties of the asteroid introduce even more uncertainty, sliding the whole line of probabilities along the asteroid's orbit.  For more details on this dynamical model check out JPL's Apophis Webpage.