Things I Don’t Understand: Mercury’s Wandering Sun

July 25, 2018

Okay, this is a thing I’ve read about multiple times, but no matter how many times it’s been explained to me I just don’t get it.  Apparently on Mercury, the sun sometimes appears to change directions in the sky.

Let me explain what I mean.  Imagine you’re standing on the surface of Mercury (and are somehow still alive).  You see the sun rise in the east, just as it does on most planets in the Solar System.  And then over the course of a long (very, very long) Mercurian day, you watch the sun slowly (so very, very slowly) travel from east to west.

But at one point, let’s say around midday, the sun appears to stop its east-to-west motion and then, for a short while (about 4 Earth days), it wanders from west to east instead.  Then the sun stops again and continues on its original westerly path.

Why does this happen?  I know it has something to do with the length of Mercury’s solar day versus its sidereal day.  A solar day on Mercury, the time it takes for Mercury to complete a rotation relative to the Sun, is approximately 176 Earth days long. But Mercury’s sidereal day, the time it takes for Mercury to complete a rotation relative to the ecliptic, equals about 59 Earth days.  Also, Mercury’s year is 88 Earth days long, so Mercury’s solar day is roughly twice as long as its year.

Obviously this all means the sun moves very slowly through Mercury’s sky, but why should it briefly stop, turn around, and go the other way?  I just don’t get it. I guess I just can’t conceptualize why this happens.  Maybe if I were better at math, all those numbers would add up for me, and I’d understand what’s going on.

Anyway, does this make sense to anyone else, or are you just as baffled by this as I am?

Update: Looks like I have a lot of really smart readers! It’s still kind of hard for me to conceptualize why this happens, but it’s starting to make a little more sense to me. The first comment from TureNorthBricks definitely cleared up a lot for me.


Art in the Wild: Mr. Sun

July 16, 2018

Of late, I’ve felt that I need to push myself a little harder with my art.  I’ve been doing lots and lots of drawings, so it’s not that I’ve gotten lazy; rather, I feel like I’ve gotten complacent.  I feel like I keep doing the same kind of drawing over and over again, without really challenging myself or stretching my artistic skills.

So to shake up my routine, I decided to take some of my art supplies “out into the wild,” so to speak.  Or at least I took them out of my art studio and brought them with me to my day job.  My hope was that I could draw something based on first hand observation, rather than from photo references, mannequins and maquettes, or pure imagination.

A year or two ago, a thoughtful friend left this Mr. Sun figurine on my desk.  Given the history of this blog, that seemed like a good place for me to start.

One of the challenges of drawing from first hand observation is that your mind plays tricks on you.  You have to get past what your mind thinks you should see and draw what your eyes actually see. I had a really tough time with this drawing because my mind kept insisting there should not be highlights and cast shadows on the Sun; the Sun is supposed to be a light source!

It’s been a long time since I’ve taken my art out of the studio like this.  I think it was good practice artistically speaking, and a surprisingly difficult mental challenge as well.  I plan to do a whole lot more of this.  Let me know in the comments if you’d like to see more of the results (good, bad, or ugly).

I love drawing almost as much as I love writing.  But when you love something, there’s a real danger of settling into a comfort zone, becoming complacent, and getting bored. And then you may start to fall out of love with that thing (or maybe even that person) that you loved so much.

So whatever it is you love, I hope you’ll keep pushing yourself, take some risks, and challenge yourself to do something more with it.


Sciency Words: Macromolecule

July 13, 2018

Today’s post is part of a special series here on Planet Pailly called Sciency Words.  Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together.  Today’s term is:

MACROMOLECULE

After all the years I’ve been writing Sciency Words, I’ve noticed something.  A lot of times it might seem pretty obvious what a scientific term means, but then you dig a little deeper and find that the term is not so clearly or precisely defined as you’d expect.

Defining macromolecule should be easy.  Macro means big, molecule means molecule; ergo, a macromolecule is a big molecule.  But after I read this paper about the discovery of organic macromolecules on Mars, I had a question: just how big does a molecule need to be to get that macro- prefix?

German chemist Herman Staudinger is credited with coining the term macromolecule.  It was a highly controversial concept at the time.  Another German chemist, Nobel laureate Heinrich Wieland, wrote to Staudinger in the 1920’s saying: “My dear colleague, drop the idea of large molecules; organic molecules with a molecular weight higher than 5000 do not exist.”  But Staudinger would later become a Nobel laureate himself for proving that they do.

I take that Wieland quote to mean that the word macromolecule was defined as any molecule with a molecular weight in excess of 5000, but I’ve seen other sources claiming it was defined as any molecule containing one thousand or more atoms, and still other sources saying it’s ten thousand or more atoms.

But those were the kinds of definitions being used in the early 20th Century.  Modern usage gets far more complicated and confusing.  As Wikipedia explains, the definition of macromolecule “varies among the disciplines.”

  • In biology, there are four kinds of macromolecule: lipids, proteins, nucleic acids, and carbohydrates. If it’s not one of those four things, it’s not a macromolecule, according to a biologist.
  • Polymer scientists go by a definition set by the International Union of Pure and Applied Chemistry (IUPAC), which states that a macromolecule is a “molecule of high relative mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass.”
  • Wikipedia also mentions a definition that involves aggregates of molecules sticking securely together due to intermolecular forces rather than covalent or ionic bonds.

It’s not unusual for one word to be defined in different ways by different fields (see my post on metallicity).  This is a big reason why some scientific terms end up being so difficult to define.

As for those organic macromolecules Curiosity found on Mars… in the context of that research, I think macromolecule simply means “very big molecule.”  Like I said on Wednesday, we don’t really know what, specifically, Curiosity found, and maybe we never will.  We just know that it must’ve had a lot of very big molecules in it.


The Big Martian Maybe

July 11, 2018

Could life exist on Mars?  There’s plenty of compelling evidence that it could, and also plenty of compelling evidence that it could not.  As a result, we’re left with a big, fat maybe. Perhaps the biggest, most frustrating maybe in all of modern science.

After last month’s announcement that the Curiosity rover had found large, complicated organic chemicals on Mars, I was initially tempted to add another point to the “yes, life could exist on Mars” column. But then I read the actual research (which is excellent, by the way).  At this point, I think the only thing we can say for certain is that the big maybe about Mars is even bigger and even more maybe-like.

The Curiosity rover dug up some samples from Martian mudstone, samples that apparently contained organic macromolecules.  What are macromolecules?  For now let’s just say they’re very big molecules.  We can dive into the technical details of what defines a macromolecule in Friday’s episode of Sciency Words.

The problem, as I understand it from that research paper, is that these macromolecules were too big for Curiosity’s instruments to analyze.  So Curiosity destroyed the molecules through a process called pyrolysis (also coming soon to Sciency Words) and analyzed the bits and pieces as they broke apart.  Even those bits and pieces were difficult for Curiosity to study because there were so many of them, but for the most part they seemed to be aromatic compounds made of carbon, hydrogen, and sulfur.

These are the kinds of organic materials that could be deposited on a planet by meteor impacts.  They could also have formed through rather ordinary geological processes.  Or they could be the residue left behind by some kind of biological activity.  And there doesn’t seem to be any way to know for sure where these organics came from based solely on the data Curiosity was able to collect.

So we’re still left with a big maybe.  However, it was once thought by some that the Martian environment was too harsh to preserve these sorts of molecules at all.  Thanks to Curiosity, we now know Mars can and does preserve its organic macromolecules.

And that means that if Mars has had any sort of biological activity, either in the past or present, the chemical record of that activity should be there for us to find.  A definitive yes or no to our question is possible!  We just have to keep digging.


The Infamous “Pluto Not Yet Explored” Stamp

July 9, 2018

A couple years ago, I needed some postage stamps.  To my delight, the local post office had a wide selection of space-themed stamps to choose from, including this four-stamp sheet commemorating the New Horizons mission to Pluto.

But it wasn’t until I read Chasing New Horizons by Alan Stern and David Grinspoon that I realized the full significance of the stamps I’d purchased.  Those stamps were, in fact, a major part of New Horizon’s history.

The story begins in 1991, shortly after Voyager 2 completed its flyby of Neptune.  The U.S. Post Office issued a set of stamps honoring the first NASA space probes to visit each of the planets and also the Moon.  Pluto was also included in the set, with the caption “Pluto not yet explored.”

Apparently some in the planetary science community took this as a challenge.  That set of stamps, including the “Pluto not yet explored” stamp, became a symbol of work that had been left unfinished, of a job that still needed to be done.  The stamp was used in proposals and presentations arguing for a Pluto mission.  It was part of the public outreach campaign once New Horizons was underway. And the day New Horizons reached Pluto, a poster-sized version of the stamp was help up for the press with the words “not yet” crossed out.

So naturally, following that 2015 flyby mission, the Postal Service had to issue new stamps.  That day when I went to get stamps, so I could pay my rent and bills and other mundane things, I had no idea how much meaning and significance was packed into that little stamp sheet.  Even the elongated dash in “Pluto—Explored!” feels significant, as though it’s a reminder of the words that were crossed out.

I’ve never been a stamp collector, but as it so happens I still have at least one sheet of Pluto stamps left, and once I knew the full story behind them I went and did a little shopping online.  Now both sets of stamps—the 2015 stamps for New Horizons and the original set of stamps from 1991—are part of my modest collection of space exploration memorabilia.


Sciency Words: Aromatic

July 6, 2018

Today’s post is part of a special series here on Planet Pailly called Sciency Words.  Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:

AROMATIC

At some point when I was a little kid, I discovered that gasoline doesn’t smell terrible.  In fact, it has an almost sweet aroma to it.  I got in a lot of trouble for this because, for obvious reasons, my parents didn’t want me sniffing gas fumes.  But still, that subtly sweet smell is there, and it’s caused by a chemical known as benzene.

Apparently I’m not the only person to take note of benzene’s smell.  German chemist Augustus Wilhelm Hofmann is credited with the first usage of the word “aromatic” to describe benzene, along with a whole host of other sweet-smelling chemicals.

Hofmann seems to have realized not only that these chemicals smelled similar but also that they had similar chemical compositions.  “Of this series,” Hofmann wrote in 1855, “few members are at present known, but the group of aromatic acids is itself very imperfect and limited.”  In other words, Hofmann predicted the existence of more “aromatic” chemicals that should fit the pattern.

And more chemicals of this series were later discovered, and we now know what they really have in common: a flattened, ring-like chemical structure, as pictured below:

As an adult, I know better than to sniff gasoline, and as an artist I know better than to sniff my art supplies.  But the xylene used as a solvent in some pens and markers does have that same vaguely sweet aroma as benzene. However, not all of the chemicals we call “aromatic” smell so nice, or smell at all.  It’s the flattened, ring-like structure that defines aromaticity today.  The odor is no longer considered relevant.

You might be wondering then why we still call these chemicals aromatic, if their aromas aren’t important.  This seems to be another case of scientists naming something before they really understood it.  The same thing happened with the word organic.  The term was used so often in scientific literature and became so deeply ingrained in the scientific lexicon that we’re now unable to change it.

The ring-like structures in aromatic chemicals are incredibly strong and unlikely to break apart during chemical reactions. That makes them really good structural components for the large, complex molecules that make life possible here on Earth—and may have once made life possible on Mars.  But we’ll talk more about that next week!


Sciency Words: Coatlicue

June 8, 2018

Today’s post is part of a special series here on Planet Pailly called Sciency Words.  Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:

COATLICUE

You may recall the famous words of Carl Sagan: “We’re made of star stuff.”  Turns out we’re not made of just any old star stuff.  No, a great deal of our stuff came from one star in particular, a giant star named Coatlicue that went supernova about 4.5 billion years ago.

I first saw this name in a recent article from Scientific American called “The New Biography of the Sun,” which in turn referenced a paper from the journal Astronomy & Astrophysics titled “Solar System Genealogy Revealed by Extinct Short-Lived Radionuclides in Meteorites.”

In short, certain radioactive isotopes found in our Solar System can be thought of as our Solar System’s D.N.A.  The authors of that “Solar System Geneaology” paper used some of those isotopes (most notably aluminum-26) to try to reconstruct our Sun’s family tree and give us some idea about what the Sun’s “mother” must have been like.

Coatlicue would have been a giant star, approximately 30 times as massive as our Sun, ensconced within a giant molecular cloud along with other giant star siblings.  This is sort of like what we see today with the stars of the Trapezium inside the Orion Nebula.

About 4.5 billion years ago, Coatlicue went supernova.  The explosion accomplished two things: it seeded the surrounding molecular clouds with heavy elements (like aluminum-26) and, because of the force of the explosion, caused those molecular clouds to compress, triggering new star formation.

I have to confess that I feel like there’s a lot of guesswork and speculation going on here about how, specifically, Coatlicue died and how, specifically, the Sun and its planets were born.  But the general idea that the death of one star triggers the formation of others is consistent with what we already know about star formation, so it makes sense to me that something like this must have happened for our own Solar System.

As for the name Coatlicue (which I believe is pronounced Kwat-LEE-kway), that comes from Aztec mythology.  Coatlicue was the mother of the Sun.  So that makes sense.  In the myth, Coatlicue was also the mother of the stars, which actually sort of matches up with the science too.  That supernova explosion 4.5 billion years ago would have triggered the formation of other stars—perhaps several hundred of them—in addition to our own Sun.

I didn’t see this in either Scientific American or that “Solar System Genealogy” paper, but I’d like to believe Coatlicue might not have been totally destroyed in that supernova.  Perhaps some remnant is still out there, living on as a neutron star or a black hole or something.  If so, I doubt we’ll ever find it, but if I know anything about mothers, I’m sure our Sun still hears from Coatlicue every now and then.