May 25, 2016: Mars Part Deux! and Atmospheric Dispersion

Mars continues in its opposition, and I continue to go out to observe it whenever possible to see if I can see surface details.  A few nights ago, it was clear again (probably a record number of clear nights in one week!), and so out I went.

It’s a little hard to overstate how tough it is to see the surface detail.  On Jupiter, the equatorial belts are dark and easy to see and jump out at you; the fading and shrinking GRS, even without the Baader Moon & SkyGlow filter, was visible in its absence – the notch it takes out of the belt it lies next to.  And on Saturn, you can often see a difference in cloud belt colors that is pretty readily apparent.

For whatever reason, probably mostly because it’s way down low in the atmosphere at this opposition (and will be even lower at the next one in 2018), Mars is loathe to give up its surface treasures.  You’ve really got to observe Mars – stare and stare at it, and then stare some more after that.  Because of this, your mind can start to invent detail that isn’t there.  This is what happened to the great Mars observer Percival Lowell around the turn of the 20th century.  He stared so long that he started to see “canals” – straight line features – on the surface of Mars.

In doing so, he was building on the work a couple of decades earlier of Giovanni Schiaparelli, who had noted in the accounts of his observations, published in Italian, natch, that he saw pretty much the same thing:  “canali” on the surface.  Due to a mistranslation, this word was rendered as “canals” in English, when the actual meaning was simply and innocuously “channels” (or watercourse).  Obviously, canals implies an artificial structure, and that leads to some intelligence to build those canals.

Lowell ran this idea.  Boy, did he run with it.  He envisioned an ancient Martian civilization, facing severe and prolonged drought.  Seeking much-needed water, they built an enormous network of canals to bring it down to them in the equatorial regions from the polar ice caps.  Alas, the great Martian civilization could stave off the inevitable for only so long, and died off, leaving the canals behind.

Obviously, this turned out to be sheer fantasy, disproven as astronomical photographic techniques improved over the 20th century.  The final nail in the coffin was provided by the Mariner probes of the late sixties and early seventies, which found no canals or channels of any sort.  Well, with the major exception of Valles Marineris, a stupendous canyon that stretches over 2500 miles long, 120 miles across, and up to 4 1/2 miles deep, it is a huge gash across Mars, the largest in the solar system.  The canyon was duly named after the probe that discovered it, Mariner 9.  But even a feature that large is not visible telescopically from earth.

Why would two presumably careful observers see channels/canals in the first place?  There are two explanations, and both depend on these being optical illusions, one created by the eye, the other by the brain.  The eye illusion creates itself when using very high magnifications.  The higher the magnification on a telescope, the smaller the exit pupil is.  The exit pupil is the shaft of light that exits through the eyepiece and enters your eye.  When the exit pupil gets significantly below 1mm in diameter, a curious phenomenon occurs – you start to see the floaters in your eyes, and even the network of blood vessels on your own retina.

Your retina on the left; your retina on drugs on the right.  (Actually that’s Lowell’s canal map.)

The second illusion occurs in the brain itself.  We humans are pattern-recognition machines.  We constantly seek out patterns, order, structure in everything we see, especially when it comes to faces.  This is why we’ll imagine seeing the image of Mary in grilled cheese, or Jesus in a dog’s butt.  We will even imagine patterns when the patterns themselves do not really exist.  Particularly, when we look at a faint group of dark smudges, the brain will intervene and connect them with straight lines.  Which is exactly what Schiaparelli and Lowell’s brains did while viewing Mars.

Lowell saw the same straight-line features again when he went on to observe and study Venus.  We now know that the Venusian atmosphere obscures all surface detail, so that this was impossible.  You would have thought that seeing the exact same features on two very different planets would have tipped him off that something wasn’t quite right.  “Hey guys, is it just me, or . . . ”  “Yeah, dude, it’s totally just you.”

In due time, Lowell actually turned out to be right.  Sort of.  Oh, not in his fantasies about ancient Martian civilizations building canals to stave off their impending waterless doom.  No, but he was completely correct that there actually is plenty of evidence on the surface of Mars that water once covered the face of Mars, far in the distant past.  Lakes, flowing rivers carving out river beds, seas of water washing up on shores.  It’s just that the evidence for this is impossible to see from earth and had to await the sending of space probes to the planet.

Getting back to what I saw, when I observed Mars previously a week or so ago, the wind was whipping by, gusting up to 20mph, and the little orange globe was bouncing all over the place, making observation difficult.  When I observed again a few days later, I was able to observe Mars a little later in the evening, towards midnight, and a little higher in the sky, getting to between 22 and 25 degrees.  More importantly, that obnoxious wind had disappeared, making observing infinitely easier.  And better.

This time, not only was I able to see Syrtis Major (and Minor), with Mare Serpentis (Sea of Serpents) trailing off to the side, I was also able to see the dark albedo feature Mare Acidalium (Acidalian Sea), on the other side of the globe.  Something like this:

mars christopher sullivan
Mars – now with twice as much detail than just three days earlier!  (photo credit – Christopher Sullivan)

Well, not exactly of course.  The photo captures much more detail than I was able to see, as that Mare Acidalium dark area on the bottom was only barely visible.  But the photo does just about does capture about what I was able to see visually.  I still haven’t seen the polar cap, though.

Tonight was our building’s annual roof party, and I did a little impromptu outreach with some of my neighbors, showing them Jupiter, Saturn, and Mars.  They liked Jupiter, Mars was its usual orange self, but jaws were seriously dropping at the sight of Saturn.  As usual.  You want to impress someone who’s never looked through a telescope before?  Show them Saturn.  Saturn never fails to please.

The seeing was definitely a bit above average a bit later in the night towards midnight.  By that time, Mars was higher in the sky at 26-27 degrees, approaching its 28 degree peak this year.  This is about as good as I think I’ll be able to see it at this opposition.  The seeing was good enough to let me pump up the magnification to my limit, 246x, by Barlowing my ortho.  The more I observe, the easier the features are for me to tease out.  Not much easier, but in those few moments of atmospheric steadiness every few minutes, the details do appear.  On Mars tonight, I was able to see Mare Cimmerium (the Cimmerian Sea; Mars features have the best names!) pretty clearly.  But still no polar cap!



Something else I’ve been seeing at Mars is bizarre and was really causing me to scratch my head.  That is, until I duly asked the wise old sages over at Cloudy Nights. As I mentioned in my last post, I’ve been seeing Mars lit up in red, white, and blue. And it wasn’t even Memorial Day! It was as if I was seeing chromatic aberration in my scope. But that isn’t possible, as I was observing with a Mak, where chromatic aberration is basically non-existent. I thought I was going nuts, but at the monthly outreach event last Friday, one of the viewing public saw the same thing I was seeing. What the heck was going on here?

He had studied optics, so we started hypothesizing. Maybe the meniscus was introducing CA the way an achromatic refractor’s objective would? Could it be some sort of optical illusion based on the eye’s relative insensitivity to light from the red end of the spectrum, and the white and blue was somehow the brain’s invented compensatory effect? Or could it be atmospheric effects from Mars being so low in the atmosphere – that the atmosphere itself was acting as an achromatic objective?

It turns out it was the latter, a seeing phenomenon known as atmospheric dispersion. This is where the thickness of the atmosphere closer to the horizon causes the light to react the same way as it would through an achromatic refractor. The atmosphere bends the light differentially, causing the colors to arrive at the scope at different focal points, leading to the multi-colored display. The top of Mars, the part furthest away from the horizon, has the bluish tint, while the bottom has a reddish tint, which is a bit distinct from the actual orange-red color of the planet itself.

It seems that this visual aberration affects all planets low in the atmosphere, including Saturn, which is pretty close to and lower down in the sky than Mars.  However, the reason that I haven’t ever noticed it on Saturn before is the same reason that you don’t always notice CA on all objects through an achromat:  At 0.24, Saturn is a full two magnitudes dimmer than Mars, which is at a very bright -1.84 during this oppositio.  Since the magnitude scale is logarithmic, that means that Mars is about six-and-a-half times brighter than Saturn.  That brightness difference means that Saturn’s just not quite bright enough to trigger the effect, or at least not noticeably so.

What would a visual aberration be without some way to correct for it?  And, of course, since there’s atmospheric dispersion, that means there’s a corresponding atmospheric dispersion corrector.  This is yet another of the many consequences of seeing and how it impacts our view of the universe through our telescopes.  Alan MacRobert has written an excellent article discussing atmospheric dispersion, along with all of the other various aspects and impacts of different seeing conditions for S&T here; it is a must-read to understand what’s going on in the atmosphere.

But this atmospheric dispersion effect is going to continue to make seeing the polar cap difficult.  Even at 26-27 degrees, the effect was still present (although noticeably reduced).  It creates a whitish/bluish spike or haze right at the top end of Mars, right where the polar cap is supposed to be.  So far it’s been tough trying to see the white cap through the blaze of color.  I’ll keep trying over the next few weeks.



5 thoughts on “May 25, 2016: Mars Part Deux! and Atmospheric Dispersion

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