How to tell time on Mars

This week, PhD Candidate Alex Innanen takes on a topic that has challenged planetary scientists and science fiction writers for decades: how do you tell time on a planet that has similarities in its revolutions to the Earth but some pesky differences? Often you'll hear of researchers working on Mars time because the length of its day is so similar to the Earth's. However, the match isn't perfect and can lead to unpleasant physiological effects for some and impractical, though hilarious, fixes. There is no 'Venus Time' or 'Jupiter Time' because their days are so different from ours that it makes no sense to try. But what to do with the extra 39 minutes in the martian day? Or the extra months needed in the martian year? 
(Image above from LMD's "Martian Seasons and Solar Longitude")

by Alex Innanen

One of my favourite things to read about are different calendars and methods of timekeeping. Here on Earth, there are all sorts of different calendars – the Gregorian, which you’re likely familiar with, the Julian calendar, and various lunar, solar or combination calendars. We also keep time within a single day in different ways – the standard now is a 24-hour clock, but at various times people have tried to introduce decimal time, where each day might have ten hours divided into 100 minutes. And this is just on our own planet, where generally the apparent movement of the sun and moon in the sky can give you a sense of when things are occurring.

Unsurprisingly, when you move to different planets things start to get more complicated.

Let’s take Mars for instance. Mars’ day (called a sol) is about 40 minutes longer than an earth day. If you want to use a 24 hour clock you either have to have a sneaky not-quite-an-hour-long 25th hour, or you have to make every hour a bit longer. Which is what the Mars clock that’s used for mission planning does. But wait, does that mean each hour has more than 60 minutes? Well, not really, if you make each minute a bit longer than an earth minute, and you can do that, not by adding extra seconds, but by making each ‘Mars second’ a bit longer than an Earth second. This way you can still use a familiar 24 hour clock.

This doesn’t mean that there’s some intrinsic ‘Martian second’ that is longer than an Earth second. While the second is an SI unit, it’s fairly arbitrary, as is splitting up a day into 24 hours. It’s just what we’re used to a second being.

One thing about round planets is that local noon – when the sun is directly overhead – happens at different times in different places. This is the reason that people on Earth invented time zones, so that local noon could line up – more or less – with noon on the clock. In reality, there’s a fair amount of variation even within a time zone. For example, there is about a 20 minute of difference between local noon in Toronto and Montreal just because of their difference in longitude, even though they use the same time zone.

There are no official time zones on Mars and missions tend to use local mean solar time (LMST), which is based on the average­ length of the sol, split into 24 hours. (There’s also local true solar time [LTST], which is referenced around the local noon, but drifts from 12:00 LMST throughout the year.) There is a generally accepted time standard for Mars, called Airy Mean Time (AMT) or Coordinated Mars Time (MTC) (comparable to Earth’s Greenwich Mean Time (GMT)/Coordinated Universal Time (UTC)) which refer to the mean solar time at Mars’ prime meridian, the crater Airy-1.

The other thing about Mars is its year is almost twice as long as an Earth year, about 668 sols or 687 Earth days. There is some familiarity, though, because Mars, like earth, has seasons! But Mars’ orbit is more eccentric than earth’s – that is to say, the path it travels around the sun is slightly more oblong. Earth also travels in an ellipse, and we do see the effects of not orbiting in a perfect circle on Earth seasons – the northern summer is about 94 days long, while the northern winter is only around 89 days – but not to so great an extent as on Mars. There the northern spring is 194 sols while the northern autumn is only 142 sols, a difference of 52 sols.

So what if we want to know what time of year it is? We can actually use the position of Mars (or any planet) in its orbit to tell this – specifically a parameter called solar longitude, shortened as Ls. We can start the year at a Ls of 0 degrees, the northern vernal equinox (the start of northern spring). Each subsequent season then starts at intervals of 90 degrees for a full 360 degree orbit. This convention is used in most scientific contexts because it can fairly easily tell you if events are occurring at around the same time year after year. It also avoids having to deal with things like leap years or leap seconds, because Mars’ orbit will always be 360 degrees – that’s just how geometry works.

All of this is what’s used currently to orient ourselves in time on Mars, but there have also been a number of attempts to create calendars and timekeeping systems that could be used by people living on Mars itself by everyone from scientists to fiction writers. The calendars divide the Martian year into months – which vary in their length, and in how many months there are in a year – and weeks – again, varying in length and quantity. Some calendars use familiar names for months and days of the week, and some make up new names, or new versions of the earth names. Getting into them all would probably take a whole series of blogposts, but it is a very fun rabbit hole to disappear down.

Mars is made of Swiss cheese

  

If the Moon is made of Green Cheese, then what cultured dairy confection makes up Mars? Why Swiss Cheese, of course! This week, Alex takes us on a tour of the pitted south polar terrain of Mars whose interplay of sunlight, water and carbon dioxide ices result in something that looks visibly similar to Swiss Cheese. Naming planetary terrains after food is not new, nor is it limited to the inner solar system. If you were putting together a platter of hors d'oeuvres, Cantaloupe makes an excellent accompaniment to Swiss Cheese. Perhaps we will have to take a closer look at Neptune's moon Triton in the future...

By Alex Innanen

Long-time PVL blog enthusiasts may recall that my planetary journey began at the Martian north pole looking at many, many HiRISE images. Over the past year I’ve returned to the Martian poles – the south pole this time.

Both poles have layered deposits of mostly water ice and dust, and residual water ice caps left behind when the winter layer of CO
2 ice sublimates in the summer. The south polar residual cap (or SPRC for the acronym fans) is mostly made up of carbon dioxide ice as well, overlying water-ice. The terrain of the SPRC is as varied as the North pole, but has some features that are unique to it. One of these are circular or circular-ish pits with steep sides and flat bottoms. The terrain they carve out is similar to a piece of Swiss cheese, giving the features their nickname. 

The distinctive pits of Swiss cheese terrain, from the HiRISE instrument.
[NASA/JPL/University of Arizona]

In Swiss cheese – the kind you can eat – the distinctive holes are formed by carbon dioxide bubbles that are released by the cheese-making bacteria. The Swiss cheese features of the SPRC are much larger than the ‘eyes’ in a piece of cheese – on the order of tens to a few hundreds of metres in diameter. No bacteria are forming these holes, instead they’re likely formed from fractures in the residual cap, which are widened into pits through sublimation from their walls. In the southern spring and summer, the steep, dark sides of the pits get more sunlight than the flat floors, causing the walls to sublimate and grow outwards by a few metres per year.

If the pits grow large enough, they can even grow into each other, creating intricate, branching features that can cover large swaths of the residual cap, like you can see in the HiRISE image here. It’s been suggested that based on this rate of growth, every century or so the entire SPRC could be entirely carved out by Swiss cheese features, causing a total resurfacing. 

[NASA/JPL/University of Arizona]

The Swiss cheese features occasionally show more ephemeral features such as bright, surrounding halos or dark fans emanating from higher standing areas. There’s a fairly clear halo around the feature shown at the top of this post – sometimes nicknamed the ‘Happy Face’. It looks almost like the feature is glowing, but what we’re really seeing is a localized region of higher albedo (i.e. more white) surrounding the Swiss cheese feature. These halos have only been observed during the Southern summer of Mars year 28 (2007, for Earthlings), and their appearance happened to follow a global dust storm. It’s likely, though, that these halos aren’t actually a ring of material getting lighter, but rather the SPRC as a whole getting darker from settling dust, except in the areas close to the pit walls. The mechanism that was proposed to explain this in a 2014 paper, is that the sublimation from the pit walls that I discussed above raises the amount of CO2 in the atmosphere and pushes the settling dust from the storm away from the edges of the pits. Lower rates of sublimation on flat areas allow the dust to settle normally.

The dark fans are much smaller and harder to pick out of even HiRISE images – on the scale of 1-10 m². They tend to appear at the edges of high-standing areas, ‘fanning’ into the lower areas. They appear in the southern spring, and unlike the halos they have been seen over multiple Mars years. Moving into the summer, as CO
2 ice sublimates, the terrain around the fans darkens until the fans disappear. Their formation is also much more exciting – they’re formed when jets of gas rupture through the CO
2 ice layer, lifting dust and depositing it outward in the fan shape. Dust can then get trapped in layers of ice, making it darker, absorbing more sunlight, and leading to more sublimation, creating more trapped gas to explode out and create more fans.

Until now I’ve been talking about CO
2 ice which makes up the majority of the SPRC. But what about water ice? The polar layered deposits are composed mostly of water ice and dust, and in the Southern summer the SPRC shrinks and exposes some of the water ice of the south polar layered deposits. It is possible that the flat floors of Swiss cheese pits also expose water ice in the summer. There have been detections of water vapour associated with the pits, but this could also be from their walls, which could be layers of CO
2 and water ice. In either event, the work I’ve been doing looks as if it is possible for the water ice in the Swiss cheese pits to have any appreciable contribution to atmospheric water vapour. The polar caps are the major source of surface water ice, and the yearly formation and retreat of overlying CO
2 ice, exposing water ice, drives Mars’ water cycle. I’m interested in finding out how much, if any, water vapour could be released from the Swiss cheese pits, and in the event of most or all of the SPRC being removed by Swiss cheese pits, whether this could have a significant impact on the amount of atmospheric water vapour.

Rainbows and Their Cousins

Here at PVL we do love atmospheric optical phenomena! I can report that, more than once, we were headed somewhere as a group and got delayed by particularly impressive parahelia, arcs or other interesting effects. While they often go unnoticed, such effects are more common than you might imagine. Robert Greenler, author of the lab-favourite "Rainbows, Halos and Glories" estimates that you can see at least one of the effects Alex describes below on about one day in three. So the next time you are outside: look up, fair reader, and take a glance a few tens of degrees from the sun! You might be surprised and impressed with what you find.

By Alex Innanen

It’s pride month, which means everything is decked out in rainbows from street corners to store shelves, and also I’ve been thinking about rainbows all month. So for your education and enjoyment some pictures of rainbows and other pretty atmospheric optics, and some of the science behind them.

 
A quick overview about the visible light spectrum: We all know that white light is made up of the colours of the visible spectrum which spans from red to violet. When Newton performed his experiments with prisms he identified seven colours – the classic ROYGBIV – but in truth the spectrum is continuous, and in fact those whose languages have fewer colour terms will actually perceive the divisions between colours differently (this is really really cool and unfortunately outside the scope of this post, and also I’m not a psychologist but there’s a nifty paper on it). IF you’ve ever listened to Pink Floyd or possibly performed an elementary school science experiment, you know that passing white light through a prism splits it up into the rainbow. This is because light refracts, or bends, when it goes from one medium to another such as air to glass. When light encounters the glass of a prism it moves slower, and the angle that it refracts is based on both the index of refraction of the material and the specific wavelength. Red has the greatest wavelength and is refracted at a smaller angle than blue. If the differences in angles of refraction are large enough, the different colours will ‘spread out’, or disperse.

How far can a golfer hit a drive on Mars… and the Moon?

Click Here to view an animated version of the figure above!

This week, undergrad Noah Stanton takes on a burning question in comparative planetology: how would a change of venue to another planet or Moon affect one's golf game? Read on for his deep-dive! Reminds me a little of some of the tangents I followed in my earlier years!

by Noah Stanton

Have you ever been watching golfers playing on Pebble Beach and thought, ‘What would happen if he or she took that shot on Mars or the Moon?’. I’d assume no, but I am here to tell you this important information. In order to figure out a way to model a golfer’s shot let’s start somewhere we know a little bit better, Earth. 

Modelling a golf shot involves bio-mechanics, aerodynamics, elasticity of the golf ball, etc., which is maybe outside the abilities of a mere blogpost. I will need to make some assumptions, focus on some parts of the swing, and ignore the rest. The two modeled parts of the swing will be:

1)    The initial contact and acceleration due to the club-head hitting the ball
2)    The flight of the ball after the initial contact