{"id":36242,"date":"2022-09-15T14:27:00","date_gmt":"2022-09-15T14:27:00","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=36242"},"modified":"2022-09-21T10:25:58","modified_gmt":"2022-09-21T10:25:58","slug":"mars-season-has-arrived","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2022\/09\/15\/mars-season-has-arrived\/","title":{"rendered":"Mars season has arrived"},"content":{"rendered":"\n

<\/p>\n\n

With Mars at opposition this December \u2013 and a rare lunar occultation expected too \u2013 now\u2019s the time to start observing and tracking its changing features, writes Paul G Abel<\/strong><\/p>\n\n

<\/p>\n\n

On 8 December 2022, Mars reaches opposition. It\u2019s going to be a splendid telescopic target, well-placed for observation in the Northern Hemisphere and well worth observing in the dark winter months. And as an added bonus, on the same night as opposition the Moon will appear to pass in front of Mars in an event known as a lunar occultation. December is set to be the month of Mars. But before then, Mars will be growing steadily larger and brighter, meaning there\u2019s plenty to see in the run-up. Here we\u2019ll explore the exciting phenomena we can observe before and after opposition. <\/p>\n\n

Opposition occurs when an outer planet is opposite the Sun in the sky. Sun, Earth and planet all lie in a straight line, with Earth in the middle, and at this time the planet is due south at midnight and visible all night. We don\u2019t get a Mars opposition every year, nor are they all favourable for UK observers. This is due to the shape of the Martian orbit. All planetary Mars orbits are slightly elliptical, but the Martian one is more so than Jupiter\u2019s and Saturn\u2019s. As a<\/span> result, Mars comes to opposition once every two years.<\/span><\/p>\n\n

The position of Mars in its orbit dictates its altitude in our skies at opposition. In a perihelic opposition, Mars is at (or near) perihelion (its closest point to the Sun). The planet is large and bright, but low down for Northern Hemisphere observers. In aphelic oppositions, Mars is at its furthest point from the Sun and has a small apparent diameter, but is high in UK skies. December\u2019s opposition is a transitional one: moving from perihelic to aphelic. In my opinion, these are the best: Mars reaches a good size and is fairly well placed.<\/span><\/p>\n\n

\"\"
At perihelion, Mars is at the closest point to us in its elliptical orbit. Aphelion is the opposite <\/figcaption><\/figure><\/div>\n\n

<\/p>\n\n

Get your bearings <\/strong><\/h5>\n\n

The type of opposition affects not only altitude and apparent diameter, but also the tilt of Mars as we see it from Earth. Mars has an axial tilt of about 25\u00b0 and the tilt, along with the orbit, means that during perihelic oppositions the southern hemisphere of Mars is tilted towards us. The tilt can be quite pronounced, meaning we have splendid views of the southern polar cap and surrounding regions. During aphelic ones, the northern hemisphere is presented and it can be almost impossible to see the far south. Transitional oppositions tend to favour the equatorial regions and so they\u2019re a chance to see both the north and south caps at the same time.<\/span><\/p>\n\n

\"\"
Red alert: there\u2019s a chance to simultaneously see both polar ice caps of a big, bright-shining Mars around opposition <\/figcaption><\/figure>\n\n

It will be helpful to learn some key Martian features, as this will enable you to identify the more obscure areas of Mars. The dark Syrtis Major is a good starting point as it has a very distinctive shape. Mars rotates once in 24 hours 37 minutes, so it takes about three weeks to view the entire Martian surface from one specific location on Earth. The USA, for example, will see a different part of Mars compared to the UK due to the time difference, and this is why international scientific cooperation is essential during a Mars opposition. <\/p>\n\n

\"\"
An areocentric longitude (Ls) map of Mars\u2019s most well-known features (30\u00b0 = 1 month); drawn with a mirror-inverting telescope (south up) <\/figcaption><\/figure>\n\n
Seasonal changes <\/strong><\/h5>\n\n

<\/p>\n\n

\"\"
A good place to start is Syrtis Major (arrowed), one of Mars\u2019s most recognisable features <\/figcaption><\/figure><\/div>\n\n

Mars is a dynamic world. Just like Earth it undergoes well-defined seasons. Although the Martian atmosphere is thinner than Earth\u2019s, it is sufficient to produce brilliant white clouds, dust storms and winds travelling half the speed of sound \u2013 and all of this is visible in our telescopes. Winds blow Martian dust about and, over many years, the new accumulations of dust can change certain surface features. <\/p>\n\n

Mars\u2019s seasons bring many different phenomena, but how do we define a season on Mars? Astronomers long ago solved this problem and invented a quantity called \u2018areocentric longitude\u2019 or \u2018Ls\u2019. We split the Martian orbit into 12 months (see the map above). Each of these months is 30\u00b0 in Ls. Spring equinox in Mars\u2019s northern hemisphere occurs at Ls = 0\u00b0 (this is also autumn in the south) and \u2018month 1\u2019 runs from 0\u00b0 to 30\u00b0. Northern summer occurs at Ls = 90\u00b0, while the autumn equinox is at Ls = 180\u00b0. Finally, the northern winter solstice occurs at Ls = 270\u00b0. Mars becomes a viable target for medium and large telescopes when it has an apparent diameter of 6 arcseconds or more. This occurs between 11 May 2022 and 11 April 2023, during which we will see winter and spring in Mars’s northern hemisphere, and summer and autumn in the south (Ls 225\u00b0 to 50\u00b0). You can check what the Ls will be on a date with the free software WinJupos and planetarium software like Sky Safari. In early September, Ls is about 297\u00b0. It is late autumn in the northern hemisphere and spring in the south. Mars has more of its northern hemisphere tilted towards us, and medium-sized telescopes should pick up the north polar cap, which is slowly melting. In contrast, the southern polar cap will be very small, having melted over the summer. September is the time to start looking for white clouds: they collect in large basins like Hellas, Argyre and Eridania. The large volcanoes in the Tharsis plateau also attract them, and here they can form distinctive \u2018W\u2019-shaped cloud patterns.<\/span><\/p>\n\n

<\/p>\n\n

\"\"
White clouds can form around some features, such as the tops of volcanoes or within valleys and ravines<\/figcaption><\/figure><\/div>\n\n
Bright clouds and dust storms <\/strong><\/h5>\n\n

White clouds can become quite brilliant and it is fascinating to watch them, particularly if they are close to the morning limb, as they often melt away during the course of a Martian sunrise. If you use optical filters, try a blue or violet filter as these will make the clouds stand out. <\/p>\n\n

By early October, Ls is about 315\u00b0 and there\u2019s a chance a dust storm might erupt. These begin life as small orange clouds but can grow to obscure whole regions or a whole hemisphere. When large dust storms occur, they throw vast quantities of dust into the atmosphere. When the dust settles, often features appear altered, which is why Mars maps from the 1950s look different to today\u2019s. Dust storms can start anywhere, but usually in the south. Watch out for small orange clouds in the basins of Hellas, Noachis and Argyre. Syrtis Major and Solis Lacus are also good places to search. <\/p>\n\n

By November, Ls is about 330\u00b0 and we are entering month 12. It\u2019s late autumn in the south and getting colder: watch out for frosts and fog appearing in Hellas and Argyre. There may be bright clouds over Edom and the volcanoes, while the northern polar cap should now be easier to see.<\/span><\/p>\n\n

Opposition\u2026 and occultation too! <\/strong><\/h5>\n\n

On 8 December opposition occurs and, remarkably, we can look forward to a rare lunar occultation of Mars on the same date. See it with the naked eye, binoculars or a telescope. At about 04:00 UT, look towards the west and the Moon will be close to Mars in the constellation of Taurus. Mars is predicted to be \u2018touching\u2019 the western limb of the Moon at about 04:57 UT, when viewed from the centre of the UK. The exact timing will depend on your location (from London it occurs around 05:00 UT; from Edinburgh at 04:52 UT), so it pays to start observing slightly earlier. The Moon will then pass over Mars, with the planet predicted to reappear around 05:57 UT (from the centre of the UK). Through a telescope you\u2019ll be able to watch the disc pass down behind the lunar limb, almost as if Mars is setting on the Moon! <\/p>\n\n

\"\"
As a remarkable bonus treat, Mars will also be occulted by a full Moon on opposition day <\/figcaption><\/figure>\n\n

Later on 8 December, turn your telescope to Mars at 19:00 UT and you will see the Solis Lacus (the eye of Mars) looking straight back at you. The north polar cap should be visible at the bottom of the disc (in a mirror-inverting telescope) and Olympus Mons will be located near the bottom right limb. If there are any bright clouds present, this huge volcano will appear quite bright even in medium-sized telescopes. <\/p>\n\n

<\/p>\n\n

\"\"
A lunar occultation of Mars, taken from Brazil on 6 September 2020. Weather permitting, the occultation on 8 December should look just as spectacular <\/figcaption><\/figure><\/div>\n\n
Hunting flashes <\/strong><\/h5>\n\n

There is a fascinating phenomenon known as \u2018Mars flashes\u2019, brilliant \u2018starlike\u2019 flashes that can last for a number of seconds. They occur in two regions: Edom and Tithonius Lacus (see map, above), and although occasional flashes have been reported elsewhere, these are the main two sites to keep an eye on. The flashes are thought to be caused by reflections from ice crystals in clouds over these regions. The geometry has to be just right. If you were standing at these sites, Earth and the Sun would have to be overhead near the zenith. <\/p>\n\n

This special alignment between the Sun, Earth and Mars is very rare, and doesn\u2019t necessarily occur at each opposition. Check our table of predicted dates and times (below) and see if you can detect these light signals from Mars. <\/p>\n\n

The spring equinox in the north of Mars (Ls = 0\u00b0) starts on Christmas Day; the onset of the warmer weather (for Mars) will see the sublimation of the north polar cap \u2013 ice changing directly from a solid state into vapour, returning volatiles to the Martian atmosphere. Brilliant fogs and hazes will collect along the boundary of the northern pole, making the whole area very brilliant. Keep an eye on Hellas, Argyre and Eridania, as bright, white clouds are likely to collect around here. Mars will still be a viable target months after opposition. Owners of small- to medium-sized telescopes can follow it until the end of February 2023, with the planet remaining above 10 arcseconds. Larger telescopes will continue to show details on Mars well into April.<\/span><\/p>\n\n

During January and February, Mars\u2019s tilt as seen from Earth means we will get a good look at the equatorial regions. See if you can observe the north and south polar caps at the same time. During these months it is autumn in the south and the great southern cap will be starting to form, continuing to grow until winter. <\/p>\n\n

There will be lots to see on Mars over the coming months, making it a spectacular sight before and after opposition. Whether it\u2019s a large global dust storm or Martian light signals, you can chart your own telescopic adventures on the Red Planet. <\/p>\n\n

<\/div>
\n

Light on Mars? <\/strong><\/h2>\n\n\n\n

See if you can observe the famed flashes in Edom Promontorium <\/p>\n\n\n\n

\"\"
Edom Promontorium<\/span><\/figcaption><\/figure><\/div>\n\n\n\n

Catching the Edom flashes will require a medium telescope (6-inch reflector or 4-inch refractor) and a clear night, but not much more! Although observing the flashes may prove a little tricky, the challenge is well worth the effort and certainly achievable through an 8-inch reflector. If you do spot a flash, keep a record of what you see and you will be able to contribute to this very worthwhile scientific endeavor, helping astronomers learn more about the reflective nature of the Martian surface. We have made predictions as to when and where Edom flashes will occur based upon when the Sun\u2013Earth\u2013Mars geometry is aligned. Try observing on the dates and times suggested below. Note: the angle of the reflective plane will have a significant effect on when flashes will occur, so we recommend monitoring Edom for 90 minutes either side of the times stated.<\/span><\/p>\n\n\n\n

For more on observing the Edom flashes, watch this month\u2019s Virtual Planetarium on our website www.skyatnightmagazine.com\/bonus-content\/IRIVOWL<\/a><\/strong><\/p>\n\n\n\n

\"\"<\/figure>\n<\/div><\/section>\n\n

<\/p>\n\n

<\/div>
\n

Sketch what you see<\/span><\/h2>\n\n\n\n

It\u2019s simple and a great way to fine-tune your Martian geography know-how <\/span><\/p>\n\n\n\n

\"\"
One of the author\u2019s sketches of a dust storm over Argyre Planitia, along with detailed notes of the observation <\/span> <\/figcaption><\/figure>\n\n\n\n

Start sketching the Red Planet long before opposition and it won\u2019t be long before you have drawings of the entire surface. Follow these five simple steps: <\/p>\n\n\n\n

1. <\/strong>First, you\u2019ll need to make a blank to draw on. Download yours in this month\u2019s Bonus Content<\/strong><\/a> or make your own by drawing a white circle 50mm in diameter. <\/p>\n\n\n\n

2. <\/strong>Spend a good 15 minutes observing Mars at your telescope before starting. What details are visible? Can you see a polar cap? Are there any prominent dark markings? What else can you see? Experiment with magnification so that the features are not too blurred. <\/p>\n\n\n\n

3. <\/strong>You\u2019re ready to draw. Start by putting in the terminator. Near opposition, Mars is 100 per cent illuminated, but either side of opposition, the planet has a phase. The terminator is not necessarily at right angles to the central meridian, so draw it carefully. <\/p>\n\n\n\n

4. <\/strong>Now add the most prominent dark features, then the polar cap if you can see it. Add subtle features like clouds or delicate streaks. The Martian limb may be quite bright if clouds are present. Complete your drawing within 12 minutes: any longer and Mars\u2019s rotation will have significantly moved the features. <\/p>\n\n\n\n

5. <\/strong>Finally, record the date, time (in UT) and telescope size and details. You can add other data like the value of the central meridian: this tells you which feature is on the north\u2013south line at the time of your drawing.<\/span><\/p>\n\n\n\n

\"\"
You can find a blank sketching\/ observing form to start recording your Mars observations in our Bonus Content section<\/a> <\/strong>online <\/span><\/figcaption><\/figure>\n\n\n\n

Now try this <\/span><\/strong><\/h4>\n\n\n\n

Once you\u2019re comfortable sketching the Martian surface, you can go on to experiment with using different colour filters. Blue enhances white clouds, orange\/red brings out any orange clouds. If your drawing was made using a filter, record that too. The more you draw Mars, the more you will see! <\/p>\n<\/div><\/section>\n\n

<\/p>\n\n

\n\n
\"\"<\/figure><\/div>\n\n

Dr Paul Abel is an astrophysicist and the Director of the British Astronomical Association\u2019s Mercury and Venus section<\/p>\n\n

<\/p>\n\n

<\/p>\n\n

Photos: PAUL ABEL, SEBASTIAN VOLTMER\/CCDGUIDE.COM, FRANZ KLAUSER\/CCDGUIDE.COM, PETE LAWRENCE <\/p>\n","protected":false},"excerpt":{"rendered":"

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