Mars is this month’s big deal. Its motion through the stars stopped on Oct. 30 and became retrograde motion as Earth began to overtake it prior to opposition. As a result, Mars moved westward from above Betelgeuse in Orion to closer to Aldebaran in Taurus. That continues into early January until Mars stops north-west of Aldebaran on Jan. 12 and begins normal prograde motion again.
Mars begins December at magnitude -1.8 and its disc at its maximum of 17.2” (arc-seconds). Its closest approach occurs at 18:00 on Nov. 30, although its exact opposition (to the sun) occurs on Dec. 7 at 22:00. (All times are Pacific Standard.) The two are a week apart because Mars’s orbit is notably elliptical; we’re currently overtaking Mars in our smaller orbit while it’s starting the outward leg of its egg-shaped orbit. By the time we catch it, it’s farther away than the week before. Mars’s orbit is also slightly inclined to our own so it moves north (its current position) and south of the plane of Earth’s orbit. The moon does the same thing above and below Earth’s equator but more so. The lunar eclipse of November occurred because the moon traversed the plane of Earth’s orbit exactly when it was opposite the sun; result – it went through our shadow. I think so anyway; I was up three times that night hoping to see the eclipse and Uranus. The next morning the clouds went away; it was clear and chilly through the next night and we missed it by a day.
All of this three-dimensional orbital mechanics stuff is very useful, of course, but the payoff for us, mere plebian observers, happens about 18:40 on Dec. 7 about 27 degrees above the eastern horizon. Mars, at opposition, disappears behind the full Moon at about its 9 o’clock position at the same time. Mars then reappears about 19:38 at the Moon’s 1 o’clock position. The attached Stellarium screenshot shows that emergence from behind the moon.
Much of our current understanding of the solar system has been a result of efforts during my lifetime. Mankind’s reach beyond Earth began with Sputnik in 1957, we’ve put people on the moon, retrieved asteroid samples and sent spacecraft to every planet and then some. Certainly, the Earth-moon system is better understood – the moon was likely formed from orbital debris following an early collision with something the size of Mars. Mars also shows very clear evidence of a substantially different past; there are six VERY big volcanoes on Mars, including the solar system’s largest: Olympus Mons. NASA describes it as “a shield volcano 624 km (374 mi) in diameter (think of Arizona), 25 km (16 mi) high, and is rimmed by a 6 km (4 mi) high scarp.” By comparison, Mauna Loa in Hawaii, measured from the ocean floor, “is a shield volcano 10 km (6.3 mi) high and 120 km (75 mi) across. The volume of Olympus Mons is about 100 times larger than that of Mauna Loa.” East of the Tharsis Bulge containing the volcanoes is Valles Marineris, a 4,000 km (2,500 mi) long valley complex, 200 km (120 mi) wide and up to 7 km (23,000 ft) deep. These are not features typical of forever dead worlds.
While Mars presently has a miniscule magnetic field and less than one per cent of Earth’s atmospheric pressure, it likely wasn’t always so. Many of the surficial features of Mars like Valles Marineris could be easily explained by depositional and erosional processes common on Earth and caused by water but not currently possible on Mars at its low temperatures and pressures. It would seem though that about 4 billion years ago, the core dynamo that powered Mars’s magnetic field began to fail. Mars is smaller than Earth, its smaller core was never as hot as Earth’s and it has cooled more quickly to immobility than Earth’s will because of the cube/square law. Before that happened, however, Mars may well have been able to hold an atmosphere; the key is to have a magnetosphere that shields the planet from the ever-present solar wind, a steady stream of charged particles from the sun. It flows outwards at millions of miles per hour and is entirely capable of blasting gas molecules out of the gravitational well of a planet. One NASA estimate is that Mars loses 2 kg of atmosphere to the solar wind every second. Give it a few billion years and your atmosphere’s history. When that goes, so does any liquid water as well as your insulation from the cold of space. Every time I see Mars shining bright red high in the sky during its close approaches, I ache a little to think of what it may have looked like once and what future it might have seen had things been just a little different.
If there is any chance of clear skies in early December we might be able to arrange a public viewing venue, either Astro-Café style or maybe at the SCAC observatory. Wish for clear skies, folks.
The Dec. 9 club meeting open to the public will be at the Sechelt Library at 7 p.m. The lecture topic will be posted at the Sunshine Coast Club website at https://sunshinecoastastronomy.wordpress.com/. We hope to see you there.
An addendum, Dec. 6
Coincidentally, the Astronomy Picture Of The Day site for Dec. 3, 2022, shows two pictures of Mars taken two days apart in late November from Singapore. The slight angular difference of Mars between photos gives a 3D effect that, as the article states, can be seen by crossing your eyes while looking at the two images. No red/green glasses required. Use this direct link: https://apod.nasa.gov/apod/ap221203.html
Lastly, while tides will be large during and following the full moon, two weeks or so later – around Dec. 23 to 25 – the new moon will occur at perigee (close approach to Earth) and the tides will be even higher. May we all have a happy and merry and not overly-salty Christmas.
