March 30, 2020

Astro event: A Close Planetary Conjunction.

The dusk planetary action continues this week with a close conjunction of the planets Mars and Venus. Our two nearest planetary neighbors in space have been playing a game of apparent cat and mouse in the dusk skies, approaching each other within two degrees of arc July 31st, receding, and then approaching again. Closest approach is around August 19th, when both planets are within 1° 45’ of each other as seen from our Earthly vantage point. This is one of the better planetary conjunctions of the year, and a good study in comparative planetary characteristics and orbital mechanics.

Both will fit well into a binocular field of view or a low power telescope eyepiece; Venus will show a thinning crescent of about 24” in size and Mars will be a tiny 4” in diameter. Both will lack appreciable telescopic detail but display a red versus silver color contrast. Watch their apparent motion and note the change from evening to evening. Mars is receding towards superior conjunction on the far side of the Sun on February 4th 2011, while Venus is diving towards inferior conjunction between us and the Sun on October 28th, 2010. And don’t forget, the most distant Full Moon on the year occurs on August 24th, as the Full Sturgeon Moon approaches apogee on August 25th, 13 hours after full.

This astro-word for this week is Barchan dunes. Geology, you ask? Isn’t this an astronomy themed blog? Barchan dunes do have an astronomical-themed tie-in, trust us… these arc-shaped sand dunes have been spotted by the HiRISE orbiter on the planet Mars and are a product of the fierce prevailing winds prominent there. These tend to sort relative to particle size and can be observed creeping downwind over time. The seasonal freeze and thaw cycle on Mars that feeds the formation of barchan dunes may even contribute to the seeding and escalation of planet-wide dust storms that are typically observed. As these regions thaw, carbon dioxide ice sublimes and causes darker areas of the exposed dunes to accelerate the process via solar heating. These sublimations may even occur in dramatic fashion as sandy gas jets exploding through the icy surface. Barchan dune monitoring on the Red Planet may prove crucial to the prediction of the onset of planet-wide dust storms for future manned expeditions. Those featureless planetary disks are becoming more like places in our scant lifetimes!

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