APOD: Star Trails for a Red Planet (2019 Nov 30)

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APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by APOD Robot » Sat Nov 30, 2019 5:05 am

Image Star Trails for a Red Planet

Explanation: Does Mars have a north star? In long exposures of Earth's night sky, star trails make concentric arcs around the north celestial pole, the direction of our fair planet's axis of rotation. Bright star Polaris is presently the Earth's North Star, close on the sky to Earth's north celestial pole. But long exposures on Mars show star trails too, concentric arcs about a celestial pole determined by Mars' axis of rotation. Tilted like planet Earth's, the martian axis of rotation points in a different direction in space though. It points to a place on the sky between stars in Cygnus and Cepheus with no bright star comparable to Earth's north star Polaris nearby. So even though this ruddy, weathered landscape is remarkably reminiscent of terrain in images from the martian surface, the view must be from planet Earth, with north star Polaris near the center of concentric star trails. The landforms in the foreground are found in Qinghai Province in northwestern China.

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RocketRon

Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by RocketRon » Sat Nov 30, 2019 5:51 am

Quite a visually striking pic.

How long an exposure is that ?
(and why are such details never included !).
Guessing that that looks like only a small fraction of a full circle (24 hours),
so maybe 10 to 15 minutes ??

Has anyone/anything yet done such an exposure on Mars of the starscape ?
Mars would seem to offer quite a range of interesting astroviewing possibilities,
the visible stereo potential and of radio astronomy observations must be quite inviting.
providing such equipment can be safely uploaded and deployed ....

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Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by JohnD » Sat Nov 30, 2019 10:22 am

Yes, it does, if you bother to read the first link in the blurb above.
J.

heehaw

Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by heehaw » Sat Nov 30, 2019 12:16 pm

Ah, but does Mars have a SOUTHERN pole star? We readers are left in ignorance....

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Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by Ann » Sat Nov 30, 2019 1:33 pm

I don't think that the landscape in today's APOD looks like a potential Martian vista. Surely Mars doesn't have such big rocky outcrops with such very long parallel deep-cut groves and such strikingly sedimentary layers?

Yes, I know Mars has sedimentary rocks and mountains, but surely not like this...?

I second heehaw's question as to whether Mars has a southern pole star.

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Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by Chris Peterson » Sat Nov 30, 2019 2:23 pm

heehaw wrote:
Sat Nov 30, 2019 12:16 pm
Ah, but does Mars have a SOUTHERN pole star? We readers are left in ignorance....
Marginal. Kappa Vel is about 3° from the southern pole, at mag 2.5 (somewhat dimmer than Polaris).
Chris

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Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by neufer » Sat Nov 30, 2019 4:41 pm

Chris Peterson wrote:
Sat Nov 30, 2019 2:23 pm
heehaw wrote:
Sat Nov 30, 2019 12:16 pm

Ah, but does Mars have a SOUTHERN pole star? We readers are left in ignorance....
Marginal. Kappa Vel is about 3° from the southern pole, at mag 2.5 (somewhat dimmer than Polaris).
https://en.wikipedia.org/wiki/Kappa_Velorum wrote: <<Kappa Velorum (κ Velorum, abbreviated Kappa Vel, κ Vel; formally named Markeb from the Arabic مركب, markab meaning "something to ride") is a binary star system in the southern constellation of Vela. The two components are designated Kappa Velorum A and B. The system is only a couple of degrees from the south celestial pole of Mars, so it could therefore be considered the southern polar star of that planet.

From parallax measurements, this system is located at a distance of roughly 175 parsecs from the Sun. The apparent visual magnitude is 2.48, making it readily visible to the naked eye in the southern hemisphere. Kappa Velorum forms part of an asterism known as the 'False Cross' along with Delta Velorum, Iota Carinae and Epsilon Carinae, so called because it is sometimes mistaken for the Southern Cross, causing errors in astronavigation.

Kappa Velorum is a spectroscopic binary system consisting of a pair of stars that complete an orbit around each other with a period of 116.65 days and an eccentricity of 0.19. Because the individual stars have not been resolved, further details of the orbit have not yet been determined. The combined stellar classification of the pair is B2 IV, which matches the class of a B-type subgiant star that has exhausted the hydrogen at its core and begun to evolve into a giant. It has an estimated size of 6.9 times the Sun's radius.

Analysis of the spectrum of Kappa Velorum shows absorption lines due to the interstellar medium between Earth and the star. Observation of these features over many years has shown that the lines vary in strength, probably caused by a small dense cloud extending 10²–10³ au moving across the line of sight.>>
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Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by orin stepanek » Sat Nov 30, 2019 4:44 pm

Ahh; it's a fake; but at least they tell us it is! It's a picture from China instead! I was excited thinking somehow they had a picture from the polar star for Mars! :mrgreen: :shock: Oh well; it is a nice photo anyway! :clap:
StarTrailsOnRedPlanet1024.jpg
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Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by neufer » Sat Nov 30, 2019 5:22 pm

orin stepanek wrote:
Sat Nov 30, 2019 4:44 pm

Ahh; it's a fake; but at least they tell us it is! It's a picture from China instead!
  • It's really all about yardangs.
https://en.wikipedia.org/wiki/Yardang wrote:
<<A yardang is a streamlined protuberance carved from bedrock or any consolidated or semiconsolidated material by the dual action of wind abrasion by dust and sand, and deflation which is the removal of loose material by wind turbulence. Yardangs become elongated features typically three or more times longer than wide, and when viewed from above, resemble the hull of a boat. Facing the wind is a steep, blunt face that gradually gets lower and narrower toward the lee end. Some geologists have suggested that the Great Sphinx of Egypt is an augmented yardang. The word itself is of Turkic origin, meaning ‘steep bank’, and was first introduced to the English-speaking world by the Swedish explorer Sven Hedin in 1903. Hedin first found the wind-sculptured "clay terraces" or yardangs in the dried up riverbed of the Kurruk-daria in Central Asia. In China, they are sometimes known as yadan from the Chinese transcription of the Uyghur form of the same name. Other names for them are "mud-lions", "mushroom rocks", "sphinx-like hills", "koukour" in Tunisia, and "kalut" (Persian for "ridge") in Iran.

Pictures from Mars show that the yardang ridges occur on a massive scale there; some individual ridges are tens of kilometers long with intervening valleys nearly 1 km wide. Yardangs on Mars are typically found in the Amazonis region but the best ones are found in the equatorial region. Yardangs on Mars demonstrate that much of the eolian erosion is recent since they are sculpted in young geologic units.

Yardangs are formed by wind erosion, typically of an originally flat surface formed from areas of harder and softer material. The soft material is eroded and removed by the wind, and the harder material remains. The resulting pattern of yardangs is therefore a combination of the original rock distribution, and the fluid mechanics of the air flow and resulting pattern of erosion. Yardangs come in a large range of sizes, and are divided into three different categories: mega-yardangs, meso-yardangs, and micro-yardangs. Mega-yardangs can be several kilometers long and hundreds of meters high and are found in arid regions with strong winds; meso-yardangs are generally a few meters high and 10 to 15 meters long and are commonly found carved in semiconsolidated playa sediments and other soft granular materials; and micro-yardangs are only a few centimeters high.
A large concentration of mega-yardangs are found near the Tibesti Mountains in the central Sahara. There is a famous yardang at Hole in the Rock in Papago Park in Phoenix, Arizona, a rock formation with a roughly circular hole in it. Another yardang in Arizona is Window Rock, near the town of Window Rock. It is a 60-meter sandstone hill with a very large circular hole in the middle of it.
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Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by orin stepanek » Sat Nov 30, 2019 5:37 pm

Yardangs: that's a new word for me! :shock: Sometimes I say dang my yard! That's cuz it grows too fast; it's full of weeds; and it is hard to keep green! :mrgreen:
Orin

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Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by neufer » Sat Nov 30, 2019 5:45 pm

https://en.wikipedia.org/wiki/Axial_tilt wrote:
<<In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. At an obliquity of 0 degrees, the two axes point in the same direction; i.e., the rotational axis is perpendicular to the orbital plane. Earth's obliquity oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle; Earth's mean obliquity is currently 23.4367° and decreasing.

Mars's obliquity [currently 25.19°] is quite variable over millions of years and may be in a chaotic state; it varies as much as 0° to 60° over some millions of years, depending on perturbations of the planets. Some authors dispute that Mars's obliquity is chaotic, and show that tidal dissipation and viscous core-mantle coupling are adequate for it to have reached a fully damped state, similar to Mercury and Venus. The occasional shifts in the axial tilt of Mars have been suggested as an explanation for the appearance and disappearance of rivers and lakes over the course of the existence of Mars. A shift could cause a burst of methane into the atmosphere, causing warming, but then the methane would be destroyed and the climate would become arid again.>>
https://www.etymonline.com/search?q=oblique wrote:
<<oblique (adj.) early 15c., "slanting, sloping, sideways; crooked, not straight or direct," originally of muscles or eyes, from Old French oblique (14c.) and directly from Latin obliquus "slanting, sidelong, indirect," which is perhaps from ob "against" (see ob-) + root of licinus "bent upward." Closest in form and meaning are līmus 'transverse' and sublīmis 'transverse from below upward', and the latter would be morphologically similar to oblīquus. Figurative sense of "indirect" is from early 15c. As a noun in anatomy in reference to a type of muscle the direction of whose fibers is oblique to the long axis of the body or to the long axis of the part acted, by 1838.>>
Art Neuendorffer

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Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by Ann » Sat Nov 30, 2019 6:01 pm

neufer wrote:
Sat Nov 30, 2019 4:41 pm
Chris Peterson wrote:
Sat Nov 30, 2019 2:23 pm
heehaw wrote:
Sat Nov 30, 2019 12:16 pm

Ah, but does Mars have a SOUTHERN pole star? We readers are left in ignorance....
Marginal. Kappa Vel is about 3° from the southern pole, at mag 2.5 (somewhat dimmer than Polaris).
The Southern Cross, the Criss Cross and the False Cross.
Source: https://skywiseunlimited.com/2016/07/24/vela/


Ah, Kappa Velorum! I thought it rang a bell! :D Of course it does, because Kappa Velorum (top star of the False Cross) is intrinsically bright, hot and blue! My favorite kind of star :D

Ann
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RocketRon

Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by RocketRon » Sun Dec 01, 2019 4:40 am

JohnD wrote:
Sat Nov 30, 2019 10:22 am
Yes, it does, if you bother to read the first link in the blurb above.
I didn't find a picture of the starscape from Mars ?
You appear to be answering a question I didn't ask....

So I'll ask again.
Has anyone done a starscape view from on Mars yet.
Is this available to view anyplace ?

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Re: APOD: Star Trails for a Red Planet (2019 Nov 30)

Post by neufer » Sun Dec 01, 2019 1:14 pm

RocketRon wrote:
Sun Dec 01, 2019 4:40 am

Has anyone done a starscape view from on Mars yet.
Is this available to view anyplace ?
  • No need to bother unless something really exciting is happening at the time:
https://en.wikipedia.org/wiki/C/2013_A1_(Siding_Spring) wrote: <<C/2013 A1 (Siding Spring) is an Oort cloud comet discovered on 3 January 2013 by Robert H. McNaught at Siding Spring Observatory using the 0.5-meter (20 in) Uppsala Southern Schmidt Telescope. Comet C/2013 A1 probably took millions of years to come from the Oort cloud. After leaving the planetary region of the Solar System, the post-perihelion orbital period (epoch 2050) is estimated to be roughly 1 million years.

C/2013 A1 passed the planet Mars very closely on 19 October 2014, at a distance of 140,497 km (0.000939 AU). After its discovery, there was thought to be a chance of a collision with Mars, but this possibility was excluded when its orbit was determined more accurately. All NASA Mars orbiters—including 2001 Mars Odyssey, Mars Reconnaissance Orbiter and MAVEN—as well as ESA's orbiter, Mars Express, and ISRO's orbiter, the Mars Orbiter Mission, reported a healthy status after the comet flyby on 19 October 2014. During the flyby, orbiters around Mars detected thousands of kilograms per hour of comet dust composed of magnesium, iron, sodium, potassium, manganese, nickel, chromium and zinc. In addition, the comet nucleus was determined to be between 400 and 700 meters, much smaller than originally assumed. The nucleus rotates once every eight hours. On the ground, the Curiosity and Opportunity rovers obtained images as well.

The main body of the comet's tail was projected to miss Mars by some 10 Mars diameters. As a result, only higher-than-average-velocity meteoroid dust, ejected earlier in the approach of the comet, allow for impacts on Mars, its moons, and orbiting spacecraft. Dust particles ejected from the nucleus of the comet, at more than double the expected velocity when the comet was 3 AU from the Sun, could reach Mars approximately 43 to 130 min after the closest approach of the comet. There is a possibility for millimeter- to centimeter-size particles released more than 13 AU from the Sun, however, this is considered unlikely, although massive ejections from farther out have been deduced.

The coma of the comet is projected to more than double the amount of hydrogen in the high atmosphere for a period of several tens of hours and to warm it by about 30 K for a few hours—the combination increasing the effect of atmospheric drag on the Mars Reconnaissance Orbiter and MAVEN spacecraft causing a measurable increase in orbital decay because of atmospheric ram pressure. These spacecraft will be approaching Mars to minimum altitudes of 250 km and 150 km and orbital periods of 3 and 4 hours, respectively. The amount of drag cannot be narrowed down greatly until the production rate of the comet is known, but it could be from 1.6 to 40 times normal drag. MAVEN, in particular, also has instruments to observe any changes to the gas composition of the atmosphere. The closer moon of Mars, Phobos, orbits far higher, at a minimum distance of 9,234 km , more than 10 times the height of Mars's atmosphere.

Estimates for the diameter of the nucleus have varied from 1 to 50 km, but now the nucleus is known to be only approximately 400–700 meters in diameter, roughly the diameter of asteroid 2010 XG11 that approached Mars on 29 July 2014. Based on early upper-limit size estimates, the resulting upper-limit energy of a hypothetical impact with Mars was 24 billion megatons. The diameter of such a hypothetical impact crater would be roughly ten times the diameter of the comet's nucleus. A 700-meter impactor would create around a 7–10 km crater.

The odds of an impact with Mars were 1 in 1250 in March 2013, 1 in 2000 in late March 2013, 1 in 8000 by April 2013, and 1 in 120,000 by 8 April 2013. The 8 April 2013 JPL Small-Body Database 3-sigma solution was the first estimate to show that the minimum approach by Comet Siding Spring would miss Mars.

MAVEN detected an intense meteor shower. Comet Siding Spring has a rotation period of approximately 8 hours. Debris from Comet Siding Spring added a temporary, but strong layer of ions to Mars's ionosphere (the first time such a phenomenon has been observed on any planet), and 85 tonnes of cometary dust were vaporized high in Mars's atmosphere. Magnesium, iron, and other metals were observed to have been deposited. An observer on the surface would have seen a few tens of meteors during the plane crossing.

During the flyby of Mars at a proximity of 140,000 km, Comet Siding Spring's magnetic field, generated by its interaction with the solar wind, caused a violent turmoil that lasted for several hours, long after its flyby. Its coma washed over Mars with the dense inner coma, reaching or almost reaching the planet's surface. The cometary magnetic field temporarily merged with and overwhelmed Mars' weak magnetic field.

As seen from Earth, on 19 October 2014, Mars was in the constellation Ophiuchus, near globular cluster NGC 6401, and 60 degrees from the Sun. Mars and C/2013 A1 were 1.6 AU from Earth. As of October 2014, C/2013 A1 had an apparent magnitude of roughly 11 and was the third-brightest comet in the sky at that time. At an apparent magnitude of 0.9, Mars was estimated to be about 11,000 times brighter than the diffuse-looking comet with a low-surface brightness. To observe C/2013 A1 visually from Earth would have required a telescope with an optical mirror at least 0.2-meter in diameter. By November 2014 the comet had dimmed to magnitude 11.6 and was only around the fifth-brightest comet in the sky.

Mars and Comet Siding Spring were visible to the STEREO-A spacecraft during the 2014 encounter. In orbit around Mars were the spacecraft Mars Reconnaissance Orbiter, 2001 Mars Odyssey, ESA's Mars Express, MAVEN, and the Indian Mars Orbiter Mission (Mangalyaan). The last two missions had arrived less than one month before the closest approach of C/2013 A1 to Mars. All these artificial satellites may have been exposed to potentially damaging particles. The level of exposure will not be known for months, but NASA had taken several "precautionary measures" as it prepared to study C/2013 A1. Two key strategies to lessen the risk were to place the orbiters on the opposite side of Mars at the time of the highest risk and to orient the orbiters so that their most vulnerable parts were not in the line of impact.>>
Art Neuendorffer