Mars's Mysterious Elongated Crater

[Turtle]

I know, I know, I'm being ornery :D

 

reasonably ornery i allow. :D

 

 

i get nothing of the sort from that quote.

You mean that it's "skipping". Yeah, I guess it doesn't say that in so many words. Here's another that says 'bouncing':

And they're not always circular, says astronomer Fred Watson.

"Occasionally you see craters that are elongated and sometimes you see canyons that have been excavated by incoming meteorites.

"You even see lines of multi-ringed craters as if something has bounced over the surface, says Watson

Are impact craters always circular?

 

:ebomb: i deduce that dear watson is fallen prey to the same errors as the rest and allowed his familiar experiences with things knocking together to misguide his speculation. :ebomb: "as if they bounced", said he; as if.

 

It doesn't seem too unreasonable to me, but it's not something I've studied.

 

Yeah. I think the minimum possible velocity with which a meteor can hit a planet is the planet's escape velocity, about 11.2 km/s for earth and 5 km/s for mars. So, yeah, it'd be 15% for earth and 45% for mars. I'm not sure how they 'scale up' their impact experiments, but I would guess that velocity would be scaled with distance so it may be appropriate that the velocity of the projectile is slower than a typical meteor. I'm not sure.

 

we have several threads on impacts here @ hypography over the last several years which have many useful links & information. these comprise my study of the subject & i have fought this battle before. :D knowledge is power. we seem not to have the search function up yet but i have a fair recall & will try to be brief in summarizing here. :ebomb:

 

so the idea of skipping doesn't seem unreasonable for the reason i ascribed to watson. however, this type of energy release is magnitudes beyond peoples' experiences and it is computer modeling that is providing the answers to how these impacts scale up from our common experiences & expectations. my touchstone guy on high energy impacts is dr. mark boslough at sandia national labs. he has the fastest gun, as well as the supercomputer. :)

 

the effect of impacts making large craters at these high velocities is to pulverize, melt, vaporize, & plasmatize the coincident materials at the point of contact. no skip of an intact impactor after impact, low angle or not. bomb! :omg:

 

[quote name='mosdest'That had occurred to me as well. The distance between the two impact points is roughly 25 kilometers. At a typical speed the meteor would span that distance in less than 2 seconds. Something, it would seem, other than gravity would be needed to change the missile's trajectory.

]

the formation you show is easily explained by multiple objects traveling together or perhaps one of those potato shaped jobbers like asteroid luetia.

Perhaps. If they do resemble the ones created in a lab with a single impactor though, then that might be the more natural explanation.

 

~modest

 

perhaps. but that experiment was in sand wasn't it? pas de touché!!

[Turtle]

mmmm... we lost several posts overnight. :D que sera sera. :blink: for now i'll just give a couple links we found; the first a paper by dr. mark boslough on impact anti-podal focussing and the second on oblique impact anti-podal focussing and the extent of impact melting at various angles of entry. the last diagram for a 15 º entry shows an elongate profile of shock effects that matches quite well the crater on mars from the opening post. no hop, no skip, no jump. :mellow:

 

>> Axial Focusing of Impact Energy in the Earth’s Interior: a Possible Link to Flood Basalts and Hotspots

 

>> shock dynamics: antipodal effects

 

[modest]

The aether took our posts :D

 

Let's see... I had... Very relevant I think, yes:

 

Here is an article advocating the skipping, or as it says "ricocheting" idea:

 

In an instantly classic study, Don Gault of NASA's Ames Research Center and John Wedekind of Caltech reported in 1978 that the shape of an impact crater and its ejecta pattern depend on the angle at which a projectile strikes a target. Using the Ames Vertical Gun Ballistic Range, they shot millimeter-size spheres and cylinders into targets of granite, quartz sand, and volcanic pumice powder at velocities up to 7 kilometers per second. Such experiments had already been crucial to understanding the physics of normal impact cratering, but Gault and Wedekind went one step further and asked, what happens when a projectile makes an oblique impact.

 

Surprisingly, not much changes until the impact angle is less than 45° (measured from horizontal). But at shallower angles the crater becomes increasingly elongated in the direction of motion, and portions of the projectile ricochet and gouge out a series of small pits downrange from the main crater. As the impact angle changes, the ejecta and rays undergo even more pronounced changes than the craters do. When the impact angle is less than 15°, the ejecta pattern becomes elongated in the downrange direction and a "forbidden zone," where no ejecta appears, develops in the uprange direction. For grazing impacts of just a few degrees, the rays go sideways only, producing a butterfly-wing pattern. Amazingly, examples of all of these exotic ejecta patterns can be found on the Moon, Mars, and Venus.

 

Oblique impact resolves the mystery of one of the most bizarre crater pairs on the Moon. South of Mare Crisium in Mare Fecunditatis are two small craters with unique parallel rays streaming from one of them.

 

Messier is very elongated (15 by 8 km), and the irregularly shaped Messier A (16 by 11 km) has long twin tails of rays that point away from Messier. Previous explanations for this crater pair have ranged from imaginative to fantastical. All were wrong.

 

<...>

 

The really remarkable fact about Messier and Messier A is that the scientists Gault and Wedekind were able to beautifully mimic every one of the pair's weird features in laboratory impact experiments. A grazing impact (1° to 5°) of a projectile coming from the east excavated Messier (explaining its elongated shape and classic butterfly-wing ejecta pattern) and another part of the projectile ricocheted downrange to form Messier A and its long rays. Bigger craters formed obliquely too...

 

Messier on the Moon

 

and, the academic article it's talking 'bout is: Experimental studies of oblique impact - NASA Technical Reports Server

 

which I hadn't read completely, but noticed the following: [...trying to recall ridiculous pun...]

 

I haven't read the article completely, but in glancing the following stuck out [ :blink: ] :

 

Except for obvious elongate craters and asymmetrical ray systems and distributions of inferred melt and ejecta deposits (e.g., Moore, 1968, 1976; Howard and Wilshire, 1975; Hawke and Head, 1977), many effects and results of oblique impact trajectories remain unrecognized or poorly defined and understood. It is commonly assumed either tacitly or for reasons of simplicity, convenience, and/or lack of information that impact craters have been formed under conditions corresponding to vertical incidence. Probably the major contributing factor in this neglect of obliquity effects (other than a paucity of experimental impact data) has been the general acceptance of explosive cratering and its associated large data base as being analogous and applicable to impact events (e.g., Shoemaker, 1962; Baldwin, 1963; Roddy, 1977). Although there is a firm basis for the analogy in many respects, the comparison neglects an important difference between explosions and impact events; i.e., explosions represent an energy release from a point source whereas impacts effectively release energy along a line source of the path of projectile penetration.

 

I personally wouldn't want to rule out the possibility of a 'skipping' or 'ricochet' type thing. It seems, at the very least, a valid interpretation of experimental results.

 

The antipodal focusing as an explanation of concentric craters, I thought was a very clever hypothesis, T :mellow:

 

~modest

[Turtle]

The aether took our posts :o

 

Let's see... I had... Very relevant I think, yes:

 

Here is an article advocating the skipping, or as it says "ricocheting" idea:

 

Messier on the Moon

 

and, the academic article it's talking 'bout is: Experimental studies of oblique impact - NASA Technical Reports Server

 

which I hadn't read completely, but noticed the following: [...trying to recall ridiculous pun...]

 

I haven't read the article completely, but in glancing the following stuck out [ :( ] :

 

I personally wouldn't want to rule out the possibility of a 'skipping' or 'ricochet' type thing. It seems, at the very least, a valid interpretation of experimental results.

 

The antipodal focusing as an explanation of concentric craters, I thought was a very clever hypothesis, T :D

 

~modest

 

 

ok; i'm a little gun-shy about making a detailed reply, so i'll try for brief reply with impact. :doh:

 

the experiments with guns come no-where near the minimum velocity a meteoroid impacting a body must have, so i don't find them valid models. the fastest gun on earth is now at sandia labs - a rail gun - that can shoot metal disks up to 20 kilometers/second. problem is projectiles liquify from the acceleration and that isn't a solid-to-solid impact so not useful. (down-n'dirty reference coming in!)

...Accelerated to 13 km/sec, the plates are neither distorted, melted, nor vaporized, as they would be if shot from a gun. When the plate is accelerated to a speed about 20 times faster than a bullet, or 20 km/sec, the more forceful acceleration needed to reach higher velocity causes temperatures of 2,500 K to occur in the flyer plate; this liquefies aluminum flyer plates.

source

 

so if sudden acceleration liquifies metal, then imagine what sudden deceleration does. i will concede the gentleman's contestation of the "bomb" analogy in your second quoted passage, but it is no ricochet either. it is destruction of the impactor through shock, melting, vaporization, & plasmatation(? :D).

 

well, i better not stick my neck out in the aether any further so i withdraw for the time being. catch you at the antipode. ;) . . . :blink: :mellow:

[Turtle]

The aether took our posts :o

 

Let's see... I had... Very relevant I think, yes:

 

 

Messier on the Moon

 

...

The antipodal focusing as an explanation of concentric craters, I thought was a very clever hypothesis, T :blink:

 

~modest

 

so i mentioned before that an asteroid, potato shaped, rotating fast, or otherwise weak, might break up shortly before impact and make twin round craters or if close enough together even an elliptical crater. seemed natural enough then to see if asteroids come in pairs to start with & not only do they, there is new evidence for how they get that way. :D pretty simple to imagine what might happen when one such pair collides with a larger body, don't ya know. ;) :( :doh:

 

Asteroids are often thought of simply as big rocks orbiting the Sun, but they can have quite exciting lives. Small irregularly-shaped asteroids can be "spun up" to fast rotation rates by sunlight falling on them -- much as the asymmetric profile of a propeller blade helps it to spin up in the wind. New results show that when asteroids spin fast enough, they can split into two pieces which then begin orbiting each other. Scientists call this process "rotational fission".

 

A new study released this week, led by Petr Pravec of the Astronomical Institute in the Czech Republic and involving many other institutions around the world, shows that many of these binary asteroids do not remain bound to each other but escape, forming two asteroids in very similar, but independent, orbits about the Sun where previously there was just one. Many such asteroid pairs have been discovered in recent years and the new work shows that their properties match perfectly with what is expected from the rotational fission model. It was predicted that in such asteroid pairs the larger one would always be at least five times more massive than the smaller object, and this is exactly what is found. More equally-sized binary asteroids remain bound to each other. ...

How Asteroids Split Up -- Mystery of asteroid pairs solved

 

i wrote to one of the antipode-shock authors to ask about echoes; will let you know if anything comes of it. :o

 

i'm outy! . . . . . .:mellow:______________ :D

[modest]

the experiments with guns come no-where near the minimum velocity a meteoroid impacting a body must have, so i don't find them valid models. the fastest gun on earth is now at sandia labs - a rail gun - that can shoot metal disks up to 20 kilometers/second. problem is projectiles liquify from the acceleration and that isn't a solid-to-solid impact so not useful. (down-n'dirty reference coming in!)

source

 

That seems like a valid concern to me :mellow:

 

~modest