APOD: Ireson Hill on Mars (2017 Jul 19)

[geoffrey.landis]

It's not vertically stretched, it's horizontally compressed.

ROFL.

[Chris Peterson]

(BTW, the correct term for material which survives ablation and is in the process of cold fall is "meteorite". Even before it hits the ground.)

• Meteoroid => Meteor => Meteorite

That's not quite accurate. A meteoroid becomes a meteorite (assuming some of it survives ablation). The ablation process produces a physical phenomenon of energy release we call a meteor.

The Wikipedia article is also not too accurate anymore. Might have to edit it.

In actual usage, for many years now, surviving meteoritic material has been called a "meteorite" (including "micrometeorite") even before hitting the ground. This is especially true considering that the old IAU definitions didn't say "ground", but "Earth". Pretty useless since we've started finding meteorites on other planets!

In any case, as noted, the actual usage is what it is, and doesn't strictly follow the old IAU definitions. There are now new definitions (I was part of the committee that created them). They are still provisional (although in wide use), but will almost certainly be approved unchanged by Commission 22 at an IAU meeting in the near future.

The relevant new definitions are:

Meteor is the light and associated phenomenon (heat, shock, ionization), which results from the entry of a solid object from space into a gaseous atmosphere.

Meteoroid is a solid object of a diameter between 30 µm and 1 meter moving in, or coming from, interplanetary space.

Meteorite is any solid object that survived the meteor phase in a gaseous atmosphere without being completely vaporized.

[JohnD]

Meteoric iron would be nice. But landing at the usual speed for a meteor, with almost no atmosphere to slow it down would crater the bedrock and spread the meteor across the Marscape as vapour, condensing to dust. Is there a mechanism for a slow speed landing?

Yeah. The atmosphere. The rovers have found many meteorites sitting on the surface. On Mars, meteoroids which survive burning up in the upper atmosphere fall with a terminal velocity on the order of five to ten times that on Earth, still below the speed of sound and too slow to cause cratering. (Recall that Martian landers typically use parachutes for part of their landing sequence.)

Chris,
But something the size of Ireson Hill?
I went away to calculate, and meanwhile you and neufer talk up a storm! But I think I still have a point to make.

Ireson is 5 x 15m, or about a 10m wide stone. If it is meteoric iron then it will have a mass of about 4,000 tonnes.
Most meteorites seen on Mars, or found on Earth, are much smaller. Why?
The American Meteor Society estimates that meteors encounter the Earth at 11-72 Kilometers/sec- assume that true for Mars, but that's almost irrelevant, due to atmopsherioc drag.
NASA calculates terminal velocity due to atmospheric drag by Vt = SQR[2W/Cd Rho A]
Where W = Mass = 4x10^6 kg
Cd = Coeff drag For a sphere, about 0.6
Rho = atmospheric density At surface, ~0.020 kg/m3. Obviously the higher the lower, but this figure will maximise drag.
A = Fontal area 10m sphere = 78m^2
So Vt = SQR(2 x 4x10^6/0.6 x 0.02 x 78)
= 3000msec = 3kilometers/sec

This is at the lower limit for a "hypervelocity" impact, that would vapourise the meteor and raise a significant crater.
But Ireson Hill is intact and there is no crater.
Therefore Ireson is not a meteorite.
JOhn

[neufer]

But Ireson Hill is intact and there is no crater.
Therefore Ireson is not a meteorite.

IMO, Ireson and the rest of the more weathered dark dunes are all formed from iron rich meteorite material left over from the formation of Gale Crater.

[Chris Peterson]

Meteoric iron would be nice. But landing at the usual speed for a meteor, with almost no atmosphere to slow it down would crater the bedrock and spread the meteor across the Marscape as vapour, condensing to dust. Is there a mechanism for a slow speed landing?

Yeah. The atmosphere. The rovers have found many meteorites sitting on the surface. On Mars, meteoroids which survive burning up in the upper atmosphere fall with a terminal velocity on the order of five to ten times that on Earth, still below the speed of sound and too slow to cause cratering. (Recall that Martian landers typically use parachutes for part of their landing sequence.)

Chris,
But something the size of Ireson Hill?

No, very unlikely. Sorry if I wasn't clear. I didn't mean to suggest I think this hill is an ancient meteorite, only that the atmosphere of Mars behaves much the same as that of Earth when it comes to meteorites surviving to the surface.

The largest meteorites on the Earth are on the order of two to three meters across. The lower limit on initial speed for a Martian meteoroid is about 5 km/s (compared with about 11 on Earth), so I imagine we might have somewhat larger bodies reaching the ground intact. But not, I think, anything the size of Ireson.