RE: BI pedestal, etc....
Over the billion year (+-) life of this landscape there have been occasional hard shakes due to tectonic or impact earthquakes. This would certainly topple any pedestal and otherwise alter the resting of BI. This factor needs to be considered. As an aside, this type of short-term-rare but long-term-likely seismic shake would affect surface cracking in general...
(another side question: why haven't we seen any more "Anatolia" types of gaping cracks snaking across the landscape?)

Sometimes strange things can happen. I recall an article about boulders creeping across a dry lake bed in the California desert. Turns out that with a little rain, the super-concentrated brine acted as a grease, and the desert winds could blow the rocks across the lake bed.

Block Island Ramblings...
Well, Block Island seems to be significantly metallic, with possible stony inclusions (I'm basing this on appearance). The many vesicles seem to be gas bubbles from the ancient solidification, although some of them could be from inclusions that have since evaporated/dissolved/eroded away. The raised rims around some of the vesicles indicates something inside the vesicles reacted with the structure of the meteorite and made that surface slightly more resistant to erosion.
The vesicles could have been refilled for a time after the meteorite fell, with ice, Meridiani sulfur-salt goo, dust, or whatever.
The elaborate filigree structure around the big pit indicates a hard material with tensile strength (metallic), and it looks like it is being slowly eaten away chemically. The filigree structure could also represent a fine-grained soft component in the metallic matrix that is being preferentially eroded.
Looking forward to microscopic images.
The presence of 'blueberries' in the hollows could be remnants of previous burial, and/or wind-driven deposits from an episode where Mars had more atmosphere.
There we go, enough opinionated ramblings to power Oppy for a day, at least!

-- MarkG

The surface of BI seems to show some signs of atmospheric friction melting (among the pits/vesicles), which puts some constraints on its trajectory. The fact that it survived impact means that it was most like a grazing trajectory (or low-angle secondary). Or so it seems. When we get some composition hints, we will know a bit about how strong this rock is.

The possibility exists of a history of ice coverage at Meridiani, where it is possible that any impact crater in that ice would have since sublimed away, leaving the meteorite intact.

Sorry to anthropomorphise...

There is a Wikipedia, but the short answer is that Vulcanoids are a purported group of small bodies orbiting inside Mercury's orbit. Messenger can look for them when at perigee -- looking at an angle away from the sun, but still look at space inside Mercury's orbit.

So, on this last leg to the Martian west, across mostly pavement, what were the Right Front wheel's currents like?

It will also be interesting if any stereo-pair frames are shceduled.


Any further word on the search for "Vulcanoids"?



--Mark G.

In trying to understand Meridiani and the Endeavor rim, I keep the following things in mind...

1) The profound ancientness of the Martian surface.
2) The significant variability of the Martian atmosphere. Perhaps a third of it freezes and sublimates yearly, plus secular orbital and axis changes indicate periods of different conditions over time, plus injections and depletions by volcanoes and large impacts, plus highly variable dust content further obscures the issue. The presence all over Mars of ventiform terrain features whose formation is beyond the transport capability of the current atmosphere underscores this.
3) Any surface water would likely be either ice or ice-covered most (if not all) of the time, even in ancient Mars. Ice-dominated or periglacial shorelines is what one is likely to see. For Meridiani, the interplay between surface ice and very concentrated brine underneath is important to consider.

So, with the above in mind, I follow the rovers and the orbiters, and try to see how various ideas fit with the unfolding observations.

One of the things I keep thinking about it the "planed-carved-like" slopes in Victoria crater. The surface around the rim was either flat, a planed-down slope of 15-35 degrees, a cliff, or debris beneath a cliff. If the surface was covered by ice sheets that were seasonally mobilized by brine at their base, these ice sheets (or their large cracked pieces) would creep down slopes and effectively plane them.

Did this ice come from seepage from below or from precipitation from the atmosphere? Was Meridiani always equatorial? Fun questions.

Seeing the rim deposits of Endeavor may shed some light on this. Or pose more questions.

If meteors (or their fragments) get slowed down to near-subsonic after hitting the atmosphere, then they just fall like rocks. They hit the ground and maybe bounce and maybe crack or chip the surface, but they don't create an explosive crater. Oppy has already passed by and looked at some of these meteorites laying about on the Meridiani ripples (like the one near the heat shield).

(Well, remembering Oppy passing by that heat shield and meteorite seems like a long time ago... and it was!)


--MarkG

I can try to offer a qualitative answer (I'd need lots of rust-remover to give a quantitative answer...).
The size of a crater is a function of the energy of the impinging object (diam ~ Energy**3 ?). E ~ M*V**2.
So a small object smashing in really fast makes the same size crater as a larger object coming in slower.
There is a dichotomy of approach velocities, with primary objects coming in at or above the Martian escape velocity (5km/sec), and secondary objects being much slower (~2-3 km/sec and under).
Atmospheric drag goes in proportion to the surface area of the impinging object, so smaller objects are more easily slowed down and/or evaporated that large objects. The effect of this is to produce a threshold of size below which crater-producing impacts become unlikely (for given velocity/density/strength/trajectory-angle).
With a large field of meter-ish sized craters, we would expect the group of crater-producing objects to be all within a factor of about 100 in mass (assuming nearly-equal velocities).

Now, I am not sure of the actual numbers, but the Martian atmosphere would essentially filter out smaller impactors. The question to ask here is whether primary impactors that would produce a one-or-two-meter-sized crater are below this threshold, and therefore much less likely?

A primary impactor group would need to split into a number of similar-sized fragments, and the effect of atmospheric drag would tend to scatter their V-squared energy for impact crater size even more.

A large impact producing showers of secondary debris, however, could easily produce a number of similar fragments, since the parent material and the launching shock forces in a given area of the source rock are similar. The ballistic launch of these fragments then has a decent likeleyhood of producing a group of similar-sizes fragments well above the threshold for significant atmospheric drag, but that produce fairly small craters.

(Note that a grazing-impact object might produce a group like this if it breaks up and falls just ballistically...., but that is also a chancey scenario.)

Thus, the secondary impact scenario at this scale seems more probable. Probabilities are the best we can do without samples to analyze. Field trip, anyone?

'Nuff said?

-- MarkG

This area seems to be a line/zone of small secondary craters, as has been mentioned before. Is there any discussion of what the primary crater might be? If the primary crater is identified, a rough date determination would be possible.

Is anyone looking into this?

-- MarkG

Yes, the Messenger site is now back up, but it was down for a bit (the New Horizons JHUAPL site was also down at the same time).

Vulcanoids -- yes, the Vokrouhlicky article implies that there could still be a remnant population, but I think the erosive effect of high-energy collisions would bring the number down.

What is the limiting magnitude and field of view of the wide-angle camera? They are expecting to be able to find a 15KM object, but given what assumptions?

Messenger JHUAPL site seems to be down.... ?

I'm not so jazzed about the West route. If you look at the pictures of when Spirit was overlooking that area, a lot of that area has dust/sand dunes, and the rover might dig in there, even if going downhill. An impression-by-looks seems like it might have soft spots.

I know the formal search hasn't started yet, but have any KBO's been discovered yet that are in the post-Pluto possible trajectory cone?

It seems not unreasonable to speculate that the very deep soft soil that foiled the latest attempts to climb atop HP was a loose-dust-filled former wind scour under an overhanging edge of the relatively hard and strong HP layered deposits. (Presumably, the HP mud volcano filled a bowl-shaped depression, either a meteor or explosion crater.) In most cases around HP, the accumulation of regolith from the eroding edge of HP provides some sort of of pavement and packing, but not in the spot Spirit was trying to climb.

Just to the E of Spirit's position is an area of horizontal-lying rock slabs that could be a deposit of "mud lava" outflowing from HP. The satellite pics also seem to show a tongue sticking out N from HP. This could represent an alternate access path onto HP. However, the slabs are quite fragmented, with large chunks of this rock, plus vesiculated boulders, so picking a way through this would be challenging.

Good luck to the Rover Drivers, I do hope to get a better look at Von Braun... (pitcher's mound).

The wire-brushed shiny surface of Santorini sure looks amorphous. Maybe some of the long-sought shock melt?

I wonder what the spectrometer results will show... (I hope they got good readings...)

-- MarkG

A 100km radius core of 3 g/cc density (i.e., a small rocky core) would only amount to 2% of Tethys' mass, and not much of a change to its overall density, but could influence the course of its dynamics.

The presence of the two trojan companion moons Telesteo and Calypso are also a smoking gun for a severe disruption of Tethys in the past.

I'll have to get me an Icarus Subscription....


Another possible source of the huge areas of "lineated surface perturbations" might be forces from tidal friction and distortion induced by the changes in the rotational and orbital motion of Tethys by the big collision. The collision almost certainly was off-center enough to impart rotational energy, and if might have been enough to induce rotation in Tethys, which might have taken a while to damp down. At least a whopping libration.

Orbitally, almost any collision would have added eccentricity, with its variable tidal bulge to dissipate. (Non-Saturn-Equatorial-plane perturbations also would yield some dissipation, but this I think is the weakest.)

As to what might have caused the actual form of the huge canyon system, one can speculate about the displacement, distortion, and subsequent relaxation of a small rocky core at the center of Tethys...

-- MarkG

Has anyone expounded on the possibility of extreme seismic disruption causing the lineated-but-chaotic features seen on Tethys? A major impact would release P, S, and surface waves of extreme magnitude, and might provide enough energy at wave reinforcement points to fracture or partially melt the material in the upper layers.