Craters on Eros

l Large (>1 km diam.) craters are roughly saturated, like Ida, more

numerous than on Gaspra. (Giant craters like Mathilde? Not

really, but...)

l Smaller craters are increasingly undersaturated (not like on the

Moon). Factor of 30 below empirical saturation at 15 m diam.

l Some regional variations in crater density. Fewer craters in

Shoemaker Regio and on steep slopes.

l Craters display a range of morphologies, indicating degradation.

Remember: Most cratering and geology probably happened billions of years ago, while Eros was in the main asteroid belt...

Boulders on Eros

l Very steep size distribution: few larger ones (possibly

consistent with Ida) but dramatic increase in numbers

at smaller sizes, unlike boulders/rocks seen anywhere

else that has been imaged at this resolution.

l Eros may have a million boulders larger than 8 meters

across (the size of a small house).

l Boulders are more numerous than craters of the same

size at diameters < 20 m.

l Some boulders appear coherent, resting on the surface.

Others are buried, still others are broken into pieces.

Occasionally, boulders are resting in shallow craters

they may have made, or are related to a groove.

Dynamical History of Eros

(Michel, Farinella & Froeschlé 1998, AJ 116 2023)

l Dynamical history of Eros is probabilistic, not deterministic.

l Eros probably formed by catastrophic fragmentation of parent

long before minor resonances evolved it to planet-crossing.

l Eros probably became planet-crossing many tens of Myr ago.

l Roughly 50/50 chance that (a) its orbit aphelion was often enough in main belt that it continued to be cratered at rates similar

to main-belt asteroids or (b) it became "detached" from main

belt (as it is today) and has had a "hiatus" in impacts --

cratering rate perhaps ~1% of main-belt rate. (Caveat:

5 Myr integrations of clones need to be extended.)

l Possibility of a cratering hiatus may explain differences between

Eros and otherwise similar main-belt S-types, like Ida.

If there has been a cratering hiatus...

...some otherwise negligible processes might

become more apparent, like:

l Thermal creep (rather than impacts) could drive mass wasting.

l Reaccumulation of short-lived satellite boulders, without

subsequent impact fragmentation.

l Internal readjustment and settling associated with minor evolution

of spin axis.

l Enhanced "space weathering" effects (bombardment by solar wind

and micrometeorites) compared with impact-driven regolith

evolution or excavation of fresh ejecta.

For example: the largest crater formed in last 50 Myr might be

< 0.5 km, so no fresh ejecta blankets from Azzurra-sized craters would have formed...perhaps explaining spectral homogeneity.

Conclusions

l Surface of Eros does not look like lunar surface at high

resolution, despite similarity at larger scales.

l Small craters are less abundant than numerous boulders

at < 20 meter diameter.

l Extrapolations to smaller sizes (which would predict

saturated boulders, hardly any craters) is pointless since

low fly-by images will be taken in a few days.

l One possible interpretation of why Eros is spectrally almost

homogeneous and why there are many boulders and few craters

is that there may have been very little impact cratering

on Eros for the last many tens of Myr.

l Fine-scale surface structure and character of regolith could

be radically different from anywhere else that we've seen.

Following charts are for DPS Press Conference and Space Frontiers Talk (Manhattan Beach, Saturday morning):

Where did Eros Come From?

...and Where has it been Lately?

(The "Dynamical History of Eros" ... per Michel et al. 1968)

l Eros probably formed many hundreds of millions to billions of

years ago when two asteroids, in the main belt, collided.

l Minor gravitational forces from planets like Mars and Jupiter

gradually jostled it into an "escape hatch" that sent Eros

careening in past the orbit of Mars toward the Earth.

l Most of the large-scale "geology" on Eros (craters, ridges, etc.)

probably was formed during the long period it was in the belt.

l Governed by chaotic dynamics, we can't know exactly how long

Eros has been in Earth-approaching orbits or where it has been.

Roughly 50% chance it has continued to penetrate the belt and been bombarded and cratered like other main-belt asteroids ...but ~50% chance it's been detached, has had impact hiatus.

If there has been a Cratering Hiatus...

What Other Processes, Operating over Tens of

Millions of Years, might have Dominated Eros'

Small-scale Surface Evolution & Appearance?

l If Eros had a temporary, steady-state "cloud" of small satellites,

many might have decayed onto the surface, but the boulders

would not have been smashed by later impacts...perhaps

explaining the abundance of boulders.

l Minor jostling, settling, and thermal creep (due to day/night

cycle) -- usually negligible compared with impacts -- might have

dominated downslope motion...maybe erasing small craters?

l Space weathering (by micrometeorites, solar wind) might have

matured everywhere...with no "freshening" by impacts: might

explain the amazing homogeneity of Eros' colors (compared

with Ida).

What is the Surface of Eros Like?

(Caveat: We'll know a lot more after Thursday's Low Fly-by)

l It could be enormously rocky, with truck- and house-sized

boulders and blocks crowding the surface cheek-by-jowl

(rocks much larger than on Mars and with few spaces

between them). (Gravity is low and highly variable.)

l Deep (hundreds of meters) regolith is seen in places, but

elsewhere there might be "bed-rock" at the surface.

l Prospecting for different types of rocks around Eros might

be fruitless -- it seems to everywhere be made of the

same stuff: ordinary chondritic meteorite material.

l Impact hazard deflection? We won't need to for a million years ...Eros exhibits structural regularity (like a solid monolith), but

there is plenty of evidence that it must be, and is, heavily

fractured in situ. Thus it might respond to a push or a blast like

a "rubble pile," not a cohesive object.

Is Eros Representative of Other Asteroids?

l Eros is one of the biggest Earth-approaching asteroids, has more

gravity than a typical kilometer-scale Near Earth Asteroid.

l Eros may have had a ~50 Myr period of lunar-like rather than

asteroid-like cratering (a virtual hiatus). Difficult to

predict what effects that might have on a human-scale. But

do not expect it to be like the Moon, which is warmer, subject

to higher impact velocities, and has much more gravity.

l Many other S-type asteroids may be like Eros. But the "S"

classification is a grab-bag of what could be very different

beasts, ranging from fractured rocky monoliths (like Eros), to

rubble piles (like Ida?), to "gravel" piles, to mostly metallic

objects (like Gaspra?).

l Most asteroids (even in Near-Earth space, but especially in the

main belt) are not S-types. Carbonaceous bodies may be

radically different places for space explorers, operations, etc.