Transcript Cratering on Mercury: Insights from the MESSENGER Flybys

Cratering on Mercury:
Insights from the MESSENGER Flybys
Clark R. Chapman (SwRI),
R.G. Strom, C.I. Fassett, L.M. Prockter, J.W. Head III,
S.C. Solomon, M. E. Banks, D. Baker, W.J. Merline
73rd Annual Meeting of the Meteoritical Society
#5325, 11:45 a.m., Tuesday, 27 July 2010
New York City, NY USA
Meteoritical Context for
Studying Craters on Mercury
Other Issues
Addressed by
Mercury’s Craters
• Large craters
penetrate deep within
the crust, revealing
layers of different, deep
geological units.
• Crater densities (esp.
of primary craters)
reveal stratigraphic
sequences in
emplacement of
geological units
expressed on the
surface.
• Cratering physics:
why does basin
morphology begin for
smaller craters on
Mercury than on other
bodies like the Moon?
 Chronology. Was the history of cratering on
Mercury the same as on the Moon and Earth?
 Impactor Populations. Was the same mix of
asteroids and comets responsible for Mercury
cratering, or was there a later bombardment by
“vulcanoids”?
 Style of Cratering on Mercury. Does the higher
impact velocity on Mercury or other factors
(e.g. those responsible for the prominence of
large secondary craters on Mercury) help or
hinder the launch of meteorites from Mercury
that could reach the Earth?
 Composition. Does the variety of
compositional units revealed by penetration of
Mercury’s crust by large craters provide clues
about how to identify meteorites that might
have come from Mercury?
Mercury has Many Basins!
 Mercury has many more basins than the Moon
 Yet the spatial density of large basins per unit area is about the
same as the Moon
 This apparent contradiction is resolved by realizing that basin
morphology appears at a considerably smaller diameter on
Mercury than on the Moon
 Data are (so far) from images with limited lighting and viewing
geometries. Once we have good global coverage, and global
laser altimetry, we can search for large quasi-circular features,
like those found on Mars and the Moon
Rembrandt
There are Multiple Potential
Sources for Mercury’s Craters
 Primary craters from the same populations of Near
Earth Asteroids (NEAs) and comets that crater the
Earth, Moon, Mars, and Venus today.
 Primary craters from the same population/s that
caused the Late Heavy Bombardment (LHB) on the
Moon ~3.9 Ga. (These may have been main-belt
asteroids and outer solar system planetesimals, if the
Nice Model is correct. Or other collisionally and
dynamically processed remnants from accretion.)
 Vulcanoids (remnants of hypothetical population of
planetesimals interior to Mercury’s orbit, which would
not have appreciably cratered other planets).
 Secondary craters from larger craters and basins.
 Endogenic craters (volcanic vents, subsidence
craters, etc.)
 Other rare causes (e.g. tidally split SL9-like bodies)
Our Working Hypothesis
 Mercury was saturated with craters and
basins during the Late Heavy Bombardment
by the same population of impactors that
struck the Moon.
 Mercury has since been cratered by the
same population of NEAs that still crater
other inner solar system bodies.
 Many-to-most craters <10 km diameter on
widespread plains units are secondary
craters.
 Endogenic craters are a small but important
contribution to negative relief features.
Interpretational Framework:
Cratering Components
25
Populations 1 and 2 (R. Strom)
 Lunar highlands are
prototype for Pop. 1
produced by LHB
 Young Mars plains are
prototype for Pop. 2
(current NEAs derived
by size-dependent
Yarkovsky plus
resonances from main
belt) plus secondaries
at D<1 km
 Caloris exterior plains
are dominantly Pop. 2
(plus large secondaries
D<10 km)
Schematic
diagram of
possible LHB
scenario
Caloris Basin Relative Ages
Important result: If exterior
plains are even younger than the
Caloris interior plains, then they
are certainly volcanic flows.
Thus Mariner interpreta-tions of
the knobby textured Odin
Formation as Cayley-Plains-like
Caloris ejecta are wrong.
Caloris is a relatively young basin,
with about half the density of
superimposed craters as the
general cratered highlands of
Mercury, but it was cratered during
the LHB, while the interior/exterior
plains mainly post-date the LHB.
Caloris Basin
Variability of
Intercrater Plains
 Bottom panel shows MESSENGER crater
spatial densities (R values) in a part of
Mercury that resembles the average
highlands measured from favorably
observed regions by Mariner 10. The
distinct deficiency of craters on Mercury
<30 km diameter was ascribed to
“intercrater plains”.
 New studies of other regions show
considerably variability. Top panel
shows deficiency extending to craters
150 km in diameter, implying a thick
sequence of intercrater plains (i.e.
volcanism).
 Middle panel shows a modest deficiency
extending to 100 km diameter, but a
prominent secondary crater branch
appears at an unusually large diameter,
~20 km.
“Twin” Young
Basins on Mercury
Raditladi Basin Seen on M1 Flyby
Rachmaninoff Basin Revealed on M3 Flyby
 Both basins ~250-300 km diam.
 Similar inner peak rings
 Lightly cratered floors with
circumferential extensional
troughs
 Similar rim morphologies
A Closer Look at the Recently
Discovered Rachmaninoff Basin
 Compare very
low crater
density inside
peak ring with
slightly higher
crater density
between peak
ring and rim
 Lighter colored
interior floor
has breached
peak ring on the
bottom
 Both basins
have fairly
young ejecta
blankets and
many surrounding secondary
craters (next slide)
Ejecta and Secondary
Craters of Raditladi and
Rachmaninoff…and a
Recently Volcanically
Active Region
Raditladi Basin
Rachmaninoff Basin
100 km
Note “orange” color within peak
ring, like other young volcanic
plains on Mercury. Also note
the proximity of Rachmaninoff
to what may be a large volcanic
vent (in the very bright region
northeast of the basin).
Relative Ages: Basin Rims
and Plains within Basins Note: Very low
 (A) Inner plains and annular plains of Rachmaninoff: Inner
plains are clearly younger than annular plains, but
apparently older than Raditladi plains (but size distribution
is not the same shape, confusing the comparison)
 (B) Rachmaninoff rim and ejecta suggests an older basin
formation age than for Raditladi
crater densities
and small
areas of
counting units
cause poor
statistics…but
it’s the best we
can do!
Basins and Plains: Approximate
Relative Stratigraphy by Crater Density
Relative Crater Density (varies by factor >30!)
 1.0:
Highlands craters
 0.5:
Caloris rim = Rembrandt rim
 0.35: Floor of Rembrandt
 0.2:
Interior Caloris plains (volcanic)
 0.15: Caloris exterior plains (volcanic)
 0.1
Rachmaninoff basin = annular plains
 0.05 Rachmaninoff inner plains
 0.03: Floor of Raditladi = rim of Raditladi (is
floor impact melt prompt volcanism?)
Mercury’s Absolute Chronology:
Raditladi Example (applying lunar chronology)
 Sequence:
Heavily cratered
highlands → Caloris basin → Exterior
plains → Raditladi basin/plains
 If lunar chronology applies, then
 If exterior plains formed early (3.9
Ga), then Raditladi is 3.8 Ga (red
arrows)
 If smooth plains formed ~3.75 Ga
Preferred!
then Raditladi’s age is <1 Ga!
(green arrows)
Two Chronologies for Mercury
Age before present, Ga
4.5
4
3.5
3
2.5
2
1.5
1
0.5
NOW
Formation to magma ocean/crustal solidification
C
A Bombardment, LHB, intercrater plains formation
L
O
R
I
S
Smooth plains volcanism
Raditladi
Cratering, rays
Lobate scarps, crustal shortening
Classical (Lunar) Chronology
Vulcanoid Chronology Example
Formation to magma ocean solidification
C
A
L
Vulcanoid bombardment, intercrater plains O
Raditladi
R Smooth plains volcanism
I
Cratering, ray formation
S
Bombardment, LHB
Lobate scarps, crustal shortening
Conclusion: We must wait for orbital
mission for good stratigraphic studies


Mariner 10 imaged 45% of surface? (I don’t think so.)
MESSENGER has almost completed coverage? Not YET for robust geological analysis
Mariner 10 Image & Shaded Relief
MESSENGER image
Abstract
Introduction: During its three Mercury flybys, MESSENGER imaged most regions unseen by Mariner 10 and viewed some
previously seen regions under more favorable lighting. The surface density of impact craters and basins on Mercury with
diameters D>200 km is comparable with that of the Moon, though possibly there are fewer large basins. The largest basin
mapped from Mariner 10 (Borealis) has not been reliably recognized in MESSENGER images. Two smaller peak-ring
basins (Raditladi and Rachmaninoff) are comparatively young. Large craters and basins have numerous secondary
craters, which generally dominate Mercury’s crater populations at D<10 km. Extensive volcanism apparently modified
Mercury’s crater populations at D <100 km, to variable degrees in different regions, but was as powerfully destructive of
craters D<40 km as the many degradation processes that affected Martian highlands.
Large Craters and Basins: The morphologies of dozens of peak-ring basins have illuminated the transition from smaller
complex craters to basins. Caloris and Rembrandt basins are fairly well preserved and formed during the later part of the
Late Heavy Bombardment (LHB); craters on their rims follow the Population-1 size-frequency distribution (SFD)
characteristic of LHB cratering throughout the terrestrial planet region (believed to be the result of direct scattering of mainbelt asteroids). Volcanic plains formation within Caloris ended well after the basin formed, close to the end of the LHB: its
interior plains are dominated by the later Population-2 craters typical of near-Earth asteroids today, chiefly derived from the
main belt by size-dependent processes such as the Yarkovsky effect. Volcanic plains formation exterior to Caloris
continued afterwards, based on a lower density of almost purely Population-2 craters. These plains clearly postdate
formation of the Caloris basin by a substantial interval and are not ejecta deposits like the lunar Cayley Plains, as had been
hypothesized after Mariner 10.
SFD’s for Mercury’s craters with D>10 km in various cratered regions of Mercury differ widely, more than was appreciated
from Mariner 10. In some regions, voluminous intercrater plains obliterated all craters with D>100 km, whereas elsewhere
plains buried only smaller craters so that many with D>40 km remain from older eras. Intercrater plains and younger, often
more spatially restricted, smooth plains both formed by volcanic emplacement.
Small Craters, Secondaries, and Young Plains: In some places (e.g. in regions near Raditladi) Mercury’s craters are
dominated by secondaries for D<20 km. In general, the upturn of the SFD at smaller sizes occurs at D<8 km, a much
larger diameter than the few km typical on the Moon and Mars. Perhaps larger secondaries are formed on Mercury than on
other bodies. The temporally sporadic and spatially clustered nature of secondaries hinders studies of relative ages of
small and/or recent units. Nevertheless, the extremely sparse densities of small craters within Raditladi and Rachmaninoff
suggest that these basins are unusually young. In the case of Rachmaninoff, volcanism continued within its inner plains
until comparatively recently, long after basin formation, and thus those plains cannot be impact melt.