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Figure 1. IODP-ICDP Expedition 364 drilled into the subsurface Chicxulub
steep cliffs and open chasms. Summit
heights vary along the circumference of
the peak ring. On the Moon, where the
erosional processes familiar on Earth do
not occur, that differential topography is a and numerical modeling of peak-ring
primary feature, caused by shear and fault
displacement during the emplacement of
the peak ring (Kring et al., 2016).
Spectral analyses of the lunar surface
captured from the orbiting Chandraayan-1
spacecraft indicate the peak ring is com-
posed of anorthositic, noritic, and olivine- ward to form nappe-like structures in a
bearing (e.g., troctolite or dunite) rocks
from deep crustal or even upper mantle
depths (Kramer et al., 2013). Those rock
types occur in spectacular outcrops (Fig.
2B). Soon after Apollo, it was common to
hear geologists lament that there are no
outcrops on the Moon because the surface
is covered with regolith. However, the
rocks exposed in the peak ring constitute
hectometer- to kilometer-size outcrops that
are now recognized as high-priority sites
peak ring at borehole M0077A (red dot), which was ~30 km northwest of Pro-
greso and the north shore of the Yucatán Peninsula. The blue circle repre-
sents the approximate diameter of the 180–200-km subsurface impact struc-
ture. The gravity signature of the structure (from lows of -16 to highs of +30
mgal) and locations of other drilling sites are shown in the inset. The only two
sites with continuous core are the ICDP Yaxcopoil-1 (Yax-1) and IODP-ICDP
M0077A boreholes. Other boreholes are Yucatán-1 (Y1), Yucatán-2 (Y2),
Yucatán-6 (Y6), Chicxulub-1 (C1), Sacapuc-1 (S1), and Ticul-1 (T1).
Figure 2. (A) The morphology of a peak ring is
evident in this view of the ~320-km-diameter
Schrödinger basin on the Moon, looking from
the north toward the south pole. NASA’s Scien-
tific Visualization Studio. (B) A close-up view of
a segment of the peak ring with rocks uplifted
from mid- to lower-crustal levels by the impact
event. The field of view is ~17 km wide through
the center of the image. Lunar Reconnais-
sance Orbiter Camera image M1192453566.
for future lunar sample return missions nearly doubling from 20 to 40 km from
(Potts et al., 2015; Steenstra et al., 2016). the east to the west and producing bilat-
eral asymmetry in the peak ring (Fig. 3).
Geologic mapping of those rock types As shown below, those types of morpho-
logical effects, visible at the surface on
emplacement (Kring et al., 2016) suggest the Moon, are mirrored in the subsurface
the rocks in the peak ring were derived Chicxulub peak-ring basin on Earth.
from mid- to lower-crustal depths on the
Moon (e.g., ~15–26 km deep). During the CHICXULUB
impact event, those rocks rose above the
lunar surface and, without the strength to The subsurface morphological charac-
maintain that elevation, collapsed out- ter of the peak ring of the Chicxulub cra-
ter is similar to that of Schrödinger,
circumferential peak-ring. Pre-impact although the topography on the upper sur-
crustal strength seems to have affected face of Chicxulub’s peak ring is more
that process. A gap in the peak ring subdued because of Earth’s greater grav-
occurs in the southeastern quadrant, ity. Thus, while Schrödinger’s peak ring
which is an area in the target that had rises up to 2.5 km above the basin floor,
been previously weakened by the seismic reflection data (Morgan et al.,
Amundsen-Ganswindt basin-forming 2000) indicate Chicxulub’s peak ring had
event. There, the peak ring collapsed ~400 m of relief before being buried.
below the level filled by impact melts and Additional seismic data suggest the peak
impact breccias. Pre-impact crustal thick- ring varied in height circumferentially
ness also varied across the target area, (Gulick et al., 2013), with reduced
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