Kuiper Belt Objects: Physical Studies 615
9.1.4 REASONS FOR COLOR PATTERNS
What could cause these color signatures? One possibility
is the radiation-reddening and impact-graying mechanism
discussed earlier in Section 1. However, such a mechanism
should result in a uniform distribution of B-R colors for
Centaurs and not two clusters of B-R colors. In addition,
gray impact craters and their ejecta blankets would be ran-
domly distributed on the surface so that one hemisphere
might have more than another, resulting in measurable color
changes as the object rotates. However, repeated and ran-
dom measurements of individual rotating KBOs and Cen-
taurs give the same B-R color. Also, extensive observations
of Pholus suggest that it has a highly homogeneous surface
color. Figures 10a and 10b show the R-band brightness and
B-band brightness of Pholus as a function of a single rotation
phase taking 9.980 hr. Figure 10c is the difference of 10a
from 10b, yielding the B-R color across the entire surface of
Pholus as it makes one rotation about its axis. The solid hor-
izontal line is the average of the points. The dashed lines are
plus or minus one standard deviation,σ= 0 .04. Any vari-
ation in the B-R surface color of Pholus must be smaller
than 0.04 magnitude (4%). Again, there is no evidence of
gray impact craters on a radiation-reddened surface.
Another possibility is that the colors of KBOs are the
remaining signature of a temperature-induced, primordial
composition gradient. The small, rocky terrestrial planets
(Mercury, Venus, Earth, and Mars) close to the Sun and
the giant, hydrogen-rich gas giant planets (Jupiter, Saturn,
Uranus, and Neptune) farther away from the Sun are the
result of such a gradient. In the inner Solar System, tem-
peratures were so high that only metal and rock forming
elements could condense from the nebular gas to form
small, rocky, and metal-rich solids. At and beyond the orbit
of Jupiter, the hydrogen-dominated nebular gas was cold
enough for the H 2 O to condense out. We may be seeing a
similar effect on the colors of KBOs and Centaurs. We now
suspect KBOs did not all form at about the same distance
from the Sun. Perhaps the red classical KBOs formed far-
ther out in the nebula where it was cold enough to hang on
to their CH 4 ice reddening agent. Perhaps the gray KBOs
formed closer to the Sun and were not able to hang on to
their CH 4 ice reddening agent.
Additional work is necessary to figure out whether the
radiation-reddening and collisional-graying mechanism,
the temperature-gradient mechanism, or some other mech-
anism is responsible for the colors of KBOs and Centaurs.
9.2 Spectroscopy
There are only a handful of KBOs and Centaurs that are
known to exhibit ice absorption bands in their spectra.
H 2 O-ice bands are seen in the spectra of Charon, 19308
(1996 TO 66 ), Varuna, Quaoar, Orcus, Pholus, and Chariklo.
CH4-ice bands are seen in the spectra of Pluto, Neptune’
FIGURE 10 Homogeneous B-R surface color of Pholus.
(a) R-band magnitude vs. rotation phase. The x-axis spans a time
interval of 9.980 hr. (b) B-band magnitude vs. rotational phase.
(c) Difference between above two panels yield B-R color vs.
rotational phase. The solid line is the average of the 94 points.
The dashed lines are plus or minus one standard deviation,σ,of
0.04 magnitude. Any variation in the surface color of Pholus as it
completes one rotation on its axis must be less than 0.04
magnitude (4%). Pholus exhibits a homogeneous surface color.