and that of the other known low-albedo families
covers the entirety of the inner main belt. This
means that all dark asteroids with D < 50 km
in that region are likely members of the primordial or other families. The implication is
that the background of unaffiliated asteroids
is represented only by the planetesimals, whereas
the smaller low-albedo asteroids could be linked
back to a handful of original inhabitants. We
suspect that this could be a general feature of
the main belt, which we have shown here only
for 2.1 < a < 2.5 AU.
Unlike other old families, the eccentricity and
inclination of the primordial family members
are not correlated, spanning the entire orbital
element space of the inner main belt (Fig. 3, B
and C). The e and i distributions of the members
of the primordial family are similar to models of
families that may have formed before the giant
planet instability (15). The precise time of onset
of the instability remains the object of discussion
and ongoing research (24), but our results indicate
that this family predates the instability regardless
of its date because the calculated Yarkovsky
drift age allows the family to be as old as the
solar system. Although there is evidence for
this instability throughout the solar system (14),
the primordial family represents a record of
the collisional evolution of the main belt before
this event. The existence of this family implies
that the main belt before the planetary instability
was sufficiently dynamically excited to produce
at least one very large family-forming colli-
Some of the largest members of the other
known low-albedo families are within the bound-
aries of the primordial family (fig. S2). There-
fore, these other parents could have been family
members of the primordial family themselves,
before suffering their own subsequent family-
forming collision. This includes families for which
spectral surveys found similarities and homo-
geneities across their members in multiple wave-
length regimes (20, 26, 27). If some of the other
low-albedo families are related to the primordial
family, then a large fraction of all low-albedo
asteroids in the inner main belt could share a
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1028 8 SEPTEMBER 2017 • VOL 357 ISSUE 6355 sciencemag.org SCIENCE
Fig. 3. Size distributions and spread in proper eccentricity and incli-
nation of the primordial family members. (A) Cumulative size distribu-
tions of low-albedo asteroids located in the section between the inward
borders of the primordial family V shape and the Polana family (open
circles), and low-albedo asteroids that do not belong to other families in
the region of (a, 1/D) space above and beyond the inward border of the
Polana family (solid circles). There are no known asteroids with D < 3.82 km in
the first population. The slope of the size distribution of the second population
changes at D 13 km. (B and C) Orbital distribution of primordial family
members, which are spread over the entire inner main belt. For the
first (open circle) population, only asteroids with D < 50 km are displayed in
(B) and (C). For the second (solid circles) population, only objects with 13 <
D < 50 km are plotted. Gray dots represent all other known asteroids,
regardless of their albedo values.
Fig. 4. Cumulative size distribution of planetesimals.
The cumulative size distribution of those asteroids that
are outside V shapes unless they are family parents,
such as (8) Flora (solid squares), is corrected for the
maximum number of objects that were lost because of
the collisional and dynamical evolution, in order to obtain
an upper limit for the distribution of the planetesimals
(open squares). Functions of the form N(> D) = N0Db,
where N is the cumulative number of asteroids, are fitted
piecewise in the size ranges D > 100 km, 35 < D < 100 km,
and 8 < D < 35 km. For the original planetesimals size
distribution, we obtain the values of b reported by the
labels in the plot. In the range of sizes between 8 and
35 km, we give the 1s and the 3s upper limits on the
planetesimals size distribution. The sizes of (8) Flora, (27)
Euterpe, and (298) Baptistina are conservatively
corrected by adding the volume of their respective family
5 50 500 10 100
Void with no original asteroids
1 upper limit
3 upper limit