Geoscience Reference
In-Depth Information
3. Asteroid SFDs
Chemical analyses of Apollo samples of impact melts point to a dominantly
asteroid reservoir for LHB impactors, rather than to cometary objects.
4
Also, recent asteroid surveys with high resolution and large sky coverage
such as Spacewatch, SDSS, or LINEAR have given us a significant degree
of understanding of the SFD of MBAs as well as of NEAs. However, we
cannot directly compare the SFD of asteroids with that of craters; it is
necessary to convert crater SFD into that of projectiles (or the other way
around). Procedures of this sort have already been established with the
help of the scaling relationship between crater size and projectile size. In
this paper, we assume the typical asteroidal impact velocity and asteroidal
density as those of crater projectiles, and convert crater SFD into projectile
SFD so that we can compare the SFD of crater projectiles with asteroid
SFD. As for the SFD of MBAs, we use the results of three survey programs:
Spacewatch,
7
SDSS,
8
and Subaru.
9
We rely on the results of the LINEAR
program
10
for the SFD of NEAs. We used the so-called Pi scaling laws
11
-
14
to derive the projectile size from the crater size.
When we draw the asteroid SFDs on
R
-plot graphs, we immediately see
their remarkable similarities to the SFD of crater projectiles. At first, the
SFD of MBAs fits that of lunar LHB crater projectiles very well over wide
diameter ranges (Figs. 2(a)-(c)). Second, the SFD of NEAs seems quite
close to the SFD of the younger projectile population that has created
craters on the younger martian plains (Fig. 2(d)). Thus, we obviously have
two distinct SFD populations among current asteroids, not only among
craters. These similarities should be more than just a coincidence, having
firm physical/dynamical reasons.
4. Discussion
The fact that the ancient LHB projectiles had an SFD almost identical to
that of the current MBAs could imply several things: First and foremost,
there was a size-independent transport process for asteroids during the
LHB period. If the LHB duration was as short as 50-200 Myr as previous
research suggests,
1
the timescale of the transport process must have been as
short. Not so many dynamical mechanisms can make this drastic asteroid
transport happen. A plausible candidate is the radial movement of strong
resonances in the main belt caused by the migration of giant jovian planets.
Currently some ideas along this line are being proposed in terms of
the late formation of Uranus and Neptune and their interaction with
