LHB - Last Heavy Bombardment

I trying to figure the level of cometary/asteroid activity, again for -
100 MY. I found one article that tells about the "Last Heavy
Bombardment", LHB, that ended around -3.8 BY, not long (all things
relative in this domain) before life appears on Earth.

I can't find any other period of LHB after that, or any other definite
level of bombardment for that matter. The asteroid/comet that hit Earth
-65 MY and contributed to the end of dinosaurs wouldn't have been part
of a noticeably more active period in that regard.

So, the model I'm building in my head is that when Earth is forming,
all sorts of planetoids, comets and asteroids are
aggregating/impacting. Once the planets and moons are formed, it gets
more or less quiet on that front.

From then on (-3.8 BY), it's relatively stable. The solar system's
various bodies exert tidal forces on the asteroid belt, the Kuiper belt
and the Oort cloud, and, with more or less regularity, new comets
detach from the Oort Cloud, new asteroids leave their old orbit, and
eventually or never impact some planet, moon or the Sun itself. There
is no lenghty gradual pattern of diminishing impacts.

Thus, I can't say that around -100 MY, there is more comets or
asteroids on a potential collision course with Earth. It's not
scientifically defendable that, let's say, the solar system gets safer
in that regard as time passes. Is that heresy ?

Does anyone know of some graphic that would show the impact/time ratio
since the birth of our planet ? I haven't found any.

Thanks!

Zague

Zague wrote:

Thus, I can't say that around -100 MY, there is more comets or
asteroids on a potential collision course with Earth. It's not
scientifically defendable that, let's say, the solar system gets safer
in that regard as time passes. Is that heresy ?

I haven't looked up references, so I'm replying from memory of
various articles I've read over the year.

The conventional view is that the solar system did get safer compared
to its early life. The disk around the sun from which the planets
formed was originally denser. As the material in the disk
accreted into planets and smaller bodies, that material came
out of the disk, so the space between planets, and especially
in the planetary orbits, became less dense. There were fewer
bodies to crash into Earth because they had already crashed
into Earth in the first few billion years. The forming planets
swept the dust and particles that were in their path, adding
them to the planetary mass.

So, just as I expect to find less dirt on my kitchen floor
after I've been sweeping it a while, I expect to find fewer
impacts on Earth after Earth has been sweeping its space
for a while. The impacts we see today are mostly from
objects in eccentric orbits that originate from further out
in the solar system.

--
Glenn Holliday

Thanks Glen. It seems that after the planets formed, impacts are high
for still some time and then we get into a rather stable state that
could only be changed by major events like a close encounter with
another star.

It could correspond to some inverse exponential curve where the number
of impacts will diminish rapidly and the system will cruise for a long
time on a somewhat asymptotic line.

I've read somewhere that there's a little more activity every 100 MY or
so, but I don't have more than that. It might as well be some confusion
with the probability of a Chicxulub size collision which is also in the
order or one every 100 MY.

Zague

Thanks Glen. It seems that after the planets formed, impacts are high
for still some time and then we get into a rather stable state that
could only be changed by a major event like a close encounter with
another star.

It could correspond to some inverse exponential curve where the number
of impacts will diminish rapidly and the system will cruise for a long
time on a somewhat asymptotic line.

I've read somewhere that there's a little more activity every 100 MY or
so, but I don't have more than that. It might as well be some confusion
with the probability of a Chicxulub size collision which is also in the
order or one every 100 MY.

Zague