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Comment on A biologist’s perspective on ice ages and climate sensitivity: Part I by gbaikie

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“And there IS a decent coating of dust on the moon, though not waist deep. But the depth of that dust should be IMHO an indicator of how much dust has arrived on Earth, too. That said, I’d like to know why anyone’s claim or argument would vary very much from that. On average, what falls on the Earth would also fall on the Moon. Sometimes one or the other screens the other, but even that should average out as a wash, I think. Yes, the Earth has a bigger gravity well, and that might make a difference. But it might also lens the paths of some dust (which is traveling probably at what? 15-30 km/sec?) and bend it around the Earth, and if the Moon is on the back side, it might get more than its share.”

A difference between the Moon and Earth, is a car size rock is stopped by the earth’s atmosphere, on the Moon it directly impacted on the lunar surface. Pea size or even microscopic dust hits the lunar surface at [as you said] 15-30 km/sec. Though the most common impact speed is about 20 km/sec, but there is a wide range of possible impact velocities. The range is +70 km/sec to somewhere around the Moon’s escape velocity [2.4 km/sec]. It somewhat rare to have low velocities of around 2.4 km/sec, but those impacting at those speeds would be affected by the Moon gravity.
To impact the Moon or Earth at these lower velocities [around their escape velocities] requires that an object be in the same orbit around the sun as the Earth.
The reason is quite simple, earth is traveling around the Sun at 29.8 km/sec, and most objects are crossing earth’s orbit. So it’s like highway where cars are traveling 60 mph, if you on bicycle going with traffic and you cross the traffic, it’s the car’s speed which mostly determines the impact velocity.
And the higher velocities involve earth hitting cometary material- if they miss earth they going go out beyond Jupiter. A lot material which hits earth or Moon comes from Jupiter gravity affecting on asteroids in the Main asteroid belt- Jupiter the reason we have a constant supply- without Jupiter impacts would be much rarer then they are.

As for lunar dust, all the impactors are “gardening” the lunar surface- the turn over or gardening of lunar regolith occurs around every few million years:
“The lunar regolith, or the uppermost few meters of the Moon, consists of soil and highly fragmented rocks formed by repeated impacts of meteoroids with the Moon’s surface. These impacts eject material from the craters that are formed, and the ejecta are usually deposited nearby. The overall effect of many such impacts is a continual overturn of the regolith, which keeps it well mixed. This can be thought of as a sort of gardening, analogous to the mixing of soil performed by a farmer’s plow.

The degree of mixing in the regolith depends on the size of an impacting body, with larger impacts mixing material to greater depths. Small impacts are much more common than larger impacts, and so mixing to small depths occurs more often than mixing to large depths. Current estimates are that mixing to a depth of 1 centimeter occurs on average every million years, while mixing to a depth of 1 meter occurs only about once every billion years.”
http://www.lpi.usra.edu/lunar/missions/apollo/apollo_17/experiments/lnp/

Regolith Depth:
“We find that median regolith depths in
the mare regions are typically 2-4 m, whereas median
regolith depths on the farside and non-mare nearside
areas are typically 6-8 m.”
http://www.lpi.usra.edu/meetings/lpsc2011/pdf/2607.pdf

The impacting also compacts regolith [or dust], and smaller impactor/micrometeorites sputter the surface. Leaving a thin layer few inches [in which astronaut made all their footprint] which is on top of meters of compacted regolith.


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