Thread by @DrPhiltill

I’m not so sure. The link to the prior estimate is a paper that measures the “blast zone”, which is the region around a lander where the reflectivity of the surface has changed. We have never known exactly what causes this change. Is it from gas blowing the dust texture flat? /1

https://twitter.com/spacedotcom/status/1769695851297788280

2/ Or is it from blowing dust plowing across the surface? Or from engine shutoff when the last sputter of the engine cause a low velocity blanket of dust to fly out to a much smaller distance than normal? The problem has always been that this blast zone is *too small* to be…

3/ …the area where the dust finally lands, because the dust in rocket exhaust is going far to fast in low gravity to travel only that far. (We think we might know the cause now, but I don’t want to tell here since we will probably write a paper on it.)

So the prior estimate…

4/ …of the blast zone around the Vikram lander called it an “erosion halo”, which is fine. That’s just a different term for the same thing. The prior paper didn’t say what caused the erosion halo. The problem is that…

5/ …this news report today says the erosion halo (or blast zone) is the area where the dust “settled down.” That is wrong. It was only 145 sq m, so the radius is only 7 meters. Rocket exhaust blows dust much farther than 7 m.

Maybe the reporter didn’t understand what was said.

6/ The linked paper estimates the mass of soil (Msoil) that was blown by plugging the lander mass (Mlander) into a relationship I derived a few years ago:

Msoil = 0.762 Mlander^1.77

The Vikram lander was 1.752 tons, so it predicts 2 tons of soil were blown.

7/ This equation doesn’t take into account the lander engine configuration. I developed it assuming a single engine under the lander, firing until about 1 m altitude. It is possible the scientists studying the Vikram’s blown dust have developed a different method to estimate the

8/ amount of dust that was blown. Maybe they used landing imagery to calculate the density of the blowing dust or something similar. But if the estimate was based on the size of the erosion halo then i have serious doubts. For example, suppose that halo is the area where dust…

9/…was being removed from the surface (not where it settled as the news article says). If you know the depth that it was removed at each radius in the erosion halo then you know the volume of eroded dust. But you can’t measure that depth without knowing the shape of the terrain

10/…before the lander arrived, because it will be measured in millimeters to centimeters and the lunar surface naturally varies by more than this.

So the news article makes me doubt what it says. Hopefully there will be another peer reviewed article coming out to explain./end