Gravitational admittance estimate for Mars

Q&A
1

Hi all,

I apologise if this isn’t the right place to post this.

I am trying to estimate the elastic thickness around Elysium Mons on Mars using the gravitational admittance method. However, my estimates, especially my observed admittance, look very off and don’t match the pattern i’d expect it to (from literature at least). I’m wondering if I have missed something in my code or if there is something wrong with my data.

Any help/tips would be greatly appreciated, thanks!
code:

Capture
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gmt set PROJ_ELLIPSOID=Mars

#resample topography so that it is at 0.05 degree increments to match FAA
gmt grdconvert mola32.nc -Gtopo1.grd -R-180/180/-90/90
gmt grdsample topo1.grd -R-180/180/-90/90 -I0.05/0.05 -Gmola32.grd

#trim the area to surround elysium mons
gmt grdcut faa_test.grd -R143/151/21/29 -Gfaa_cut.grd
gmt grdcut mola32.grd -R143/151/21/29 -Gtopo_cut.grd

gmt gravfft topo_cut.grd faa_cut.grd -Iwkt -T60000/2900/3500/0.01 -Z48000 -N+a -Gadmittance_60km.txt

2

Hi,
I’m afraid gravfft (and grdfft, btw) were written before the discovery of Mars and have hardwired the Earth gravity. If you can rebuild GMT, you try to set the appropriate value for Mars.

3

Ah thank you so much!!

Are there by any chance any instructions on how to rebuild GMT so that i can set the appropriate values?

4

We have this

5

thank you!!

6

If you manage to build GMT, build from master is almost not more work at all. And master (the dev version) has now a modification that let gravfft set the gravity_at_45 via a new -M option.

1 Like
7

Ah just did it, it works, thank you so much!!

8

Hey,
Would it be possible to add a similar modification to the grdflexure module as well?
It would be helpful to be able to scale the gravity to compute deformation on other planets.

Thank you in advance!

9

Yes, probably. But would you like to try doing it (in case you know some C, ofc)?

10

But we have some annoyances. There is only one uppercase free letter (other than the global ones) in grdflexure. The I, which is different than the one used in gravfft & gtdfft (-M). Shall we use that one that is an almost global or, for example, -O that is global but only used in postscript producing modules?

11

Thanks for your reponse.
I can’t say im very familiar with C, but I can have a try.

Adding the -I would be very helpful, but i understand the annoyances and inconvenicences it may cause, so please don’t worry too much.
Even if it was just a temporary solution for a month or so, I’m doing a planetary geophysics dissertation so it would make scaling the deformation easier.

12

For a quick temporary change you can just edit line 162 of grdflexure.c.
And, another thing. My previous changes to gravfft & grdfft missed doing all the modifications to allow providing a different gravity because that was stored too in a second variable. I fixed that too in master but you’ll need to rebuild your GMT binaries.