The following is excerpted from from this answer to the Space Exploration SE question "Do Curiosity's reported measurements of Mars' surface gravity (~3.717 m/s^2) include centrifugal effects?".
It essentially answers my question. I would appreciate if someone gave the info about the gravimeter on the Phoenix lander too.
So the surface gravity on Mars is:
- 3.70703 m/s² (0.378 g) on the equator
- 3.71683 m/s² (0.379 g) on the midlatitudes
- 3.73493 m/s² (0.381 g) on the poles
So here's a complete tally using
$$a_G = -GM \frac{\mathbf{r}}{r^3}$$
From Geopotential_model; The_deviations of Earth's gravitational field from that of a homogeneous sphere:
$$a_{J2x} = J2 \frac{\mathbf{x}}{r^7}(6z^2 - 1.5(x^2+y^2))$$
$$a_{J2y} = J2 \frac{\mathbf{y}}{r^7}(6z^2 - 1.5(x^2+y^2))$$
$$a_{J2z} = J2 \frac{\mathbf{z}}{r^7}(3z^2 - 4.5(x^2+y^2))$$
$$a_C = \mathbf{r_{xy}} \omega^2 $$
magnitudes shown only (sign indicates generally "up" or "down")
at the equator at the pole
h = 0 h = -4500 m h = 0
GM -3.71317 -3.72303 -3.75729
J2 -0.01092 -0.01088 +0.02236
centri +0.01706 +0.01697 0.0
vector sum -3.70703 -3.71683 -3.73493
Do Curiosity's reported measurements of Mars' surface gravity (~3.717 m/s^2) include centrifugal effects?
Yes they do! At 5.2 degrees south latitude and -4500 meters altitude (bottom of Gale Crater) the acceleration is -3.7168 m/s taking into account GM, J2, and centrifugal effects.
So as Cheap Trick tells us and Meatloaf reiterated more elegantly in Roadie:
Everything works if you let it!
Here's some Python for double-checking my math:
def accelerations(rr):
x, y, z = rr
xsq, ysq, zsq = rr**2
rsq = (rr**2).sum()
rabs = np.sqrt(rsq)
nr = rr / rabs
rxy = np.sqrt(xsq + ysq)
rrxy = rr * np.array([1.0, 1.0, 0.0])
nxy = rrxy/rxy
rm3 = rsq**-1.5
rm7 = rsq**-3.5
acc0 = -GM_mars * rr * rm3
# https://en.wikipedia.org/wiki/Geopotential_model#The_deviations_of_Earth.27s_gravitational_field_from_that_of_a_homogeneous_sphere
acc2x = x * rm7 * (6*zsq - 1.5*(xsq + ysq))
acc2y = y * rm7 * (6*zsq - 1.5*(xsq + ysq))
acc2z = z * rm7 * (3*zsq - 4.5*(xsq + ysq))
acc2 = J2_mars * np.hstack((acc2x, acc2y, acc2z))
accc = nxy * omega**2 * rxy
return acc0, acc2, accc
import numpy as np
halfpi, pi, twopi = [f*np.pi for f in [0.5, 1, 2]]
degs, rads = 180./pi, pi/180.
R_mars = 3396200.0
GM_mars = 4.282837E+13 # m^3/s^2 https://en.wikipedia.org/wiki/Standard_gravitational_parameter
J2_mars = GM_mars * R_mars**2 * 1960.45E-06 # https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html
Req = 3396.2 * 1000. # meters https://en.wikipedia.org/wiki/Mars
R = Req - 4500. # https://en.wikipedia.org/wiki/Gale_(crater)
Rpo = 3376.2 * 1000. # meters https://en.wikipedia.org/wiki/Mars
sidereal_day = 1.025957 # https://en.wikipedia.org/wiki/Mars
T = sidereal_day * 24 * 3600.
omega = twopi/T
print "omega: ", omega
print ''
aaacs = accelerations(np.array([Req, 0, 0]))
print "Req: ", Req
for thing in aaacs:
print thing, np.sqrt((thing**2).sum())
print "total: ", sum(aaacs), np.sqrt((sum(aaacs)**2).sum())
print ''
lat = rads * -5.4
aaacs = accelerations(np.array([R*np.cos(lat), 0, R*np.sin(lat)]))
print "R: ", R
for thing in aaacs:
print thing, np.sqrt((thing**2).sum())
print "total: ", sum(aaacs), np.sqrt((sum(aaacs)**2).sum())
print ''
aaacs = accelerations(np.array([0.0001, 0, Rpo])) # avoid divide-by-zero (lazy!)
print "Rpo: ", Rpo
for thing in aaacs:
print thing, np.sqrt((thing**2).sum())
print "total: ", sum(aaacs), np.sqrt((sum(aaacs)**2).sum())
>
and if it already contains a block quote I just add a 2nd one (i.e.>>
). It's always good to add a bit in your own words to address anything specific to your new question. $\endgroup$