[X] My Assistant

[Guest_AlexBlackwell_*]

I thought I'd mention that Alex Konopliv of JPL and his colleagues have a paper in press with Icarus detailing MGS95J, an updated gravity field model of Mars to degree and order 95. It's really interesting, at least for those who like that sort of thing.

A global solution for the Mars static and seasonal gravity, Mars orientation, Phobos and Deimos masses, and Mars ephemeris
Icarus, In Press, Corrected Proof, Available online 20 March 2006
Alex S. Konopliv, Charles F. Yoder, E. Myles Standish, Dah-Ning Yuan and William L. Sjogren
Abstract

[Guest_BruceMoomaw_*]

Actually, once you hack your way through all the cobwebs of differential equations, the nitty-gritty from some gravity-field papers can be fairly interesting, given what they say about the internal structure of a world.

[Guest_Richard Trigaux_*]

Actually, once you hack your way through all the cobwebs of differential equations, the nitty-gritty from some gravity-field papers can be fairly interesting, given what they say about the internal structure of a world.

... for instance diametre of an iron core (if one) and main mantle layers. Eventually if there is a magmatic layer.

Seasonal changes? What could cause seasonal change in gravity field? Swap of ice in polar caps? Or movement of a solid core in a liquid layer, like on Earth?

[SteveM]

... for instance diametre of an iron core (if one) and main mantle layers. Eventually if there is a magmatic layer.

Seasonal changes? What could cause seasonal change in gravity field? Swap of ice in polar caps? Or movement of a solid core in a liquid layer, like on Earth?

You're right; the paper says:

3.3.2. Seasonal changes in Mars rotation
The seasonal mass transfer between the Mars polar caps causes small periodic variations in the spin (Yoder and Standish, 1997).

On another matter, I notice that their solutions for the Mars ephemeris do not include any Rover data. I presume that's because:

1. They have a better fix on the position of the orbiters in relation to Mars's center of mass.
2. The rovers move erratically and their geodetic positions are ill-defined -- pending further study

-- Yep, the paper explains the omission because of "the continual movement of the rovers." It also notes that "the tracking data from MER to Odyssey may significantly reduce the Odyssey orbit errors, and would provide a better measurement of forces normal to the orbit plane,..."

Is that a first: spacecraft tracking from another planet?

[Guest_AlexBlackwell_*]

Yep, the paper explains the omission because of "the continual movement of the rovers." It also notes that "the tracking data from MER to Odyssey may significantly reduce the Odyssey orbit errors, and would provide a better measurement of forces normal to the orbit plane,..."

Note, though, that William Folkner of JPL is a MER Participating Scientist whose investigation is entitled "Measurement of Mars Rotation Changes with the Mars Exploration Rovers." Although the MER tracking data may not be accurate enough for MGS95J, they are, apparently, good enough for, say, moment of inertia measurements like those Folkner performed with the stationary Mars Pathfinder lander.

[SteveM]

Note, though, that William Folkner of JPL is a MER Participating Scientist whose investigation is entitled "Measurement of Mars Rotation Changes with the Mars Exploration Rovers." Although the MER tracking data may not be accurate enough for MGS95J, they are, apparently, good enough for, say, moment of inertia measurements like those Folkner performed with the stationary Mars Pathfinder lander.

Thanks for that.
Apparently he hasn't published anything on it yet -- at least there's nothing in the Astrophysics Data System. Perhaps it will have to wait until they get good enough results from the Rover localization process so they can incorporate the daily movements into the analysis.

Alternatively, he can do analyses for those periods where one of the Rovers is stuck in one place for a while -- purgatory, McCool Hill,...