Saturday, August 16, 2008

Mars- Dwarf planet until death?

Last week, I showed how the back of the envelope planethood calculations on Hal Levinson’s website can be solved for time to determine when a dwarf planet reaches planethood.

I then mentioned how planetary migration can move planets into uncleared orbits, or scatter stuff into an orbit that had been previously cleared.

I will now combine these ideas with the geologic history of Mars to show that the red planet may have only achieved planethood after its magnetic dynamo shut down, its oceans dissipated, and the volcanic activity subsided to a few isolated vents.

Here’s the Wikipedia version of Mars geology (corrections from anyone who knows better are welcome):

There are two ways to divide up the geological history of Mars. Crater counting gives us three ages- the Noachian (heavily cratered southern highlands), the Hesperian (less cratered northern plains), and the Amazonian (recent features such as some of the newer volcanoes).

Alternatively, mineralogical mapping shows that the Noachian can be divided into the Phyllocian- dominated by clays and water-bearing minerals, and the Theiikian, dominated by sulphate salts. The younger areas of Mars are classified as Siderikan- iron oxide dominated- in this classification system. I have no idea how they assign numbers to the relative ages, but both systems are shown below.

So how do these timescales compare with the time needed by Mars to clear its orbit?

As I mentioned last week, the time needed to scatter objects out of the orbital neighborhood is dependent on the angle of inclination between those objects and the planet-to-be, in this case Mars. So, we will plug in the orbital inclination for 3 of the 4 original objects in the asteroid belt, and see what happens.

Vesta has an orbital inclination of 7.1 degrees. Plugging this into the formula gives a clearance time of 358 myr. On this time scale, Mars clears its orbit, enjoys a few hundred million years as a planet, then has to do it all again when the Late Heavy Bombardment scatters asteroidal riff-raff back into the newly cleaned neighborhood. Mars is a planet for about one quarter of the Noachian, and is a Dwarf planet the rest of the time.

If we increase the orbital inclination of the debris field to that of Ceres- 10 degrees- then Mars no longer has time to clear the neighborhood before the LHB messes it up again. The clearance time is almost a billion years, so Mars is a dwarf planet until the mid-Hesperian, or about a third of its life.

Finally, if we further increase the inclination to 13 degrees- similar to 3 Juno- then the clearance time blows out to 1.8 gyr, as shown below. This results in Mars being a dwarf planet for half of its life, and only acquiring planetary status towards the end of the Hesperian, when most planetary processes have ceased functioning. This would make Mars a posthumous planet.

The red planet has been the centre of the public’s interest in planets since the days of Edgar Rice Burroughs and H. G. Wells. It currently has more interplanetary missions operating on and around it than do the rest of the planets combined. Most of this scientific interest revolves around past habitability, specifically during the earliest, wettest eras. If Mars is demoted to dwarf planet during this period of time- as the above calculations suggest the current planet definition requires- then how useful is this definition?

The current Eriphobic planet definition didn’t just demote Pluto. It also forced Mars to die before it could join the planet club.

1 comment:

Anonymous said...

You make an interesting argument to the planet debate. However, I think I have a counter-argument: a planet need not have cleared its orbit completely, but only be the "absolutely dominant" object in it. (Otherwise Neptune would not be a planet, since it shares its orbit with Pluto and many asteroids.) Even if you put Mars into the present-time asteroid belt, it would still be the dominant object in its orbit: its mass is about 200 times the combined mass of the asteroids in the main belt (according to Wikipedia data). This would be the lowest mass ratio of the 8 planets, but still much larger than the corresponding values for the currently acknowledged dwarf-planets: Ceres' mass is only one third the mass of the remaining asteroid belt objects, and for the plutoids the corresponding ratio is even lower. Thus, you could still consider Mars a planet.