Do comets ever run out of water?

Question

Does all the water eventually boil out of old or short period comets?

If so, do they then turn into asteroids?

Are there any?

Answer 1

Yes, they are called Extinct comets.

The Jupiter Family comets are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages. Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid. Some asteroids in elliptical orbits are now identified as extinct comets. Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.

Answer 2

Yes, comets can and do become extinct. It likely depends on the initial size of their nucleus.

The defining property of a comet is that it can develop a gas and dust tail in the vicinity of the Sun - thus it needs to have enough volatile material which can still evaporate.

Depending on the size and composition this process may find an end when the volatile material at the surface is all gone and the evaporation is not strong enough any longer to drag along any dust which forms the visible tail.

Gundlach and collegues made lab experiments, calculations and comparisons with the in-situe measurements by the Rosetta spacecraft and found that this may depend on the size of the comet's nucleus: for larger bodies it needs a bigger pressure to overcome gravity and tensile strength of the surface material. Thus it is harder to drag along further dust - and they form quicker an inactive surface layer.

In the figure taken from their work you see the tensile strength of the typical material due to compression of its own gravity for bodies of the size indicated. . The grey-shaded area indicates the typical tensile strength as measured in-situ by Rosetta on 67P/Churyumov-Gerasimenko. It shows that from a 30km pristine comet you can erode a few 10m of surface as the lower layers are already compressed to a too high tensile strength to be eroded away by comet activity. These body will then eventually be known as extinct comet or 'asteroid in cometary orbit' (ACO). However a body of only 1km is small enough and has low enough gravity that it might completely evaporate over time when it nears the Sun repeatedly (this has e.g. been observed by SoHo).

Mind though, that such an extinct comet might still have water below the surface: The heat conductivity of the material is very low. Thus the heat does not penetrate deep, and the resulting slight temperature variation at depth doesn't suffice then anymore for any significant amount of water or CO2 to evaporate or even lift-off any dust/rock/carbon grains from the surface. Thus the surface is all solid grains sticking to eachother, and a meter below this surface you still have the pristine material.

Answer 3

As described in other answers, the answer is "Yes; they then become (like) asteroids". This answer describes asteroids that appear to have been comets in their former days.

Such identifications must be provisional unless we were lucky and smart enough to have followed the object from the time it had ices and showed cometary activity. But certain orbital characteristics are suggestive of a cometary origin, and are shared by most apparent extinct comets:

• High eccentricity: objects that have permanently been asteroids follow less eccentric orbits; the extinct comets generally keep the more eccentric orbits typical of comets.

• Close perihelion compared with most asteroids: this favors faster evaporation and exhaustion of the ices. Live fast, die young.

We look at the perihelion (P), aphelion (A), and eccentricity (e) for the first four candidates listed in the article referenced by Nilay Ghosh. Data are from the respective Wikipedia articles for these comets; P and A are in astronomical units.

(137924) 2000 BD19: P = 0.0921, A = 1.6609, e = 0.8950

14827 Hypnos: P = 0.9481, A = 4.7318, e = 0.6659

2101 Adonis: P = 0.4414, A = 3.3069, e = 0.7644

2015 TB145: P = 0.2941, A = 3.9073, e = 0.8600

Answer 4

Sort of; once enough volatiles leave the surface, the leftover non volatile material acts as a barrier to prevent any further loss of volatiles, including water!

Answer 5

Yes. Its only logical, but the real corollary to this is the 'lie' that comets have precise orbital periods. They simply cannot because they lose mass over time. Haley was wrong.