# Will Neil Armstrong's moon boot marks really last for thousands of years?

This question concerns the residual heat (if any) contained within the Earth's moon.

At the time of the Apollo moon landings, it was widely reported that the boot marks left by the astronauts would last "forever", as there is no atmosphere to disturb them and because the moon was geologically "dead."

Ignoring cosmic dust, moon quakes (which have since been confirmed) and the blast created by the Eagle lander top section as it left the moon, is this idea of extremely long term preservation of the boot prints really valid?

Do we know if the moon is capable of creating convection cells, carrying heat to the surface and gradually erasing the boot prints and tracks left by the astronauts by stirring the dust particles surrounding them?

Does the Earth create gravitational forces within the interior of the moon, even though I appreciate they may be a low order compared to the extreme example of Jupiter on Io?

• One can certainly imagine multiple effects that redistribute moon dust on relatively short time scales. Some may require the lunar atmosphere, some (like lunar quakes and thermal expansion) will not. As to the reality of it... only an experiment can tell. – CuriousOne Jun 23 '15 at 1:16
• Depending on global warming some of the things that I left in glaciers personally (aka scientific littering) will last up to 100,000 years according to some geologists... we shall see if Armstrong's footprint will make it that long. IMHO a "collector" will get there with some resin and fill that thing in within 30-40 years... :-( – CuriousOne Jun 23 '15 at 1:30
• @CuriousOne I'd be sad to see that happen. Personally, I believe that site should be left pristine as historical treasure. I'm also of the opinion that we need to go back to the moon. Soon. So much wonderful science came from it... – CoilKid Jul 15 '15 at 23:38
• I wasn't sure I wanted to ignore moonquakes, as I was thinking that they would probably not be good for the long-term survival of the marks and a bit too important to pass up. – CoilKid Jul 15 '15 at 23:42
• Two minor issues: 1) the boot print seems to be Aldrin's, not Armstrong's. 2) It may not even have lasted a single day. Depending on how far from the LEM it was, it may have been blown away shortly after firing the ascent engine. – Jens Aug 4 '15 at 7:08

Lunar soil or lunar regolith, is mostly created by meteorite and micrometeorite impacts which directly pulverize the rock, or from the ejecta from the impact. Some amount (I can't seem to find any figures) is also created from high-energy particles in solar wind causing bits of rock to spall.

In theory, the bootprints would last until the soil turns over from the space-debris impacts, or the ground shifts due to an moonquake or thermal expansion.

According to this paper, there is a 99% probability that the upper 0.5mm will be turned over 100 times in $10^6$ years. That works out to about once every ten thousand years for the top 0.5mm of the soil. It goes on to say that for a depth of 10cm (probably slightly deeper than the bootprint depth) you'd have to wait on the order of $10^9$ years to see it turn over. However, this is if you worry solely about impacts. Let's move on to moonquakes.

The Wikipedia entry on moonquakes suggest that there are four different kinds of moonquake. Deep moonquake, occurring ~700km below the surface. Probably nothing to worry about there. Meteorite impacts, which as we just saw are pretty rare. I'd say the chances of an impactor large enough to destroy a bootprint probably around 5-10cm deep, are very low. Thermal moonquakes, and shallow moonquakes. Of the four, probably the second two are the most likely to cause damage to the bootprint.

Thermal moonquakes are caused by the temperature shift between the day side of the moon, and the night side of the moon. With an almost nonexistent atmosphere, the temperature differences would be huge. However, according to these sources, (1)(2) thermal moonquakes are very mild, and therefore would probably not threaten the bootprints. Maybe some of the soil around the edges would fall in, but nothing major.

That leaves shallow moonquakes. Recorded to be as powerful as a 5.5 on the richter scale, if moonquakes threaten the prints, this type would the kind to watch for. However, I can't seem to find any studies on regolith turnover caused by quakes. Wikipedia suggests that an earthquake of a 4.5 or higher on the richter scale can damage buildings, but I doubt that even a 5.5 moonquake would do very much to the bootprints. I would lean toward what I said regarding thermal quakes and say the edges might fall in, but probably no major damage.

I would say that depending on the distance from the lander, the most damage to the bootprints probably came when the upper section of the LEM blasted back into orbit. If the prints were close, the exhaust almost certainly stirred up dust which would settle in the print. Or, if the print was directly next to lander it may have been filled in or otherwise destroyed much the same way an outflow from an shop-vacuum can stir up dust.

Ultimately though, given the number of marks they made, if you include the buggy tire marks and other bootprints, the chances that there will be evidence that those men walked on another world will, assuming they don't get disturbed by another mission/collectors, almost certainly exist for at least several millennium.

Related:

Moonquake and lunar regolith turnover. (Google scholar search)

Regolith shifting from thermal effects.

Lunar seismology.

*also:

as to convection cells and gravitational forces, I didn't see those on the search either. It's worth a little more poking around though. I'll add any info I find as I find it. :)

• thanks for taking the time to answer that, you did a good bit of digging (sorry:) for that comprehensive answer. I would imagine, if we / when we do go back on a permanent basis, it will become a national "park" status type area. – user81619 Jul 16 '15 at 11:06
• @AcidJazz No problem, I actually had a lot of fun researching this. Thank you for posing an interesting question :) – CoilKid Jul 16 '15 at 15:02

yes it would, because of the moons atmosphere, even the craters have not faded away so the footsteps would not fade away. Best type of legacy one can leave

• I don't think comparing > 1 meter sized craters to a ~ 1 cm deep footprint is convincing. – Kyle Kanos Jul 28 '15 at 21:02
• Also, craters blasted by rocks travelling 5300 mph are of a different nature than a 100 kg person stepping into dust. – Bill N Jul 28 '15 at 21:24
• @CoilKid, (a) the ~ means 'order of' here (b) still nowhere near 100+ cm, no? – Kyle Kanos Jul 28 '15 at 22:54
• @KyleKanos No conflict here, just thought it was an interesting point. I suppose leaving out my comment would clarify the situation though. :) – CoilKid Jul 28 '15 at 23:04