# How could InSight's seismometers be intentionally and meaningfully “pinged”?

I had a friend whose summer job, a long long time ago, was to travel around an unsuspecting rural area, lay a heavy steel plate on the ground, and whack the plate with a sledge hammer. Apparently he also had some kind of seismometer or accelerometer nearby. This was apparently a cheap and dirty way to get some idea of the soil and rock below the surface.

This was on Earth, not Mars.

The question InSight and active pinging of Mars asks about generating artificial seismic events on Mars for InSight's seismometers to listen to. @DrSheldon's answer mentions that this is not part of the plan; that InSight is expected to return meaningful data without this.

It doesn't mean that it wouldn't benefit from it though.

What kinds of schemes might a frugal and/or clever space agency use to generate seismic events that would be meaningful and useful for InSight measurements of Mars?

I can imagine two classes, one to help verify nothing is wrong, so something local perhaps, and the other far enough away to give geological or even planetary information.

• Maybe inSight could use its own self-hammering mole to generate very small local seismic events – Dragongeek Nov 19 '18 at 10:35
• @Dragongeek I think this question can end up with several answers. I didn't know InSight had a mole, but that might be better than a woodpecker! ;-) – uhoh Nov 19 '18 at 10:37
• @Dragongeek - My idea was to lay down a stick of dynamite, light the fuse and run. But yeah, a hammer works, too. :) – Don Branson Nov 19 '18 at 17:15
• @DonBranson was the running part of your ideas as well? Very clever! :O – uhoh Nov 19 '18 at 17:24
• @uhoh Not at first. It's all about learning from your mistakes. – Don Branson Nov 19 '18 at 17:31

The traditional method (as used in the Apollo project) was to crash used SIVB stages into the Moon.

• Since there may not be too many of those left, nor (at least) an easy way to get one to Mars, I wonder what other item might serve as an SIVB proxy? – uhoh Nov 19 '18 at 11:01
• There are a bunch of landers on Mars due in 2020. digitaltrends.com/cool-tech/future-mars-missions/2 apart the landers, each will probably have at least a heatshield and backshell impacting independently – Steve Linton Nov 19 '18 at 11:18
• There are a lot of spacecraft orbiting Mars right now. It's likely at least one of them will fail during InSight's lifetime, and so the scenario in this answer is quite plausible. +1 – Dr Sheldon Nov 19 '18 at 14:51
• This has been tried on Mars with the Mars Climate Orbiter. :-) – Martin Schröder Nov 19 '18 at 15:13
• @MartinSchröder But the timing was terrible. – Russell Borogove Nov 27 '18 at 1:35

Let's examine what's already on InSight:

• A seismometer (SEIS). It's so sensitive that it is expected to be able to sense windstorms, dust devils, and the tidal forces of Mars' moon. To isolate the sensors from motions of the main body of InSight, SEIS is in its own pod that will be placed a few feet away by a robotic arm, and attached by an umbilical.

The sensors can be recorded during the "impact" of this placement process, and there is a small chance that something useful could be learned from the results.

• A burrowing temperature probe (HP3). It will be placed by the robotic arm, and then dig with an impact hammer up to 5 meters depth. The head of the "mole" is attached with a cable that has temperature sensors along its length.

The vibrations caused by dropping the mole, as well as its digging, could possibly help the seismometer map out the nearby subsurface geology.

• An X-band radio (RISE). This transmitter and transponder will work with antennas in the Deep Space Network to locate the position (within 2 cm) and velocity of Mars. It doesn't seem to have any moving components to create a seismic "ping".

• The aforementioned arm. This could be manipulated to "thump" the surface around the lander.

• Other sensors include pressure, temperature, wind direction and speed, and magnetic field. There is a laser retroreflector on the deck, a color stereo camera on the arm, and a color panoramic camera below the deck. I don't see how any of these could be used to create a seismic "ping".

• Finally, the usual spacecraft components: landing legs, solar panels, computer, antennas, etc.

Therefore, the spacecraft itself has a limited capability to produce some seismic "pings", which might reveal the nearby subsurface geology. The most revealing observations will be by passive seismology and the other instruments.

• It's so sensitive that it is expected to be able to sense ... the tidal forces of Mars' moon. Considering Mars' moons are only 7.7 and 14 miles across, that's amazing! – Andy Nov 20 '18 at 16:21

To address the problem with martian atmosphere - maybe an impactor with high explosives could be better. Something like Tallboy bomb.

I think energy of the impact will not be higher than from a meteorite impact. Probably even much less. But intended impact have some big advantage - we know where and when exactly it happened. It can be very useful for calibration of seismic velocities model of Mars.

The main problem is cost, of course. The impactor should be specially designed. Stuff like old satellites or rocket stages is not dense enough and not stiff enough to impact martian surface with enough energy after encounter with the atmosphere. The impactor mission would cost at least 100-200 mln \$ including launch.

• I think impactors (kinetic devices) and explosives can be completely separate and alternative solutions and could be discussed individually. – uhoh Nov 19 '18 at 15:31

Active seismology was actually used by Apollo 16. Small explosives were detonated to produce artificial seismic waves.

https://www.lpi.usra.edu/lunar/missions/apollo/apollo_16/experiments/as/

The mortar portion of the Apollo 16 Active Seismic Experiment.

Two experiments, the Active Seismic Experiment on Apollo 14 and 16 and the Lunar Seismic Profiling Experiment on Apollo 17, were performed to determine the detailed structure of the upper kilometer of the lunar crust. Both experiments involved detonation of a series of small explosives. The seismic waves or ground motions caused by these explosions were measured by a network of geophones. On Apollo 14 and 16, up to 19 explosions were detonated by an astronaut using a device called a "thumper" along a 90-meter-long geophone line. On Apollo 16, three mortar shells were also used to lob explosive charges to distances of up to 900 meters from the ALSEP. On Apollo 17, eight explosive charges were positioned during the three EVAs at distances of up to 3.5 kilometers from the LM. These charges had masses of 57 grams to 2.7 kilograms. Both the Apollo 16 mortar shells and the Apollo 17 explosives were detonated by radio control after the astronauts left the lunar surface.

These experiments showed that the seismic velocity (P wave) is between 0.1 and 0.3 kilometers per second in the upper few hundred meters of the crust at all three landing sites. These velocities are much lower than observed for intact rock on Earth, but are consistent with a highly fractured or brecciated material produced by the prolonged meteoritic bombardment of the Moon. At the Apollo 17 landing site, the surface basalt layer was determined to have a thickness of 1.4 kilometers, slightly higher than the 1 kilometer thickness determined from the Traverse Gravimeter Experiment.