Why are there no methane detectors with "the mole" of the Insight lander?

Question

A possible geophysical source of methane on Mars could be ancient methane trapped in clathrate hydrates in the subsurface that may be released occasionally.

In a not secured (!) article of ESA about methane on Mars it is suggested by an image of the Elysium Planitia region that methane has also been observed in that region where the Insight lander is.

The Heat Flow and Physical Properties Package is a science instrument onboard this lander that features a self-penetrating probe to determine how heat flows inside Mars.
Nicknamed "the mole" it trails a tether containing sensors to measure the temperature profile of the subsurface.

Question: Why has this unique opportunity not been used to detect methane in the subsurface behind the probe with semiconductor gas detectors or with some other type of methane detector using the shaft made by the probe ?

Answer 1

The reason is because it wasn't part of the science objectives. It is a Discovery Mission, which has a constrained total budget to start with, and what was selected was very close to a build-to-print version of the Phoenix Lander, which is a big selling point in terms of cost as well as flight heritage of the instruments.

As easy as it sounds to just add on an experiment, in practice is isn't a small effort. For these kinds of high profile missions, there are pretty rigorous systems engineering that has to happen, such as size-power-weight constraints, compatibility with the infrastructure of the spacecraft, extensive environmental testing and integration, etc.

Answer 2

In part because an actual methane only sensor is hard to build. The standard ones you can buy have a range of things that make them false positive*, and the older hot wire ones need oxygen at a known level to operate. Mars exploration has plenty of instrument results including the positive test for life on Viking where the results are less useful because they could indicate a range of things and there is no other data to rule one one way or the other.

In this case a basic methane sensor would most likely indicate for any form of hydrocarbon compound, with the reading dependent on which type was in the sensor. Really useful chemical detector instruments use several different sensors on the same sample to quantify the various interactions and sensor quirks, which is pretty much why MSL/Curiosity is the size it is.

So strapping a basic hydrocarbon sensor to an InSight style drill does not do much science until you have had a MSL style probe drill as a reference sample.

Certainly the InSight drill technique may allow interesting subsurface samples to be gathered cheaply once a sensor package small enough to go down the hole can actually be calibrated by a larger mission.

*Most of the things they false positive on are dangerous/explosive hydrocarbons so this is a feature not a bug in most cases