The Insight meeting (“Mars Interior and Geophysics after Insight”) was this week (Jul 16-18). Yes, on the Mars Insight lander, which I will discuss on this blog for good reasons.
The Mars Insight (Interior, seismic investigation, geophysics, and heat transfer) mission had two major investigation themes, and two minor, related ones. In no particular order, the ultrasensitive seismometer(s!) (named SEIS, Seismic Experiment for Interior Structures) would read ‘Marsquakes,’ which would vary depending on the structure of the rock at local, regional, and global scales. If the planet has a thick versus thin crust, then it would eventually show up in its effects on Marsquakes. If the planet has a soft, warm mantle vs. cool, stiff mantle, that would have some effect. And if the iron core were near (large) vs. deep (small), then that, too would show some imprint on distant Marsquakes.
Also, the ultrasensitive heat probe (HP3) would measure the heat balance (positive, zero, or possibly negative) escaping the planet. All planets formed with radioisotopes, which get variously hot when they decay. The very formation of a planet- infalling, crashing material- generates heat in the process of this falling and crashing. Therefore any sort of planet already has a basic heat source- the Kelvin-Heimholtz mechanism. Between these two, all planets formed with some warmth; the better question is how much is still there, based on how fast it escapes. In other words, ‘Is Mars cold and stiff yet?’
In answering these two lines of questioning, we wade into two others: is Mars’ weather confusing our readings, when wind gusts, or just pressure fronts, blow on the instruments? Weather, or meteor impacts. Do impact events (including near-impacts: the passage of Phobos as a tidal disturbance) also confuse our readings? And secondly: does the presence of a soft mantle or liquid core (still hot enough that the iron core is molten, even partially) show up as a wobble of Mars? You can spin two eggs, or two soup cans. One fresh egg, one boiled. Or one intact soup, one empty can. Which one do you think will wobble, vs. spin straight? The same principle holds for a planet, except the spin is slower, the wobbles subtler. Conversely, measuring such ‘wobbles’ (precession and nutation) lets us know of the ‘fresh’ vs. ‘firmed’ state of Mars’ interior.
The impact data is already a measure of the asteroid flux past Mars, and adjacent to the Main Asteroid Belt. And so- it turns out- is the seismometer. We can read the passage of Phobos in the Mars Insight data from its tug, and to a lesser extent a sensitive instrument will see large asteroids as they pass. If it needs to be said: the Mars Insight seismometer is sensitive. Very sensitive. And so are the instruments that would register wobbles, and impacts. Insight contains a weather station, which can record pressure waves (basically, sound) from large-enough meteors; the RISE (Rotation and Interior Structure Experiment) instrument can track Insight (and therefore Mars- the lander is firmly planted) with precision to many decimal places.
Which brings us to this week’s meeting. The primary requirement of the mission- and therefore, the primary focus of the meeting- was the interior structure of the planet. Issues of Phobos, and of large meteors and passing asteroids, are secondary science and secondary discussions just now. Still, they happened. At least one impact was detectable, solidly measured, and reported at the meeting. This is because we have two datasets: direct seismic shock, and meteorology as an independent sign. (We can also cross-cross-confirm: other craft, Mars orbiters, have spotted fresh craters, in the course of the Insight mission.) The Insight heat probe was not so lucky, and the repeated hammering/forcing attempts added noise (literally) to the seismometer data. The RISE instrument is pretty much solid state, and old-school: there’s nothing bleeding-edge about putting a radio on a spacecraft, even a highly stable, frequency-locking radio. Viking did that, and so did the rovers (except the rovers don’t have nice, stable landing feet, making them jitter slightly).
Oh well, the asteroid topics are secondary, as a reasonable person might expect. We know the instruments worked (except HP3, which is not germane here). Therefore, the data is physically feasible (Phobos shows up) and likely taken and saved. Since the distant (compared to Phobos, certainly) asteroids have long time constants on their flybys and gravimetry signals, the derivations require broader looks.
The data is there. Who will put out the paper on Insight asteroids? Granted, the Gaia mission coincides with Insight. Given a choice, Gaia has the better, more extensive dataset. But the two are parallel and independent: Gaia uses direct optics, Insight is (by comparison) blind and ‘feels’ them pass by. Independent datasets are complementary because they confirm each other, just as orbiters may see the craters that Insight felt. If nothing else, knowing the state of Mars to more decimal places makes it a better ‘tugger’ (pedagogically). Who will put out the confirming paper?