Benjamin P. Weiss

I am the Deputy Principal Investigator and the Magnetometry Investigation Lead for the NASA Discovery mission Psyche.  The mission will orbit the ~100 km radius asteroid (16) Psyche.  Psyche is the largest known metallic asteroid in the solar system and is thought to be the core of a planetesimal whose mantle was stripped away by collisional events in the early solar system (Elkins-Tanton et al. 2020, 2022, Dibb et al. 2024).  It is humankind's only opportunity to visit a planetary core up close.  We are seeking to determine whether Psyche once generated a magnetic field, which would confirm its origin as a differentiated asteroid (Weiss et al. 2023). Click here to learn more.

I am a Co-Investigator on NASA's Europa mission, which will conduct multiple flybys of Europa and possibly also incorporate a lander.  I am team member of the Europa Clipper Magnetometer whose main goals are to constrain the thickness and salinity of the Europan ocean and thickness of the overlying ice (Kivelson et al. 2023, Cochrane et al. 2023, Cochrane et al. 2023, Roberts et al. 2023). We are developing Bayesian inverse methods for retrieving these parameters from magnetic field measurements (Biersteker et al. 2023 )

I am a Co-Investigator for Returned Sample Science on the Mars 2020 rover mission. The goal of the mission to select and cache rock samples for eventual return Earth. During our exploration of an ancient lakebed in Jezero crater., we have identified minerals formed by liquid water, indicating it was likely a habitable environment (e.g., Farley et al. 2022, Scheller et al. 2022, Wiens, et al. 2022, Bell et al. 2022, Simon et al.. 2023). We have acquired nearly two dozen absolutely oriented samples of Martian bedrock (Weiss et al., 2024). Future laboratory studies of these samples will search for potential signatures of past Martian life, characterize past aqueous processes and paleoclimate and to establish whether there were habitable environments, and study the thermal and aqueous evolution of the Martian interior to understand the history of igneous differentiation and tectonics (Mittelholz et al. 2018, Beaty et al. 2019).

Chondrites are traditionally thought to be samples of bodies that never melted. However, we have suggested that CV chondrites (Carporzen et al. 2011; Elkins-Tanton et al. 2011, Weiss and Elkins-Tanton 2013, Gattacceca et al. 2016) and IIE iron meteorites were magnetized by a core dynamo. External field sources like solar nebular field or early solar wind (Oran et al. 2018) have difficulty explaining the young age and strong intensity of the CV chondrite magnetization. This suggests that some chondrite parent bodies may be a partially differentiated, with a melted interior overlain by an unmelted crust. Although such partially differentiated asteroids have not yet been unambiguously identified, asteroid 21 Lutetia, encountered in July 2010 by the Rosetta spacecraft, might be just such an object (Weiss et 2012). We also found paleomagnetic evidence that the parent body of IIE iron meteorites is partially differentiated (Maurel et al. 2020; 2021) .

Our paleomagnetic studies of Apollo samples indicate the at the Moon generated a dynamo from least 4.2 until 2 billion years ago (Ga) (Garrick-Bethell et al. 2009, 2017; Shea et al. 2012; Suavet et al. 2013, Tikoo et al. 2014; Buz et al. 2015). Tikoo et al. 2017, Mighani et al. 2020).  Our magnetohydrodynamic simulations indicate that the field cannot be explained by amplification of the interplanetary magnetic field by large impacts (Oran et al. 2020). Nevertheless, the apparent great intensity of the dynamo field may be incompatible with a conventional dynamo driven by convection of the lunar core and so may require unusual power sources like mantle precession (Weiss and Tikoo, 2014). The long lifetime of the dynamo supports the hypothesis that the late field was powered by core convection (Mighani et al. 2020, Wieczorek et al. 2023).

I am on the science team of the Mobile Asteroid Scout (MASCOT) magnetometer (MasMag). MasMag measured the magnetic field while hopping across the surface of asteroid (162173) Ryugu in October 2018 (Hercik et al. 2016, 2020). This was the first time a mobile platform conducted surface operations on an asteroid. We found that the asteroid has no detectable remanent magnetic field, perhaps indicating that it formed in a region of the solar nebula where the ambient magnetic field was very weak. We are now making laboratory magnetic measurements Ryugu samples returned to Earth by Hayabusa2 in 2020.

A key unknown in Mars science is how the planet evolved from a warmer, wetter state billions of years ago to the current cold, dry state (Ehlmann et al. 2016).  Our meteorite studies indicate that for the last 3.5 billion years, ambient surface temperatures in were on average well below 20 °C (Weiss et al. 2002b, Shuster and Weiss, 2005; Cassata et al. 2010). Our data also indicate that atmospheric pressures did not exceed 1.5 bar during the first 400 million years of the Noachian era, and dropped below 400 mbar by 4.16 billion years ago (Cassata et al. 2012). The loss of the early atmosphere may have been linked to the death of the Martian dynamo (Weiss et al. 2002a, 2008). Samples to be cached by the upcoming Mars 2020 mission could provide a unique opportunity to test this hypothesis (Mittelholz et al. 2018).

Planetary differentiation comprises the processes by which large-scale melting gives rise to a compositionally layered structure. We have been studying meteorites from early asteroids (planetesimals), which served as building blocks of the planets. We have found that the parent bodies of the several igneous meteorites generated magnetic fields in their advecting metallic cores (Weiss et al. 2008, 2010; Fu et al. 2012; Scheinberg et al. 2015, 2017; Harrison et al. 2017, Bryson et al. 2017). Core dynamos may even have occurred on chondrite parent bodies (Weiss and Elkins-Tanton 2013).

We have shown that Martian meteorite ALH 84001 acquired much of its magnetization 4.2-3.9 billion years ago in the presence of a field with intensity roughly equal to that at the Earth's surface today (Weiss et al. 2002; 2008). This is the oldest magnetization known in any planetary rock, and shows that Mars had generated a dynamo and global magnetic field within the first 500 million years of its existence. The subsequent decay of the magnetic field may have been responsible for drastic climate change on Mars (see our Mars paleoclimate work). Samples to be returned by upcoming Mars missions should enable us to rigorously test this conclusion (Mittelholz et al. 2018)

We have been studying the paleomagnetism of impact craters on Earth to constrain the rigorously test the hypothesis that meteoroid impact-generated plasmas can magnetize rocks. This has been proposed as a key alternative to core dynamos as the source of lunar and meteorite magnetization. We have found no evidence of impact-generated fields from neither the recently formed the 1.8 km diameter Lonar crater (Louzada et al. 2008; Weiss et al. 2010) in India nor the giant ~300 km diameter Vredefort crater in South Africa (Carporzen et al. 2012). Our studies of the effect of shock on the paleomagnetism indicates that many rocks and meteorites should be capable of retaining records from magnetic fields on their parent bodies (Gattacceca et al. 2006, 2010; Louzada et al. 2010, 2011; Tikoo et al. 2015).

I was on the science team of the former Google Lunar X-Prize mission Beresheet, the first private mission to make a (hard) landing on the lunar surface in 2019. SpaceIL carried a magnetometer for measuring lunar magnetic fields (Aharonson et al. 2020)