# A Possible Explanation for the Blue Spectral Slope Observed on B-type Asteroids

**Authors:** Mark J. Loeffler, Beau S. Prince  
**arXiv:** [2201.02459](https://arxiv.org/abs/2201.02459)  
**Date:** 2022-01-07  
**Summary page:** https://www.summarizepaper.com/en/paper/2201.02459v1/

## Key points

- Researchers aimed to understand the role of dark material in the reflectance spectrum of carbonaceous asteroids
- Laboratory experiments were conducted using relevant dark materials such as graphite, magnetite, and troilite
- All three dark materials darken the reflectance spectrum of forsterite samples and cause a decrease in spectral slope (become blue)
- Spectral changes can be caused by both nm- and micron-sized grains
- Graphite is most efficient at altering the spectral slope in the ultraviolet-visible region, while magnetite is most efficient in the near-infrared
- Troilite alters the slope and albedo of their samples, suggesting that these spectral changes may not be unique to graphite and magnetite alone
- Scanning electron microscopy was performed on each dark material to better constrain grain size
- Synthetic troilite's spectrum was relatively featureless, dark, and positively sloped in the near-infrared region but contained an absorption band centered around 0.35 µm attributed to Fe-S charge transfer and crystal field transitions of Fe2+
- Mixing experiment showed good agreement with those observed on Bennu which suggests that fine-grained dark material including sulfides present in regolith can cause negative (blue) slopes found on B type asteroids providing insight into their composition and evolution processes

## Summary

In this study, the researchers aimed to understand the role of dark material in the reflectance spectrum of carbonaceous asteroids. They conducted laboratory experiments to investigate how the addition of relevant dark materials such as graphite, magnetite, and troilite can alter the ultraviolet-visible and near-infrared spectrum of a neutral silicate mineral. The team found that all three dark materials darken the reflectance spectrum of forsterite samples and cause a decrease in spectral slope (become blue). These spectral changes can be caused by both nm- and micron-sized grains. The researchers observed that graphite is most efficient at altering the spectral slope in the ultraviolet-visible region, while magnetite is most efficient in the near-infrared. However, they also noted that troilite alters the slope and albedo of their samples, suggesting that these spectral changes may not be unique to graphite and magnetite alone. To better constrain grain size, scanning electron microscopy was performed on each dark material. The team made rudimentary estimates of average grain size by counting grains on a given line and dividing by its length. They found that nm-sized graphite powder is darker than other powders used in this study. The absolute reflectance spectra of each component were analyzed. Forsterite had a largely featureless spectrum with an absorption band at lower wavelengths due to low amounts of iron impurities likely present as Fe2+ or Fe3+. Graphite had a relatively featureless, dark, positively sloped spectrum in the near-infrared region with a sharp rise below 0.5 µm due to a Fresnel peak near 0.25 µm caused by strong absorption resulting from π-π* electronic transition. Magnetite had a red and dark overall spectrum containing broad absorption bands near 1 µm as well as weak features centered around 0.55 µm due to Fe2+ and Fe3+. Synthetic troilite's spectrum was relatively featureless, dark, and positively sloped in the near-infrared region but contained an absorption band centered around 0.35 µm attributed to Fe-S charge transfer and crystal field transitions of Fe2+. The researchers also conducted a mixing experiment to simulate how different types of dark materials can affect asteroid surfaces. They poured a known amount of dark material into a glass vial, closed it tightly before shaking it vigorously for one minute before homogenizing it with gentle stirring using a spatula. Comparing their mixtures' spectral slopes with those observed on Bennu showed good agreement which suggests that fine grained dark material including sulfides present in regolith can cause negative (blue) slopes found on B type asteroids providing insight into their composition and evolution processes..