# Impact Disruption of Bjurbôle Porous Chondritic Projectile

Tomas Kohout, Maurizio Pajola, Assi-Johanna Soini, Alice Lucchetti, Arto Luttinen, Alexia Duchêne, Naomi Murdoch, Robert Luther, Nancy L Chabot, Sabina D Raducan, et al.

► To cite this version:

Tomas Kohout, Maurizio Pajola, Assi-Johanna Soini, Alice Lucchetti, Arto Luttinen, et al. Impact Disruption of Bjurbôle Porous Chondritic Projectile. The Planetary Science Journal, 2024, 5 (5), pp.128. 10.3847/PSJ/ad4266. hal-04904997

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# Impact Disruption of Bjurböle Porous Chondritic Projectile

Tomas Kohout<fnref n="1" />,<fnref n="2" /> ID, Maurizio Pajola<fnref n="3" /> ID, Assi-Johanna Soini<fnref n="1" />,<fnref n="4" /> ID, Alice Lucchetti<fnref n="3" /> ID, Arto Luttinen<fnref n="4" /> ID, Alexia Duchêne<fnref n="5" /> ID, Naomi Murdoch<fnref n="5" /> ID, Robert Luther<fnref n="6" /> ID, Nancy L. Chabot<fnref n="7" /> ID, Sabina D. Raducan<fnref n="8" /> ID, Paul Sánchez<fnref n="9" /> ID, Olivier S. Barnouin<fnref n="7" /> ID, and Andrew S. Rivkin<fnref n="7" /> ID

Received 2023 November 22; revised 2024 April 19; accepted 2024 April 19; published 2024 May 31

## Abstract

The ~200 m s⁻<fnref n="1" /> impact of a single 400 kg Bjurböle L/LL ordinary chondrite meteorite onto sea ice resulted in the catastrophic disruption of the projectile. This resulted in a significant fraction of decimeter-sized fragments that exhibit power-law cumulative size and mass distributions. This size range is underrepresented in impact experiments and asteroid boulder studies. The Bjurböle projectile fragments share similarities in shape (sphericity and roughness at small and large scales) with asteroid boulders. However, the mean aspect ratio (3D measurement) and apparent aspect ratio (2D measurement) of the Bjurböle fragments is 0.83 and 0.77, respectively, indicating that Bjurböle fragments are more equidimensional compared to both fragments produced in smaller-scale impact experiments and asteroid boulders. These differences may be attributed either to the fragment source (projectile versus target), to the high porosity and low strength of Bjurböle, to the lower impact velocity compared with typical asteroid collision velocities, or potentially to fragment erosion during sea sediment penetration or cleaning.

*Unified Astronomy Thesaurus concepts: Planetary geology (2288); Meteorites (1038); Chondrites (228); Impact phenomena (779); Asteroids (72)*

## 1. Introduction

Understanding of the impact disruption mechanism of planetary bodies and knowledge of fragment size and shape distributions are important to planetary defense mitigation efforts. They enable the prediction of fragment populations from natural planetary collisions, asteroid deflection, or disruption efforts using kinetic impactors, as well as from impacts of small asteroids on the Earth. They also enable us to determine if an observed fragment population originates from a single impact event and to detect any subsequent fragmentation (e.g., thermal cracking) or sorting (e.g., gravitational transport or rotational ejection) processes altering the original fragment population.

The knowledge of the impact disruption process comes from both observational and experimental studies with the caveat that there is a 2 orders of magnitude gap in the spatial scale that these studies typically cover. The debris of both targets (Fujiwara et al. 1977; Capaccioni et al. 1984, 1986; Davis & Ryan 1990; Durda et al. 2015; Michikami et al. 2016, 2018) and projectiles (Avdellidou et al. 2016; Daly & Schultz 2016; Wickham-Eade et al. 2018; Kline & Hooper 2019) studied during impact experiments are typically in micron–centimeter scale with either mass–frequency distribution or size–frequency distribution functions available, while global observations of boulders and their size–frequency distribution on asteroid

surfaces cover sizes of 10⁰–10¹ m (Michikami et al. 2010, 2019; DellaGiustina et al. 2019; Michikami & Hagermann 2021) with smaller decimeter-sized boulders limited to a few areas with high-resolution imaging available (e.g., Burke et al. 2021). Because a significant fraction of small boulders is partly buried within regolith, their dimensions (especially their major axes) are not well constrained. In contrast, the larger boulders may be positioned by seismic shaking to rest with their major *a* axis being parallel with the surface (Michikami et al. 2010, 2019; Michikami & Hagermann 2021). While the two shorter *b* and *c* axes of big boulders can often be distinguished from 3D models, small boulder *b* and *c* axes are often not properly distinguished due to a lack of sufficient 3D imaging resolution with respect to their scale. Thus, the observed size–frequency distribution function of the small size fraction often apparently deviates from the global value of 10⁰–10¹ m-sized boulders (Michikami et al. 2019). This, together with boulder thermal cracking, micrometeorite bombardment, migration, or ejection processes occurring on asteroids, often creates bias in these studies.

In this study, we aim to present missing information on impact disruption of chondritic material at decimeter scales in order to fill the existing gap in the experimental and observational data. In contrast to previously published work, we will investigate relatively weak and porous chondritic material and focus on catastrophic disruption of the projectile rather than the target. For this purpose, we present data on a unique historic impact disruption event of a single, 400 kg Bjurböle L/LL ordinary chondrite projectile producing a significant fraction of fragments in the decimeter range

<figure><img src="image_3.png" /></figure>

Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ Department of Geosciences and Geography, University of Helsinki, Finland; tomas.kohout@helsinki.fi</footnote>

<footnote n="2">² Institute of Geology of the Czech Academy of Sciences, Czech Republic</footnote>

<footnote n="3">³ INAF Astronomical Observatory of Padova, Padova, Italy</footnote>

<footnote n="4">⁴ Finnish Museum of Natural History, University of Helsinki, Finland</footnote>

<footnote n="5">⁵ Institut Supérieur de l'Aéronautique et de l'Espace (ISAE SUPAERO), Université de Toulouse, France</footnote>

<footnote n="6">⁶ Museum für Naturkunde, Berlin, Germany</footnote>

<footnote n="7">⁷ Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA</footnote>

<footnote n="8">⁸ Space Research and Planetary Sciences, Physikalisches Institut, University of Bern, Switzerland</footnote>

<footnote n="9">⁹ Colorado Center for Astrodynamics Research, University of Colorado Boulder, Boulder, USA</footnote>
(Ramsay & H., 1902). As the mass, volume, and 3D shape of fragments produced by this impact can be fully measured in the laboratory, the uncertainties in fragment dimensions and their orientations are reduced, avoiding the usual uncertainties in these parameters derived from remote sensing.

## 2. Methods

### 2.1. Bjurböle Meteorite Properties

The Bjurböle meteorite is classified as an L/LL4 ordinary chondrite of the S1 shock stage (Database 1994-2017, 2005-2023; Catalogue of Meteorites (MetCat) 2022). Bjurböle is a friable meteorite of high porosity (~20%; Tables 1 and A1; Pesonen et al. 1993; Flynn 2005), above the L/LL average of 5%–9% (Consolmagno et al. 2008), and of low longitudinal wave velocity $v_p$ (1030 m s⁻¹; Flynn 2005), pointing to a relatively low Young's modulus and strength.

### 2.2. Fall Circumstances

Bjurböle meteorite fall occurred in the evening hours of 1899 March 12 and was witnessed by many residents across the Baltic region. Ramsay & Borgström (1902) provide a detailed description of the fall circumstances, of which we provide here a brief summary. The trajectory roughly followed the southern Finnish coastline from west to east. Several breakup events were observed along the luminous fireball trajectory. The terminal mass fell south of the Finnish city of Porvoo (approximately 50 km east of Helsinki) on a frozen sea bay. A single 4 m-sized hole was found in the ice surrounded by an 8 m circle of fractured ice and a 20–25 m rim of splashed water and sediment (Figure 1). The fractured ice and ejecta rims are slightly extended toward the west, indicating an inclined impact from the eastern direction. This is consistent with a deceleration of the terminal mass toward freefall in the dark flight phase and a reverse of its trajectory from westward to eastward by prevailing eastern low-level winds reported on the evening of the fall. The fall area consisted of a ~40 cm thick sea ice sheet over a ~1 m layer of water, followed by an ~8 m thick layer of unconsolidated muddy sea sediment.

Recovery efforts were initialized with the aim to recover the terminal mass from the sea. A wooden casting was constructed around the impact hole, and water with mud was partly pumped out and filtered. The largest fragment recovered from the impact site weighed 80.2 kg and was located within the bottom sediment approximately 6–7 m below the ice surface. Several other fragments in the range of 10–20 kg were also recovered along with countless smaller fragments. Original detailed records of fragment counts and sizes are missing; however, the total recovered mass is reported to have been 328 kg. Ramsay & Borgström (1902) list the four largest recovered fragments as 80.2, 21, 18, and 17 kg. The first two correspond to our Helsinki A3747 (4) and Stockholm 990433 cataloged fragments and clearly appear in multiple historical photographs taken through the years and decades after the fall. However, any other historical or current information about the latter two fragments was not located, nor can their presence be inferred from any historical photographs. Thus, we lean toward the possibility that these fragments never existed and their listing may be erroneous. Ramsay & Borgström (1902) also admit that many small (most likely centimeter-sized or smaller) fragments were missed during the recovery efforts, and additional fragments may have been taken as souvenirs by hired staff

and not reported. Thus, the estimated total mass of the terminal meteorite prior to its impact with ice is possibly around ~400 kg. Based on available records, no meteorite fragments were observed on the sea ice in the vicinity of the impact nor were searched for along the fireball trajectory.

The abovementioned historical evidence implies that all recovered meteorites were part of a single ~400 kg terminal body that decelerated toward freefall velocity and stayed intact upon its impact with the sea ice. The fragmentation occurred only upon initial contact with sea ice and thus can be considered as a single impact/collision event. This fragmentation mechanism leading to the origin of recovered meteorite fragments differentiates Bjurböle from other meteorite falls, where the recovered meteorites result from one or multiple atmospheric impact events and form a strewn field. As mentioned above, atmospheric fragmentation of Bjurböle did occur, but the “atmospheric breakup” fragments most likely ended up on distant sea ice and were not searched for.

### 2.3. Impact Velocity

The freefall impact velocity of the Bjurböle terminal body can be estimated from Equation (1), where the force of gravity on the object on the left equals drag force on the right:

$$mg = \frac{c_d \rho v^2 A}{2}, \quad (1)$$

where m is the object mass (400 kg), g is the acceleration of gravity (9.81 m s⁻²), c_d is the drag coefficient (0.47 for a sphere, 1.05 for a cube), ρ is the medium density (1.280 kg m⁻³), and A is the object cross section (0.322 m² or 0.272 m² for a 400 kg and 2840 kg m⁻³ dense sphere or cube, respectively; Table A1).

Solving Equation (1) for v results in

$$v = \sqrt{\frac{2mg}{c_d A \rho}} \quad (2)$$

and returns ~200 m s⁻¹ for a sphere and ~147 m s⁻¹ for a cube.

### 2.4. Peak Shock Pressure

Peak shock pressure was estimated using the planar impact approximation (Melosh 2013; Figure 2). The Hugoniot pressures are determined for ice using the linear relation between shock-wave speed U and particle speed $u_p$:

$$U = C + S^*u_p, \quad (3)$$

with C being 1317 m s⁻¹ and S being 1.526 at -15°C for the target (ice; Melosh 2013). For the Bjurböle meteorite, the linear relation is not known, but C can be assumed as the value of the bulk speed of sound, 1030 m s⁻¹ (Flynn 2005). S was assumed as 1.42, similar to sandstone, which has similar values of porosity and speed of sound. Larger values of S like for basalt do not significantly affect the result.

### 2.5. Fragment Characterization

For the size and shape distribution study, we gathered information on the size and mass of nearly all Bjurböle meteorites larger than 1 kg reported to be present in collections worldwide. For this, we used the Finnish Natural History
<table><caption>**Table 1**
The Physical Properties of the Studied Bjurböle Meteorites (Mean Values Highlighted in Bold) Together with Information on Fusion Crust Presence</caption><thead><tr><th>Meteorite</th><th>Mass (g)</th><th>Bulk Volume (cm³)</th><th>Bulk Density (g cm⁻³)</th><th>Porosity (%)</th><th>a (mm)</th><th>b (mm)</th><th>c (mm)</th><th>b/a</th><th>c/a</th><th>c/b</th><th>Eq.d. (mm)</th><th>Fusion Crust</th></tr></thead><tbody><tr><td>Helsinki A3747 (4)</td><td>80,200</td><td>29,172</td><td>2.75</td><td>22.1</td><td>518</td><td>385</td><td>335</td><td>0.74</td><td>0.65</td><td>0.87</td><td>378</td><td>One face with irregular patches</td></tr><tr><td>Stockholm 990433</td><td>20,900</td><td>7302</td><td>2.86</td><td>18.9</td><td>344</td><td>290</td><td>199</td><td>0.84</td><td>0.58</td><td>0.69</td><td>241</td><td>Two faces with irregular patches</td></tr><tr><td>Helsinki D6147 (AL79)</td><td>13,480</td><td>4861</td><td>2.77</td><td>21.4</td><td>260</td><td>223</td><td>212</td><td>0.86</td><td>0.82</td><td>0.95</td><td>209</td><td>...</td></tr><tr><td>Helsinki combined D6164 + D6152<sup>b</sup></td><td>10,643</td><td>...</td><td>...</td><td>...</td><td>238</td><td>195</td><td>133</td><td>0.82</td><td>0.56</td><td>0.68</td><td>193</td><td>Two faces</td></tr><tr><td>Helsinki D6149 (AL86 / 33 5413)</td><td>5450</td><td>1877</td><td>2.90</td><td>17.8</td><td>179</td><td>167</td><td>148</td><td>0.93</td><td>0.83</td><td>0.89</td><td>154</td><td>7 cm patch</td></tr><tr><td>Helsinki D6150 (TK1)</td><td>5300</td><td>1863</td><td>2.85</td><td>19.4</td><td>203</td><td>186</td><td>116</td><td>0.92</td><td>0.57</td><td>0.62</td><td>153</td><td>10 × 3 cm patch</td></tr><tr><td>Helsinki D6148 (AL81)</td><td>4350</td><td>1518</td><td>2.86</td><td>18.8</td><td>200</td><td>170</td><td>131</td><td>0.85</td><td>0.66</td><td>0.77</td><td>143</td><td>5 cm patch on one face</td></tr><tr><td>Kettunen-Moilanen RH80 (Haag 107)<sup>b</sup></td><td>3300</td><td>...</td><td>...</td><td>...</td><td>160</td><td>142</td><td>116</td><td>0.89</td><td>0.73</td><td>0.82</td><td>130</td><td>...</td></tr><tr><td>Helsinki D6163 (3130 / 26)</td><td>3113.7</td><td>1083</td><td>2.88</td><td>18.5</td><td>163</td><td>132</td><td>118</td><td>0.81</td><td>0.72</td><td>0.89</td><td>128</td><td>...</td></tr><tr><td>Chicago ME 1428 #4</td><td>2995.5</td><td>...</td><td>...</td><td>...</td><td>155</td><td>120</td><td>110</td><td>0.77</td><td>0.71</td><td>0.92</td><td>126</td><td>One face</td></tr><tr><td>St Petersburg Univ. Geol. 4371</td><td>2693</td><td>...</td><td>...</td><td>...</td><td>160</td><td>120</td><td>100</td><td>0.75</td><td>0.63</td><td>0.83</td><td>122</td><td>3 cm patch</td></tr><tr><td>Oslo MET 5</td><td>2095</td><td>...</td><td>...</td><td>...</td><td>160</td><td>115</td><td>85</td><td>0.72</td><td>0.53</td><td>0.74</td><td>112</td><td>One face + 4 cm patch</td></tr><tr><td>Vienna NHMW-H1462</td><td>1939.4</td><td>...</td><td>...</td><td>...</td><td>140</td><td>115</td><td>80</td><td>0.82</td><td>0.57</td><td>0.70</td><td>109</td><td>...</td></tr><tr><td>Helsinki D6151 (AL2 27)</td><td>1879</td><td>665</td><td>2.82</td><td>20.0</td><td>149</td><td>116</td><td>87</td><td>0.78</td><td>0.58</td><td>0.75</td><td>108</td><td>...</td></tr><tr><td>Helsinki D6146 (AL68)</td><td>1809.4</td><td>639</td><td>2.83</td><td>19.8</td><td>153</td><td>133</td><td>88</td><td>0.87</td><td>0.58</td><td>0.66</td><td>107</td><td>...</td></tr><tr><td>Killgore CH-1-C</td><td>1788</td><td>...</td><td>...</td><td>...</td><td>166</td><td>130</td><td>62</td><td>0.78</td><td>0.37</td><td>0.48</td><td>106</td><td>2 cm patch</td></tr><tr><td>Helsinki D6144 (AL32 / 143)</td><td>1704.8</td><td>590</td><td>2.89</td><td>18.1</td><td>161</td><td>97</td><td>90</td><td>0.60</td><td>0.56</td><td>0.93</td><td>105</td><td>...</td></tr><tr><td>Chicago ME 1427 #1</td><td>1611.82</td><td>...</td><td>...</td><td>...</td><td>140</td><td>110</td><td>80</td><td>0.79</td><td>0.57</td><td>0.73</td><td>103</td><td>...</td></tr><tr><td>Oslo MET 6</td><td>1570</td><td>...</td><td>...</td><td>...</td><td>135</td><td>120</td><td>110</td><td>0.89</td><td>0.81</td><td>0.92</td><td>102</td><td>One face</td></tr><tr><td>Senckenberg MPI 235/10</td><td>1495</td><td>...</td><td>...</td><td>...</td><td>135</td><td>105</td><td>80</td><td>0.78</td><td>0.59</td><td>0.76</td><td>100</td><td>...</td></tr><tr><td>St Petersburg mining museum 92-1</td><td>1441</td><td>...</td><td>...</td><td>...</td><td>140</td><td>100</td><td>80</td><td>0.71</td><td>0.57</td><td>0.80</td><td>99</td><td>3 cm patch</td></tr><tr><td>Porvoo 9184</td><td>1432.8</td><td>504</td><td>2.84</td><td>19.4</td><td>126</td><td>124</td><td>86</td><td>0.98</td><td>0.68</td><td>0.69</td><td>99</td><td>Two 2 cm patches</td></tr><tr><td>Aalto 4606<sup>a</sup></td><td>1364.7</td><td>469</td><td>2.91</td><td>17.5</td><td>126</td><td>98</td><td>92</td><td>0.78</td><td>0.73</td><td>0.94</td><td>97</td><td>7 × 5 cm patch</td></tr><tr><td>Rose D5504</td><td>1365</td><td>...</td><td>...</td><td>...</td><td>110</td><td>90</td><td>80</td><td>0.82</td><td>0.73</td><td>0.89</td><td>97</td><td>...</td></tr><tr><td>Helsinki D6145 (AL67 / 152)</td><td>1358.6</td><td>474</td><td>2.86</td><td>18.9</td><td>124</td><td>117</td><td>70</td><td>0.94</td><td>0.56</td><td>0.60</td><td>97</td><td>...</td></tr><tr><td>Cluj 1.308</td><td>1300</td><td>...</td><td>...</td><td>...</td><td>120</td><td>110</td><td>80</td><td>0.92</td><td>0.67</td><td>0.73</td><td>96</td><td>One face</td></tr><tr><td>Helsinki D6143 (AL31 / 144)</td><td>1237.9</td><td>442</td><td>2.80</td><td>20.7</td><td>109</td><td>104</td><td>90</td><td>0.95</td><td>0.83</td><td>0.87</td><td>94</td><td>...</td></tr><tr><td>Stockholm 540022</td><td>1176.7</td><td>405</td><td>2.91</td><td>17.7</td><td>94</td><td>93</td><td>93</td><td>0.99</td><td>0.99</td><td>1.00</td><td>92</td><td>...</td></tr><tr><td>Oxford OUMNH-Mt006</td><td>1168</td><td>...</td><td>...</td><td>...</td><td>104</td><td>89</td><td>92</td><td>0.86</td><td>0.88</td><td>1.03</td><td>92</td><td>10 × 2 cm patch</td></tr><tr><td>Bartoschewitz BC8.2</td><td>1157</td><td>...</td><td>...</td><td>...</td><td>130</td><td>90</td><td>70</td><td>0.69</td><td>0.54</td><td>0.78</td><td>92</td><td>...</td></tr><tr><td>Vernadsky 87</td><td>1135.5</td><td>...</td><td>...</td><td>...</td><td>110</td><td>100</td><td>80</td><td>0.91</td><td>0.73</td><td>0.80</td><td>91</td><td>One face with irregular patch</td></tr><tr><td>Cureton 1113<sup>a</sup></td><td>1113</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>91</td><td>...</td></tr><tr><td>Vernadsky 15155</td><td>1090</td><td>...</td><td>...</td><td>...</td><td>110</td><td>100</td><td>75</td><td>0.91</td><td>0.68</td><td>0.75</td><td>90</td><td>...</td></tr><tr><td>Porvoo 88-38</td><td>1069.6</td><td>377</td><td>2.83</td><td>19.7</td><td>117</td><td>96</td><td>62</td><td>0.82</td><td>0.53</td><td>0.65</td><td>90</td><td>9 × 1 cm and 2 × 1 cm patches</td></tr><tr><td>Smithsonian USNM695</td><td>1045</td><td>...</td><td>...</td><td>...</td><td>75</td><td>72</td><td>69</td><td>0.96</td><td>0.92</td><td>0.96</td><td>89</td><td>...</td></tr><tr><td>Paris 2259don</td><td>1043.2</td><td>...</td><td>...</td><td>...</td><td>100</td><td>90</td><td>70</td><td>0.90</td><td>0.70</td><td>0.78</td><td>89</td><td>...</td></tr><tr><td><strong>Mean</strong></td><td></td><td></td><td><strong>2.85</strong></td><td></td><td></td><td></td><td></td><td><strong>0.84</strong></td><td><strong>0.67</strong></td><td><strong>0.80</strong></td><td></td><td></td></tr><tr><td>s.d.</td><td></td><td></td><td>0.05</td><td>1.4</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Helsinki D6164 (AL80 / 5318)</td><td>5150</td><td>1819</td><td>2.83</td><td>19.8</td><td>238</td><td>195</td><td>125</td><td>...</td><td>...</td><td>...</td><td>...</td><td>Two faces</td></tr><tr><td>Helsinki D6152 (AL80)</td><td>4500</td><td>1586</td><td>2.84</td><td>19.6</td><td>220</td><td>187</td><td>133</td><td>...</td><td>...</td><td>...</td><td>...</td><td>Two faces</td></tr><tr><td>Kettuen RH80 (Haag 107) without corner</td><td>2977.6</td><td>1010</td><td>2.95</td><td>16.5</td><td>148</td><td>142</td><td>116</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td>Moilanen RH80 (Haag 107) detached corner</td><td>N/A</td><td>N/A</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td></tr><tr><td><strong>Mean</strong></td><td></td><td></td><td><strong>2.85</strong></td><td></td><td></td><td></td><td></td><td><strong>0.84</strong></td><td><strong>0.67</strong></td><td><strong>0.80</strong></td><td></td><td></td></tr><tr><td>s.d.</td><td></td><td></td><td>0.05</td><td>1.4</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Notes. The bulk volume, bulk density, and porosity are only available for the laser-scanned samples.

a Current location and information on dimensions is unknown.

b Sample split. Individual fragments are listed at the end of the table.
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Sketch of the fall area (left), photo (center), and a vertical section (right). From Ramsay & Borgström (1902), modified.</figcaption></figure>

Museum (LUOMUS) historical records as well as the meteorite list in the MetBase database with 36 samples reported. The meteorite list is presented in Table 1. The size and mass of the meteorites in the LUOMUS collection as well as of these from the Aalto University, Kettunen collections, Porvoo Museum, Swedish Museum of Natural History (17 in total) were determined from 3D models and using calibrated scales (mass reported with valid digits of the calibration), respectively. For 3D model measurements, a NextEngine 3D Scanner Ultra HD model 2020i laser scanner was used, and for shape reconstruction, NextEngine ScanStudio software was used. The volume and bounding-box dimensions were measured in Geomagic Verify Viewer software. The dimensions of the bounding box were used as fragment major *a*, minor *c*, and intermediate *b* perpendicular axes (*a* ≥ *b* ≥ *c*). Axis dimensions and masses of the other samples dispersed around the world were measured manually and provided upon request by the museum curators or owners.

The largest 80.2 kg Bjurböle sample is currently partly embedded in a plaster support (lower ~5 cm). Thus, its vertical dimension and mass were not possible to determine. However, a high-fidelity plaster model of the meteorite exists. We scanned both the real meteorite and its plaster model and verified that both horizontal dimensions agree within 1.8 (*a*) and 6.6 (*b*) cm, resulting in estimated measurement error of ~3% (*a*) and 17% (*b*). Horizontal dimensions were derived from the real meteorite, while the vertical (*c*) dimension and volume were derived from the plaster 3D model. As the mass of the sample, we used the 80.2 kg value found in the LUOMUS catalog.

In some of the experiments or the asteroid observations reported in the literature, the fragment dimensions are measured from 2D images by fitting a bounding box or ellipse where it is difficult to distinguish the intermediate *b* and the minor *c* axes from each other as well as to estimate the distortion of the major *a* axes due to fragment tilt. In this case, we refer to these values in the discussion as to apparent *a'* and *b'* dimensions and their ratio as to apparent aspect ratio *b*/*a*'. To simulate the apparent dimensions in 2D with Bjurböle samples, we determined the apparent aspect ratio (apparent *b*/ *a*' ratio) as the average bounding-box width/length of three random 2D orthogonal projections of the 3D Bjurböle shape models.

To mimic asteroid boulder observations, the random 2D orthogonal projections of the 3D Bjurböle shape models were also used to analyze several nondimensional shape and morphological parameters such as the roundness, circle ratio

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Peak shock pressure estimation using the planar impact approximation. The red curve shows the Hugoniot curve for ice. The blue curves show the Hugoniot curve estimates for the Bjurböle meteorite and are plotted for impact speeds of 147 m s⁻¹ (cube) and 201 m s⁻¹ (sphere).</figcaption></figure>

sphericity, circularity, solidity, and equivalent diameter using the 2D shape processing pipeline described in Robin et al. (2024). These morphological descriptors are defined in Table 2. The circle ratio sphericity is the radius ratio of the maximum inscribed and the minimum circumscribing circle. The roundness measures the curvature of corners in a particle; a value of 0 and 1 indicates a very angular and rounded particle, respectively. The circularity and solidity indicate the large- and small-scale roughness, respectively. The circularity is a perimeter ratio, and the solidity refers to the convexity of the particle. Using the morphology analysis derived from the 2D projections of the shape models enables removing the bias of the 3D representation compared to other results obtained from 2D morphology analysis.

Some of these parameters are dependent on the resolution (e.g., the particle is resolved by an insufficient number of pixels), as for the roundness (Zheng 2015) and circularity (Nagaashi et al. 2021). Both the 3D representation and its 2D projections have been acquired at high resolution
<table><caption>**Table 2**

Morphological Descriptors Computed for the 2D Projection Analyses with Their Equation, Definition, and Reference (if Applicable)</caption><thead><tr><th>Morphological Descriptor</th><th>Equation</th><th>Variable Definition</th><th>Reference</th></tr></thead><tbody><tr><td>Circle ratio sphericity</td><td>R<sub>ins</sub>/R<sub>cir</sub></td><td>R<sub>ins</sub>, the radius of the maximum inscribed circle; R<sub>cir</sub>, the radius of the minimum circumscribed circle</td><td>Riley (1941)</td></tr><tr><td>Roundness</td><td>ΣN/2r<sub>i</sub>/N</td><td>N, the number of corners; r<sub>i</sub>, the radius associated with each corner</td><td>Wadell (1932)</td></tr><tr><td>Circularity</td><td>R<sub>ins</sub>/4πA</td><td>A, the projected area; P, the perimeter of the particle</td><td>Cox (1927)</td></tr><tr><td>Solidity</td><td>A/H</td><td>H, the area of the convex hull</td><td>Mora & Kwan (2000)</td></tr><tr><td>Apparent aspect ratio</td><td>b'/a'</td><td>b', the width (smaller axes of the 2D bounding box); a', the length (longest axes of the 2D bounding box)</td><td>...</td></tr><tr><td>Equivalent diameter</td><td>2√A/π</td><td>A, the area of the projected shape</td><td>...</td></tr></tbody></table>

(0.1–0.01 mm pixel⁻¹ for the 2D projections) and enable the morphological analysis to be free of any biases resulting from the low-resolution effect.

The volume and mass of the meteorites were used to calculate their bulk density. We report new bulk density measurements of 27 Bjurböle f meteorites ranging in mass from 12.25 g up to 80.2 kg (Table 1 with meteorites larger than 1 kg; Table A1 with all meteorites and density statistics). Four smaller Bjurböle meteorites (Helsinki small 1–4) ranging from 17.27 g up to 21.62 g were also used for grain density measurement using a Quantachrome Ultrapyc 1200e gas pycnometer and N₂ gas. The average grain density of 3.53 g cm⁻³ of these small samples agrees within the standard deviation (s.d.) with the mean grain density of L and LL chondrites (3.51 ± 0.11 g cm⁻³ and 3.48 ± 0.08 g cm⁻³, respectively) and was used to calculate the porosity of the larger Bjurböle samples that did not fit the gas pycnometer chamber.

For comparison, additionally, we used the average bulk density of 3.29 g cm⁻³ and grain density of 3.47 g cm⁻³ of 40 smaller Chelyabinsk samples (average of all lithologies combined) ranging in mass from 4.35 g up to 310.60 g with an average mass of 54.40 g reported in Kohout et al. (2014) together with bulk volumes and masses of three additional larger samples presented in Kohout et al. (2017; Table A1).

## 2.6. Power-law Fit

A power-law fit of the major *a* axis and mass values was done following the Clauset et al. (2009) technique, which validates the existence of the power law. The method returns a scaling parameter *α* (power-law index) and a completeness limit *x*<sub>min</sub>. The estimation of *x*<sub>min</sub> is done through the Kolmogorov–Smirnov (KS) statistic and allows one to find the *x*<sub>min</sub> value minimizing it. Afterward, the parameter is determined through the maximum-likelihood estimator. The uncertainty for both *α* and *x*<sub>min</sub> is then derived through a nonparametric bootstrap procedure that generates a large number of synthetic data sets from a power-law random generator and performs a number of KS tests to verify if the generated and observed data come from the same distribution. This technique returns a *p*-value that can be used to quantify the plausibility of the hypothesis. Considering the significance level of 0.10, if the *p*-value is >0.1, then it is possible to conclude that any difference between the empirical data and the model can be explained with statistical fluctuations. On the contrary, if the *p*-value is <0.1, then the data set does not come from a power-law distribution but, instead, from a different one.

# 3. Results

## 3.1. Impact Circumstances

Freefall calculation for the ~400 kg terminal Bjurböle body estimates an impact velocity at the end of dark flight in the approximate range of 145–200 m s⁻¹ for cubic and spherical shapes. The associated peak shock pressure reached within the Bjurböle projectile is ~140–200 MPa. The ratio of the largest surviving fragment (80.2 kg) to the projectile mass (~400 kg) is ~0.2. There is no indication of significant bulk density and porosity variations with fragment size from ~10 g up to the largest 80.2 kg, indicating high homogeneity within the Bjurböle material at a wide size range. Such high homogeneity in physical properties is similar to what is observed in the Chelyabinsk LL fall, known for the coexistence of various shock lithologies, where a comparably wide size range of fragments was studied (Kohout et al. 2017; Figure 3).

## 3.2. Fragment Dimensions and Shapes

The results on fragment dimensions, mass, and bulk and grain density are summarized in Table 1. We managed to confirm masses of 36 and dimensions of 35 (to us) known Bjurböle fragments larger than 1 kg (totaling 188 kg), out of which 16 were laser-scanned and 19 were measured by hand with a measurement tape. Additionally, we laser-scanned three fragments listed at the end of Table 1 that were accidentally broken during curation, as detailed below.

In one case, it was inferred from LUOMUS historical records that the fragments Helsinki D6164 and D6152 are broken pieces of a larger fragment with a catalog mass of 10.643 kg. In the dimension ratio and frequency mass–size distribution analyses, we list this sample only once, with each dimension represented by the estimated combined value and combined catalog mass (being approximately 1 kg larger than the current mass of surviving fragments, most likely due to additional mass loss). For bulk, density, and porosity statistics, we use individual fragment masses and volumes.

In another case, the sample RH80 (Haag 107; Robert Haag being the previous owner) had a recent corner split, and Jarkko Kettunen and Jarmo Moilanen are the current owners of the two parts. Thus, we similarly used the original combined dimensions and mass prior to the split for dimension ratio and frequency mass and size distribution analyses, while the new laser scan of the current larger part of Kettunen RH80 was used for density and porosity statistics.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Bulk densities and porosities of Bjurbôle samples as a function of mass. Chelyabinsk data (Kohout et al. 2014, 2017) are plotted for comparison.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Dimension ratio *b/a*, *c/a*, and *c/b* (left) and apparent aspect ratio (right) distribution histograms of Bjurbôle fragments larger than 1 kg.</figcaption></figure>

The Porvoo 9184 fragment also has a detached corner. We were able to perform its laser scan with the corner in its original position, obtaining representative results for its original shape.

Besides these three documented cases and the occasional observed detachment of corners, we did not find a record of significant fragment splits. We cannot rule out such historical occurrences. However, if these occur, it will happen along a preexisting loose crack on the verge of natural fragmentation anyway. We do not assume this occurred on large scales in the past, and possible mass loss on the order of a few percent does not significantly affect our results.

The sample Cureton 1113 is listed in the LUOMUS historical catalog to be transferred to the Curetons collections. Unfortunately, we did not find any information about the current location of the sample, and thus we cannot measure its dimensions. However, we use the listed mass value in the fragment cumulative mass distribution statistics.

We did an extensive search for additional fragments in private collections, including requests for information directed to renowned meteorite collectors and public announcements in the fall area through press releases and local social media groups. Despite these efforts, we were not able to locate additional fragments.

The dimension ratios *b/a*, *c/a*, and *c/b*, together with apparent aspect ratio distribution histograms, are presented in Figure 4. The mean *b/a* ratio is 0.84 with an s.d. of 0.09, the mean *c/a* ratio is 0.67 with an s.d. of 0.13, and the mean *c/b* ratio is 0.80 with an s.d. of 0.12. All dimension ratios show unimodal distribution, and the range of the apparent aspect ratio values is narrow centered toward left (lower values). The other shape and morphological parameters such as apparent aspect ratio, circularity, equivalent diameter, roundness, circle ratio sphericity, and solidity determined from 2D surface projections of the 15 laser-scanned Bjurbôle fragments are listed in Table 3.

### 3.3. Fragment Cumulative Size and Mass Distribution

For consistency with earlier asteroid boulder studies or impact disruption experiments, we choose the meteorites' major *a* dimension and mass, respectively, to plot these against their cumulative number in a log-log space (Figure 5) and use the Clauset et al. (2009) technique to validate the applicability of the power-law fit. In the case of the major *a* dimension, the resulting “size” scaling parameter *α* (also called the power-law index) is −2.7 ± 1.4, and the completeness limit *x*<sub>min</sub> is 130 ± 23 mm. In the case of mass, the “mass” *α* is −1.0 ± 0.4,
<table><caption>**Table 3**

Shape and Morphological Parameters Determined from 2D Surface Projections of 15 Laser-scanned Bjurböle Meteorites (Mean Values Highlighted in Bold), Together with Asteroid Range (Highlighted in Bold) from Robin et al. (2024)</caption><thead><tr><th>Meteorite</th><th>Equivalent Diameter (mm)</th><th>Apparent Aspect Ratio</th><th>Roundness</th><th>Sphericity</th><th>Circularity</th><th>Solidity</th></tr></thead><tbody><tr><td>Helsinki D6143 (AL31 / 144)</td><td>100</td><td>0.92</td><td>0.34</td><td>0.73</td><td>0.85</td><td>0.97</td></tr><tr><td>Helsinki D6144 (AL32 / 143)</td><td>108</td><td>0.69</td><td>0.31</td><td>0.60</td><td>0.77</td><td>0.96</td></tr><tr><td>Helsinki D6145 (AL67 / 152)</td><td>101</td><td>0.68</td><td>0.36</td><td>0.60</td><td>0.80</td><td>0.98</td></tr><tr><td>Helsinki D6149 (AL86 / 33 5413)</td><td>161</td><td>0.78</td><td>0.36</td><td>0.70</td><td>0.87</td><td>0.98</td></tr><tr><td>Stockholm 540022</td><td>95</td><td>0.99</td><td>0.37</td><td>0.82</td><td>0.86</td><td>0.98</td></tr><tr><td>Stockholm 990433</td><td>260</td><td>0.80</td><td>0.30</td><td>0.63</td><td>0.80</td><td>0.97</td></tr><tr><td>Helsinki D6151 (AL2 27)</td><td>113</td><td>0.69</td><td>0.29</td><td>0.62</td><td>0.79</td><td>0.97</td></tr><tr><td>Helsinki D6163 (3130 26)</td><td>134</td><td>0.84</td><td>0.35</td><td>0.72</td><td>0.84</td><td>0.98</td></tr><tr><td>Porvoo 88-38</td><td>88</td><td>0.66</td><td>0.29</td><td>0.66</td><td>0.80</td><td>0.98</td></tr><tr><td>Porvoo 9184</td><td>103</td><td>0.71</td><td>0.24</td><td>0.59</td><td>0.80</td><td>0.96</td></tr><tr><td>Helsinki A3747 (4)</td><td>407</td><td>0.69</td><td>0.33</td><td>0.63</td><td>0.82</td><td>0.97</td></tr><tr><td>Helsinki D6146 (AL68)</td><td>116</td><td>0.65</td><td>0.27</td><td>0.58</td><td>0.78</td><td>0.96</td></tr><tr><td>Helsinki D6147 (AL79)</td><td>226</td><td>0.90</td><td>0.32</td><td>0.76</td><td>0.83</td><td>0.97</td></tr><tr><td>Helsinki D6148 (AL81)</td><td>154</td><td>0.87</td><td>0.38</td><td>0.66</td><td>0.83</td><td>0.97</td></tr><tr><td>Helsinki D6150 (TK1)</td><td>161</td><td>0.70</td><td>0.36</td><td>0.64</td><td>0.84</td><td>0.98</td></tr><tr><td><strong>Mean</strong></td><td><strong>155</strong></td><td><strong>0.77</strong></td><td><strong>0.32</strong></td><td><strong>0.66</strong></td><td><strong>0.82</strong></td><td><strong>0.97</strong></td></tr><tr><td>s.d.</td><td>86</td><td>0.11</td><td>0.04</td><td>0.07</td><td>0.03</td><td>0.01</td></tr><tr><td><strong>Asteroid boulder range</strong></td><td>...</td><td><strong>0.70–0.75</strong></td><td><strong>&lt;0.56</strong></td><td><strong>0.61–0.67</strong></td><td><strong>0.84–0.90</strong></td><td><strong>0.95–0.96</strong></td></tr></tbody></table>

and the completeness limit $x_{min}$ is 1135 ± 435 g. Considering the significance level of 0.1, the *p*-values we obtained computed from 5000 KS statistical tests are 0.78 and 0.75, respectively. This means that the data set comes from a power-law distribution, and any difference between the empirical data and the model can be explained solely with statistical fluctuations. Figure 5 also depicts the frequency plot of the two data sets, together with the mode, median, mean, and other parameters of the distribution.

The $x_{min}$ of both data sets corresponds to a cumulative count of ~22–24, meaning that in terms of dimension as well as of mass, ~80% of the meteorites fit the power-law distribution.

## 4. Discussion

Bulk density measurements of all Bjurböle meteorites (Table A1) reveal that they are homogeneous from centimeter (10 g) up to decimeter (80 kg) size ranges (Figure 3) with the mean value of $2.84 \text{ g cm}^{-3}$ (s.d. 0.05) being lower than the mean L ($3.36 \text{ g cm}^{-3}$) and LL ($3.54 \text{ g cm}^{-3}$) values (Consolmagno et al. 2008) due to Bjurböle's above-average porosity of 19.6% (s.d. 1.5). The ratio of 0.2 of the largest surviving Bjurböle meteorite to the projectile (terminal body) mass meets the catastrophic disruption threshold of 0.5 (Wickham-Eade et al. 2018). Thus, impact conditions such as impacting velocity (even though it is subsonic with respect to material $v_p$ of $1030 \text{ m s}^{-1}$) and peak shock pressure were sufficient to catastrophically disrupt the projectile represented by the Bjurböle terminal body in a single event, and the studied Bjurböle meteorites are representative of projectile catastrophic disruption.

There are three unique factors in our Bjurböle fragment study where current data sets are sparse or nonexistent. First, the Bjurböle meteorite breakup represents an opportunity to study collision fragments in the decimeter size range positioned in between experimentally produced fragments (below ~centimeter) and asteroid boulders (above ~meter). Second, Bjurböle is a chondritic impactor, more relevant to planetary collisions when compared to basaltic, shale, plastic, or metallic impactors used in experiments. Third, the Bjurböle material is

relatively weak and porous compared to other rocky materials (e.g., basalt, shale) used in most previous projectile breakup studies providing data on friable material breakup.

Nevertheless, we are aware that two factors may limit the applicability of our results. First, the estimated terminal impact velocity of $145–200 \text{ m s}^{-1}$ is roughly 25–35 times lower than the typical $5 \text{ km s}^{-1}$ collision velocity within the asteroid belt (Bottke et al. 1994). In our case, this is advantageous, as it is sufficient to achieve catastrophic mechanical breakup of the terminal body and at the same time avoid internal pore collapse, impact melting, or other shock features associated with higher impact velocities (Guldemeister et al. 2022). Second, there is an uncertainty in the Bjurböle impactor original shape. However, Durda et al. (2015), Michikami et al. (2016), and Wickham-Eade et al. (2018) reported that the projectile or target shape have little effect on fragment shapes or sizes, if any at all.

An interesting aspect of our results is the shapes and dimension ratios of the Bjurböle fragments. The mean *b/a* and *c/a* values (0.84 and 0.67, respectively) are slightly higher compared to those reported from target catastrophic disruption experiments (~0.7 and ~0.5, respectively) for a wide range of material compositions and porosities (Fujiwara et al. 1978; Lange & Ahrens 1981; Capaccioni et al. 1984; Michikami et al. 2016), meaning that our projectile fragments are closer to being equidimensional than typical target disruption fragments. This trend is also supported by a slightly larger mean apparent aspect ratio (0.77 with an s.d. of 0.11) compared to the mean values of boulders observed on the (65803) Didymos I Dimorphos (secondary), (25143) Itokawa, (162173) Ryugu, and (101955) Bennu asteroids (compare to the asteroid range of 0.70–0.75 in Table 3 based on bounding-box-derived data by Robin et al. 2024). The apparent aspect ratio of these boulders on the surface of asteroids has been suggested to be formed by catastrophic disruption (Michikami et al. 2010, 2019; Robin et al. 2024).

Morphological parameters such as circularity (~0.81 with an s.d. of 0.05, large-scale roughness indicator) being slightly smaller, and solidity (~0.97 with an s.d. of 0.02, small-scale
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Power-law fit (left) and frequency and cumulative plots (right) of the major *a* dimension (top) and mass (bottom) distribution of the Bjurbôle meteorites larger than 1 kg, together with the best-fit parameters.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Comparison of Bjurbôle fragment compactness and solidity to those observed on asteroid boulders (Robin et al. 2024).</figcaption></figure>

roughness indicator) being slightly higher compared to the mean asteroid values, reveal rather less concave shape of the Bjurbôle fragments (Figure 6, Table 3; asteroid range based on data by Robin et al. 2024). However, the mean circle ratio sphericity of the Bjurbôle fragments is in the same range as

boulders on the surface of asteroids (Table 3), indicating a similar boulder elongation. The mean roundness of the Bjurbôle fragments indicates a rather angular medium (0.33 with an s.d of 0.04), which suggests that the fragments underwent limited alteration processes after the fragmentation
at impact. The comparison with asteroid boulders is difficult, as the roundness value obtained for these boulders only provides a higher limit (<0.56; Robin et al. 2024) due to the lower image resolution of asteroid surfaces.

Asteroid boulders are most likely predominantly composed of target fragments but may contain a significant fraction of projectile fragments depending on the collision event circumstances. The fragment apparent aspect ratio derived from random 2D projections is a suitable proxy to similar asteroid boulder measurements from 2D images. Indeed, the Bjurböle mean apparent aspect ratio value of 0.77 is slightly higher than those for asteroid Ryugu (~0.70), (433) Eros (~0.72), Itokawa (~0.62–0.74, depending on methodology), Bennu (~0.72), and Dimorphos (~0.60–0.70), boulder values reported in Michikami et al. (2010, 2019), Michikami & Hagermann (2021), and Pajola et al. (2024), despite the asteroid boulders being 1 (Ryugu, Itokawa, Bennu, Dimorphos) or 2 (Eros) orders of magnitude larger than our Bjurböle meteorite fragments. The abovementioned asteroid values are in between the Bjurböle apparent aspect ratio value of 0.77 and the mean true c/a ratio of 0.67 and often show a bimodal apparent aspect ratio value distribution (Michikami et al. 2010; Pajola et al. 2024), similar to our Bjurböle c/a histogram showing a long asymmetric shoulder toward larger values (Figure 4). This observation suggests that some boulders on these asteroids may be resting on their a–c plane. On the contrary, boulders on the asteroids with higher apparent aspect ratio values and unimodal distribution (similar to that of the Bjurböle b/a values) may rest rather on their a–b plane. Asteroid (65803) Didymos (primary) shows a high boulder (<16.5 m) mean apparent aspect ratio value of 0.86 (Pajola et al. 2024), slightly higher than that of the mean Bjurböle b/a ratio, meaning that Didymos's boulders are more equidimensional compared to Bjurböle or the boulders of Dimorphos or other asteroids. This is an interesting observation given the generally accepted conclusion (Barnouin et al. 2024) that Dimorphos was formed from mass shedding and gravitational reaccumulation of material originating from Didymos, itself a member of the Baptistina asteroid family, and may indicate the presence of a fragment sorting mechanism influencing Dimorphos's formation (Pajola et al. 2024).

We admit that the abovementioned interpretations are speculative and require future detailed 3D study of asteroid boulders. We note that Michikami et al. (2010, 2019), Michikami & Hagermann (2021), and Pajola et al. (2024) observe a systematic boulder apparent aspect ratio increase with decreasing size and attribute this to reorientation of smaller boulders on the surface due to impact shaking. The relatively high Bjurböle apparent aspect ratio values that we observe at decimeter scales highlight a potentially important difference between collisional target and projectile (Bjurböle meteorite) disruption. This is supported by the Bjurböle mean apparent aspect ratio value being a good match to the mean apparent aspect ratio values of submillimeter projectile fragments (10¹⁻¹⁰ µm) reported by Wickham-Eade et al. (2018) for impact velocities below 1 km s⁻¹. The general trend of higher apparent aspect ratio is more pronounced for the weaker shale material compared to the stronger basalt. Our results validate this trend up to the decimeter size range. This observation is further supported by Bjurböle's relatively higher sphericity and lower eccentricity. Michikami et al. (2010) point out that boulders in the 0.1–5 m range identified in high-resolution Itokawa images (being similar in size to our fragments) appear less elongated

with statistically higher apparent b/a ratios compared to the global average of larger boulders, indicating that a similar trend may exist with ordinary chondrite asteroid boulders.

Another effect impacting Bjurböle meteorite morphology and shapes can be either the presence of a fusion crust or fragment erosion during penetration of the sea sediment or subsequent fragment cleaning. Bjurböle meteorites contain very few irregular fusion crust patches, mainly on the larger fragments (Table 1 and photos in online repository), as the breakup occurred during collision with sea ice rather than during hypervelocity atmospheric entry. Thus, atmospheric ablation cannot itself explain the observed differences. The fragment erosion may cause an observed change toward a smoother and rounder appearance. However, its extent is unknown and impossible to quantify.

The Bjurböle meteorite cumulative “size” distribution (a axis) power index α of –2.7 is below the value of –2, which is considered to be indicative of prime catastrophic disruption with little subsequent fragmentation (Michikami & Hagermann 2021) and is comparable to experiments with basalt projectiles impacting at velocities below 1 km s⁻¹ (Wickham-Eade et al. 2018) or slightly steeper compared to the one reported in Avdellidou et al. (2016) for the impact of rocky projectiles onto ice targets. Our Bjurböle meteorites are naturally free of thermal cracking, fatigue, or other natural fragmentation or accumulation processes besides accidental or intentional specimen breakup during handling consistent with shallow α values even at small sizes. Similarly, cumulative “mass” distribution α values around ~1 are reported for target catastrophic disruption in impact experiments (Fujiwara et al. 1977; Davis & Ryan 1990; Durda et al. 2015; Michikami et al. 2016). The break in the power-law fits at 130 mm or 1135 g as indicated by x_min may be caused either by the incompleteness of our fragment set below this limit or by the change in impact regime (Takagi et al. 1984).

From asteroid observations, a similar cumulative “size” distribution α value of –3.05 ± 0.14 was reported for global Itokawa (Michikami & Hagermann 2021), –2.62 ± 0.19 for the Itokawa “head” (Michikami & Hagermann 2021), –2.66 ± 0.05 for asteroid Ryugu (Michikami & Hagermann 2021), –2.9 ± 0.3 for asteroid Bennu (DellaGiustina et al. 2019), and –3.4 ± 1.2 for Dimorphos (Pajola et al. 2024). The shallower Itokawa “head” α value is explained by fine-material migration occurring on Itokawa toward accumulation areas on its “neck” and “body” (Michikami & Hagermann 2021), collisional size segregation (Shinbrot et al. 2017), or seismic segregation (Sánchez et al. 2022).

Higher cumulative “size” distribution α values are associated with higher projectile impact velocities (Wickham-Eade et al. 2018) or are indicative of other processes responsible for subsequent fragmentation or material sorting. Some asteroids indeed show higher values, such as –3.25 ± 0.14 for Eros (Michikami & Hagermann 2021) or –3.6 ± 0.7 for Didymos (Pajola et al. 2024). The origin of the detected large boulders on Eros is assumed to be related to the large impact creating Shoemaker crater (Michikami & Hagermann 2021). On the other hand, while there is evidence of thermal cracking on Bennu (DellaGiustina et al. 2019), and it may also occur on Ryugu, the resulting fragments are likely below the 30 cm image resolution and thus do not steepen the observed global power index.

The binary asteroid Didymos–Dimorphos is a peculiar case. Didymos's cumulative “size” distribution α value of
$-3.6 \pm 0.7$, with a 22.8 m boulder size limit, is similar to Dimorphos's $\alpha$ value of $-3.4 \pm 1.3$ with a 5 m boulder size limit (Pajola et al. 2024). One possible explanation is that Didymos was formed from an impact-disrupted rubble-pile asteroid and that Dimorphos inherited to a large extent its fragment size distribution from parent Didymos through shedding and reaccumulation (Barnouin et al. 2024). However, the Dimorphos boulder size-frequency distribution resemble rather a Weibull distribution than a power law at boulder sizes smaller than 5 m (Pajola et al. 2024), which is indicative of some subsequent process affecting the ~meter boulder population. Additional impact fragmentation or thermal cracking may be responsible for the destruction of boulders smaller than a few meters being below current imagery resolution and thus shallowing the distribution. There is evidence of thermal cracking from close-up observations of the boulder morphologies of Dimorphos (Lucchetti et al. 2024). However, with respect to the young 0.03–13.3 Ma age of Dimorphos (Barnouin et al. 2024), the thermal cracking simulations (Lucchetti et al. 2024) require longer timescales to destroy small boulders and thus may not be primarily responsible for the observed depletion of ~meter-sized boulders. Our results indicate that the shallow $\alpha$ of $-2.9$ at submeter sizes is also compatible with a single impact disruption at collision velocities below ~1 km s$^{-1}$. Unfortunately, no similar high-resolution observations are available on Didymos, so one cannot detect or discard similar depletion in ~meter-sized boulders on the primary asteroid.

## 5. Conclusions

The relatively slow subsonic (~one-fifth of material $v_p$) impact velocity of the Bjurbôle projectile was sufficient to cause its catastrophic disruption in a single impact event. This was followed by little additional fragmentation, as indicated by the 0.2 mass ratio of the largest surviving Bjurbôle meteorite to the projectile (Bjurbôle terminal body), by a moderate cumulative size distribution power-law index $\alpha$ of $-2.7$ and cumulative mass distribution $\alpha$ of $-1.0$. The Bjurbôle meteorites representing the projectile fragments are homogeneous from centimeter (10 g) up to decimeter (80 kg) size ranges, as revealed through the bulk density measurements. They are rather equidimensional compared to fragments typically originating from target material as observed in impact experiments or on asteroid surfaces (with the asteroid boulder origin being assumed to be predominantly of the fragmented target material). When compared to the boulders observed on visited asteroids, the Bjurbôle projectile fragments are also characterized by slightly lower roughness at large scales and greater roughness at small scales. These differences can be attributed either to different fragment sources (projectile versus target), to the high porosity and low strength of the Bjurbôle material, or to a lower subsonic impact velocity compared to typical impact velocities within the asteroid belt.

Our results imply that efficient catastrophic disruption of projectiles can be achieved even at sub-km s$^{-1}$ velocities, especially with weak materials. Projectile fragments are rather small compared to the original projectile size and may be well mixed within the target fragment population. On the contrary,

projectile fragments tend to be in certain cases (especially for weak projectiles and lower collision velocities) more spherical with smoother large-scale morphology compared to target fragments. No additional significant differences were observed between our decimeter-sized fragment properties and experimentally produced smaller fragments or naturally occurring larger asteroid boulders.

## Acknowledgments

The authors would like to thank the following individuals (listed in alphabetic order) for providing information on or access to meteorite samples in worldwide collections: Dmitry Badjukov, Solveig Bergholm, Natalia Bezaeva, Chloé Brillartz, Rainer Bartoschewitz, Neil Bowles, Kelsey Falquero, Henrik Friis, Matthieu Gounelle, Victor Grokhovsky, James Holstein, Svetlana Janson, Juha Jämbäck, Jarkko Kettunen, Kitty Killgore, Andrei Kosterov, Nikolai Kruglikov, Jörgen Langhof, Jussi Leveinen, Jarmo Moilanen, Emma Nicholls, David Rose, Mikhail Shabalov, Reidar Tronnes, Grigoriy Yakovlev, Luminita Zaharia, and Jutta Zipfel. Information on the location of Bjurbôle material in worldwide collections was gathered from the LUOMUS and MetBase databases. This work was supported by the DART mission, NASA contract 80MSFC20D0004, Academy of Finland project No. 335595, and institutional support RVO 67985831 of the Institute of Geology of the Czech Academy of Sciences. This work was supported by the Italian Space Agency (ASI) within the LICIACube project (ASI-INAF agreement No. 2019-31-HH.0) and HERA project (ASI-INAF agreement No. 2022-8-HH.0). A.D. and N.M. acknowledge funding support from the European Commission's Horizon 2020 research and innovation program under grant agreement No. 870377 (NEO-MAPP project) and the Centre National d'Etudes Spatiales (CNES), focused on the Hera space mission. A.D. acknowledges PhD funding from the University of Toulouse III.

T.K. led the work, participated in the meteorite measurements, and did the majority of data interpretation. A.S. participated in the meteorite measurements and contributed with data interpretation (density and porosity). A.L. contributed with the gathering and analysis of Bjurbôle historic records. A.D. and N.M. contributed with fragment shape analysis. M.P. and A.L. contributed with the fragment SFD and MFD, derived the best fit, and contributed to part of the manuscript and interpretation. R.L. contributed with the peak shock pressure estimate. N.L.C., S.D.R., P.S., O.S.B., and A.S.R. contributed to data interpretation.

## Data Availability

Meteorite measurement data sets, 3D shape models, and metadata are available in the Zenodo repository at doi:10.5281/zenodo.10062980.

## Appendix

This appendix contains Table A1 with physical properties of all Bjurbôle and Chelyabinsk meteorites presented in this study.
<table><caption>**Table A1**
Bulk and Grain Density and Porosity, Together with Global Mean of All Laser-scanned Bjurböle Meteorites (Highlighted in Bold) Including Additional Smaller Meteorites</caption><thead><tr><th>Meteorite</th><th>Mass (g)</th><th>Bulk Volume (cm³)</th><th>Bulk Density (g cm⁻³)</th><th>Grain Volume (cm³)</th><th>Grain Density (g cm⁻³)</th><th>Porosity (%)</th></tr></thead><tbody><tr><td>Helsinki A3747 (4)</td><td>80,200</td><td>29,172.40</td><td>2.75</td><td>...</td><td>3.53</td><td>22.2</td></tr><tr><td>Stockholm 990433</td><td>20,900</td><td>7302.16</td><td>2.86</td><td>...</td><td>3.53</td><td>19.0</td></tr><tr><td>Helsinki D6147 (AL79)</td><td>13,480</td><td>4860.55</td><td>2.77</td><td>...</td><td>3.53</td><td>21.5</td></tr><tr><td>Helsinki D6149 (AL86/33 5413)</td><td>5450</td><td>1877.31</td><td>2.90</td><td>...</td><td>3.53</td><td>17.9</td></tr><tr><td>Helsinki D6150 (TK1)</td><td>5300</td><td>1862.90</td><td>2.85</td><td>...</td><td>3.53</td><td>19.5</td></tr><tr><td>Helsinki D6164 (AL80/5318)</td><td>5150</td><td>1818.94</td><td>2.83</td><td>...</td><td>3.53</td><td>19.9</td></tr><tr><td>Helsinki D6152 (AL80)</td><td>4500</td><td>1585.59</td><td>2.84</td><td>...</td><td>3.53</td><td>19.7</td></tr><tr><td>Helsinki D6148 (AL81)</td><td>4350</td><td>1518.40</td><td>2.86</td><td>...</td><td>3.53</td><td>18.9</td></tr><tr><td>Helsinki D6163 (3130/26)</td><td>3113.7</td><td>1082.62</td><td>2.88</td><td>...</td><td>3.53</td><td>18.6</td></tr><tr><td>Kettuen RH80 (Haag 107) without corner</td><td>2977.6</td><td>1009.85</td><td>2.95</td><td>...</td><td>3.53</td><td>16.6</td></tr><tr><td>Helsinki D6151 (AL2 27)</td><td>1879</td><td>665.49</td><td>2.82</td><td>...</td><td>3.53</td><td>20.1</td></tr><tr><td>Helsinki D6146 (AL68)</td><td>1809.4</td><td>638.79</td><td>2.83</td><td>...</td><td>3.53</td><td>19.8</td></tr><tr><td>Helsinki D6144 (AL32/143)</td><td>1704.8</td><td>589.85</td><td>2.89</td><td>...</td><td>3.53</td><td>18.2</td></tr><tr><td>Porvoo 9184</td><td>1432.8</td><td>503.64</td><td>2.84</td><td>...</td><td>3.53</td><td>19.5</td></tr><tr><td>Aalto 4606</td><td>1364.7</td><td>468.56</td><td>2.91</td><td>...</td><td>3.53</td><td>17.6</td></tr><tr><td>Helsinki D6145 (AL67/152)</td><td>1358.6</td><td>474.43</td><td>2.86</td><td>...</td><td>3.53</td><td>19.0</td></tr><tr><td>Helsinki D6143 (AL31/144)</td><td>1237.9</td><td>441.98</td><td>2.80</td><td>...</td><td>3.53</td><td>20.7</td></tr><tr><td>Stockholm 540022</td><td>1176.7</td><td>404.84</td><td>2.91</td><td>...</td><td>3.53</td><td>17.8</td></tr><tr><td>Porvoo 88-38</td><td>1069.6</td><td>377.45</td><td>2.83</td><td>...</td><td>3.53</td><td>19.8</td></tr><tr><td>Helsinki 1002</td><td>856.5</td><td>300.77</td><td>2.85</td><td>...</td><td>3.53</td><td>19.4</td></tr><tr><td>Helsinki AL63</td><td>754.4</td><td>263.77</td><td>2.86</td><td>...</td><td>3.53</td><td>19.1</td></tr><tr><td>Copenhagen 947</td><td>465.6</td><td>159.84</td><td>2.91</td><td>...</td><td>3.53</td><td>17.6</td></tr><tr><td>Helsinki AL34</td><td>151.75</td><td>53.59</td><td>2.83</td><td>...</td><td>3.53</td><td>19.9</td></tr><tr><td>Helsinki small 4</td><td>21.62</td><td>7.83</td><td>2.76</td><td>6.13</td><td>3.53</td><td>21.7</td></tr><tr><td>Helsinki small 3</td><td>20.89</td><td>7.51</td><td>2.78</td><td>5.92</td><td>3.53</td><td>21.1</td></tr><tr><td>Helsinki small 2</td><td>20.56</td><td>7.41</td><td>2.78</td><td>5.8</td><td>3.54</td><td>21.7</td></tr><tr><td>Helsinki small 1</td><td>17.27</td><td>6.26</td><td>2.76</td><td>4.88</td><td>3.54</td><td>22.0</td></tr><tr><td>Helsinki SC57</td><td>12.252</td><td>4.28</td><td>2.87</td><td>...</td><td>3.53</td><td>18.9</td></tr><tr><td><strong>Mean</strong></td><td>...</td><td>...</td><td><strong>2.84</strong></td><td>...</td><td><strong>3.53</strong></td><td><strong>19.6</strong></td></tr><tr><td>s. d.</td><td>...</td><td>...</td><td>0.05</td><td>...</td><td>0.01</td><td>1.5</td></tr><tr><td>Helsinki A3747 (4) plaster model</td><td>80,200</td><td>29,376.51</td><td>2.73</td><td>...</td><td>3.53</td><td>22.7</td></tr><tr><td>Helsinki A3747 (4) real watertight approximation</td><td>80,200</td><td>28,968.30</td><td>2.77</td><td>...</td><td>3.53</td><td>21.7</td></tr><tr><td>Chelyabinsk small average</td><td>...</td><td>...</td><td>3.29</td><td>...</td><td>3.47</td><td>6.0</td></tr><tr><td>Chelyabinsk big</td><td>502,000</td><td>156,122.2</td><td>3.22</td><td>...</td><td>3.51</td><td>6.6</td></tr><tr><td>Chelyabinsk S</td><td>22,750</td><td>6989.1</td><td>3.26</td><td>...</td><td>3.51</td><td>6.1</td></tr><tr><td>Chelyabinsk V</td><td>5130</td><td>1581.61</td><td>3.24</td><td>...</td><td>3.51</td><td>7.2</td></tr></tbody></table>

**Note.** Additionally listed are the values of the Helsinki A3747 meteorite plaster model, the Helsinki A3747 real meteorite with a watertight approximation of its invisible bottom part, and the Chelyabinsk meteorites from Kohout et al. (2014, 2017) presented for comparison in Figure 3. Note that in the case of Bjurböle, the grain density mean is calculated from the meteorites Helsinki small 1–4 only, and this value is used for other Bjurböle meteorites (indicated by italics) to estimate their porosity. Similarly, the mean grain density of the Chelyabinsk small meteorites is used to estimate the porosity of the three large meteorites.

## ORCID iDs

Tomas Kohout https://orcid.org/0000-0003-4458-3650

Maurizio Pajola https://orcid.org/0000-0002-3144-1277

Assi-Johanna Soini https://orcid.org/0000-0002-3865-2886

Alice Lucchetti https://orcid.org/0000-0001-7413-3058

Arto Luttinen https://orcid.org/0000-0002-3129-0392

Alexia Duchêne https://orcid.org/0000-0002-0385-5755

Naomi Murdoch https://orcid.org/0000-0002-9701-4075

Robert Luther https://orcid.org/0000-0002-0745-1467

Nancy L. Chabot https://orcid.org/0000-0001-8628-3176

Sabina D. Raducan https://orcid.org/0000-0002-7478-0148

Paul Sánchez https://orcid.org/0000-0003-3610-5480

Olivier S. Barnouin https://orcid.org/0000-0002-3578-7750

Andrew S. Rivkin https://orcid.org/0000-0002-9939-9976

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